bcarpman

Just wondering if anyone out there has experience modifying the HP500 EFI manifold for peak HP?

This manifold has runner lengths tuned for right around 4000-4200rpm. This is a great all around manifold for someone looking for a broad torque curve right in the operating range most people use, and probably doesn't hurt peak HP too much up to about 5000rpm.

However, for my application, I already have more torque than I can use even with a short runner single plane carb manifold. I would far rather tune the manifold for 5000-5300rpm. To do that I would have to take 3-4 inches of runner length out of the manifold. There are several ways to do this and I'm curious if anyone has tried it, and what results they've seen?

Also, any thoughts on the stock HP500 throttle? Seems a tad on the small side for a 550hp motor. I saw someone refer to it as 1200cfm, but the total flow area is smaller than the throttles on a 800cfm carb. Anyone messed around with bigger throttles?

Thanks

bcarpman

Wow, no one wants to comment on this? Has anyone even heard of someone doing work on this manifold?

I took some more detailed measurements, and I think it's probably tuned for 4200-4300rpm.

RichardCranium572

You can shorten the runners MAYBE 3", still not enough, pretty sure you can buy a single oval butterfly TB, but that isn't the problem for higher rpm.

ezstriper

do yourself a favor at this point...put a carb and good intake on it....you will beat your head against the wall and spend $$$$ to gain little using that setup and wanting it to run in the upper rpm levels..

45SS

I spoke with Jim V. about porting these manifolds. He has done them and knows what works, but if you are trying for anything over 600 to 625 you need to go to another set up. He said they have design problem that limit what you can do in the porting process.

formula370ss

contact Mark at Precision Marine 504-469-7463 . He's suppose to be the man when it comes to EFI Merc engines.

bcarpman

The engine made 550hp with a carb. 45ss is right and I would have fabricated a completely custom manifold if I wanted to run bigger displacement or higher rpm. If I end up wanting more than 600hp, I'll be stepping up to a procharger along with this manifold.

The runners on a single plane carb manifold are too short to make best power at 5300rpm, but the AZM are a little too long. My calculations (which are usually pretty close) indicate needing to take out 2-3 inches, but those are just calculations, and hence my interest in seeing if anyone has actually done this.

Sorry, but I do not agree with a lot of people around here about how hard it is to tune EFI. I've been doing it for over 20 years. There are a so many things you can do with EFI that are just impossible with a carb. And making a change on a laptop is SOOO much easier than taking apart fuel bowls, or in many cases taking the carb back to the shop to change air bleeds or PVCR sizes.

I spent two years tuning the carb on this engine, and although it ran better than any other carbed boat on the lake, it still had major issues in my opinion. The biggest being fuel distribution, and hence the amount of fuel that needed to be dumped out of the exhaust just to keep the leanest cylinders lit, and hence a good 10-20% fuel consumption penalty at cruise.

The compromise between throttle size and idle quality is also not there with EFI. There are also tricks you can play with spark to tremendously improve idle and the potential for reversion. Cold start and idle quality during warmup in spring and fall should also be vastly improved.

But unlike most people, I have the equipment to tune it right, including Wide range O2's on both sides, and fuel control systems that I understand and can easily tune. This boat is also my toy, and hence I really just want EFI as that is what I am comfortable with. I do not think this change is going to make a huge difference. It won't, but it was a free upgrade for me (traded the carb setup straight up for everything including the injectors), and I am betting it will make a small but noticeable difference

thanks for the phone number. I hate calling people that I'm not going to buy stuff from. I was just hoping for some free OSO knowledge

bcarpman

That started to look like it was going to be a REALLY helpful thread. Too bad they never followed up with the promised data.

My power peak with the carb was 5300-5400, and I would hope to get around 560-570hp with the HP500 manifold reworked

thanks

RichardCranium572

That hp level is definately doable, it gets really tough to make more than ~630 with that manifold.

H2Xmark

tyler crockett is building a 500efi intake, i think it uses the bottom and he makes a new top for it, early dyno numbers from what i have heard are very strong