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.

No, actually I didn`t rebuild at all (engine has 250 hrs). I basically just removed the intake and all associated fuel injection parts, installed the Edelbrock intake and Holley carb. I plumbed in an electric Carter Racing fuel pump and fired it up. I ran the Delco distributor for a few weeks, but recently swapped it for a MSD Pro Billet Marine distributor, box, coil, etc...
The MSD doesn`t help much except to give a better throttle response and crisper idle.
It was pretty easy.........
I am swapping in a bigger cam this Fall along with some head work.

Pat, I did that exact project for a customer this winter. The L29 has flat top pistons and a 9:1 compression already. I ported the heads & installed larger valves and a mild cam. We messed with the injection for a long time. We reprogrammed the ECU several times. The stock injectors are 22 lbs and didn't flow enough for the modifications. Switched to 40 lb. injectors and reprogammed the ECU. Finally got it sorted out and running very good. Net gain??? 4 MPH! The stock EFI just doesn't flow enough to support any more HP. We did pick up a ton of torque though. It just died at about 4500 RPM. Tyler Crockett told me he had done a lot of dyno testing on one of these motors and could not get over 375 HP on the dyno. He swapped to a carb and picked up about 75 HP! So, based on these experiences I would not do anything to it unless you're will to change the fuel system.

Hey LFarr, a roots type supercharger makes boost depending on LOAD.
If you are under full acceleration out of the hole, you may see full boost at 2000 rpms. If you are cruising at 3500 rpms, you will see about half of that.
I see the boost readings every time I go out. Also have had SC'd ProStreet cars that would make 10#'s all the way down the 1/4 mile. Yet driving them at 3400 rpms on the road without a LOAD you would see 2#'s.

OK Guys,
The fuel injection is coming off, an Air-gap intake and four barrel carb going on. I bought another set of L-29 heads that are going to the machine shop next week. I also bought a set of used Stainless marine manifolds for her.
Any suggestions on the cam??? I plan to install captain's call when I pull the engine so I don't think I can go that big. Any body know if the 425HP/454HO cam would be the right call? I also think Crane makes some Gen 6 marine cams.
Thanks for all the help.

Patrick,
I have purchased a camshaft for my engine. I plan on installing during the winter. It is a GM Performance Parts hydraulic roller. It is the camshaft for the ZZ502 crate engine, also known as the 502/502. I don`t have the specs in front of me, but it is something like .544"/.544" with something like 230* duration. I was told by experts that it is the biggest cam you can run with the L29 non-adjustable valvetrain. If I remember correctly, I paid $125 for it from Scoggin-Dickey. It was a new "pullout". I also bought matched springs, brand-new, still in box for $75 (General Motors). You can go bigger on the cam by converting your valvetrain to a fully adjustable one. Crane sells the studs for $80.

Guys,

A Whipple is not a roots blower. It is a screw blower (or more specifically, a Lysholm Screw air pump). It physically pulls air in from one END of the case and discharges it out the other END (as in back to front). It is adiabatically more efficient than any other form of positive displacement blower. With that said, it IS indeed a positive displacement pump and can be thought of as a very very efficient roots pump with none of the drawbacks or heat problems.

Now, a positive displacement blower WILL make almost full boost at low rpms. Theoretically, they "should" make full boost at 1000 rpm, but operating clearances and allowance for thermal expansion dictate an "engineered" loss of efficiency at very low rpms. These blowers pump a given VOLUME of air for each turn. This differs from a centrifugal pump (turbo, procharger, vortech, paxton, etc) which uses inertia to move air. Centrifugal blowers will make more boost with more rpm - until they reach the design limit for the flow ratings for the volute and turbine.

Now, for those who see less than max boost on their roots blower at mid rpm - this is due to the throttle plates being partially closed. Not saying that you will see 5 pounds of boost at steady 1700 rpm and 5 pounds of boost at steady 4500 rpm. What you WILL see is if you slam the sticks wide open from idle - the boost will jump to 90% of max boost and will stay there all the way to the top end. Not telling you to do it, just saying that it will. Centrifugal guys will see a different tale. Centrifugal guys will see the boost hop to 2psi by 1500 rpm and then it will continue to climb all the way to top end (turbos are different and we aren't talking about them cause they are exhaust driven and engine LOAD determines the quantity of heat energy present in the exhaust stream to drive the pump).

This heavy boat "may" do okay with a centrifugal system but my guess is that it would be best served by a low boost whipple setup. It needs guts coming out of the hole to turn the steeper prop he will be running.

5 pounds of boost on a cast 454 at 91 octane is pushing the limits. I don't know that I would advise that. 3 1/2 pounds would be trouble free.

Whipples are more expensive than most anybody elses blower kits. I feel that a whipple would be the best blower solution for this motor.

A 174 blower and a carb would be the most affordable solution and would be "almost" as good.

I like prochargers, but don't feel that they are the best option for this heavy boat and cast motor.

Bob, I've read all these expert's opinions and all have their merits. Having been around this arena for a while and tried most all the suggestions plus port nitrous, I would suggest the least attractive of the options: Opt for a 540 crate motor from a variety of manufacturers and use an Arizona Speed and Marine fuel injection system. Mercruiser uses their system on the 525 EFI motor. All the components other than the intake manifold and plenum are GM Delco Remy, available at your local Chevy dealer. www.azspd.com is their website. Talk to Dan Peloso @ 480 753-0208. They sell complete motors with all the necessary stuff. 540 CI 620 hp/650 ft lbs 9:1 CR $17,000 =/-. Ought to be worth 25 mph on Tahoe. Or... keep your short block and make the modifications mentioned previously and add the AZSPD system. This will solve the EFI problem. AZS sells a 502 motor, a little bigger than yours, that starts with the turn of the key, idles at 700 rpm, runs on 87 octane and make 520 hp @ 5000. If you used your short block should get you close to the high 400's. The low and mid range torque of these systems is impressive. I wanted the above and purchased a system this year.

