Compression Ratio on Blown Alcohol ???

What would the advantages/ disadvantages be on a 10.5:1 compression motor and taking it to 11:1 or 11.5:1 ??? What can I expect to gain ??? Other than being harder on the bearings, what else would I chance damaging for the benefits ??
Regards, Alcoholic

 

I'm just starting out with the screw blower myself, and am going to run 10.8 compression. If you're running 10.5, it may be OK with a screw, but not with a roots blower. If you're running a roots, you should be up at 11.2 compression with a really good blower, or at 11.5 with something that is not so good. Raising compression kind of compensates for lack of boost, if you run too low of compression with no boost it's not going to go very fast.

When you raise compression (beyond reasonable levels), you increase your risk of detonation, which could lead to black deathing the piston skirts and cylinder walls, breaking and flaking of piston rings, cracked crank shafts, pre-mature damage or sudden failure of rods, blackened or damaged rod and main bearings, and on and on. The correct amount of compression is hard to tell, as it depends on how good your ignition system and your blower is; if you run a 44 amp magneto and a newer blower, you probably shouldn't go over 11.4 with a roots; in the past I have run as high as 12.2 with a Mallory 3 mag and an old retrofit blower.

 

Ken

Ive always wondered how people could just say X or Y compression ratio is reccommened. Wouldn’t DA be a variable as well? For example.. in 4000 feet of air, a motor with 12:1 is really only like 11:1.

Ive seen this calculations before.


Corrected Compression Ratio = FCR - [ (altitude÷1000) x 0.2 ]

 

compression

you can either add blower or add compression and spin the blower slower. The trick is to not get "over center" on the cylinder pressure. Off course more power is more wear, I myself run 12 to 1.... but I don't run much timing, I tune with my fuel curve.....others may disagree but it seems to work for me.

 

What happens though when you decide to race at a lower altitude track?

Are you planning on compression ratio changes with head gaskets?

So long as it's not beating the air to death, it would seem that a boost change would be one hell of a lot simpler than pulling top end of the motor to do a head gasket change.

 

There's definitely a lot of variables to consider, some setups are inherently safe, and others work very well and are safe within a narrow tuning band. If you stray out of that band, there could be serious damage.

 

meth compression real high

One EPA test engine we discuss in our methanol book runs over 19 to one static compression and 15 psi boost. The science says you can go real high as long as you control vaporization with the proper enrichment and temperature; and do not exceed the strength of the engine: heads, head gaskets, head studs, etc.
bob szabo, author "5000 Horsepower on Methanol" and "Fuel Injection Racing Secrets"
www.racecarbook.com

 

Theres therotical and real life. "Devils in the details"

If this is a 24° big block chevy than dome height vs spark plug location gets ugly when big compression is used. Than you have to use more ignition timing than you would need when comparing less dome height.
If this is the case I'd turn up the blower and not surpass about 11:1.

 

Going by what most of you all have said, it appears something in the vicinity of 11 would be optimal. I'm building a 354 hemi (budget operation) and had been thinking of something about 10, allowing me more adjustment with the blower, just to keep it safe. Your thoughts?

 

I know of BAE guys that run 10-11:1 with really good 14's and a lot of OD & engine RPM.

 

Are you talking about a steel motor???

 

I am indeed! Yeah, I know, NO ONE runs steel motors anymore. I'm a solo parent paying a mortgage, etc, so it's a real tight budget. If it's any consolation *sic*, only the block is original. One thing you guys forget, everything up there is cheap, so anyone can afford to build a decent motor/car. Down here in New Zealand, our dollar buys 75 US cents on a good day, AND we have to pay heaps for shipping as well. It's tough, and because of all that (AND low wages) drag racing is not as big as we wish it to be.

 

I think you took what he said the wrong way. Everyone has a budget. Not all of us are sponsored by Castrol or Budweiser.

But, I think Eli is right, a 14-71 will be way too much blower for the cubic inches. Seems like a good 8 or 10 would be more appropriate... But I'm far from an expert. Maybe you could call SSI, Littlefield, Mooneyham, etc, and get their professional opinion?

If it would help you out any, I can call them for you to save you some $$$ on the phone bill. But you'll need to provide me specifics with the motor.

 

Umm, yes, I agree, a 14 would be WAAAAY too big for it. I don't know where you got the idea I'm putting a 14 on but I do have a 10 with strips and everything. I figure it's big enough that with 10 - 11 comp I will be able to run it slightly under or even, somewhere in that range. The theory is, a slower blower will be more reliable (there's the 'budget factor' creeping in again).