Help required with compression ratio...

[Andy302]

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 (2.02” 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.:thumb: :thumb:



Cheers

[kdavies3]

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.

[markn]

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.

Mark

[Andy302]

Thanks for the replies.

I think.....do you know I dont know what to think anymore! 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!

Cheers

[kdavies3]

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.

[Andy302]

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!

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!!)

Originally Posted by kdavies3

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.

[wilf]

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.

[callum]

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!

[mikey]

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.

Mike.

Mike.

[mikey]

Sorry I meant cam not carb.
Mike.