Devs93
OBS 4 EVA
Copied and pasted this from another site tons of awsome info
I’ve been on the forum long enough to notice the same questions popping up again and again. Although the forum has a search feature, it is often easier to just ask your question. That's what the forum is for. Well to lessen repeat questions, I've attempted to gather together some great discussions on TBI- and Vortec-related performance topics into one big post – perhaps the ultimate post. I have not limited myself to this board and have included very useful discussions/info from other websites. I have used a running FAQ-type commentary to minimize reading through a ton of posts and so that I can share with you my experiences and expertise. Hopefully, this thread will serve as an important resource for those of you who are new to OBS trucks and serve as future reference for the FSC “old timers”. Feel free to offer suggestions or add your own Q/A entries to the appropriately entitled...
ULTIMATE 88-98 OBS PERFORMANCE TOOLBOX
What is TBI?
TBI stands for throttle body injection. This was one of the first mass-produced fuel injection systems on GM trucks starting in 1987 for the final year of the square box body style. GM introduced a redesigned body style the following year in 1988, which would last 11 years from 1988 to 1998. TBI-equiped motors included the 4.3L V6, 5.0L, 5.7L and 7.4L engines. The 7.4L version aptly resided in 454SS trucks and production ran from 1990-1993. The 5.7L TBI engines were produced from 1987 to 1995, with power levels that ranged from 185-210hp. These pre-Vortec engines used hydraulic non-roller camshafts.
What’s a Vortec engine?
In 1996, GM introduced the 5.0L and 5.7L Vortec engines were that included several key enhancements: hydraulic roller camshaft, new higher flow Vortec cylinder heads, central sequential fuel injection (CSFI), OBDII, etc. These enhancements allowed the 5.7L Vortec engine to produce 250 hp and 320 lbs-ft of torque. Production of these engines ran until 1998 when GM redesigned the Chevy and GMC trucks for the 1999 model year, ending the smooth body style now referred to as the old body style (OBS).
Are there any cheap ways to make more power and get better fuel economy?
Well it depends on how much of the work you can do yourself. There are free or relatively cheap mods for sure, like the infamous reverse air lid trick on TBI engines, the ultimate TBI mods, and making your own AFPR for your TBI engine. You can also learn how to burn your own PROMs for a small investment that will pay you back huge as you keep upgrading your motor with better fuel economy and power. But unfortunately, more power requires a significant investment in time and money. Speed costs money…How fast do you want to go?
Better fuel economy can come with more power. How? Because it takes less throttle to get going and less pedal to keep you going. On the highway, you’ll especially notice the difference when your engine doesn’t have to downshift in order to get up hills. It is power that you can feel and it will save you money at the pump. The other factor is driving-style. Light throttle saves you money at the pump. Not always easy, but that’s the key to saving money on fuel costs. Let’s be honest here. V8 engines, despite their “advanced” fuel injection, are only capable of about 17-18 miles per gallon on the highway. If you wanted fuel economy, you would have bought a hybrid car instead.
What should be the first thing I do to my TBI/Vortec engine?
The first thing you should do to your engine is to make sure it has a complete tune up and is in proper running condition. This means replacement of the spark plugs and wires along with a new distributor cap, and rotor. The TBI or throttle body unit should also be cleaned thoroughly. This requires taking it off and cleaning all of the small passages that run along the bottom side of it. Cleaning or replacing the PVC valve is also crucial. You will also want to check your serpentine belt and coolant and vacuum hoses. They get old and crack and break down over time and this would be a good time to replace them. Finally, you will want to check your timing. The factory setting for the L05 (TBI) 5.7L engine is 0* but check your engine’s emission sticker under the hood for the factory timing setting for your truck. You can advance the base timing if you want, but timing is mainly controlled by the engine’s computer. A custom chip will add more timing under the curve to improve low RPM torque, so adjusting the base timing is pointless. You may have to run a higher octane fuel though so you will have to consider that. After your truck has received a full tune up you can start to consider your first modifications.
What is involved in a performance tune-up? What are some regular maintenance items to replace?
A complete tune-up includes: new air filter (cleaned reusable filter), new distributor cap/rotor, new spark plugs set at the appropriate gap, test and replace faulty spark plug wires, check timing, check and/or replace PCV valve, check and replace faulty vacuum hoses, check coolant level, oil/oil filter change, check transmission fluid level, check steering pump and brake fluid levels, check windshield washer fluid, check and readjust TV transmission cable (pre-93 trucks), remove slack from throttle linkage, clean terminals on battery.
