How much compression ratio can you get away with?

[350matt]

As the title says gents,
I've used 10.5:1 in the past with a fairly long 280° cam and it gave no problems, admittedly I de-burred the combustion chamber when I ported it but I still ended up at 32° of advance on full load at 6K.

Should also say that this was with fully distributor less mapped ignition.

I'm considering 11:1 for the next build

thoughts?

[sidecar]

As the title says gents,
I've used 10.5:1 in the past with a fairly long 280° cam and it gave no problems, admittedly I de-burred the combustion chamber when I ported it but I still ended up at 32° of advance on full load at 6K.

Should also say that this was with fully distributor less mapped ignition.

I'm considering 11:1 for the next build

thoughts?

In my humble 11:1 is too much, the chamber design is very old with hardly any squish. Of course the CR depends on the fuel and the cam that you intend to use.

I read somewhere that if two similar engines are setup but one with a higher CR and less ignition advance than the other, the one with the lower CR and more advance will produce more power and a better spread (more area under the curve) than the higher CR engine. I guess its all about peak chamber pressure at the right number of crank degrees past TDC.

I personally think that 10:1 in a RV8 is high enough for today's pi55 poor fuel. Assuming that we are talking about an RV8!!!!

EDIT...As your logon is "350" I bet you are not talking about a RV8, in which case I ain't got a clue!

Anyway, you have asked a good question, I'm sure that the "bun fight" will be fun!

Pete

[kiwicar]

Hi
I presume you are talking 350 chevy here?
It depends alot on the cam (as you have sussed) and the fuel, I presume you want to run normal unleaded of about 95 octane? I have a cam with 252 at 50 thou of inlet timing on 112 deg LSA (quite alot of dration but reduced overlap all mechanical roller valve train) timed about 2 deg late of straight up and 11.4 :1CR which I recon will be pretty safe (on RHS ally heads and flat top pistons).
If you are running iron heads I would take off .5 CR. a full spec of the cam would help, but I think you are pretty close to as much compression as you can run on ally heads and a tad high for iron.
Best regards
Mike

[350matt]

Sorry should have said good old rover is the candidate, the 350 part of the moniker is from when I owned a 350i TVR

and the previous cam was Iksy 282 hydraulic grind

I was coming down in favour of V8 developments Stealth grind for this build

[kiwicar]

Ah
Please ignore my last post I shall go crawl back under my rock
As a general guide about 9 to 9.25 :1 once the inlet valve has closed on a fairly middle range tune on ally heads, (tubular headers, ported heads, decent inlet side etc) as was said before be a bit conservative with the rover as it has very little squish.
Best regards
Mike

[350matt]

hang on tho' I was running 10.5:1 and the engine was making 273bhp and 280 ft /lb on 4ltr with worked heads and intake, why go backwards?

[kiwicar]

Hi
I think you missed the bit "once the inlet valve is closed"
Secondly if you were getting 273 bhp out of a 4 litre Rover I would say you were running a fairly high state of tune, no matter which direction the car was traveling at the time. . . . .
Look at your cam chart, work out when the inlet valve will close fully, work out how much stroke is still to go and using that as your cylinder volume calculate your CR using the correct chamber volume, aim for that to be in the region of 9 to 9.25 to 1. Now there are things that will mean there is some leaway on this, it assumes the cylinder filling at peak torque is about 95% (well that is what I used for my chevy), with the rover you may want to use the figure more like 85 or 90% as the heads are more restrictive, it assules some exhaust scavenging effect. I think Ian Stewart is using a static CR around 14:1 but he has a pretty wild cam and fuel injection (and I suspect some trick heads).
You need to look at the whole package in deciding CR, if the heads are biased to the inlet flow at the expence of exhaust then you may want more exhaust cam timing OR alot of static CR a late closing inlet valve with a lot of overlap, and try and get more sqush and use fuel injection. though the first will tend to make the power "hang on" after peak power at a slight cost to overall numbers the second may make the engine hit a bit of a wall after peak horspower. It is all part of planning out an engine.
Best regards
Mike

[ian.stewart]

mmmmm, I am running far more static comp than 11-1, it was so high when I calculated the comp, I asked for a 2nd and even 3rd check on here, works out at about 13.8-1 !!!! BUT, this is all lowered by using a long duration cam with LOTS of overlap,
If you have ever heard my car run, you would know it has to be something odd with the way it sounds,, but it deffo works, just shy of 290hp on Dave Walkers rollers, and about 275ft/lbs thats from a 3.9 p5 block

[sidecar]

mmmmm, I am running far more static comp than 11-1, it was so high when I calculated the comp, I asked for a 2nd and even 3rd check on here, works out at about 13.8-1 !!!! BUT, this is all lowered by using a long duration cam with LOTS of overlap,
If you have ever heard my car run, you would know it has to be something odd with the way it sounds,, but it deffo works, just shy of 290hp on Dave Walkers rollers, and about 275ft/lbs thats from a 3.9 p5 block

When you say a "3.9 block" is the capacity 3.9 or has it been stroked?

[ian.stewart]

bore, 93.5mm