need More power for 7.4MPI
 

I have a heavy Cruisers 2870 28.5' X 10' wide. I have a stock 7.4MPI/Bravo III with 22P props it will do 39MPH @ 4400RPM sealevel. I live and often opperate @ Lake Tahoe 30MPH @ 4200RPM 20P props 6200' MSL. Please select any or all of the following questions:
1. What can I do to effect performance short of super charging, cam.chip.timmimg, etc.
2. What can I do that would eventually apply to supercharging.
3. I want to keep this rig reliable as I travel great distance's in this rig.
4. If I supercharge what PSI should I stick to inorder to keep it reliable?
5. Might I have to beef up certin parts like head gaskets?
6. Procharger vs whipple vs Vortec.
7. Can I expect problems with the out drive?
8. Will a drive shower help?
Thankyou

BaronBob

baronbob

the 7.4 is all cast. therfore not a good option for the blower..

I feel that drive Showers always help. but not in the speed catagorie.

To gain any real, usable power from this engine, you will have to supercharge. As mentioned, it has cast pistons and cast steel crank. Plan on low boost to keep engine reliable. You can re-build engine with a good crank and forged pistons and then add supercharger.

I was hoping to hear a little more on this subject as I have a 7.4MPI as well.
From what I have read about these engines, they can be modified to make allot more power. I am just not sure about the fuel injection system.
The newer 7.4MPI engines have the L-29 Vortec heads with large oval port intake runners and the high flow "D" shaped exhaust ports. From what I have read, they are much better than rectangular port heads on a 454 running below 5600rpm. GM also builds a 500HP/502 crate engine with the L-29 heads. The hart shaped combustion chamber is smaller than an open chamber, so with flat top pistons the compression ratio is nearly 9:1.
The weakness of these engines, are the 8:1 dished cast pistons, small rods, and wimpy valve train.
If I was to rework an L-29 454, and I will likely down the road, I would do the following:
Change the pistons to Flat Tops to increase the compression ratio.
Install a good set of Rod Bolts.
Rework the heads with larger valves, stronger springs, and roller rockers.
Install a hotter roller cam; maybe the 425HP/454HO GM cam.
I don't think we are breaking the bank here...

But what to do with the fuel injection system? Are there modifications out there? Bigger injectors? Bigger throttle body? Any ideas?
Would be a shame to chuck it and bolt on a carb...

Why go through all that headache. Just put in a bigger used engine.

No actually it would not be a shame...
I just did exactly that on my 1998 7.4 MPI.
Yanked the injection and bolted on an Edelbrock Air-Gap intake, Holley 800 double-pumper carb and replaced the Delco distributor with an MSD set-up.
Incredible mid-range power increase, 300 rpm gain on top and 4 mph increase in top speed. I have not seen a noticable increase in fuel consumption... :)

If you wanted to do this on a budget, buy a Performer intake, Edelbrock 750 carb and keep the Delco ignition (it will work without the ECU).

If you want to solve the "getting on plane problem" then 3-5-7 #'s boost will (most probably) do it, whipple / B&M / Weiand each are "roots type" superchargers and have static boost at all rpm's +/- some percent, i.e. 5 #'s at 1000 rpm & 5#'s at 4500 etc., and are "better" than centrifugal superchargers, like Procharger, out of the hole.

Your stock motor is , in my opinion, quite capable of handling 5 #'s and running for extended periods at 4800 rpm's with the following:

91 octaine,
total advance 28-30 degrees ((maybe 32)),
very efficient intercooler bigger is better,
adequate "fuel curve"

You should also consider an additional or larger oil cooler, adding an oil temperature guage, and a drive shower.

Roots blowers do not have "static boost" It just increases faster with a load. If you've got 5# at 1000 rpms, there is no way you'll have 5# at 4500.

If you want to add power and still maintain the same reliability, I would go with near new stock power, like a 496 Mag HO. Think about the future also. How hard would it be sell a cruiser with a blower engine???

Hey DonMan,

Did you rebuild with higher compression pistons and a hotter cam???

Please tell me more...

Hey Griff "way"

The roots type supercharger is known for its ability to produce large amounts of boost while spinning at very low speeds. On an automotive application, a roots type supercharger can often make it's full (peak) boost by 2000 engine rpm. taken from http://www.superchargersonline.com/content.asp?id=22

That "peak" is generall maintaned throughout the rpm range.

also see Whipple product info and hp charts for mercrusirer
http://www.superchargersonline.com/p...ber=WIP-MERC62

hp at 2500 rpm 180 w/sc 280 = +56% hp
hp at 4700 rpm 300 w/sc 460 = +53% hp

suggesting that the boost is virtually constant (static) throughout the hp range shown, you'll see this on the graph.

COMPARE to turbo / centrifugal

Rayay Turbo on small block ford

rpm boost

2000 1
2500 4
3000 8
3500 12
4000 14
4500 14

see "Turbochargers by Hugh Macinnes" p. 148

If you've got better info on positive displacement systems, roots or screw, and their boost at various rpm's, I'd be interested to review it.