Some common replacement items in general and preventative maintenance: Fuel filter, O2 sensor(s) (replace after 65,000-80,000 miles), PVC valve, belts, wiper blades.
I want to clean my IAC because I’ve heard that it can affect engine idle. How can I do this?
The quick way: remove the IAC and lightly spray carb cleaner on it and inside the chamber. Wipe the residue off with a shop towel. Reinstall.
The detailed step-by-step, careful approach:
1). Remove the IAC. It can be found on the pass side of the TBI.
2). After you have done so, reconnect the IAC connector and place a bag or towel around the IAC so the pintle doest go flying off. Once this is done, jumper pins 'A' + 'B' in the ALDL connector. Once jumpered, turn the key to the ON position, but don't start the car. The ecm will eject the IAC pintle. Turn off the ignition and remove the IAC and its components.
3). Next step is to clean the parts. I like using lighterfluid or naphtha. Cleans and leaves no unpleasant odor once it evaporates. Use a toothbrush to remove all the carbon deposits from the guide, spring, and the threads on the pintle shaft. Once dry lightly coat the pintle shaft and guide with a light grease or oil.
4). Reassemble. Place the spring over the pintle and place the pintle shaft in the guide slot in the IAC. It wont just go in as there is an armature with gear teeth on it. Screw the IAC pintle in until the guide slots on the pintle shaft just meet up with the guides in the IAC main body. From there gently rock the IAC pintle back and fourth while applying light pressure until the pintle gets partially back into the IAC.
5.) Reinstall the IAC. Screw it in by hand and if you feel any sort of resistance, STOP!!! The pintle has bottomed out. Reconnect the IAC and pull the jumper out of the ALDL if you have not yet done so. Lightly depress the gas pedal and start the car. Let it run for about 30 secs and shut down. Hand tighten the IAC and add about 1/16 additional turn is about all that's necessary. And you’re done. The vehicle is ready for use.
How do I adjust my TPS?
On TBI engines, the TPS is not adjustable. Just leave it alone. You can test it for proper operation using a volt meter, but there is no need to adjust it. Here’s a link that will help you to test your TPS using a digital volt meter: http://autorepair.about.com/cs/faqs/l/aa042603a_1.htm. It also includes instructions on how to test other sensors for proper operation and may help to fix common engine problems.
Will using higher octane fuel add power?
No. If your 350 TBI is stock and running the stock base timing setting of 0* advanced, then there is no need to run higher than 87. Even with 0* you could possibly still hear knock. This could be caused by excess carbon deposits that are creating detonation usually found in high mileage motors. Seafoam top engine cleaner or equivalent (GM top engine cleaner) can be used to remove these deposits and prevent detonation. Higher octane fuel may be required for engines that are running high compression ratios or advanced timing curves from a custom chip or tune. Running higher octane fuel than is needed for your application will not clean your engine or add power to it. In some cases it may actually decrease power because of an incomplete combustion.
Now that my engine has a tune up, what areas should I start to mod/upgrade?
An extremely popular first mod is removing the stock air intake snorkel assembly and replacing it with an open air element or cold air intake (CAI) kit from K&N, AirRaid, etc. An open element will sit flat on top of your TBI unit and allow you to remove the spacer ring that surrounds the perimeter of the top of your TBI unit. Before you look into serious mods like heads and a cam you need to address the other weak points of the truck, such as the exhaust system and fuel system. A complete exhaust upgrade from the exhaust ports back should be installed as the first major mod to take the full advantage of future upgrades. Also, the stock fuel pump will not be able to effectively support the power your motor will make with a new cam and heads. Installing a larger fuel pump will not affect the fuel pressure of your current setup since the pressure is modulated by the fuel pressure regulator, not the pump. An adjustable fuel pressure regulator and a fuel gauge should be added to adjust fuel to match your power upgrades as you go along. Lastly, you will want a performance chip/tune. The best approach is burn your own chips, but there is a steep learning curve. The next best option is to have tuners such as Wester’s Garage, Wait4Me, TBI Chips, etc, burn you a custom PROM for your application, particularly if you’ve changed the displacement, heads, and/or cam on your engine. On a mostly stock engine, chips and tunes from Jet Performance, Hypertech, etc. will work fine.
