I'm thinking the intake closing (and to a lesser extent, the opening event) are the same for both cams. I would expect the cranking compression to be the same. It would be interesting to see them both on a dyno to compare the torque curves. I wouldn't call that a "totally different" cam, just different LSA.

It would be a big difference on a 340 though.

Tech Archives?...


Chris..

I'm not really satisfied that event locations (IVC, etc.) are anything more than markers established after the fact, and are faux science as predictors.
The engine cannot see any part of the valve train, including the valve.
What does it see?
Pressure differentials (is the door open or not, and which way is the traffic going - not what the door looks like, or how big it is):
IVO = vacuum begins
IVC = flow reversal point
XVO = residual combustion pressure (balanced against pumping loss)
XVC = after the wave return has peaked

The sole purpose of the position of each event is to anticipate when this pressure change will occur (or reach an acceptable level such as EVO), and if the cam gets it wrong you have no power. Opening the door to no traffic (.800" lift and no vacuum) does nothing. Leaving the door open after traffic starts moving the other way is worse.

A prediction of where X" Hg. will occur in degrees based on a cam card is not going to work, except as a coincidence.

well it is good to know i can get what ever Cam I need as long as i am willing to pay for the Billet stuff. I was kind of worried about that as i am going to be running a 50mm bearing cam and MaMopars stock of Blanks is limited from my understanding. Leon gave me 2 of his cams that I think will be pretty close to what I need for a Turbo SB, BUT I want to know what my options are if they happen to be way off.

Back on subject!!I am no where close to understanding Cams by any means but what I have seen and learned over the years. You really can't talk about any Cam Spec with out mentioning the others as well.

It seems companies like Comp-Cams and others have tried to simplify Cam Terminology for those of us with simple minds by reducing the terms to Duration and LSA. In reality there is a lot more to it than that.

Even though it confuses me to no-end the bare bone basics are IVO, IVC,EVO,EVC. With those set you dictate your In & Ex Durations, In & Ex Cetnerlines and LSA numbers.

IVO
IVC = IN DUR. & IN Centerline > lsa

EVO
EVC = EX DUR. & EX Centerline > lsa

Changing any 1 of the above has it's consequences on the others. And the consequences are even great as you get farther away from the basics of IVO,IVC,EVO,EVC.

Now back to Turbo cams. Turbos tend to like a wider LSA as well as more Duration in the EX for the simple fact they like an exhaust that is forced out of the cylinder. Try to force the ex out with overlap and you'll be forcing it back into the intake.

Optimally THEORY would say you want to open the intake as soon as possible @ TDC on the Int. Stroke an hold it open as long as possible until BDC and allow BOOST to fill the cylinders. NO OVERLAP at all and also open the exhaust as soon as possible @ BDC and hold it open until the piston has reached TDC on the ex stroke.

I wouldn't even try to guess what the numbers for that kind of cam would be.

I know there are some guys using a bit more overlap than you would think on Turbo Engines for racing Just to keep the EGT down.

I'm definitely keeping my eyes open and paying attention best as I can. A lot I need to still learn before I put mine together.

I like it , would it run?

Thanks MIKE!!!

Yeah I figured there was a reason!! or everyone would have already been doing it. I never really seen the numbers.

That would probably make a good grunt/trq low speed cam. But not much else.

Now correct me if I'm wrong again!! But wouldn't you really want the Cam to be the last thing in your selection of parts when building a motor?

Wouldn't you want to get your bottom end and heads and intake all worked out, then select your cam?? Let the cam bring it all together, as long as the parts were made to work together in the beginning.

This is my thoughts on it. Just seeing if anyone agrees or if I'm off in left field somewhere.

Here is some more info to throw at you guys. My turbo cam was spec'd by Kenny Duttweiler with a 242/242 dur @ .050 W/113 LSA.
Int opens @ 8 BTDC
Int closes @ 54 ABDC
Exh opens @ 54 BBDC
Exh closes @ 8 ATDC
With the above info, it looks like there is 16 degrees overlap. It also appears that the most that I should consider advancing this cam is 4 crank degrees with a resultant intake centerline of 109 degrees. This would put the exhaust closing event @ TDC.
Assuming that there is a 1:1 relationship between boost and exhaust backpressure, and also assuming that there will be no exhaust scavenging, what are your opinions on this cam ? Advance, Retard, start with a new grind ? Add more overlap ? This cam was installed straight up @ 113 intake centerline. My ultimate goal is to enhance turbo spoolup since I'm running a large turbo. My previous performance has been 9.86 @ 137 MPH in a 3700# car with a small 76 mm turbo. Engine pulled to 7500-8000 RPM so I have room to sacrifice some top end.
Allan G.

Your cam does have 16* of overlap and is mild for the combo. With turbos IMO mild is all you need.

Advancing the cam 4* would put it at a 109* ICL, but the exhaust closing point would be 4* ATDC not at TDC as you stated. Crank * not Cam *

Your interests are Spooling up the Turbo faster, So IMO Id get a Split duration cam that opens the exhaust valve sooner before BDC Or maybe Advance the cam you have now even more then its 4* Maybe like 8* for a 105* ICL

IMO even with boost a little bit of overlap scavenging isnt a bad thing. mike