Good, honest advice wanted.
 

Over the winter, I would like to perform a few modifications to my 7.4 MPI (310hp). Last winter I installed Stainless Marine exhaust, K&N arrestor and an adjustable fuel psi regulator from Vortec. I did not gain any rpm or speed whatsoever. A couple experts have advised I am tapped out with the MPI set-up.
I would like to scrap the fuel-injection ( I have a buyer for it) and install the following pieces:

Edelbrock 750 cfm Marine Carb
Edelbrock Performer RPM intake
Holley 110 gph electric fuel pump
GM ZZ502 (502/502) camshaft
.527"/.544" 224*/234*
GM ZZ502 valvesprings
Mercruiser Delco distributor (from carb engine)

My Questions:
Do you think I`m crazy for walking away from the injection?
Will I gain any rpm or speed from these mods?
Would you do anything different with what I have to work with?
...Please don`t tell me to sell the 7.4 and buy a 502. I want to work with this engine before I swap power.

As always, any comments good, bad or other are always appreciated!!!!

I would use a Merlin manifold, it has the brass inserts in the water passages. As for the rest, not sure.

DO NOT USE THAT CAMSHAFT!!!!! Get a cam that is for a marine application!!! Call Teague marine or someone that builds marine H.P. engines. If you go with that cam you will be sorry.

Also I recommend going with the Edelbrock Air Gap manifold. Makes better power at the mid and top end.You also might want to look at doing some port work on the heads as well.

leave this engine stock...well i guess if would rather spend more time working on it than using it????

i think your going to run into problems somewhere between the heads on the motor and that cam your trying to install.but i do know that you dont have much farther to go with the original setup....already been through that.........

one word ........Pro charger!

Isn`t the Merlin manifold an open plenum? Would this really be a good idea? I always run in fresh water, no salt ever. Do I really need those brass inserts?

The ZZ502 cam was actually recommended by a expert who frequents this forum.

As for the Air-Gap intake, just read a dyno report on one. I think I will go that route, looks like a great manifold!

I have spoke with a reputable performance engine builder, they say it will work just fine. I can even retain my factory non-adjustable valvetrain.

FOUR WORDS......

I CANNOT AFFORD ONE! :D :D :D :D

hey man buy the time you do a 330 on roids you could buy a used Procharger off the board!Take my word for it Ive done several.....To do it right you need heads, exhaust,ignition, carb , intake, roller rockers, and a rebuild kit, with all that your better of saving for the Charger...my .02

For most people, it would probably be better to spend a little more and get the Procharger. But for me, I think my plan is better because I already have most of the parts needed. As I stated in my first post, I have a buyer for my MPI. Total cash outlay will be under $500.
No one has answered either of my first two questions.
1. Is this a good idea to swap the MPI for a carb?
2. Will I actually gain any RPM or speed from these mods?

with out changing the heads I think your better of leaving it alone.......go with a holley 800 carb if you switch my .02

if you do a search on" 300's on roids" you can see what I did a couple years ago...proven package with lots of Hp (505) lots of input from Kaama,Crazyhorse and Bruce and other Motor guys!!!

Thanks Dyno!
I will check it out!

I currently have VORTEC heads. Aren`t these supposed to be good mild-performance heads?

you better ask JimV about those heads....I've never heard that anyone was having any good #'s from the stock heads!!!

In answer to your two questions:

I feel that the MPI will hold you back from more power. It still works in a supercharged setup because they all have fuel enrichment capabilities but that does not apply to your setup so I think you are doing the right thing.

As far as RPM/Speed you will change/move the power band up with a cam change but be careful to not go over 5000 rpm with that motor because it will come apart. As far as speed increases go you can expect about 1 mph for every 20 hp increase on a Baja.

Also don't make the mistake of over carbing that motor.

Hope this helps !!!!!

SuperV turns his 330's to 5700 never came appart yet! 7 years and counting!!!!good machine work is the key!!!

Ask Brad Perry ,pm or e-mail him, He has a recipe that will gain you 7-10 mph and yes it does include a carb

Dyno,
Couldn't agree with you more but remember he is dealing with a STOCK bottom end here!!!

I have a 2 bolt main small block that I run to 6500 but as you said good machine work!!! My engine is all balanced as an assembly and revs beautifully!!!!

Oh! I thought we were going to open her up!!!!

