Little help needed
 

A buddy of mine has a 26ft sonic prowler. When he bought the boat it had a stock 502 mpi. He had the lower end redone last year with what he says a blue motor or 500hp long block. The stock 502 fuel injection set up was used with no changes to it he said. He now can't get over 4200 rpm's with a 24p bravo 1. Now there is an aftermarket lower on the boat that the guy before him put on after blowing the original lower, which looks like an imco lower. He is concerned it could be different ratio then the 1.5, can you just bolt on a lower with a different ratio? I am not super familiar with drives, but figured you would need a new gear set for upper and lower right?

My thoughts are not enough fuel or maybe airflow with the stock 502 set up and it needs to be tuned, more fuel pressure or ecu upgrade. What do you guys think?

did he ever drive the boat b4 the engine was rebuilt? what were the old engines wot rpm's.and yse on a bravo you have to add the upper and lower gear rati's together to figure out what the final gear ratio is..so it could vary..that boat should run about 68 mph with a 22 p bravo one prop..

Yea he drove the boat before, and he didn't mention any issues with how it ran prior to the rebuild. Where do you think he should start looking to get it to run the way it should? I mean a 500hp should spin atleast 4800! How can you tell what ratio the drive is? Is it necessary to have fuel pressure adjusted when making that kind of hp increase and have ecu programmed? Is the 24p bravo 1 prop too big even with a 500hp? I would think its fuel and running a little lean. Biggest reason I think that when I first bought my bought it had a small carb on it a 600 quadrajet, and I would only spin 4200 at 58mph GPS. Put a 6inch air gap on it, a Holley 850 jetted it right and picked up to 4800-4900 and 65mph GPS.

I had a 26 Prowler. Bone stock 502 mpi 415hp and came with a 24 b1 on it. It would spin it 4900-5000 depending on trim. The boat only did about 60 mph but again, this was an engine with no mods at all.

I am sorry, looking back those rpm #'s were with a 22p bravo 1, not a 24p.

A 500hp bottom end is basically the same as 502mpi bottom end and will not add any HP.
If he used a 500hp long block with 500hp cam, then he needs to have the ECM reflashed or he will be way lean and probably will melt down the engine.

right that is what I was thinking, but also I was reading the if it is a true 500hp long block then it was designed for Carb application, so the cam is all wrong, 112 lob center where is fuel injection wants 114 lob center. Carb designed cam will cause a vacuum issue on fuel injection. He may have more of an issue then he even realizes

I am not thinking that the cam profile is it. If it your worried about vacuum and it were an oldschool mechanical fuel injection then maybe, but not with efi. And an HP500 cam is a 110* LSA not 112.

Your friend, I am positive, is not the first person to have a different cam than the stock 502 with that fuel injection. While the computer program does control alot I would think it would still be able to make up a bit on it's own. I couple be 1000% wrong.

Can I sound stupid for a minute? Does it sound like a 502 mag or a 500 at idle? Are you sure your friend got what he paid for?

Next, ign timing where it needs to be? That can have a HUGE effect on power and WOT rpm.

Last, who put this bottom end together? Just asking as I actually had a friend install a cam and he had no idea that it had to be done in any certain way. He lined up dots in the timing gears and that was it. Well, they were the wrong dots. The thing was 15 degrees or so retarded. Sounded nasty as hell but was a total turd!!!

i have a friend that has one with a 496 ho spinning a 24 bravo one prop,,he see's 70 on his gps...it sounds like it could be a retarded timing problem..it could also be a ecm program issue but you would think how fat the old ecm was truned it would run as good as the old engine..if it were me i would bring the boat back to the builder....was this boat in a older thread that was rebuilt a few times?????

Yep, 1000% wrong. LOL!!!!! The ECM tune is a fixed fuel and spark curve. It does not adjust itself.
I agree, if the timing isn't set correctly and it could be a big factor in power loss.

I gave him the info i found out, and the main reason I was looking into this cause he was focused on the drive hurting him and I just don't think thats the case. I put him in touch with the guy who I do my stuff with, and he is going to get the guy who built his engine to talk to him as well. The guy that built his motor is a very trustworthy guy, and a big drag racer in the area that build fantastic car motors. Now with that said that doesn't mean he can build a good boat motor as we all know that a great engine builder for cars doesn't make a great engine builder for boats. The motor is a boat motor and was a long block that was rebuilt by the engine builder. So more then likely the engine builder just rebuilt the long block, which was said that was in good condition and only needed to freshen cylinder walls not bore them, and all new bearings. So he probaly just reassembled everything and sent it on its way. The guy that I do my stuff with said it probably will need a cam swap, fuel pressure adjutment, and a tune. But without really seeing the boat and be able to diagnose it that is just a determination based off of a conversation on the phone.