Carb SBC vs TBI SBC vs Gen 3 LS reliability

[slow_c1500]

On average, how much of an mpg/hp gain are we talking with shorty headers?

 

[RanchWelder]

My TBI Vortec L31 rig has the Sanderson shorty headers. Milage really depends on how hard you drive it and your cam profile. Cam is everything. Start with a good cam or waste your time and money.

Headers do one thing really, and that is allow flow, rather than restrict it. Too much flow is bad too, so finding the sweet spot for TBI headers is tough. Most cheapies flow poorly. $350.00 was a lot of money for my budget for headers and the Sanderson's run and fit great.

You might try to call them and ask if they can FAB you a set for L31 Vortec straight plug heads... They are in San Fran.

They are the only header company I could find with AFFORDABLE off the shelf 1-5/8" primaries to 2-1/2" collectors.

The ones from Sum are not really for straight plugs and you'll want a thicker flame resistant crush gasket to get the extra 1/8-1/4"so your wires run without melting. Lil'bit longer ARP manifold bolts too.
Copper manifold sealer spray... whatever floats your boat.

You'll have to ignore the scary story from Sanderson regarding never using a gasket though...
Guys running circle tracks run them wide open with the gasket setup I run with zero issues or header damage. If I had known they are not for straight plugs and I could afford Sanderson custom's, maybe I would have considered them... It works great as it is. ACDelco OEM wires really makes a difference...

2-1/2" exhaust will sound sweet with high flow headers. You'll want the Vortec L31 heads to breath all the way out the end. 3 wire O2 mod works well off a relay too. Much higher quality sensor and lasts a lot longer than 1 year the one wire is rated for. (Old Technology).

OBD1 is simple and cool to work out with a new harness and a good tune.

Check out Mort for a tune or emissions related advice and a nice clean harness:

CHEVROLET Legacy Complete TBI Kit – Affordable Fuel Injection

affordable-fuel-injection.com

 

[slow_c1500]

When you say mileage can depend on cam profile, does that mean I can get a smaller cam to improve mpg? Or stick with the stock one? Mine is a vortec so it already has a roller cam.

 

[Schurkey]

I don't know where you'd find a "smaller" cam than what's already in the Vortec.

Might be you could find a "better" cam, though.

 

[Scooterwrench]

What your trying to achieve is best cylinder scavenging for a given RPM and cylinder volume. Under scavenging leaves burnt exhaust in the cylinder reducing VE which effects power and economy. Over scavenging pulls incoming air/fuel down the pipes wasting that part of the charge with the same effect. Header tube length is tailored to to the RPM you intend the run the engine at and dia. is tailored to cylinder volume and exh. port size. Exhaust pulses flow through the pipes at the speed of sound. When the exhaust valve opens it creates a pressure wave that starts down the pipe. As that wave travels down the pipe it creates a negative pressure behind it. That negative pressure is what scavenges the cyl.
It has been determined long ago that 1-5/8" primary tubes work best for the size of the SBC exhaust port so that equation has been done for you. Now you need to determine primary tube length. The exhaust valve is going to be open for a certain amount of time(duration)for a given RPM. The RPM at the peak of the torque curve is a good number to shoot for as this will give you a broader range of scavenging for that cam. Short tubes work better at higher RPM because the exhaust pulse frequency is faster whereas long tube primaries work better at lower RPM because the pulse frequency is slower.
If you were building headers from scratch you would do the math to determine what the optimum primary tube length would be for the RPM range your going to run and how long the exhaust valve is open at that RPM multiplied by the speed of sound.
Because most of us don't build our own headers we're stuck with either long or short. For real world daily driving with the occasional passing of a slow poke 38-42" primaries works best for us.
Shorties are a little better than iron logs but most of us aren't going to be turning the motor fast enough to make them work to their full potential. Dyno numbers can be impressive but rarely achieved out there on the road.

 

[slow_c1500]

I have heard that shorty headers work better for lower rpm, and long tubes work better in the higher rpm’s. Is that what you meant? In the post you had them switched.

 

[RanchWelder]

Scooterwrench has good explanation to the dynamics at work, either way you want to go.

He explained basic theory and physics, so your brain would hopefully stop over-thinking what can become years of expensive research and development. Length tuning gets into a lot of math and money for small gains, which you may never need, unless you get nerded out with thousands of dollars spent at the dyno and start a youtube channel racing for money. $$$$

He confirmed the "tubing diameter" recommended for a good flowing SBC. Few manufacturers offer the correct size and now you know basically why; diameter, above every other option, matters to your street truck exhaust flow. (...at street-able performance levels.)