DonMan:

I went the same route you have gone and picked up an honest 6mph. Note however, this was switching from peanut port heads to the vortec heads. I would guess the simple cam and intake swap would be significantly less. I used the Edelbrock performer RPM, 454HO cam instead of the 502/502 cam, Gil exhaust, Vortec heads, K&N filter. Changed the ignition module from the stock 454 to a 454mag (Thunderbolt V) to reset the rev limiter to 5050rpm. I will be really curious as to how it runs with the 502/502 cam. Actually, i will be even more curious to see if it clears the pistons (check it!). Mine has a nice lumpy idle, but doesn't pull on the top end quite like i expected. Low end from 2500-4000 is phenominal. I would guess what most have said here, that the cam will be 2 much for the heads. You will have some fun getting the cam to work anyway. You will need to replace the stock valve rotators with spacers on the intake and exhaust. The intake guides for some reason are bigger, so you'll hafta drill out the spacers. The springs will then drop right on (with the proper seals and retainers). You'll prob hafta screw around with shimming them to achieve the right pressures. Stock valvetrain will work fine.

Carb-wise, i'm running a stock Mercruiser/Carter/Weber/AFB. I'm still playing with jetting, so I can't really help ya too much for a baseline. I'm currently running 107 jets on the primary and secondarys with a 68/42 primary rod. Install a 5" spring for the metering rods, or you'll be beating your head on the dash trying to figure out why it doesn't run. The cam won't pull enough vacumn to keep the metering rods pulled down into the jets, so it just dumps fuel and drowns the engine out.

I hope you have luck with the cam, and if it does work out, i'll prob be switching. I've been waiting for someone to try it. You are definately goin the right route getting rid of the MPI for small time mods.

To wrap this long saga up, i'm now looking for a 174 blower, so maybe what everyone is telling you is correct. Leave the fulie and put a blower on........:confused: :D

You wont have any clearance issues with that cam..but check it anyway. Make sure the springs are setup right!!!! and for piece of mind I would switch over to adjustable rockers. Easy to do and cheap. The Air gap works good..I got one on mine. Mallory makes a marine pump cheaper and better then holley. Mine is 4 years old and still puts out 8lbs a full throttle. And you dont need a regulator. Id go with a Holley carb. But thats a personal prefrence. I understand them..I dont an AFB
Dan

Dan what Mallory pump are you running????And yes its still cold as a ***** up here!and no I cant come down there......yet! maybe in Feb. or March.....I'm working on it!

The 310 HP MPI 7.4 is the Vortec engine. The heads are actually very good. Just have larger valves installed and a good bowl blend and they will support well over 400 HP. I would use a much smaller cam though. That's only a 454. You want to build peak HP right around 5000-5100 to keep the engine alive. The heads will breath well, you don't need a big cam. The MPI is a problem. Changing to the carb will get you where you want to be. As long as you design the engine to make a lot of torque and keep it at those RPM it will live just fine. If you go over about .550 lift you'll have to flycut the pistons. That 502 cam will be pretty tight. I like something around 216/220 on 110 degree LSA for that package.

I would not be looking to get too much more power from this motor. Remember, the bottom end has cast parts.

DonMan,

I see Bobl already replied to you, and that you're in Texas too. Bob has mine in his shop right now (7.4 MPI) working on it.

You see the name of Bob's shop in his sig (near Lake Travis). I brought mine down from the DFW area. If you want someone to do it right, give him a call. Once the parts are all back and machine work is done, you'll be able to get an idea of what gains to expect once mines done; a case study if you will. How restrictive is the GM MPI on our's, we'll see.

There are good options for staying with EFI as well going carb; we're reserving options for mine once we see what we get. It all depends on what you want, how you'll run it and how much $$ you want to sink into it. Ouch, bad choice of words. How about "...how much $$ you want to "invest" in it." :D

Give Bob a call.

Been running mine all summer with the Merlin. No problems and lots of power. I'm in fresh water too, was recommended to me to use the brass inserts because of aluminum and water combo. Not sure why but when you are talking to someone with experience, I usually go with there opinions. Later, 88 Mach 1

BobL:

not to step on any toes here ;) , as far as i know, larger valves can't be installed in vortec heads. If you look at the chamber, you'll see what i mean. The heart-shaped design, doesn't really allow for any increase. Maybe some head experts can jump in here and disprove me. I'd love to get some bigger valves in there.!:eek:

