High Compression, Carb.....and Pump Gas

[PTAddict]

Chevy big blocks have a less efficient chamber design, and are thus more detonation-prone than small blocks (and the LS motors are far better yet). So normally, 11.5 static compression without electronic spark advance control is asking for major trouble.

But "normally" does not include your situation. That cam is huge, even by the standards of 500+ inch motors. More to the point, the intake valve closing point is very late, which will bleed off a lot of cylinder charge through reversion at lower, more detonation-prone RPM levels. So the compression ratio that the motor actually "sees" - often called "dynamic compression ratio" - will be much less than you would think by looking at the 11.5 static number. So I think you'll likely get away with it just fine, assuming aluminum heads, conservative spark timing, and a good cooling system.

As an aside, I think you may have issues trying to get those "self-learning" type of EFI systems to work with your cam. The large overlap cycle will dump so much oxygen into the exhaust at lower RPM that the wideband sensor will still be reading dead lean even when the actual A/F is very rich. You'll probably need to have a controller that can be manually programmed, and it is very useful to have a CO/HC exhaust analyzer available to tune idle and lower RPM mixtures, since they are still reliable indicators of A/F ratio even in the presence of excess O2.

[GregWeld]

In other words.... don't rely on the CR number of the pistons in the box. That number is complete BS (for this discussion anyway) and is only a number to use for comparison against another piston while they're still in the box. That number will tell you a little bit but won't answer the critical question of what fuel you can use. And it sure as hell isn't the magic bullet to tell you if a motor is going to make any power compared to a "lower" compression (SCR) motor. As you know - it's all about the right COMBO.

I don't want to sound like a know it all here... but I'm trying to answer an often asked question in a way that will allow "others" to read it and maybe start to ask the RIGHT questions should they be in the same spot some day.

[GregWeld]

Originally Posted by PTAddict

As an aside, I think you may have issues trying to get those "self-learning" type of EFI systems to work with your cam. The large overlap cycle will dump so much oxygen into the exhaust at lower RPM that the wideband sensor will still be reading dead lean even when the actual A/F is very rich. You'll probably need to have a controller that can be manually programmed, and it is very useful to have a CO/HC exhaust analyzer available to tune idle and lower RPM mixtures, since they are still reliable indicators of A/F ratio even in the presence of excess O2.

So true! And even more true with "oxygenated" fuels these days. And I've helped tune guys cars that SMELLED (nose test) pig fat - when they were really LEAN. A guy could write a book about what the O2 sensor "sees" and what you see when tuning with a laptop etc. But that's for a different thread.

[GregWeld]

Sorry for too many posts....

What is your intent for this motor/car combo? Cause IMHO that motor will suck for street use with that cam but then again -- I'm a bottom end torque motor guy. I want to FEEL power every time I barely tickle the throttle. So it's all good as long as YOU are getting what you're looking for.

[Vegas69]

I think you'll regret putting a flat tappet cam in that motor. The spring pressures quoted look week for 7k to me. I'd spend the money and go with a roller. It will make more power.

I also agree that combination will be rowdy on the street and a poor choice if you want to inject it down the road. It would be a fun racer though!

[g356gear]

It's a motor that I found and was looking at as an option. I would really like to stay around 10.0:1 if possible with a new motor but I wanted to get some feedback on this set up. Thanks guys!