How do I do the ultimate TBI mods and are they worth it?
The ultimate TBI mods are a set of modifications that improve the airflow and fuel into your engine. They include: removing the ridge on the injector side of the TB unit, shaving the throttle shafts, installing an injector pod spacer. There are two great tech articles on how to do this below and they are pretty straight forward once you have a grinder and TBI unit in your hands. It will improve airflow by about 5%, but more importantly unshrouds the injectors for a cleaner air entry + fuel atomization. This added flow will help at higher RPMs. Companies like CFMTech and XtremeFI also manufacturer TB with larger than stock 46mm holes (48mm and 50mm). These units already include the ultimate TBI mods mentioned performed on the TB.
Ultimate TBI mods instructions: http://www.thirdgen.org/ultimatetbi and http://www.thirdgen.org/ultimatetbi2
Pics of the ultimate TBI mods: http://www.thirdgen.org/techboard/tb...owing-tbi.html
Will a larger TB make more power?
IMO this should be your last upgrade. I’ll say it one more time: most people overestimate the CFM requirements of their engine. For a more accurate estimate: use the Wallace Racing CFM calculator: http://www.wallaceracing.com/intakecfm.php. I like it because it also takes into account the type of intake manifold. Engine CFM requirements highly depend on the Volumetric Efficiency (VE) of your engine at red line. It automatically calculates the compensatory 120% to 150% of the engine's base requirement using the dual plane intake. If you were to use a single plane intake (yes one exists - the Edelbrock MPFI unit is a single plane design), the percentage over the engine's base CFM requirement is between 110%-130%. Most factory engines are between 65-75% VE at redline. On a mildly upgraded 350 engine (i.e. headers, exhaust, chip) with stock heads and cam, the VE hits about 72%. If you were to change cams, you're looking at about 78% VE. Change heads and cam, about 85-88% VE.
Stock TB cfm on a 350 engine is 490 cfm at 1.5inHg. Redline on the stock engine (or mine at least) is 4500-4750 RPM. Like I said before, the stock engine's VE is about 66%. Input the numbers into the calculator and what do you get: 381 to 476 cfm. So that's why the engineers at GM chose the size of TB that they did. Now let's now say you've changed the cam plus all the other performance goodies, redline now at 5000 RPM, stock heads, VE now at 78%: 474 to 592 cfm required. You could still use the stock TB easily with no major bottleneck in power. Ultimate TB mods would give you 520 cfm. Even with a cam change and other performance goodies, the stock TB is still pretty decent. You could upgrade to the CFM-Tech unit rated at 590-620 cfm if you wanted to.
What if you changed heads/cam now and redline at 5600 RPM, VE now at 88%: You'd need 613 to 766 cfm. The CFM-Tech TB would work, but the larger 50mm TB would be better. It's rated at 690-750cfm, depending if you believe the upper figure quoted by XtremeFI or not. The 454 TB (also 50mm) is rated at 670 cfm at 1.5inHg w/o the injector pod according to airdeano's tests. I'd tend to believe airdeano's numbers more because they're pretty bang on: http://www.fullsizechevy.com/forums/...d.php?t=145234.
A 383, stock heads, with cam, Redline at 5000 RPM, VE again at 78%: 519 to 648 cfm. Either the CFM-Tech or XtremeFI TB would work. The stock TB even with the ultimate TBI mods (520 cfm) would be too small IMO.
The bottom line is: calculate the CFM requirements of your particular engine. Most likely, you won’t need to buy a larger TB unless you’ve made extensive upgrades to your engine. The ultimate TBI mods are relatively easy to do yourself and will increase your TB airflow to about 520 cfm. More than enough to support your performance upgrades. (Note that the required cfm of your engine has nothing to do with hp!!)
What is a TBI spacer and does it add power?
