Thanks, as I said I am just putting a good used 350 back on line, as the timing set is stock and worn I will be replacing it and as the engine is roller cam ready block even drilled and tapped for the spider, I want to slightly improve it as a low RPM truck cam (G20 Van) engine with a ROLLER CAM.
As far my 383 it is has been built for a special use, and a high MPG low RPM van engine.
This is my write up on my 383: The ideas I give are based on all my years of reading and from a Book called:
RX for RV performance and Mileage By John Geraghty and Bill Extes published by Trailer Life in 1986.
In is in great depth as to how to maximize MPG and power to move Motor Homes and trucks towing RVs. Although old the only thing missing is fuel injection, but their chapters on engine tuning covers the subject well with old timing systems and carbs. These “Facts” can be translated to tuning a fuel injection system easily and what is true for a carb is also true for an injection system.
It is my basic bible and all data have been cross checked and I have found that no article has ever disproved what they wrote, FOR MAX MPG in a RV. In these vehicles Torque is KING.
The big changes I made is to have my cake and to eat it too: Rhoads lifters giving me a Clark Kent cam at 1500RPMs and Superman at 3000RPM (well a Mild power cam by hot rodding standards)
Then as for gearing I am running a Motor Home 4L80E with a NOS US gear Dual Range Overdrive allowing me to run my stock 3.43 gears or even go to 3.73 if it checks out and still cruse at my cams lower torque peek at 1500/2000 at 80/85…This combo is strong enough for me to even tow..
AND because nearly everything I have read and talked with people whom run them I will be running an 87 TPI intake. These are reported to add around 30% MORE torque, MPG, and HP by themselves alone.
My target is 20MPG or better, and to ad that I will be running an OBDI PCM with Lean Burn Cruse or Highway Model, extra EGR, and perhaps water injection.
And before someone says, “You cannot hope to get a big van to 25MPG, It is a BRICK…” Chevy 2020 Express Vans ARE Doing 29/30MPG at highway speeds, so that myth is dead.
SO now the cam: the choice of a Cam had become a nightmare obsession. The cam is THE MOST IMPORTANT part of an engine. The cam controls what kind of an engine you building. Cam grinders and makers’ 90% of the time give more power by shifting the power band up. Beware this is often true for so called RV cams…
So a mild stock engine can run from 1500 to 5000. Move that to 2000 and 5500 more power, move it to 2500 and 6500 MUCH more power.
BUT to get that power costs fuel, more is needed with each power shift.
To get better Miles per Gallon, the idea is to keep the low RPMs power band by not shifting the power curve upward, and setting the whole car/truck drive line up to cruse near your engines torque peak.
I have been working on getting an engine set up to run at a torque peek of 2000RPMs. And gearing to have that RPMs at 80/85MPH.
Sadly it seems this will limit other factors as passing and mountain climbing.
Problem I was facing is going all out for torque at 2000 RPMs means the HP will also be limited to around 3500/4000 and she will not make much, but IF I run Rhoads V-Max roller lifters, and with Rhoads Lifters running at 10% reduction I can have a mild Cam for low RPMs and a wild Cam for higher power RPMs.
So here are the numbers: I order Green Cam Grinding’s #806 roller running stock 1.5 rockers.
So at low rpms she will be:
Rhoads Lifers: Less 10% Duration 185.4/191.7 Lift 371.7/385.2 lobe sep. 114 = torque peak should be right around 2000/2200
At higher RPMs A Power cam: Duration 206/ 213 Lift 143/428 A near top CAM with stock heads and valve train. I believe the torque peak should now have moved to 2800/3000 and HP up to around 4500/4800 RPMs.
So NOW I am very close to the low RPM cam I modeled this from:
Low RPM 350 Cam: 179.5/192.2 .350/381
And again everything I read was NOT to do head work…what is great for HP is not great for MPG, and I am running the 14102193 head (64cc combustion chambers) cylinder heads with swirled intake ports - the intake ports were designed for fuel economy (the design was also shared with the 103 heads used on the 4.3L with TBI). The swirl ports (known to GM as a vortex chamber) along with the irregular shape of the combustion chambers limit the airflow, SO the engine is built mainly stock and will not run over 5000RPMs.
And again keeping in a tight budget and not needing any special head work I am running stock valves, stock springs etc, so kept max lift under .450.
I have read reports by users of these Rhoads Lifters of a nearly flat torque curve from 1500 to 3500/4000 RPMs…
Here is the original target Cam out of a low rpm engine with a torque peek at 2200 RPMs
I also based my thinking on the Chevy 400 and OLDS 403, both had Cams that torque peaked at 2000RPMs.
So at 1500 to 2500 My new cam’s intake valve will be opened a just little longer from 179 to 185 and open slightly more from .350 to .371 and the Exhaust Valve will be 192.2 to 191.7 and opened a little more at .381 to .385.2.
This should give me the MAX Torque at 1500 to 2500 RPMs which should product enough power to easily pull the van at 75/85MPH.
Whereas at 2500 RPMs the Rhoads Lifters will start to pump up and cut in my hotter part of the cams profile so instead of a drop in power my engine should just keep on making more torque and HP all the way to 4500 RPMs.
I am very well aware this runs contrary to everything a hot rodder engine builder knows and believes to be true.
As the engine began in 2002 I even committed farther sins I used an old school SBC 400 Crank and RODS in my build, I did Teflon coat the skirts…
I hope to get the transmissions mated this summer (It is a shotgun wedding) and to test run the 383 this fall on a engine test stand and install all of this insane setup into my 93 Star Craft G20 van.
AS I am building an engine test stand for this test firing of the 383, I decided to test the test stand on the old 350 I am asking for the cam for…killing two birds with one run. That way any mistake(s) that might risk an engine can be tested on the 350 first.
Rich
