Reversion possibility question on SBC with XM270HR cam
 

Hello, I basically have a stock L31 (5.7 vortec truck motor) with aftermarket valve springs to handle a higher lift cam. Motor has a dual plane intake with a quadrajet carburetor and HEI ignition. Alpha drives.

I am wondering if a Comp Cams XM270HR cam would cause resonance reversion with the stock mercruiser manifolds and 4" mercruiser risers, standard through hull exhaust?

The next cam up, the 276, says it needs better exhaust. Wondering if this one is ok with the stock exhaust?


http://www.compcams.com/Company/CC/c...?csid=202&sb=2


Intake Duration at 050 inch Lift:218
Exhaust Duration at 050 inch Lift:224
Duration at 050 inch Lift:218 int./224 exh.
Advertised Intake Duration:270
Advertised Exhaust Duration:276
Advertised Duration:270 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:0.495 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:0.503 in.
Love seperation is 112
Intake centerline is 110

I saw no signs of reversion on my stock merc manifolds with the 270HR. I have since upgraded to stainless marine manifolds.

It can revert and it has on some. Most small block guys running this cam (or larger of course - but tons run this cam) with stock or GLM exhaust weld on 10" or so 'inner' pipe. Do a search and you'll see.

BTW: Hard to find a better shelf cam for your application than this. You can go smaller but you will not be as happy.

Where is your idle set leave your employee to work till 9:30PM while your at home Donzi Matt, LOL.

Only a 4º increase over stock roller cam duration for a 6.2 MPI. Doesn't seem like a lot.

Well, a 502 Mag has 224/224 and HP500 has 222/228 so........can't always go by that.

Anyway, been apart of a quazzilion sbc builds and have had some (okay, just a few) report reversion with that cam. I can't tell you if they had too low of an idle or what. The one's I was part of hands on did not, but only a few of those had stock exhaust. Most had Stainless Marine or Eddie Marine's.

So...thus why I put out the warning. Keep an eye out.

I idle mine at 750. Shifts well at that RPM. It will idle down around 600 when cold, but again I have never had an issue that I have noticed. I did the stainless exhaust in preparation of the 383 it will be getting soon and to lose some weight out of the back of this pig.When I do the new motor I will probably sell this cam and go with a cam from Bob really optimized for the combination, whatever that ends up being.

As far as working until 9:30, he was working on his own car. We typically work as late as 7-7:30, but have on occasion pulled some late nights in the summer. We ain't afraid of work around these parts.

I stopped by on my way home from work to congratulate you on working late like I do. But you weren't. LOL.

As a heads up about the 383, there is a good one for sale complete. Contact Member Ryan00TJ .
It pushed his 21ft Challenger to 80mph. Bob M cam, AFR 190 heads, etc,etc,etc,etc.

I've know Ryan online/e-mail/phone for a ton of years now. I have bought props from him and just recently the Alpha SS off his boat and another prop. All good stuff , he know's what he's doing. I have his phone # if you ever need it.

I've known Bob M for years too. Met him down in Florida for a conference once too. Great guy.

He (Ryan) bought another boat (Velocity) and is selling off all his old killer stuff.

Here's a link to one of the things he's selling so you can find him faster:
http://www.offshoreonly.com/forums/s...ss-risers.html


http://www.offshoreonly.com/forums/s...ss-risers.html

I saw that motor and it looks like a hell of a deal. I don't really want to convert back to a 2 piece motor though. Also, between the drive, mooring cover, and everything else I put into that boat this year, I have spent my budget on it.

On the flip side, there is a lot of good stuff in that motor, probably better than what I would put together for my boat and my Alfalfa drive would probably hate life, but the boat would go like hell. I am almost thinking about going to an MPI setup. My wife drives the boat and making the boat completely turn key and simple has quite a bit of appeal to me.

Sorry to derail the thread.

I run a slightly larger/bigger cam and have no reversion with stock manifolds and risers.
(Crower #00483LM Dur @ .050” Lift: 221°/230° RR: 1.5/1.5 Gross Lift: .525”/.545” LSA: 114° )