Brad Christy

Guys,

So, we're in the planning stage of rebuilding a 496HO after a piston failure. New pistons, rods, the whole shebang. We are rebuilding pretty much everything from the tank to the intake manifold, addressing the fuel delivery issue we believe to have been the culprit of the failure. We are also brooming the PCM555 ECM, and replacing it with a Holley HP, so we will have easy access to the tuning. Now we're discussing cam options to hopefully unleash some of the benefits of the ProCharger (M1). We are retaining the stock heads, exhaust and the SilentChoice (CFO's orders). I initially looked at the Raylar 211 cam, which is for mild boost applications, but have been talked out of it, for fear of reversion, particularly with SilentChoice. But, looking at the numbers between the two, there seems to be a fair bit of room, specs-wise, for some gains from the something in between. We can get pretty much anything we spec out from one of two companies that the builder uses for custom cams. He's got a software program that allows him to plug in numbers and predict HP/torque curves and RPM limits and all that good stuff, but it doesn't predict reversion; that comes from experience-gained intuition. While I trust him, as he comes very highly recommended, and he is, in fact a big power marine engine builder, I am just paranoid of reversion with our particular setup, given some of the "chocolate milk" horror stories I've read here on OSO. I want/need to get it right the first time. I'm not looking for anything dramatic, but if there are gains to be had safely, I'd like to render them. Hell, if I can just open up some top end RPM, I'd be happy, since we now have the parts that will withstand it.

Anybody got any suggestions? Feel free to throw out full spec sheets and I'll run them by the builder.

Thanks. Brad.

underpsi68

Did you ask Raylar about reversion with their cams?

I would look into a set of his heads at this point. Big hp to be had and aluminum for weight savings.

Brad Christy

UnderPSI,

No, I have not. I called Mark Boos about doing the tune after rebuild, and, as soon as I mentioned the stock exhaust, he expressed reservations. When I mentioned retaining the SilentChoice, he outright denounced it.

I hear ya about the heads, but it's jut not in the budget. The CFO is already sh!tting bricks at $5K, let alone the potential for $10K. She initially wanted to know why we had to replace all eight pistons. Even mentioning adding another $3-5K would probably get me introduced to a divorce lawyer.

I'm just looking to squeeze what I can out of what I've got, and with as few bruises as possible. If I end up sticking with the OEM cam, so be it. But I thought it was worth the ask.

Thanks. Brad.

Tartilla

Brad, what are your performance goals?

No sense bench racing without a proper goal for guys to make suggestions etc.
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What were you making pre-issue? Educated guess of course.

What performance do you want in Phase 2 a few years after the new Phase 1 build and Holley setup?

Brad Christy

Tartilla,

I learned in my days of playing RPGs that the quickest way to get the perfect monster killed is to give it stats. I can tell you that ProCharger advertised their kit at 615HP on the HO. I can also tell you that I’ve been told by a couple reliable sources that I was likely not getting much over 500, realistically. With that said, I can tell you we were turning a Bravo28 prop to ~5200 behind a 1.5 ratio drive, pushing a PQ280, 4800lbs dry, to 76-77GPS, 78 under perfect conditions. My fantasy is 85, but anything between is welcomed.

I was fairly specific, even if in a rather vague way, as to my performance goals. I want to get it apart and back together, without breaking the bank. But I know that there is more than a handful of HP left on the table in the initial build, and I’d like to extract them while we’re selecting parts. We will have a stout engine, with reliable parts that are not likely to fail under the load we will be putting them under. While I don’t HAVE to replace the cam, it’s a relatively inexpensive upgrade that can be tossed in the basket while we’re selecting those parts, that can eliminate some of the inherent restrictions keeping those hidden HP from being rendered.

I know there are some guys on here that really know cams, of which you may be one, and I’d like their (your) guidance on cam specs that will provide gains over the OEM HO cam, without inducing reversion.

There is no “phase 2”. The CFO has already discovered how white her knuckles need to be. There will be no further rebuilds unless something else catastrophic happens.

Thanks. Brad.

underpsi68

My next call would be to Raylar. They know what their cams/ combo do and need.

Honestly I think you are crazy not buying the heads. I completely understand about the budget BUT the stock heads aren't performance in any way. I would find things to sell (body fluids if needed) to make the heads happen. I would rather keep the stock cam and buy heads if it was one or the other.

Tartilla

The 496HO cam isn't bad for a SC BBC. INT 224° @0.510 lift and 115° LSA. Something like that.

The a new cam change would likely need spring a change. Maybe even lifters. Adds up.

The stock 4stHO heads have value...what are they worth used?
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Get some Promaxx aluminum heads. Keep the cam. Reduced pumping loss from a far better exh port in the head. 3/8 raised exh port only lifts it up.

Stock 496HO heads seem to have a rough go with porting. Maybe just do a bowl blend.
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Button up the stock cam. Focus on the new EFI and tune it well. If you want more HP....turn up the ProCharger a bit.

Gentle on the Bravo and keep it propped on the higher rpm putting less tq on the drive.

87MirageIntruder

Unfortunately there is only 1 head currently on the market for the 496 8.1 Gen 7, and that is the aluminum offering from Raylar @ $4,259. Dart stopped making their gen 7 iron heads years ago.

My notes show the stock HO cam as: intake 222 / .510, exhaust 232 / .510, 115.5 separation

Brad Christy

50HP for $400 or 150HP for $4K The HP/$ math is just not in the favor of the heads.

I’ve spoken with Larry at Raylar. The only one they’ve got that would be of any benefit to me is the 211, which he suggested against in light of the stock exhaust and Silent Choice. The whole point behind this thread is that there ARE no off-the-shelf options that do me any good. But we are not limited to what’s off-the-shelf. We can spec any cam profile we want, and there is plenty of gap to close between the stock cam and the Raylar 211. There’s where I want to be. I’m looking for cam numbers to consider.

New springs are already on the docket, if for no other reason that the OEMs just suck. We will play the lifters by ear. I’m hoping we will be able to keep them.

You are right about the OEM heads. Builder says there’s just not enough wall to do anything without getting into the water passages. He did mention installing bigger valves, which would assuming would mean opening up the ports and larger valve seats. Well outside my wheelhouse.

Builder advises against spooling up the ProCharger. The boost curve is already pretty non-linear as it is. We will be stretching the limits of the fuel pressure regulator as it is.

Thanks. Brad.

Tartilla

Rock and a hard place for sure Brad.

Apparently the stock 8.1l heads had issues trying to get larger exh seats for 1 88 valves. Cutting large pockets for the seats were getting into the water jackets.

Seems they really cast these paper thin all around.
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MIke Jones from Jones Cams is around the best for custom cams. He has a full page questionnaire for cam recommendation. Custom Rollers are $541

Marine | Jones Cams

Focussing on the complementing the ProCharger is likely the key.