to bleed off sufficient cylinder pressure on a 383 sbc with 11:1 compression and aluminum heads to keep detonation away? Its a Lunati Voodoo hydraulic roller with 231/239 duration @ .050, 110 LSA, 106 ICA, 15 degrees of overlap.

 

My 383 sbc is close to this set-up....maybe it will help you judge; cam is a Comp Cams hyd. roller, 230/230 duration @ .050, 110 LSA, .598 lift w/ 1.6 rockers. Compression is 10.9:1 with aluminum heads (AFR 195's). Total timing is 38 degrees. I have no detination using Sunoco 94 octane. It will run on 93 octane, but not as well, still with no detination. 92 octane requires drastic timing reduction to run at all.

 

Do either one of you run a vacuum advance ? Mine wil run on 91 octane, haven't tried regular, 11:1 CR, 35 deg advance. In my opinion a cam in the duration range you are looking at will not help in reducing cylinder pressures

 

U need to know compression press with old cam to compare overlap values from old to new cam and determine what to expect for comp press with new cam. Like if old cam had 200psi with 70* overlap then new cam with 60* overlap will have much more compression. How much u ask? I really don't know but u now would have 15% less overlap flow area.

BTW u have to calcutate overlap from full duration (whatever that is) - not from .050" lift. Big difference. Also the full duration numbers are different from different mfr as each uses own amount of vlv lift to determine duration start and stop lobe locations.

But my guess is with aluminum heads and any large street cam (> 275* full duration) u should be fine. And if the eng builder built in any quench then all the better to prevent detonation - better quench can handle more compression without detonation.
cardo0

 

Is that what you decided to go with or are you still comparing? Did you ever figure out the other issues you were having?

ltlevil

 

I do not run a vacuum advance.....your 35 deg. advance vs. my 38 total seems to help you use 91 octane.

 

Elevation has a lot to do with it. At nearly 4000 feet I run 91 octane without a hint of detonation, regardless of where the timing is at, and I run it locked out.

 

Not encouraging info but useful.

 

Yes, I have a vacuum advance.

 

The cranking compression is 200 with a 242/248@.050, 25 degrees overlap (294/300 advertised, 77 degrees overlap) The proposed cam is 282/290 advertised, not sure of the overlap with the advertised duration. I'm guessing 65-70 degrees based on a COMP XE282HR that has 282/290 with 65 degrees of overlap. Quench should be good, .003 deck, flattops w/2 valve reliefs, .036 compressed gasket.

 

That cam was the leading candidate until it suddenly came to me to consider the compression ratio before committing. It already has 200 psi cranking compression with a 242/248@.050 cam.

I am still working on the other problem but I am confident I will eventually get it solved. I will definitely let ya'll know when I find out the problem.

 

I'm about 10-20 ft above sea level.

 

My 406 is quite safe at 185 pounds of cranking pressure. In a discussion on the Hot Rod magizine forum some of the racer's involved in the drag week event that were running any gas that they wanted, but were primarily using pump gas to travel between the events, were claiming 210 to 225 pounds of cranking pressure.
The shape of the cylinder quinch area will have a lot to do with how much compression your engine can take. Keep your valve to head clearance below .040 to take advantage of the best your cylinder heads can offer.

 

Nastee,
I am a fan of the dynamic compression ratio calculator for determining the best cam timing for a given mechanical compression ratio. The software is free for you to download. The spread sheet type program uses geometry and details about your camshaft to calculate the amount of mixture that is trapped just as the intake valve closes. This volume vs the amount of mixture compressed at TDC is used along with empirical evidence (experience) to determine if a cam timing will match the rest of the engine with respect to available fuel octane.

Check it out at:
Dynamic Compression Ratio Calculator

-Mark.

 

I have to run 100 octane and mine is 11.5:1.

 

I feel you should call Lunati and talk with them about it. Since they design the cam I am sure they can give you the info you need. Asking people here is more speculation than anything else.

 

Probably a good idea.

 

You are very welcome. . I feel it is a good anytime you pick out a cam to call the company that produces it. As they do allot of testing and evaluations before they produce the cam. So they can tell you what works and what does not work for your application. Sorry I can not for the life of me remember the persons name that designs the VooDoo line but I do know if you talk to him he knows his stuff and will steer you in the right direction.

 

Calling the manufacturer is always a option, it has been my experience that the manufacturers techs lean toward the conservative side so as not have you phone back and give em shnit.

As far as speculation on the cam choice is concerned, some of it is speculation but some of it based on experience and the fact that this is not rocket science. You are looking for a dynamic compression range to run pump gas without detonation. If nastee383 enterd the correct perameters in he calculator above he should be able to figure out if he can run on pump gas.

 

MotorHead yes if he puts in the right information to a point. You also have to consider vehicle weight,Timing and curve,rear end gears,size of tires,transmission among other things. Even if one person has this very same cam and it is installed exactly the same they may like the cam or dislike the cam depending on the other variables with there application. I do not know about the other manufacturers of cams but I do know the person that is designing the VooDoo line is very good as I have run his designs when he was with General Kinetics way back when. And he also is more than willing to help with information to help a customer. I am sure someone here might just know his name. And I think he posts on a Chevelle forum. Someone in the past mentioned this on this forum.