A TBI spacer is just what it sounds like. It is a piece of plastic, wood, phenolic resin, or aluminum that bolts in between the TBI unit and intake manifold. It helps increase the intake manifold plenum size and is designed to add power. It also acts as a thermal insulator between the TBI unit and intake manifold. Resin and plastic materials may be more effective than aluminum and since they will not conduct as much heat into the TBI unit. There are two paper gaskets between the TB and the intake manifold so you be the judge. Most people claim that they have improved throttle response and mid-range torque after installing their spacer. A TBI spacer usually requires a little tweaking of the throttle brackets and TV cables (for pre-93 trannys) to keep it from pulling the throttle open. Make sure you properly adjust your TV cable. Failure to do so could result in transmission failure. Any service manual has an instruct set on how to properly adjust your cable. An on-line service manual is available from Autozone’s website: http://autozone.com/servlet/UiBroker...Parameters=005
Also, keep in mind that if you upgrade to a larger TB, it is a good idea to have your throttle body spacer and intake manifold machined to match your intake tract.
What is an injector pod spacer and do they add power?
An injector pod spacer is a plastic spacer that raises your injector pod up 1/4" to unshroud the injectors for better air and fuel delivery. They can be found on ebay and are also sold by Jegs and Summit Racing. This mod is considered a component of the ultimate TBI upgrade. Do they add power? No dyno tests have ever proved it but when combined with the Ultimate TBI mods, your truck engine will gain both fuel economy and power: http://www.cfm-tech.com/trucksarticle.htm
How much power can my stock fuel pump support?
The answer is not much. On the typical 350 TBI engine with an AFPR, up to 240 hp. Past this, and you need larger injectors, more fuel pressure, and/or a larger fuel pump. A high flow unit will supply enough power to your TBI system no matter what mods you go with. Popular choices include, stock GM TPI pumps, Walbro 190lph and 255lph, and the Holley 255lph. Each of these pumps can support gobs of power but will work safely with your TBI set-up because your fuel pressure regulator will keep the fuel entering into your TBI unit where it needs to be. For Vortec engines, fuel injectors are rated at 19#/hr at 42.5 psi, but the stock fuel pressure is actually at 60 psi (22-23 #/hr). Max hp is about 330 hp because of the limitations of the fuel injectors. There is some discussion that some Vortec injectors make be underrated and may flow up to 28#/hr at 60 psi. This would support around 400 hp. Unfortunately, there are no larger replacement fuel injectors for Vortec engines or direct-fit AFPR that can boost fuel pressures to support more hp. Some Vortec owners have gone so far as to replacement their Vortec induction with the marine version, which uses 24#/hr fuel injectors. This way, the induction system won’t be the bottleneck in making big power.
I’ve been on the forum long enough to notice the same questions popping up again and again. Although the forum has a search feature, it is often easier to just ask your question. That's what the forum is for. Well to lessen repeat questions, I've attempted to gather together some great discussions on TBI- and Vortec-related performance topics into one big post – perhaps the ultimate post. I have not limited myself to this board and have included very useful discussions/info from other websites. I have used a running FAQ-type commentary to minimize reading through a ton of posts and so that I can share with you my experiences and expertise. Hopefully, this thread will serve as an important resource for those of you who are new to OBS trucks and serve as future reference for the FSC “old timers”. Feel free to offer suggestions or add your own Q/A entries to the appropriately entitled...
ULTIMATE 88-98 OBS PERFORMANCE TOOLBOX
What is TBI?
TBI stands for throttle body injection. This was one of the first mass-produced fuel injection systems on GM trucks starting in 1987 for the final year of the square box body style. GM introduced a redesigned body style the following year in 1988, which would last 11 years from 1988 to 1998. TBI-equiped motors included the 4.3L V6, 5.0L, 5.7L and 7.4L engines. The 7.4L version aptly resided in 454SS trucks and production ran from 1990-1993. The 5.7L TBI engines were produced from 1987 to 1995, with power levels that ranged from 185-210hp. These pre-Vortec engines used hydraulic non-roller camshafts.
What’s a Vortec engine?
In 1996, GM introduced the 5.0L and 5.7L Vortec engines were that included several key enhancements: hydraulic roller camshaft, new higher flow Vortec cylinder heads, central sequential fuel injection (CSFI), OBDII, etc. These enhancements allowed the 5.7L Vortec engine to produce 250 hp and 320 lbs-ft of torque. Production of these engines ran until 1998 when GM redesigned the Chevy and GMC trucks for the 1999 model year, ending the smooth body style now referred to as the old body style (OBS).
Are there any cheap ways to make more power and get better fuel economy?
