View Full Version : Help required with compression ratio...

19-08-04, 06:02 PM
Hi, I know this has probably been asked before.......

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. 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 (2.02Ē 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 donít think I want to use forged pistons as everywhere I look says donít 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.:thumb: :thumb:


20-08-04, 07:19 AM
Hi Andy,
unless you want to run octane boosters or use super unleaded, I would advise to not go higher than 10.5 or 10.75:1 even at that you may suffer some mild detonation if you pick up some crap fuel.

20-08-04, 11:16 AM
You should choose your compression ratio based on your cam choice. Comp Cams will give you the best range for that cam. It's a bit chicken and egg really but you should be fine with 10.5:1.


20-08-04, 02:14 PM
Thanks for the replies.

I think.....do you know I dont know what to think anymore!:o I get my head round one possible combo and then something makes me think again and now my head really hurts!!

Using UEM's calculators the KB115's give 9.1:1 static and 7.53:1 dynamic using the 268H cam, so do I deck the block to go higher? Can I do this and not get a piston, valve clash? Arhgg! Why the hell didnt I buy a turn key!!!

Now Im 2nd thinking my cam choice as Ive received varying feedback on the 268H & XE268H, also putting them into Desktop dyno shows that theu give around the same performance as the performer plus cam, infact the performer plus is better than the 268H!?!:angry:

Edelbrock naturally emailed to say use the rpm cam with the rpm heads but Ive read on this site that people have removed that one in favour of the 268H or similar due to the lumpy idle, then if you ask in the US they say go aggressive after all its supposed to be a muscle car not a Nissan Micra!

So I just dont know anymore, Im going back to the rear axle rebuild as it requires less thought!


20-08-04, 04:16 PM
I run the full Monte Edelbrock Performer RPM package and I fing the idle OK at around 700 ish. It can be a little lumpy to drive at slow speeds but it's acceptable if you stay in a low gear to keep the revs up a bit. :D :D

20-08-04, 05:25 PM
I spoke to Brain at Repower earlier regarding cams etc and he basically said youve brought the rpm heads so put the rpm cam in to make the best use of the heads! Cant argue with that really!! I suppose Edelbrock have done the hard work by comming up with the cam profile to suit the rpm heads.

He aslo suggested the Keith Black KB116 pistons with the .200 dome, this will give CR around 10.2:1 according to UEM's calculator, although the piston down bore value cant be right due to the pop up.

Anyway Im now feeling more like the full rpm kit coming on! :D

One draw back is the head gaskets in my Fel-pro kit arnt suitable for the rpm heads so Ill be flogging them off then!! The head gaskets will cost more than the whole engine kit cost me!!

Cheers for the help Guys, what this space (currently filled with a oily engine block!!)

I run the full Monte Edelbrock Performer RPM package and I fing the idle OK at around 700 ish. It can be a little lumpy to drive at slow speeds but it's acceptable if you stay in a low gear to keep the revs up a bit. :D :D

20-08-04, 05:37 PM
Andy - the RPM cam has had a "bad press" as far as I can see. Don't forget that one man's "streetable" cam is another's "unstreetable".

Don't forget that this cam is freely available to (and much used by) a litiginous bunch of folks called "Americans".

If it was as bad as you seem to think, Edelbrock would have withdrawn it years ago. The specs for the rpm cam show that it is hardly radical. ANY cam that improves upper-rpm performance WILL affect idle. Brian's advice is spot on - go with the matched components.

Lumpy idle is a "badge of honour" amongst most US petrolheads. In fact they go out to find a cam which specifically gives that idle. I like a lumpy idle myself as it happens. Who cares? It's a totally unpractical car anyway for goodness sake!

The only exception to this is the blower-brigade. They can run wimpy cams and still pull 600hp. Too easy. Sorry Rob.

EDIT: BTW - the advice to match CR to your cam is spot on.

20-08-04, 05:40 PM
My SBC 350 is fine with 9.5 and the full rpm package, if a little peaky. For road use I would avoid 'magnumitis' - you'll lose tractability and mid range torque and end up in trcation bragging about the bhp that fired you through the hedge backwards! Unless you spend your life over 5,000 rpm on track days I suggest a little moderation - you will not feel inadequate in the trouser department!

20-08-04, 07:22 PM
Different things for different people

If anybody wants it I have a performer RPM carb that did 1200 miles.

For me it was a pig to run at low speed and a real t--- when queing into car shows etc. At 30 mph anything but 3rd gear or below and it felt like a ferry in bad weather. Interestingly enough Kenny has stopped using or reccommending them for this reason.

I swapped for the Comp cams 270 and found that in anything other than a straight line the car was much quicker and more responsive due to the better torque characteristics.

In fact in a straight line the only difference was around the 5000 RPM mark where the RPM cam had slightly more left, but not enough to choose over the much better manners of the 270.

I think much better gains can be achieved by getting the flow characteristics and decking/preparation right. I found benefits from improved quench Also 9.5 to 1 is fine with both the RPM and the 270.



20-08-04, 07:23 PM
Sorry I meant cam not carb.

20-08-04, 07:26 PM
God I missed it

Wilf you are back. Thats really brightened up my day and no I am not taking the -iss.

Great to have you back on the patch,

ps still havent forgotten the party

20-08-04, 09:13 PM
Party was great. Back in July as I recall. Sorry you missed it.

22-08-04, 07:25 AM
Thanks agian for the replies everyone, only just picked them up due to total ADSL failure! Now up and running again.

