Pat McPherson

Over the last few weeks, I have been reading everyones threads and posts about how to get more out of these low power engines. I have asked a lot of questions and have receive a lot of great input.

I bought an all most complete MPI system from a racer down in Fla., his name escapes me. His engine builder had tried all kinds of tricks to make more power, before switching to a carb. I only wanted the bigger injectors and reprogrammed brain, but I got everything.
I trip over the intake last night in my basement so that is why I'm bringing up the subject again.

I think everyone agrees that the GM MPI system on these 7.4s will not flow enough air to make power at higher RPM.
Oh, unless you bolt a $5,000 supercharger on it.

I have another question though...

Why Not?

Is it the size of the throttle body?
Is it the size of the intake runners?
Has anyone thought about modifying the intake castings?

I know that there are engine builders that have done testing on 7.4MPIs, and the conclusion is to dump the MPI and bolt on an intake and carb.

I am going to do that to mine at some point.

Before I throw this stuff in the recycle bin, I thought I would bug everyone one last time.

robyw1

I have a lot of experience with the GM EFI systems. I don't like the idea or going back to the carb. Now I have the 7.4 MPI in my 22' Scarab but have not had much time to look into any similarities with the GM systems I have worked with. However yes you can install larger injectors, improved throttle-body, ECM and even a cam upgrade. The thing is though you really can't exceed 5000-RPM s unless you upgrade the internals. (i.e crank, rods, pistons and maybe main caps) I will be looking into modifying this engine in the near future. I still believe that this engine with its current configuration can be massaged to perform better than any carb and intake combination with the same internals.

The first and foremost thing you can do is get rid of that stock throttle-body gasket that closes up a third of the opening. I modified my gasket and it helps because it slaps the rev-limiter a lot easier now and has better mid-range power.

Roby

bobl

The throttle body is very small for one thing. Secondly, pull the plenum off of the intake and look at how the runners are made. They make a complete 360 degree loop thru the intake. Granted this makes for a long runner, but really kills high speed air flow. That's why they make a ton of torque, but die at higher RPM.

Pat McPherson

Thanks for the e-mail bobl,

I took another look at the GM intake

I wonder what one of those Arizona Speed and Marine intakes goes for??

How about swapping the GM electronics onto one of those?

Dennis Moore

I would suggest adding a camshaft with more duration and a better exhaust system. The block has four bolt mains, steel connecting rods, a strong cast iron crankshaft and hyper-u-tectic pistons. If you can find someone that can reprogram the computer for the better camshaft and exhaust I am pretty sure the stock EFI would flow enough extra air for a nice performance increase.
Dennis Moore

robyw1

I'm going to look into reprogramming those ECMs. We used to do it with our GMC Typhoons & Syclones. I just need to get some time and work with the engine.

Roby

bobl

Pat, you have the MEFI 3 ECU so it would work with any fuel system. Tyler Crockett did the reprogramming for me and he should still have the mapping I used.

I shared this story some time back, but I'll tell it again. When I started the 7.4 project Tyler Crockett told me he had built a 7.4 with the GM MPI and thrashed it on the dyno. The best HP he could get was 375. That was with the ECU programmer right there setting it up on the dyno. He removed the EFI and bolted on a good intake and carb setup(don't remember which one) and made 500HP!!! Coincidentally, after I finshed the same modifications on the boat and tested it, we gained 4 MPH. That would equate to just about a 60 HP increase, putting us almost exactly where Tyler got on the Dyno. Hmmmmmm. In conclusion, if 375 HP is the goal, then the GM MPI can get you there....If you want more, better look elsewhere.

Pat McPherson

OK Fellows,

I all-ready bought a set of Stainless Marine exhaust manifolds and the plan for a better valve train is already in motion.

Why not give the MPI system another try before recycling it. My MPI engine does run a lot smoother than any other boat engine I ever had.

Don't know if 375HP will get me to 70mph or not, but for the cost of reprogramming the ECU it's worth a try

Thanks!

Eddienel

Found this thread and thought it was interesting. Yes it’s 17 years old. Wish the last poster would have posted his results and not left it unfinished.

Cerberus66

i did the tb gasket ecu flash, k&n airfilter. And got more power same rpm and was able to pull a 25p prop from a 23 in my Baja hammer. 71mph.