Compression and Timing Q

I understand that when you overdo the CR (Compression Ratio) bit on an engine you can avoid detonation by use of higher octane gasoline - which is in short supply most places - and retarding the timing.

My 462" Buick at 9.0/1 CR runs well at 32* BTDC all-in.

Has an easy going curve as well with all-in coming in around 2600-2800 rpm.

A touch more aggressiveness with timing coming in at a lower RPM could help, but so far it's doing ok and running 87 octane summer and winter.

Fwiw - 32* all-in is usually what's recommended for late Buick 455's whether they are stock or modified.

I do realize that a more radical cam will cut the dynamic CR.


So, going in the opposite direction, if I went from a stock 8.7/1 CR to 7.9/1 CR it looks like I could get away with 2-4* more of BTDC all-in timing.

Sounds weird to me you say?

Me too.
The 455 Buick in the 31 roadster has 7.9/1 CR heads that I'll probably use on the initial fire-up.

When the "Built" engine comes to be, it'll have a bigger cam, more carburetion and a CR of about 9.5/1 to 10.0/1 and revert back to the 32* all-in figure.

Thinking too, that dialing in about 2* more initial may help power and MPG on the 32's 9.0/1 engine.

Fwiw - manuals for stock Buick 445 engines call for 32* all-in and most built engines call for the same or less.


I have read of some serious, naturally aspirated, BBC's that are running as low as 28* all-in.

Any experiences with using a touch more timing on a low CR engine?

 

Why would you want to lower compression in order to run more timing advance?
The compression loss would offset any gain you might make,,also,,the lower comp. would hurt fuel milage as well.
Doesn't make sense to me?

Tommy

 

Granted, sounds wrong, but the dummy 455 I'm using came with 7.9/1 CR heads.

It appears to be a very low mileage engine that was stored in a dry building at the junkyard, virtually no ridge on the cylinders.

Right now, the thinking is to use this engine to drive the car for a while and then take my time building the "real" engine.

Just a temporary expedient.


Going off on a tanget from the basic question, I asked some of the Buick V8 board guys what they thought about using these heads with a blower.

Their thinking was the 7.9's didn't breathe well enough to warrant using even with a blower.

Personally, I'm of the opinion that they would breathe just fine on a street runner.

Without a blower, the proposed - mainly cuz I have most of the parts - engine on the drawing board looks like it would come in at about 460 HP.

 

Actually the motor parts, pistons and Heads, do not control the compression of the motor. The cam and its timing events is what controls the compression. Most performance cams change those events to a point that the motor is actaully not producing enough compression to completely ignite the air fuel mixture.There fore high dome pistons and smaller head chambers are use to help compensate for this loss. Also what is commonly known as "Clatter" or Ignition "Ping" most of the time is Not cause from too much timing advanced but lack of fuel or lean cylinder condition.

 

I dont know if its the right thing to do, but I have done it. I had a 327 with a big solid cam @ 9 to 1 static compression that ran on pump gas with 44* total and it ran very good
I put alot of miles on it too

 

Change and control are two different words with different meanings.I did not say those things would not change the compression. The final out come is "controled" by the camshafts grind characteristics.

 

After perhaps 2,000 RPM with a stock or mild cam (and certainly 4,000 RPM regardless of cam timing) the cam has no effect on compression ratio. DCR is off the table, by definition, before the torque curve starts.

 

Russco, that is exactly what I am talking about. As long as the fuel curve is right, advanced ignition timing does not hurt. It actually increases the efficency of the motor by allow more burn time.

 

It seems like cylinder pressure is being confused with static compression. Yeah, a cam will not change the static compression but it can make a big difference on effective cylinder pressure.

In the hemi I'm building for the digger I plan to run alcohol which likes cylinder pressure. My pistons will only make 9.25:1... maybe. I'm actually running a milder cam with less duration and overlap to help keep cylinder pressure up. I'm also going to overdrive the 6/71 11% but that's another story.