Modifying AZM/HP500 manifold
 

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

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.

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.

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..

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.

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

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 :)

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

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

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

also arizona speed has a mono blade throttle body that flows around 1200 to 1300 cfm

I thought of doing that. It wouldn't be very hard, and starting from scratch with the top would be the way to go. I've got a TIG in my shop and have made complete manifold in the past.

This may sound silly (it is), but I think the reason I haven't decided to go that route is that the stock manifold looks really good, and everyone recognizes it as an HP500 manifold.

May still go that route.

I spent the money on those and saw nothing

Again I spoke to one of the engine builders already mentioned in this thread. He said that the AZ mono blade does not work well on the engines in the 500 CID range it kills the port velocity in the engine. (no bottom end or strong mid-range). When you install it on 540CID you will start to see gains.

nice to know.

When I had an 800cfm holley, I had 1.4 inhg manifold vacumn at full throttle. I put an 850 on it and it went to .9 inhg. I will be interested in seeing what it's at with this setup and throttle. I'd like to see around .5, which is probably another 5-8hp and probably the point of diminishing returns.

Was just talking to a tech at K&N about a flame arrestor for this throttle. He was asking me questions and said: "have you removed the walls between the runners in the top plenum?"

Apparently the "easy" mod I was thinking about is pretty common to this manifold.

I'm surprised no one around here has messed with it.

if you remove the upper plenum walls that would make your runners shorter and give you more top end power would it not? what about the added space in the upper plenum now, how many people have done this and what are the results?
Mark

I have two 500 EFI that we did the top and the bottom and cut the TB and installed the single blade got the cubic inch up to 572 with all new stuff and crank made 679 hp at marks precision marine with stop K&n filters and they act like super charged motors with the excelleration 169 hours no problems. Motors look stock except for the aluminum heads sure does help when buying insurance :rolleyes:

Any pics/more info on what you did to the plenum? My 500EFI's are 540's with heads, cam, monoblade TB, etc and made 630hp at Precision Marine.....about 90 hours so far without issue. We've known the plenum is holding back airflow and I've considered trying to open them up. Pics would be great!!!!
Thanks

I went more ratical than they wanted me to and the relabilty might not be there but I dont drive wide open alday

by the way which props u running and do u have shorties

If you have a 35 Fountain then we have completely different hulls. I have a 2002 382 Formula, as you know they are considerably slower than your fountain. Everything I've read suggests shorties and extension boxes produce very minimal gains on this hull (and the cost isn't worth it to me). I'm running regular length XR's w/o boxes.

While we "only" made 630 hp at 5,200.......they made 700 tq at 4,300. My goal was cruise speed and the high tq number was perfect. But always wanting more! :)

With 500EFI's the boat comes stock with 28 Bravos. I have several props that all provide good cruise speed and need a set of "top-end" props. Currently running 28 MAX that are full 15 5/8 and labbed (still have the diffuser ring). I also have a set of 30 MAX that are 15 1/8 and thinned at the leading edge but not truely "labbed". These perform nearly identical to the 28 MAX's and have always been within 100 rpms and .5 mph of the 28's (WOT numbers were 0.1 different). With 1/2 fuel and all gear at 3,500 rpms I'm seeing anywhere from 54 to 58 GPS. WOT has been 5,300 at about 83 with either of these props. The fastest props I ran were labbed 30 Bravos, these turn 5,600 at about 86.

How's your setup run? Props etc. In a 35 I suspect your over the century mark!!!!

I run 6 blade with a long defuser ring 32x15 runs a trun 97 miles but I would be more than happy to hang out with u and your family and have a beer on any lake we got a poker run on chicamauga lake in may would u guys please come

I'd love to hear more about what you did!!! Where did you cut the divider? Did you stay with the o-rings, or just make gaskets? What did you do to the lower?

I can almost garrantee the plenum is too small for peak power. I've seen bigger plenums on small blocks. The only thing you lose with a bigger plenum is throttle response, but even that isn't much of an issue till you get really big.

Like I said earlier, the only things that stops me from making a new plenum for it is not knowing what I'd really gain from all the work, and the look: everyone recognizes the HP500 manifold, and I know whatever I came up with would be a lot less "slick" looking.

It would be pretty easy to impove on this plenum: Just start with a 1/4 inch base plate cut bigger than the lower runners on all sides (you want the runner to be able to breath on all four sides). Add another 1/4 inch plate around the top of the runners to provide a generous 180 deg. radius at the top of each runner. Then weld up a box around the plate. Wouldn't need to be any thicker than the stock plenum, as the increased length and width would provide the extra volume.

Of course then it would just look like a big box on top of the engine. Something about the look of those runners sticking out the side of the plennum that just looks mean! :)