Well it depends on how much of the work you can do yourself. There are free or relatively cheap mods for sure, like the infamous reverse air lid trick on TBI engines, the ultimate TBI mods, and making your own AFPR for your TBI engine. You can also learn how to burn your own PROMs for a small investment that will pay you back huge as you keep upgrading your motor with better fuel economy and power. But unfortunately, more power requires a significant investment in time and money. Speed costs money…How fast do you want to go?
Better fuel economy can come with more power. How? Because it takes less throttle to get going and less pedal to keep you going. On the highway, you’ll especially notice the difference when your engine doesn’t have to downshift in order to get up hills. It is power that you can feel and it will save you money at the pump. The other factor is driving-style. Light throttle saves you money at the pump. Not always easy, but that’s the key to saving money on fuel costs. Let’s be honest here. V8 engines, despite their “advanced” fuel injection, are only capable of about 17-18 miles per gallon on the highway. If you wanted fuel economy, you would have bought a hybrid car instead.
What should be the first thing I do to my TBI/Vortec engine?
The first thing you should do to your engine is to make sure it has a complete tune up and is in proper running condition. This means replacement of the spark plugs and wires along with a new distributor cap, and rotor. The TBI or throttle body unit should also be cleaned thoroughly. This requires taking it off and cleaning all of the small passages that run along the bottom side of it. Cleaning or replacing the PVC valve is also crucial. You will also want to check your serpentine belt and coolant and vacuum hoses. They get old and crack and break down over time and this would be a good time to replace them. Finally, you will want to check your timing. The factory setting for the L05 (TBI) 5.7L engine is 0* but check your engine’s emission sticker under the hood for the factory timing setting for your truck. You can advance the base timing if you want, but timing is mainly controlled by the engine’s computer. A custom chip will add more timing under the curve to improve low RPM torque, so adjusting the base timing is pointless. You may have to run a higher octane fuel though so you will have to consider that. After your truck has received a full tune up you can start to consider your first modifications.
What is involved in a performance tune-up? What are some regular maintenance items to replace?
A complete tune-up includes: new air filter (cleaned reusable filter), new distributor cap/rotor, new spark plugs set at the appropriate gap, test and replace faulty spark plug wires, check timing, check and/or replace PCV valve, check and replace faulty vacuum hoses, check coolant level, oil/oil filter change, check transmission fluid level, check steering pump and brake fluid levels, check windshield washer fluid, check and readjust TV transmission cable (pre-93 trucks), remove slack from throttle linkage, clean terminals on battery.
Some common replacement items in general and preventative maintenance: Fuel filter, O2 sensor(s) (replace after 65,000-80,000 miles), PVC valve, belts, wiper blades.
I want to clean my IAC because I’ve heard that it can affect engine idle. How can I do this?
The quick way: remove the IAC and lightly spray carb cleaner on it and inside the chamber. Wipe the residue off with a shop towel. Reinstall.
The detailed step-by-step, careful approach:
1). Remove the IAC. It can be found on the pass side of the TBI.
2). After you have done so, reconnect the IAC connector and place a bag or towel around the IAC so the pintle doest go flying off. Once this is done, jumper pins 'A' + 'B' in the ALDL connector. Once jumpered, turn the key to the ON position, but don't start the car. The ecm will eject the IAC pintle. Turn off the ignition and remove the IAC and its components.
3). Next step is to clean the parts. I like using lighterfluid or naphtha. Cleans and leaves no unpleasant odor once it evaporates. Use a toothbrush to remove all the carbon deposits from the guide, spring, and the threads on the pintle shaft. Once dry lightly coat the pintle shaft and guide with a light grease or oil.
4). Reassemble. Place the spring over the pintle and place the pintle shaft in the guide slot in the IAC. It wont just go in as there is an armature with gear teeth on it. Screw the IAC pintle in until the guide slots on the pintle shaft just meet up with the guides in the IAC main body. From there gently rock the IAC pintle back and fourth while applying light pressure until the pintle gets partially back into the IAC.
5.) Reinstall the IAC. Screw it in by hand and if you feel any sort of resistance, STOP!!! The pintle has bottomed out. Reconnect the IAC and pull the jumper out of the ALDL if you have not yet done so. Lightly depress the gas pedal and start the car. Let it run for about 30 secs and shut down. Hand tighten the IAC and add about 1/16 additional turn is about all that's necessary. And you’re done. The vehicle is ready for use.
How do I adjust my TPS?
