Mopar Muscle Association UK

Yes, compression ratio is mechanically fixed (well in most engines to date anyway!). Cylinder pressure is what you really want to know because that is what affects the real performance or detonation resistance. However because that is not easily measurable CR is often quoted as the nearest available data. Still of use but big cams etc bleed off pressure so do affect what you can run. You want as much as possible but not so much you get detonation. In reality comparing your combo with other similar is a decent way to make an estimate of what CR is acceptable.

I think your Q stems from the fact that aluminum conducts heat away more quickly than iron which reduces the pressure. So conventional wisdom is that with ali heads an extra 1 point of CR is required, it's a decent rule of thumb. Note though that this is often mistakenly touted as an advantage of ali heads - more CR is good, I can run more CR with ali. Actually you need the extra CR to get back what you are losing over iron heads, so no power advantage.
Good weight savings though!

MrNorm wrote:
I think your Q stems from the fact that aluminum conducts heat away more quickly than iron which reduces the pressure. So conventional wisdom is that with ali heads an extra 1 point of CR is required, it's a decent rule of thumb. Note though that this is often mistakenly touted as an advantage of ali heads - more CR is good, I can run more CR with ali. Actually you need the extra CR to get back what you are losing over iron heads, so no power advantage.
Good weight savings though!

Cheers MrNorm, that is exactly what I read too. Bacca has since informed me that he expects the 340 to make around 10-1 CR and will require Super Unleaded to run. But if what you are saying above it true then the CR will be more like 9-1 correct? I mean is this how it works?? So normal Unleaded should be fine,yes?

If you are on rig money I'd run it on super anyways. Better to be safe than sorry....Ping! Ping!

I would say 10:1 with alli heads is on the low side.

I would use 11:1 and use super unleaded.

But 10:1 with normal unleaded sounds right for a Scotsman.

Dave wrote: But 10:1 with normal unleaded sounds right for a Scotsman.

Sorry! I forgot to factor in the tightness!

As ever it is a number of factors.
Cam is a big one - lots of overlap helps but is more strip orientated.

I could run normal unleaded on a 12.3:1 ally headed motor 'cos of the large cam. I ran super or C12 at the track.

Ignition timing is another key factor, I am a convert to running loads of initial

Yes. The cam (valve timing) is the main dictator of cylinder pressure after compression ratio. Bigger cams need more compression ratio than smaller cams.

on the other hand and in general
I'd hazard a guess that.....................

massive overlap should not be used as a band aid to cover up the fact that you have too high a CR for your intended application and/or fuel availability

PS you don't, which is why i said in general

anyway on standard motors by the time you take production line tolerance into account
an advertised CR of 10:1 on a production line engine can be anything from 8:1 up....they tended to err on the side of caution with deck height and head thickness....but not necessarily replicated on aftermarket parts....they are probably worse

Dave

Yes, I agree 10:1 with iron heads, you should be able to go to 11:1 with ali....but again it does depend on the combo as everyone keeps reinforcing. With a stockish cam even that might Bananarama! a bit. So with ali heads your 10:1 should be fine, not sure you will need the super unleaded. Frankly I would build it to run on regular, otherwise you take a hit every time you use the car.
Also as said (some good advice in this thread) you should measure everything to be sure. With your CR you probably have enough safety but it just makes sense to know rather than guess.

I'll be running 10:1 with iron heads and plan to use regular...we'll see if that works but I'm not expecting a problem in my application.

Finally if you can possibly build it to run with quench you definitely should - it is free detonation resistance, and/or more power & economy. Depends on whether your Eddie heads are closed chamber or not, but if they are run no more than 40 thou clearance and you will be safe as houses with that CR.

Anymore than 10.5:1 on Alu heads is asking for trouble when running pump fuel , short duration cams will build more cylinder pressure than long duration cams , another critical factor is lobe sep , wider lobe seps create less cylinder pressure for a given duration.

I think valve timing plays a bigger part in regards to detonation than quench #s , an example is the 605" , quench #s are around .300" , lobe sep 114* , total spark lead is 35* running on 97 ron.

Running high initial figures is sometimes a band aid for an overly rich carb.