Supercharging is great if you have a forged piston/rod/crank motor. Otherwise you end up with problems, i.e. lack of reliability, low power increase at low boost (3.5 lbs), lots of work to install, not that inexpensive to install ($5,000+) and in your application, in my humble opinon, the wrong option. Go with a big motor with AZS and you'll smile all the way across Tahoe.

Wow!! I am impressed with all the knowledge and experience expressed on this subject of pumping up this low performer. I have talked with both Procharger and Whipple and they both believe that I will not have a problem with my cast componets if I keep my boost under 5-6 #s. I know they are both in the business of selling boost devices but they say they have plenty of examples of reliable installations as long as I stay low in boost and RPM's (mid 4's). There are things I can do such as cam duration and timming for futher reliability.
So here is what I am going to do. By the way thanks for all the input to you-all. Nice to hear from you Don. Operating elevation change is a big factor in my decision. Not necessarly going that fast but improving hole power are also factors here. I think I may be able to purchase a used Whipple and bolt it on to my existing EFI. Here is my reasoning. My yearly travels take me to various atmospheric conditions and I would like a computer to figuar out mixture, timming ant to watchout for detionation. I appreciate the discussion about better flow with a carburator, clean manifold anb better flowing heads however I believe that a positave displacement system covers alot of the performance gains of better flow at atmospheric conditions. I also remember the hassel of re-jetting as temp and elevation changes. Being an aviator I watch over-all manifold pressure and note that 5-6#s oh boost equates to 10-12" of manifold pressure. A hot day on Tahoe can result in a 9000' density altitude and a hot day on shasta can be in the 6000' range. Durring operation on a hot day @ Tahoe Converting boost to "s and then taking into consideration density altitude(1" per 1000') I will net only a 1/2#-1 1/2# over normally aspirated performance @ sea level (STP). Shasta would be 2-3# same conditions. I will have to watch WOT deuration durring colder and lower operation.

The whipple is a good choice if supercharging is your decision. My only caution is to stay on the light side. Whipples make a lot of torque, can break things and have an incidence of burnt/broken pistons when they get too lean. Good luck, Bob.

If you setup for your low altitude operation, then you will not have any worries at higher elevations.

With an MPI you can adjust fuel mix with fuel pressure adjustments. Higher elevations will result in richer performance (which is safer then the alternative).

Don't confuse your MPI setup with a closed-loop automotive FI setup. The boat unit does have knock sensor, but does not have the ability to sniff the exhaust to properly adjust fuel mix at altitude. It uses a MAP and BARO sensor to adjust to where it "thinks" it should be. Your MPI brain might need some programming tweaks to adjust for altitude properly once boosted. (but you ARE correct in that it DOES have the capability to adjust automatically, it just won't know if it adjusted correctly or not - that will be up to the fuel curve map that whipple burns into the ECM).

Since you clearly understand the relationship of density altitude and supercharger effectiveness, you may even want to talk to Whipple about buying a "high altitude" pulley for operation in the mountains.

Thanks for the detailed response. I will look into your suggestions and may be able to visit Whipple since they are less than an hours flight from Tahoe. I see your salutation on allot of correspondence. You must like this stuff. What do you think about the longevity of my cast parts? should I change them prior to supercharging?

Detonation will kill your cast motor.
Detonation is the evil for all motors, especially blown ones, but where it is "HARD" on a forged motor, it is DEATH to a cast one.

Your setup is safe to 4800 rpm. Brief runs to 5000 rpm are your upper limit. Your rod bolts are a weak link for high rpm operation, but you aren't going to run high rpm with a properly setup blower so no biggie.

Boost builds cylinder pressure and heat, but in a fairly linear fashion. Detonation spikes both of these values.

Keeping your effective compression ratio within a sensible range for the available fuel will keep detonation away. Plan to set your motor up to the fuel that will be available where you boat.

If you do plan to utilize Whipple's services, he will be able to set up your ECM for safe timing and fuel curves with the parameters you give to him.

You will certainly need to add a fuel pressure gauge and a low fuel pressure warning light/buzzer. This can be done by adding a pressure switch to the fuel system. You do NOT want to run a blower motor on low fuel pressure - especially a cast one. A few seconds at fast cruise can kill your motor. A big nasty buzzer will ensure that you get out of the throttle quickly and then determine the cause.

m

Baronbob,

What bobl said is right on. How do I know, it was my boat he worked on :D

I got hung up on keeping the MPI even after Bob described it when he pulled it off. It was a gamble, and I found out. Bob wanted to yank the MPI and go carb but I wasn't ready. He knew what the result would be. ;) With what he did, it sounds bad a$$ with the exhaust opened up and turns a lot of heads (it's a big ol' Sea Ray 240 Sundeck) and is really strong but it flat lays down on the top. In the high North Texas heat and a normal load I'm getting about 3 MPH more than before. Is all that work worth that top end gain? Probably not, but it also came with some added learn'n as a bonus.

On the up side, I know that with the head work, cam etc., there's quite a bit more power hidden in there and all I need to do is take it back to Spicewood and let Bob do surgery and remove the MPI tumor!

Add another one, Ive got 2 bbc in mine. Cruisers Rogue 2800, virtually the same hull without the arch, windshield and bow rail.