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

So having most of my engine experience coming from Fuel injected motors, I used to play around with high compression quite a bit. Mainly high compression, naturally aspirated set-ups. 11.0+ set ups mainly. With my interests shifting to old school Chevy big blocks, I have been looking at upgrading to something new. The feedback I need is based on what kind of compression can you run with pump gas on a higher compression motor (11.5:1) with a carb on a big block. I may go EZ-efi on this motor down the road but for now the carb will be the ticket. Input would be appreciated. Thanks.

Steve C

From what I've read it seems the consensus is generally <11:1 and really more like 10.5:1 or less. I was thinking of shooting for 11:1 myself but the more I read the more I'm thinking 10.5:1 or less is where I'll end up.

Now if you extend your "pump gas" to include e85 then you can jump the compression up quite a bit, e.g. 13:1+ :).

John

I would love to run E85...especially with boost, but we don't have it around here yet. Nearest source is around 500 miles away.:(

I've heard of being able to run 11.5:1 with aluminum heads but I would shoot for 11:1 to be safe.

Stick with <11:1, and 10.5-10.75 is the sweet spot for premium pump swill and aluminum heads. You only gain ~4% HP per full point of compression, so the very small power gain isnt worth the considerable (and ultimately expensive) risk. Even at the 10.5-10.75 ratio, you need to get all the details handled.

Build for worst case scenario at ALL times. Compression heavily relies on the camshaft. What cam are you running? My ZL-1 has 10.3:1. What if you get a slightly bad batch of gas or race the car and induce higher temps? Play it safe.

Its a forged internal ZL1 using new generation block and heads and running a COMP cams nostalgia ZL1 camshaft. 11.5 compression ratio.

What are the specs? Roller or flat tappet?

http://www.compcams.com/Company/CC/c...ookieSupport=1

Now I understand your talk of compression...:D

Static "compression" and actual CYLINDER PRESSURES are two different animals and now you can toss in DYNAMIC CYLINDER PRESSURE -- and it's the later that will make a difference in what fuel you can run and your timing. Timing is the killer if you're on the verge. The right timing curve will get you more power than will one or two points of octane.

Dynamic cylinder pressure is the calculation of the ACTUAL swept area of the cylinder. In other words -- where the INTAKE VALVE closes in relationship to the piston coming off BDC. You can loose an INCH off the stroke if the valve closes late compared to closing "earlier". HUGE difference in the calculation and DCR is ALWAYS lower than SCR.

Then factor in Cylinder pressure - which is constantly changing with RPMS etc. DCR can be calculated -- and CP can be measured. I'd be looking for CPs under 200 with a carb. Or a DCR of 8.25:1 or less but no less than 7.5:1 on 91 octane.

Modern EFI cars can pull timing out with the knock (detonation really) sensors... thus they can run on the ragged edge and be able to "bail out" (pulling timing out) when knocking is sensed. EFI can also "add" one point to the compression number -- as does the aluminum heads. That's why you see LS motors (etc) up there in the numbers game.

So --- There are aftermarket knock sensors. And you can get a dial back timing module (MSD) to use when you'd need it. You'd need the knock sensors because by the time you might notice the problem - you may already have a problem! But I agree with everyone else here... WHY would you purposely build a grenade. Keep the cylinder pressures down in safe zone and drive and enjoy the car all day every day. Nothing is worse than a poor running, overheating POS. And we've all had those! They suck.

My advice --- and you can do this easily before you ever fire off the motor -- is to check the numbers calculating the DYNAMIC COMPRESSION... or at the very least - do a cylinder pressure check and compare those numbers with known 'limits'. Because it's the combo of the cam and head chamber size - gasket thickness - and static CR number of the pistons - and where they TDC in the bore (quench area) as installed etc. that will determine what you really need to know.

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.

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.

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. :lol: But that's for a different thread.

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.

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!

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!