I think Im going to give the rpm a go and see what happens.:thumb:


22-08-04, 01:12 PM
hi andy.
i would advise on flat top pistons, deck your block so as the pistons are level with the top of the block. this will help with you squish clearence, and will enable higher compesion ratio to be run.i would use a felpro head gasket these normallly work to 40 thou compressed, and if you reqire more compression skim your heads.
i would also look at the forged pistons as they are not much dearer and are that much more stronger.

22-08-04, 01:36 PM
Decking as you describe would effectively bring the piston down bore at TDC to zero I pressume? Given the combo of rpm heads & big valves will I see any clearance problems when using the rpm cam? Or is it a case of buy the bits and see (something I would very much like to avoid, Id like to buy the bits knowing they are going to fit!!) UEM's calculator give 9.3:1 using 60cc heads, a 0.039" gasket thickness and zero deck clearance using the KB115 pistons.

How does that sound?


22-08-04, 01:45 PM
Just located the KB281's, has anyone used them instead of 115's? they seem to give same CR but are lighter, is that better?

22-08-04, 03:20 PM
decking will bring the piston level with the top of the block at tdc. this will help with your squish , which will give you give you more power , and enable you to run higher compressions on lower octane fuel.
you ask about clearence problems, i would always recommend you check when you dry assemble the engine for any interferance problems. if you were to use pop up then i think you may encounter , valve hitting the pistons , which would mean notcing the pistons.
if you would like me to put your combo on desktop dyno for you i am happy to do so if you could give me all the details.

if you can lighten up your pistons rods etc this will help with less stress on your rotating assembly, and you should be able to run more rpm.if you lighten it up more than stock weights then you will need to have the engine balanced, if you were not going that route already.

22-08-04, 04:33 PM

Thanks for the offer, Ive tried running my set up through desktop dyno (2000 version) and I get results as per the attached picture, hope you can read it, with the setup as per the 2nd attached picture. I dont know if I have everything correct for the rpm packagae, I have been fairly careful checking things out but would be grateful if you could check it over, do you have enough information there?

Setup is:

1971 302 bored to 4.020 (not decided yet but block is stock size at moment.)
Eagle SIR5090FP rods
Stock replacement crank rated to 500 bhp, is this better than my seasoned crank Ive got in my engine?!?
KB115 or KB281 pistons, (281's are lighter) compression height for both is 1.608" valve relief is 6.5cc for both.
Fel-pro 1011-2 head gasket as recommended by Edelbrock, compressed thickness 0.039"
performer rpm heads, 2.02" intake version.
performer rpm cam
performer rpm intake
Performer 600 cfm carb
1.6 ratio roller rockers

Just noticed that the CR is set at 10.5:1, that may be a bit high with the 115's or 281's.

I think thats the important bits, thanks for your help.

Cheers :thumb:

22-08-04, 05:56 PM
Hi Andy,

I am building my 350 chevy at the mo and I was gonna use the rpm cam but decided againist it due to the fact you have to rev the tits out of it. I have chosen the XE274H. Gives loads more torque low down and fits between the 268 and rpm cam. Don't forget Andy that torque throws you back in your seat not HP. Its a V8 not Lotus 7. If you look at your power graph you will notice that your engine will peek its torque at 5000rpm which I think this is too high. You want it all in at about 4000 to 4500rpm. Another thing that you wanna think about, is your diff ratio, your going to need at least 3.6:1 for a rpm cam to work in a 302. bear that in mind! See the com cams web site and read the articles.Your right you are building a musule car, but its no good if it drives like shit on the road. As for compresson ratio no need for anything more than 9.5.1. My little combo in my chevy will give me about 380bhp and 420IBFT of torque.

Hope this helps

22-08-04, 06:49 PM

Thanks for the input, your comments put me back looking at the 268H or XE268, both of these cams get good comments both here in the UK and on the american sites. The graph I have attached is for these two cams side by side, this time using 9.3:1 CR as given with the KB281 piston. I guess not using the rpm cam wont make the best of the rpm heads, perhaps thats just a trade off for driveability.
The diff I have is a 3.07 powerlock, hopefully that will be OK.

What do you think of the XE268 curve vs 268, I feel the XE would be better although the 268H give slighty more low down torque. Plus less valve lift than the rpm so lees chance of clearance issues.
Again I throw the floor open to comments!! :thumb:


22-08-04, 07:27 PM

That looks much better,now you have said you have a 3.07 diff, the rpm cam won't work in a 302, you would need a 350 0r a 383 to pull that diff. I have been down this road bofore. I think your right with the XE268 it would work much better. Why are you using rpm heads? have you already bought them? anyway the XE268 and the rpm heads will work great together.This is comp cams best selling street cam. Hope this make sence as I have had a few beers.


22-08-04, 07:29 PM
i have run that through dyno2003, , also i have tried a slightly larger cam, try a comp cams 270h,and small tube headers, you might be surprised . also there seems to be no or little loss in using a single plane manifold , a lot more top end power. you can always use a spacer plate under the carb to bring the power in sooner.
the cam you are using has only lift at 450 thou that seems a bit on the low size for the rpm heads , well thats my 2 cents .

22-08-04, 08:37 PM

You are making perfect sense, I find a few beers helps clear the mind of non essential thoughts, too many however also does for the essential ones like "how to stand up"!!:D
Rightly or wrongly I have already purchased the rpm heads and intake, may have a been a hasty decision but hey Im stuck with them now so Ill make the most out of them. I am now warming to the XE268 as the 268H was my choice for a long time the XE is a bit better.


Ill try playing around with the settings you mentioned, Ill also look into the 270H cam as I asked for help Im not going to discount anything until Ive checked it out fully.

As usual this forum is really helpful!! Many thanks to everyone.:thumb:

22-08-04, 08:50 PM
no worries.