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View Full Version : Upgrading power- 7.4 MPI to HP500 carb



lildogcustoms
08-12-2008, 09:51 AM
OK- So I have an opportunity to pick up a HP500 for a pretty good price...What would be the biggest obstacles in pulling 7.4 MPI and dropping in HP500. I've never done a repower on a boat...:hyper::hyper::hyper:

roadtrip se
08-12-2008, 10:07 AM
I split a set of 500EFis with a 26ZX guy when I did mine in 2002.

The only suggestion I might make is to hold out for a 500EFI or a 525EFI set-up, as opposed to the carb motor. But this is just a preference, as I have run both. Better starting, smoother torque curve, no vapor locking, and much better headers.

The results in the 26ZX were 76mph, which is honking with such a big boat on mild power.

Biggest hassle will be getting the tails on the headers done. The rest is cake.

lildogcustoms
08-12-2008, 10:20 AM
I would rather have the EFI but just getting impatient and wanting to start lining out things for winter project. Darn EFI are so high $$$$ I found the HP500 for a bargain....What type header/tail issues am I going to be looking @?

VetteLT193
08-12-2008, 10:24 AM
OK- So I have an opportunity to pick up a HP500 for a pretty good price...What would be the biggest obstacles in pulling 7.4 MPI and dropping in HP500. I've never done a repower on a boat...:hyper::hyper::hyper:

Usually fuel injected engines have a return line to the gas tank, that will need to be capped off.

Is the engine complete with carb, distributor, fuel pump, exhaust, etc? How many hours are on it since last build?

I can tell you that the HP 500 top end parts don't last. 200-250 hours is pretty much it. you might get more in a lighter boat but it depends on how you run it etc. The cam is pretty radical

The parts for rebuilding the top end are NOT cheap by any stretch. The Cam is reasonable ($450ish), the lifters and springs are special. Lifters are about $600 for a set. Springs are in the $200 range. Everyone says that on each top end rebuild you need to do the Cam, Lifters, and Springs... plus you know you'll need misc parts so that's an easy $1500 in parts to do the top end.

The good news is they run 87 octane and the bottom ends are pretty much bullet proof.

roadtrip se
08-12-2008, 10:31 AM
He is right, but usually the necessary "re-build" is a set of valve springs at 200-250 hours. The rest is check and go, unless you do some real damage.
Ironically, the springs they recommend for the carb motor replacements are the ones in the EFI motor. I had mine checked right at 200 hours and they were fine. As Vette said, I would check it out and ask for sure.

No fuel return lines on 500EFIs that I am aware of. None on the 500 carb either.

As for the headers issues, it is just a matter of getting them altered to fit your boat. I insisted on keeping my Corsa Q&Q, which represented some engineering opportunities, but alignment of the tails may take some trial and error, no matter how much you measure.

The deal? Don't know what you have the carbed motor lined up for, but 10-12K will get you a 500EFI all day long. It is worth it.

VetteLT193
08-12-2008, 11:04 AM
do you know if the 7.4 MPI has a fuel return line? curious more than anything:wink:

lildogcustoms
08-12-2008, 11:25 AM
Thanks for all the input- Engines have around 180 hours on them so sounds like they are just short enough of needing rebuild to sell....Information much appreciated...By the time I purchased and then rebuild wouldn't be to much more to jump to EFI. As far as fuel return line- not sure. If it ever stops raining before it starts snowing and I get to take boat out again I'll have a look. :smash::smash: This summer has been so wet it's really cut into my boating...

The Hedgehog
08-12-2008, 11:42 AM
I have a few friends that have had great luck with HP500 Carbs. Setup right they will run well.

These guys are right about the top end. Pretty much anything with 500 hp and up will need top end work at the 250-300 hour mark. They are also right about making sure the your have the exhaust set up right.

For the right price I would own one. Good motors. Make that 26 move nice. If you ever got bored with that kind of power, they respond well to a little boost.

glashole
08-12-2008, 11:59 AM
I am running a carbed 500 in my 22

runs real nice

lots of power :)

blackhawk
08-12-2008, 12:05 PM
The valve springs are the issues with the 500 carbs. If it were me I would just replace them for peace of mind. That motor should not need a rebuild at 180 hrs. Have a leakdown test done and if the results are good then buy it and run it.

I think Vette is referring to your current motor possibly having a return line that will need to be capped. I would also upgrade to bigger fuel lines. You probably have 3/8" and I would run 1/2" to play it safe. The swap is actually very simple and as others have stated the exhaust will be the only possible issue. But if you don't have a form of silent choice that should be simple as well.

BigGrizzly
08-12-2008, 01:05 PM
The only issue with with the top end is mainly the springs. Of all the hp 500 that I have seen Most of the springs are inconsistent even in new in the crate motors.v There is a good deal of difference between Carb and EFI HP500 engines cans, intake track length and torque vs RPMs on the dyno and in the boat. I am not going to side with Road Trip on this which I would prefer. I am not going to stat the reasons because the thread would be hijacked by people who do more reading than actual engine work and it would make prop threads envious.
What you need to consider is the 200 to 250 hour longevity. Road trip has had his engine together for over 5 seasons and he does not baby it. He goes through nor outdrives then engines. There are several people who don't get 2 seasons out of mild engines. The real fact it is as close to bolt in as they come. If you have someone build it you could get into Silver back's dilemma. The HP 500 is a tried and true engine. Some of the fastest and most reliable boats on the board are HP500 or Hp525s.

ITTLFLI
08-12-2008, 02:45 PM
Thats a great engine with all good parts. IMO i would leak her down and if that looked good change the springs and go! If you wanted you could upgrade the cam a bit but would not be necessary. To me it would depend on how good of a deal.

I did see 2 on OSO that kinda got my attention.............:eek!:

On the header note.....you should have "stock" bravo dimensions. I think CMI's website has this technical info on their website. The 454 and hp500 all bolt on the same place.

The Hedgehog
08-12-2008, 02:57 PM
What these guys said.

Scrap that silent choice, throw on my Dana's, use tips if you need and be done with it!