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Old 08-04-2005, 09:44 PM
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Default A slightly different 7.4 question...

My boat has some mutant engines - they are 7.4 LX MPI's. From what I understand, these motors have the cast internals of the carbed 330 hp 7.4, but use the good MPI fuel injection that the 454 and 502 Mags use.

I have a tentative plan to have the top ends freshened this winter. I'm thinking new exhaust (for the weight and bling factor if nothing else), new valve springs, good port-n-polish on the heads, 3-way valve job, (anything else Kaama recommends ), that sort of thing.

I figure this should be worth 50 hp a side, maybe more depending on the exact work. Enough to feel it, anyhow. My question is, assuming I have this stuff done, will the stock injection be OK? Should I consider selling the MPI units and going with carbs?

The goal is not to build a top end terror - I'm much more interested in drivability and mid-range/cruise.

Thanks for the thoughts!
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Old 08-04-2005, 09:58 PM
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Default Re: A slightly different 7.4 question...

I would use the efi the mods you are talking are not that radical but i would send the comp to Arizona speed
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Old 08-05-2005, 07:45 AM
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Default Re: A slightly different 7.4 question...

Those engines are a strange combo.
Rectangular port intakes bolted to oval port heads.
That miss match can't be vary efficient. Carbs on a set of Performer intakes would likely make more power.
Those MPI units do have some value though. It would probably be a wash if you sold them and bought new intakes and carbs.

If you wanted to do a complete rebuild with pop up pistons, new cam, and Rectangular port heads, you could keep the MPIs and turn the engines into 385HPs no problem...
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Old 08-05-2005, 07:52 AM
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Default Re: A slightly different 7.4 question...

I was reading on here somewhere that they do the rectangle port/ oval port mismatch intentionaly sometimes to add more turbulance to the flow. I think that was on a much higher flowing engine though.
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Old 08-05-2005, 08:08 AM
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Default Re: A slightly different 7.4 question...

Actually the rectangular intake and oval heads work very well on a NA engine. Keep in mind the head is sucking the air into its port. Now on the other hand ...put a blower on this combination and it really sucks. Now the colume of air is being pushed through and not sucked toward the port. It now runs into the wall formed by the smaller oval port heads
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Old 08-05-2005, 08:12 AM
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Default Re: A slightly different 7.4 question...

Originally Posted by mopower
Actually the rectangular intake and oval heads work very well on a NA engine. Keep in mind the head is sucking the air into its port.
I don't doubt you, but why would people take great pains to port match an intake to heads?
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Old 08-05-2005, 10:15 AM
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Default Re: A slightly different 7.4 question...

This is good info, always interested in learning.

Suppose I kept the pistons as is and just replaced the cam and heads (plus exhaust)? I'm really trying to stay away from tearing apart the bottom end if I can avoid it. First, I don't think the boat is worth that kind of investment, and two, it's all in good shape right now (knock on wood).
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Old 08-05-2005, 10:42 AM
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Default Re: A slightly different 7.4 question...

Originally Posted by dockrocker
This is good info, always interested in learning.

Suppose I kept the pistons as is and just replaced the cam and heads (plus exhaust)? I'm really trying to stay away from tearing apart the bottom end if I can avoid it. First, I don't think the boat is worth that kind of investment, and two, it's all in good shape right now (knock on wood).

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Old 08-05-2005, 11:18 AM
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Default Re: A slightly different 7.4 question...

That EFI actually works pretty good. It makes tons of low end torque but runs out of grunt at higher RPM. I built a 540 out of that motor once. It ran great up to 5000 RPM, then it would just lay down. The torque was incredible. We switched to a Dart intake and picked up about 200-300 RPM but lost a ton of low end and drivability. The problem is getting the ECU programmed correctly. It definitely needs to be richened up if you make a head and cam change. You could put a set of the Edelbrock heads with small chambers on it and get some compression back. Add a mild cam and you'll pick up about 75 HP.

Concerning the rectangle to oval port mismatch. On an EFI engine it doesn't create a big problem since it is just air moving through the intake. The fuel is injected directly into the port. If it were carbureted the port mismatch would tend to knock the fuel out of suspension with the air and cause a much worse problem.
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Old 08-05-2005, 11:26 AM
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Default Re: A slightly different 7.4 question...

Originally Posted by dockrocker
This is good info, always interested in learning.

Suppose I kept the pistons as is and just replaced the cam and heads (plus exhaust)? I'm really trying to stay away from tearing apart the bottom end if I can avoid it. First, I don't think the boat is worth that kind of investment, and two, it's all in good shape right now (knock on wood).
I don't think a bigger cam and rec heads will work that well with only 8:1 compression. Now if you bolt on a blower...
If you don't want to spen big $$ to make lots more power, the exhaust is a great place to start.
I saw a 200rpm increase form 4400 to 4600rpm with Stainless Marine manifolds on my 1999 7.4MPI...
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