On TBI engines, the TPS is not adjustable. Just leave it alone. You can test it for proper operation using a volt meter, but there is no need to adjust it. Here’s a link that will help you to test your TPS using a digital volt meter: http://autorepair.about.com/cs/faqs/l/aa042603a_1.htm. It also includes instructions on how to test other sensors for proper operation and may help to fix common engine problems.
Will using higher octane fuel add power?
No. If your 350 TBI is stock and running the stock base timing setting of 0* advanced, then there is no need to run higher than 87. Even with 0* you could possibly still hear knock. This could be caused by excess carbon deposits that are creating detonation usually found in high mileage motors. Seafoam top engine cleaner or equivalent (GM top engine cleaner) can be used to remove these deposits and prevent detonation. Higher octane fuel may be required for engines that are running high compression ratios or advanced timing curves from a custom chip or tune. Running higher octane fuel than is needed for your application will not clean your engine or add power to it. In some cases it may actually decrease power because of an incomplete combustion.
Now that my engine has a tune up, what areas should I start to mod/upgrade?
An extremely popular first mod is removing the stock air intake snorkel assembly and replacing it with an open air element or cold air intake (CAI) kit from K&N, AirRaid, etc. An open element will sit flat on top of your TBI unit and allow you to remove the spacer ring that surrounds the perimeter of the top of your TBI unit. Before you look into serious mods like heads and a cam you need to address the other weak points of the truck, such as the exhaust system and fuel system. A complete exhaust upgrade from the exhaust ports back should be installed as the first major mod to take the full advantage of future upgrades. Also, the stock fuel pump will not be able to effectively support the power your motor will make with a new cam and heads. Installing a larger fuel pump will not affect the fuel pressure of your current setup since the pressure is modulated by the fuel pressure regulator, not the pump. An adjustable fuel pressure regulator and a fuel gauge should be added to adjust fuel to match your power upgrades as you go along. Lastly, you will want a performance chip/tune. The best approach is burn your own chips, but there is a steep learning curve. The next best option is to have tuners such as Wester’s Garage, Wait4Me, TBI Chips, etc, burn you a custom PROM for your application, particularly if you’ve changed the displacement, heads, and/or cam on your engine. On a mostly stock engine, chips and tunes from Jet Performance, Hypertech, etc. will work fine.
How do I do the ultimate TBI mods and are they worth it?
The ultimate TBI mods are a set of modifications that improve the airflow and fuel into your engine. They include: removing the ridge on the injector side of the TB unit, shaving the throttle shafts, installing an injector pod spacer. There are two great tech articles on how to do this below and they are pretty straight forward once you have a grinder and TBI unit in your hands. It will improve airflow by about 5%, but more importantly unshrouds the injectors for a cleaner air entry + fuel atomization. This added flow will help at higher RPMs. Companies like CFMTech and XtremeFI also manufacturer TB with larger than stock 46mm holes (48mm and 50mm). These units already include the ultimate TBI mods mentioned performed on the TB.
Ultimate TBI mods instructions: http://www.thirdgen.org/ultimatetbi and http://www.thirdgen.org/ultimatetbi2
Pics of the ultimate TBI mods: http://www.thirdgen.org/techboard/tb...owing-tbi.html
Will a larger TB make more power?
IMO this should be your last upgrade. I’ll say it one more time: most people overestimate the CFM requirements of their engine. For a more accurate estimate: use the Wallace Racing CFM calculator: http://www.wallaceracing.com/intakecfm.php. I like it because it also takes into account the type of intake manifold. Engine CFM requirements highly depend on the Volumetric Efficiency (VE) of your engine at red line. It automatically calculates the compensatory 120% to 150% of the engine's base requirement using the dual plane intake. If you were to use a single plane intake (yes one exists - the Edelbrock MPFI unit is a single plane design), the percentage over the engine's base CFM requirement is between 110%-130%. Most factory engines are between 65-75% VE at redline. On a mildly upgraded 350 engine (i.e. headers, exhaust, chip) with stock heads and cam, the VE hits about 72%. If you were to change cams, you're looking at about 78% VE. Change heads and cam, about 85-88% VE.