I`ve been doing a little research on all of this, asking several different resources of their opinion. Of course, I`ve received several different answers. I don`t want to throw money at this and see what happens. I would like to see good results on first try. I know this has cast pistons and crank, so I want to keep it at 5000 rpm or less. I also don`t want to invest a ton of money or else I would just step up to a Procharger or a 502. I feel like I have outlined a pretty good plan. I am still reluctant on 2 items: The camshaft and carb choice. As far as cam, I wanted to run this 502/502 cam mainly because I thought it was in the ballpark of what this engine would like and the fact that I can get a crate motor "pull-out" camshaft for $100. Do you guys have a particular cam that you would recommend? What are the specs? The Edelbrock 750 Marine carb was recommended to me because of it`s low price (compared to Holleys) and it`s reliability. I was also told it would get better gas mileage as compared to a Holley. I am familiar with the AFB design and know how to tune them. I am also quite familiar with Holleys. If not Edelbrock, what would you run? Is the Merc distributor a good choice or should I just get a Mallory or MSD? HEI? I kmow where I can get an ignition from a `97 7.4 Carb engine. Would that be OK?
THANKS FOR ALL THE ADVICE GUYS!!!!:)

If I were you I would stick with a 800 holeey then when your ready to upgrade you already have the carb......I run a MSD but the thunder bolt works fine....cams????

this is a gm stocker engine...with stock merc parts that surround it. meaning your gimbal and transom ass...my guess is you do not have offshore engine mounts, nor external steering?

you have not listed what application this is for yet? or have not made mention of any testing? Who will build this engine?...Who will dial it in? what are you going to do for an ignition system? the distributor is only one of the parts you will need. what ignition curve will you use.

as i told Jay prior to his 330 mods....your on a budget. you do not have the first hand knowledge of how to get this done. this may just return to bite you in the ass...i guess you really have to look at if enjoy boating more or working on the boat? have you assessed the bottom of the boat??? measured your X-dimension?

read this about how jay 330 project went down! LOL

http://www.offshoreonly.com/forum/sh...ghlight=jayl13

800 Holley double pumper? Or vacuum secondarys? What`s your list number?
Thanks!

Machanical.....I'd look for a used one off a 500hp....

Comp cams makes a couple in their extreme marine series that would work well. I think they are XR-264-HR or XR-270-HR. Going to a larger cam won't make any more power at 5000 RPM and under. The Edelbrock carb is fine. If that Merc distributor is a Thunderbolt V it will have a rev limiter and you may have to change the module out to get 5000 RPM unless it's from a magnum. If it's a Thunderbolt IV it will work fine.

Madmaxx, you're are correct in that the larger valves are tight. You can, however get them in just fine by doing a little grinding in the chamber. They are more shrouded than an open chamber head for sure, though. You can lay the chamber back some and get them opened up. You give up a little compression, but the air flow increase more than offsets it. You also must replace the exhaust seats for the larger valves.

Hey Joey, I`m not out to run this thing in some offshore competition. This is a recreational boat for God`s sake. I`m just looking to get rid of the fuel injection and maybe add a few ponies at the same time!
You don`t know what my budget is and you have no freakin idea of my mechanical abilities. So how can you tell me I have no knowledge of how to get this done? You don`t know me....
I never said I was rebuilding this engine. It has very low hours on it now. For the record, I honestly can`t tell you how many engines I have built over the past 20 years, more than enough to get this job done with no sweat! I was merely on this forum to get opinions/ideas. You come on and make me out to be some uneducated moron. I`m not building a race boat or a race engine and will never pretend to be doing so. So lighten up a bit please.

sorry if i'm coming off the wrong way...

"So how can you tell me I have no knowledge of how to get this done? You don`t know me.... "...just from what you have actually posted...you never answered the question(s) what application is this going to be it?...what were your #'s from your pre mods and after...why is it you have spent $$ and time and seen zero results?...what do the #'s tell you? ...you say... "I don`t want to throw money at this and see what happens. I would like to see good results on first try."...so why didn't you apply this theory to your first mods?

I NEVER questioned your mechanical abilities WHATSOEVER...knowledge of how a system works in a real world marine enviorment and mechanical abilities are very different.

really...HONESTLY.... i'm not trying to be a dickhead....i'm just trying (maybe verbalizing the wrong way) to save ya the time and $$$...

think about this.....if there was a race class for your exact boat configuration....what would you be wlling to bet that the racers would get better numbers out of it that the mods your willing to try? all at zero compromise to the engines integrity.

ps...i'm in houston TX all the time! maybe i'll see ya out there??!!! have fun!

DonMan you have a PM...