Compression ratio help...

[Andy(UK)]

Hi,

Im looking for some help on a topic Ive never really got to grips with. Compression ratio.

Im trying to decide what CR to aim for given the grades of fuel available in the UK, 95 & 98 RON unleaded. Im building up a 1971 Ford 302 hopefully at stock or the lowest overbore possible. Ive got Higher performance stock size crank and rods, the cam is still up for discussion (cue another posting) but probably a comp cams High Energy 268H or the XE268H. Ive got a new set of Edelbrock rpm heads (2.02” intake set) performer rpm intake and I will get either a 500 or 600cfm Edelbrock carb to sit on top.

So what compression ratio would I be best aiming for given UK fuel grades and to make the best out of my engine bits?

I don’t think I want to use forged pistons as everywhere I look says don’t use them on the street (various websites, and my Des Hammil book).

So if anyone has any words of wisdom or experience they can offer I would greatly appreciate it.

Cheers

Andy

[MOTORHEAD]

Based on what I've read on this forum, 11:1 should work with 98 oct and aluminum heads. If it doesn't, a little octane booster or avgas should make things all right. My 427 with edlebrock heads is set-up for 9.5:1, which I'm doubtful I'll be happy with, as the previous owner was complaining about needing more power, but I'm going to drive it that way to get a feel for the car, asess the engine performance, and make changes later. If I had it to do over, I'd go for 10/11:1, but my engine is already together.
TTFN
Ted

[wilf leek]

Andy - also ask Comp Cams what CR they recommend for teh cam you are considering.

Cam and CR need to be matched.

BTW, 9.5 :1 with iron heads is "safe" for Uk fuel, 10.5 :1 with ally heads. Just a rule of thumb, there are other factors.

[Andy(UK)]

Back here again.....

Ive emailed comp cams to ask that question but as yet they have not responded, time to ask again I think! I did a bit more working out yesterday and woulnd up with between 9.5 & 10:1 using the KB pistons (cant remeber exactly thats why the range!) they also figured dynamic compression to be around 7:1?!?

Cheers

Andy

Originally posted by wilf leek


Andy - also ask Comp Cams what CR they recommend for teh cam you are considering.

Cam and CR need to be matched.

BTW, 9.5 :1 with iron heads is "safe" for Uk fuel, 10.5 :1 with ally heads. Just a rule of thumb, there are other factors.

[69 Mach I]

I am running 11:1 compression on 98 RON and alum heads with no problems. Is the shell V Power available in the UK? That is 100 RON or about 95.8 PON (US grade of Octane).

[brainsboy]

As one person suggested your cam is going to have alot to do with it. Looking at those cams you will never get away with an 11:1 compression, iron or aluminum heads, your cylinder pressure will be to the sky. Compression ratio is not as important as the amount of cylinder pressure you will develope. You can run 12:1 on the street at low rpms with no problems with a cam that has alot of valve overlap because the overlap keeps the motor from building alot of cylinder pressure, of course when you jump on it you will detonate but you can get my meaning here. In your case those cams are probably designed to build alot cylinder pressure at low rpms and would work best for 8.5 to 9.5, running 11:1 is looking for trouble.
I would say no more then 10:1 with those cams


Ben

[69 Mach I]

Originally posted by brainsboy


As one person suggested your cam is going to have alot to do with it. Looking at those cams you will never get away with an 11:1 compression, iron or aluminum heads, your cylinder pressure will be to the sky. I would say no more then 10:1 with those cams

Ben

When I was building my engine, I was hesitant to run 11:1 even though my builder kept assuring me it was ok. I studied up on the subject and found that the consensus opinion is that alum heads allow you to run a full point higher than iron cyl. heads. I went with it and so far have been ok.