All we answered with certainty, is where to find 1-5/8" headers that work and will last and how to mod them so they work without burned wiring on your 5.7L Vortec SBC.

If you extrapolated the fluid dynamics flow numbers, using 4 cylinders and 1-5/8"primary tubes, you now have a clue why 2-1/2" collectors and exhaust tubing matches mild to medium cam street engines, as well.

Short headers are plenty for most street engines, so don't get hung up with hard to fit long tubes, unless you have a special tuning demand proving they are needed.

Pulling or installing your engine with headers mounted and torqued can be accomplished a lot easier with short headers.

Until you select your cam and rpm range, the other differences are for the most part, unknowable.

Step one: Go select your machinist... (speak with a few of them).
Your machinist will determine your cam selection based upon the answers you provide to his/her questions, (...not so much yours...).

Your cam is the most single important decision you make together.
Choose wisely and double your research before you start.
You must pass emissions, so keep it real...

Your stock roller cam might be able to be re-ground, but not if it's in your daily driver engine....
Blanks are getting hard to come by and prices have gone up with quality declining.
Re-grinding is becoming very popular.

It sounds like you should consider asking your new machinist team to select you a good quality inspected used block.

The sound of building your spare engine in your garage makes sense to me, if you need to keep your rig on the road. Have them source you a good engine, if you are uncertain what to look for or do not have a trusted source worked out.

The junk yards might try to push you around or sell you a dud.

Many yards sell the quality used blocks to professional engine re-builders; exclusively.
The last thing you want is to find a broken bolt or casting, cracked head, or internal damage hidden somewhere. Blocks with heat damage can be warped. If the one they offer you was rebuilt, you have no idea what you are getting into.

You can drive hundreds of miles and find really expensive OEM blocks (which still require a lot of work) or everything in your price range worthless with scored cylinders and bent valves. The thirty day warranty means driving the damn thing back IF you do not find internal damages before the end of the month.

Unless you have the calipers and dial indicators, you can get skunked.

Your relationship with your machinist might last for decades, so choose wisely.
Select only good to very good quality parts, after careful research.
Re-using your existing parts can save money but comes with some added risk.
(Mean time before failure is always an issue to be contended with, on any budget build).

Find your new engine machine team, then come back and let us know what you have come up with... and watch the comments carefully for clues to save a buck or two without sacrificing quality and improve your odds of success.

 

[Pinger]

Exhaust pulses flow through the pipes at the speed of sound. When the exhaust valve opens it creates a pressure wave that starts down the pipe. As that wave travels down the pipe it creates a negative pressure behind it. That negative pressure is what scavenges the cyl.

Only when the pulse encounters a change (increase) of pipe diameter is there a negative pulse.

 

[Scooterwrench]

Only when the pulse encounters a change (increase) of pipe diameter is there a negative pulse.

Negative pressure,not negative pulse. I think what you are thinking is reversion when the pressure wave moves back up the pipe.
The negative pressure is easy to prove. Take a water hose and blow down through it then quickly stop up the hose with your tongue. You will feel the suction on your tongue caused by the negative pressure. You will also feel the reversion. It's kind of like the air is bouncing on a rubber band but it's actually the air compressing and contracting. Air has weight so moving air has inertia. The inertia of the air moving through a pipe is what creates the negative pressure(vacuum)behind it. The inertia of moving air is also what helps fill the cylinder during the intake stroke. Many people don't know it but an internal combustion engine has two compression ratios. The static compression and the dynamic compression. The static compression is cyl. volume at BDC vs volume at TDC which is an easily calculable figure. Dynamic compression is created by the inertia of air filling the cylinder on the intake stroke. This is where long intake runners and velocity stacks really pays off,gets the air moving fast,creates a lot of inertia and packs the cylinder tight. Long tube headers do the same thing with scavenging.

 

[Erik the Awful]

I have heard that shorty headers work better for lower rpm, and long tubes work better in the higher rpm’s.

Mostly, no. Shorties are compromise headers. Our trucks aren't hurting for room for headers, so your debate is really stock manifolds versus long tubes. If you go with headers and you're not building a max hp engine, go with 1 5/8" primaries and a 2.5" collector.