Do MPI motors have lots of vacuum at idle and therefore might be more susceptible to reversion??

An MPI motor (like a Mercruiser with tuned port injection) has a huge amount of plenum volume between the throttle butterfly and the intake valve. Intake manifold vacuum is much lower because of this.

On the other hand, the new small block EFI's, with the dual plane intake manifolds, have a very small plenum similar to a carb.

I have always wondered why Mercruiser used the Dart single plane intake manifold and not a high performance dual plane intake manifold on the 500 HP carb engine. I would be willing to bet that a modern dual plane would make the 500 HP carb engine more powerful at all rpm ranges.
I suspect that the engine didn't make more horsepower with the single plane but, because of the open plenum and lower manifold vacuum at idle, the engine sucked less water into the cylinders at low speeds (during valve overlap).
Do you think Mercruiser High Performance was smart enough to understand why it caused less water ingestion?
Sincerely
Dennis Moore

You'll have to excuse me, Jack. I'm taking every opportunity to gather information so that SOME DAY I can perhaps feel confident about having some build-up configuration done on my 454 Mag MPI (yes, w/o swapping for a carb).

So far, I've got:

Cam: Crane 168731
Springs, retainers, locks, etc: Isky
Rocker studs: ARP
Rockers: Comp Cam Pro Magnum
Heads: worked over stock (or maybe Dart Iron Eagle if I can get comfortable that I won't have valvetrain geometry problems)
ECU re-programming?? Who the hell knows.

I got all of this just from listening to you guys debate and drawing my own conclusions. Am I dangerous or what??

Dennis, not sure where I remember reading this, but what i remember was mercruiser went with the single plane because they thought it was worth losing a lil low end torque in order to achieve better cylinder balance. Supposedly they determined balance was better than having more low end torque and having some cylinders running lean/richer than others.........

just my 2 cents...... like i said, can't remember where i read that.:confused:

Jack, were you able to dyno those motors? Were they 454 or 502?

Pat:

If you aren't too impatient, I am about to water test a pair of 454 HOs that I changed the cams out using the 502/502 cams. I have been waiting for some tranny coolers to come in, and they are supposed to be here tomorrow. If they do, I'll be on the h2o by this weekend. I have cranked the motors, and they sounded great, but the real test will be on the water with the GPS.

KS30

KaamaScarab30,
My engine mods. are a Winter project.
Please let us know how she runs.

Wow, you guys are getting steamed!

Overlap is overlap, whether it comes from tight LSA and shorter duration, or wider LSA and longer duration.

Max had a good point on checking valve to piston clearance on the lo-po 7.4L. I did a Gen V 7.4L a few years ago and discovered that installing larger valves (going from the stock 2.06" intake to 2.19") created a clearance problem. The heads (peanut ports) were also milled .020. The pistons had a small notch with a radius to match the 2.06 valves. When I mocked up the motor with the new larger intake valves, I discovered that the valve head extended beyond the radius of the notch, which, even with a relatively small cam (218/222, 112), reduced the v/p clearance to dangerous levels. I had to cut the notch on a larger radius. I also cut the notches deeper while I was at it. The exhaust valve clearance is generally excessive to begin with so a bigger cam will not cause any problems on that side.

Also, valve to piston clearance is most affected (on the intake) by cam changes that in one way or another start the intake valve opening sooner. That could be by more duration, tighter LSA or simply advancing the existing camshaft. A trial and error test that I did showed that for every degree the intake opening point was advanced, the intake valve to piston clearance was reduced by .005".

Rich

I'm running the GM cam zz 502/502 no problems.I made my choise from Dennis Moore's book and I'm glad I did.:cool:

HeyGuys,
There is an engine builder, bobl, that posted on a previous thread about mods. he did to a L-29 7.4MPI.
He said he installed larger valves and a different cam into one of these engines.
Anybody know him?
Maybe we could get him to comment....:)