bcarpman

Ok, I'm finally crawling out from lurker mode. Thanks to everyone here for all the help so far in all the old threads.

I'm doing my own version of 330 on roids and have probably read through 200 old threads over the last month. The one problem I'm still having is cam selection. I've seen so many different recommendations, and many of them don't have cam specs.

Here's what I'm building:

496 Stroker for a 260 Powerquest
Forged Pistons 9.5:1
Roller Cam
L29 heads (For fast burn and ability to run higher compression on same octane)
496 HO Exhaust
Edelbrock air gap dual plane
750 or 800 Holley

My desired outcome:
Increase maximum efficient cruising speed
Keep close to factory idle (I do a lot of meneuvering in tight conditions)
Avoid reversion
Peak power at 5500rpm (Hopefully around 475hp)
Durability (I don't want valvetrain problems due to high lift or heavy spring pressures)

With the mixture motion from the L29's, the increased displacement, and the change to a roller cam, I should be able to bump up the cam considerably without killing my idle and getting into reversion problems, but the question is, how much?

I've had several people suggest CompCams 270HR with 218/[email protected] and .51 lift, but several others have said I'd be way undercaming a 496 with that cam.

Anyone run a similar combination and have a roller cam they really like?

Thanks

cstraub

What are you shooting for for max RPM?
Have the L29's been ported?
What kind of lift and seat pressure have the heads been set up for?

SB

iTrader: (1)

475hp at 5500rpm for a 496cid at 9.5:1 is no sweat. In fact 500hp at 5000rpm will be easy, using good cyl heads

Tough part may be your cyl head choice, if left stock.
L29 flow numbers at 28"H20 :

Lift-----Intake-----Exhaust
.100----63---------49
.200----121--------91
.300----165-------120
.400----198-------133
.500----226-------140
.600----242-------142

Talk to JimV here on the board about what should be done to those heads to support max power at 5500rpm and weigh the $$$ vs a better aftmkt head. He can help you there too.

Also, if your goal is just 475hp, and since you can make this at a much lower rpm than 5500, this will support your 'valvetrain dependability' by not having to rev as high.

bcarpman

I don't have the heads yet. I've got a line on a pair of stock heads as well as a pair of slightly reworked heads with stainless valves. I'm holding off on a decision until I decide on a cam.

Problem with porting the L29 heads is that most people end up killing the mixture motion in the process, which negates the value of using them. I did some of the intial developement work on these heads years ago at GM, and they really are light years ahead of the oval ports in terms of cyl/cyl consistancy and combustion efficiency. My guess is that given my conservative HP goals, I'll gain more in cruising speed and efficiency than I'll loose in top speed. Given that this boat is already a handfull on a good chop, I'm not sure I want to see anything over 70mph, which I should be able to get with 475hp. Let me know if you think I'm going to lose more with these heads than I think I am.

Thanks

cstraub

I would choose heads and then let the camshaft be last. I don't know what the runner volume is on these L29 nor cross sectional area. I'm a firm believer in not over heading a boat engine for the simple fact we don't have the luxury of gear changes and or clutches/stall converters to bring the engine into its power.

I don't think your goals are out of reach especially since you are looking for midrange punch, my concern is the CID and the crosssection of the head compatability. Moving air for 454 running down the highway at 2800 rpm is no sweat. Moving air for a 496 at 4000 rpm maybe a different story.

SB

iTrader: (1)

This is kinda ruff, but good for knowledge when putting a combo together.

I used an equation that helps us see if we are in the ballpark for valve lift vs head flow vs CID vs peak hp rpm.

Using 454cid with cfm I posted above for these heads:
.500 lift cam = 4283 rpm
.600 lift cam = 4587 rpm

Using 496cid with the cfm I posted above I get:
.500 lift cam = 3921 rpm.
.600 lift cam = 4199 rpm.

Again, this is taking flow of head, cid of engine, lift of cam, to come out with what rpm your peak hp SHOULD (not will) for best effeciency, etc. Basically, before head becomes a good sized restriction.

So, just using it as a 'tool' we can see that the parts selction doesn't marry that well together. Pretty close though for a 330HP 454 - as Merc used it.

==================
Anyway - I could be talking out my 'azz' since I actually have no hands on experience with these heads and just threw out mathmatics which I do use but don't heavily rely on. Not too mention - these are flow figures I copied from Scoggin-Dickey, so since not mine I can't vouch how accurate they are.

BTW: Thumbs up for being a part of the development team. That's super cool in my book.

bcarpman

I like to look at engine component selection "holistically". I agree one of the biggest mistakes people make when putting together an engine is selecting any one component that's way over done for the rest of the engine: too big a cam, huge heads, too much carb, etc.

I'm still flexible on head choice, but at this point, short of spending $$$$ on aluminum heads, the L29's seems the best compromise for the goals I'm shooting for. Now what kind of work do I want to put into them is another question.

Thanks again for all the help. Keep it coming.

PS. I'm sure someone is getting ready to chime in with "just buy a crate motor". I'm still looking for this mythical cheap crate motor everyone is talking about (in every post about similar topics). For what I'm looking for, i'd have to spend an extra $3k for an equivalent crate motor, and it still wouldn't be what I really want. This is my first marine motor build-up, but I've done plenty of auto and bike motors, so I really do know what I'm getting into. Yes, sometimes a motor buildup goes bad (usually when you use a cheap machine shop), but when it goes good, it's REALLY good, and you get that extra bit of pride everytime you hit the throttle.

cstraub

you may want to go to www.racejunk.com and see if you can find a set of Oval Merlins. Bang for the buck these will make any 496 run. This may be more "bang for the buck" in the long run.

bcarpman

Are you sure those flow numbers aren't for the old L-29? I think there was an L29 years ago that was a real pig. I do know that out of the box, the new L29 head should flow a little better than stock oval ports on the intake, and significantly better on the exhaust. I don't have the flow numbers myself, and my budies that are still at GM are out of town, so I'll have to get back to you with the "official" flow numbers.

Thanks

SB

iTrader: (1)

29 yr old 'Lucy' from dateonline.com ? Yes, she was.

Different forum though.

Seriosuly, it could be - it's the only flow #'s I have in my notes from these heads. Again, from Scoggin-Dickey in their catalog. I have no 'confirmed info' on them.

There are some great sources here (not me, sorry) that will probably help you more. It's Friday and all my customers are headed to the racetracks, the water, or jail probably, so here I sit sputtering my lips...I mean fingers.

Many good members here that can really help. Hopefully they'll chime in.