Compression Ratios

jeboa · Post by **jeboa** » Sat Aug 10, 2013 12:41 pm

Just a general question.

I was trying to read up on compression ratios for Rover V8's.

What's regarded as the maximum you would build an engine to run (if using 99RON fuel)?

mgbloke · Post by **mgbloke** » Sat Aug 10, 2013 4:30 pm

A lot depends on which cam you would be using.
My CR is 10.8:1 with a mechanical 238 cam and it will happily run on 95 with normal road driving, but I use 97+ for thrashing it round the track.

Mark

sidecar · Post by **sidecar** » Sat Aug 10, 2013 5:37 pm

I run 10:1 and I use a Piper 285 cam. Personally I think that 10:1 is high enough for a road car with a 'hot' road cam. I know that a lot of people with the old P5 (or is it 6) engines have problems with the standard 10.5:1 that they run. (It will be running this high CR with the standard mild cam I guess)

BISH V8 · Post by **BISH V8** » Sat Aug 10, 2013 7:16 pm

My Compression is 11.3 i run a 238 Cam but have to use Shell V Power fuel
for best results

jeboa · Post by **jeboa** » Thu Aug 15, 2013 8:02 pm

Thank you. I have a second hand 4.6 engine built by V8D some time back. It has a 248 mech Cam. Speaking to Rob, he confirmed that usually they build to around 9.75:1 CR.

It has a later (x-bolted) bottom end with early 14 bolt heads (36cc comb chambers) so I'm guessing that its using 4ltr pistons to get this sort of CR.

The CR is probably a bit low for the Cam that is in it currently. It's low on torque (260lb/ft peak), but is 280ish bhp and a nice revving engine to drive.

ian.stewart · Post by **ian.stewart** » Fri Aug 16, 2013 7:19 am

Depends on cam, If you use an overlap cam, you bleed compression off, I was running a healthy 13.9 ;1 cr on my last engine. that on paper is far too high to be safe on the street, It worked!!, although the static comp was 13:9, the dynamic compression was nearer 11:1 (as far as I could work out) and ran quite happilt on 99ron

jeboa · Post by **jeboa** » Fri Aug 16, 2013 8:47 am

I'm not 100% sure on the overlap, but judging by the maximum vacuum I can get at idle (<>900rpm), which is 75kpa and the strong petrol smell from the exhaust I'm guessing it's reasonably high.

kiwicar · Post by **kiwicar** » Fri Aug 16, 2013 9:13 am

Hi
as a rough guide if you take the mechanical CR and multiply it by (1+Cos Ic)/2 where Ic it the point where the intake valve is at .02" lift, effectivly closed, that can be a good guide to the effective compression ratio for that cam/engine combination. If you cant get the .02" lift point in degrees take the average of the fully closed point (from the advertised duration) and the .05" lift point (more commonly quoted). You want this number to be somewhere between 9.25 and 10.25 for a road engine.
If you know a cam CR combination that already works then you can just use the .05" point for both cams and both CRs to compare both combinations.
Best regards
Mike