Stock TB cfm on a 350 engine is 490 cfm at 1.5inHg. Redline on the stock engine (or mine at least) is 4500-4750 RPM. Like I said before, the stock engine's VE is about 66%. Input the numbers into the calculator and what do you get: 381 to 476 cfm. So that's why the engineers at GM chose the size of TB that they did. Now let's now say you've changed the cam plus all the other performance goodies, redline now at 5000 RPM, stock heads, VE now at 78%: 474 to 592 cfm required. You could still use the stock TB easily with no major bottleneck in power. Ultimate TB mods would give you 520 cfm. Even with a cam change and other performance goodies, the stock TB is still pretty decent. You could upgrade to the CFM-Tech unit rated at 590-620 cfm if you wanted to.
What if you changed heads/cam now and redline at 5600 RPM, VE now at 88%: You'd need 613 to 766 cfm. The CFM-Tech TB would work, but the larger 50mm TB would be better. It's rated at 690-750cfm, depending if you believe the upper figure quoted by XtremeFI or not. The 454 TB (also 50mm) is rated at 670 cfm at 1.5inHg w/o the injector pod according to airdeano's tests. I'd tend to believe airdeano's numbers more because they're pretty bang on: http://www.fullsizechevy.com/forums/...d.php?t=145234.
A 383, stock heads, with cam, Redline at 5000 RPM, VE again at 78%: 519 to 648 cfm. Either the CFM-Tech or XtremeFI TB would work. The stock TB even with the ultimate TBI mods (520 cfm) would be too small IMO.
The bottom line is: calculate the CFM requirements of your particular engine. Most likely, you won’t need to buy a larger TB unless you’ve made extensive upgrades to your engine. The ultimate TBI mods are relatively easy to do yourself and will increase your TB airflow to about 520 cfm. More than enough to support your performance upgrades. (Note that the required cfm of your engine has nothing to do with hp!!)
What is a TBI spacer and does it add power?
A TBI spacer is just what it sounds like. It is a piece of plastic, wood, phenolic resin, or aluminum that bolts in between the TBI unit and intake manifold. It helps increase the intake manifold plenum size and is designed to add power. It also acts as a thermal insulator between the TBI unit and intake manifold. Resin and plastic materials may be more effective than aluminum and since they will not conduct as much heat into the TBI unit. There are two paper gaskets between the TB and the intake manifold so you be the judge. Most people claim that they have improved throttle response and mid-range torque after installing their spacer. A TBI spacer usually requires a little tweaking of the throttle brackets and TV cables (for pre-93 trannys) to keep it from pulling the throttle open. Make sure you properly adjust your TV cable. Failure to do so could result in transmission failure. Any service manual has an instruct set on how to properly adjust your cable. An on-line service manual is available from Autozone’s website: http://autozone.com/servlet/UiBroker...Parameters=005
Also, keep in mind that if you upgrade to a larger TB, it is a good idea to have your throttle body spacer and intake manifold machined to match your intake tract.
What is an injector pod spacer and do they add power?
An injector pod spacer is a plastic spacer that raises your injector pod up 1/4" to unshroud the injectors for better air and fuel delivery. They can be found on ebay and are also sold by Jegs and Summit Racing. This mod is considered a component of the ultimate TBI upgrade. Do they add power? No dyno tests have ever proved it but when combined with the Ultimate TBI mods, your truck engine will gain both fuel economy and power: http://www.cfm-tech.com/trucksarticle.htm
How much power can my stock fuel pump support?
The answer is not much. On the typical 350 TBI engine with an AFPR, up to 240 hp. Past this, and you need larger injectors, more fuel pressure, and/or a larger fuel pump. A high flow unit will supply enough power to your TBI system no matter what mods you go with. Popular choices include, stock GM TPI pumps, Walbro 190lph and 255lph, and the Holley 255lph. Each of these pumps can support gobs of power but will work safely with your TBI set-up because your fuel pressure regulator will keep the fuel entering into your TBI unit where it needs to be. For Vortec engines, fuel injectors are rated at 19#/hr at 42.5 psi, but the stock fuel pressure is actually at 60 psi (22-23 #/hr). Max hp is about 330 hp because of the limitations of the fuel injectors. There is some discussion that some Vortec injectors make be underrated and may flow up to 28#/hr at 60 psi. This would support around 400 hp. Unfortunately, there are no larger replacement fuel injectors for Vortec engines or direct-fit AFPR that can boost fuel pressures to support more hp. Some Vortec owners have gone so far as to replacement their Vortec induction with the marine version, which uses 24#/hr fuel injectors. This way, the induction system won’t be the bottleneck in making big power.