Reducing compression ratio

[lindemans]

Dear fellow Pelicans,

Another item I consider doing in my 1989 3.2 rebuild is reduce the compression ratio from 1:10.3 to 1:9.5. The fuel required for the high compression version engine becomes harder to obtain.

With 1:9.5 I should be able to run commonly used reguar fuel instead.

First I thought about swapping the pistons, but I calculated that an extra spacer of around 0.75 mm or 0.03 inch between the cylinder base and the case would also give this reduction in compression ratio.

Anyone experience or recommendations with doing this ?

Thanks again for thoughts !

[cgarr]

cant you dial back the timing a bit for different fuels using the rotary switch in the ems?

[lindemans]

I know I can indeed easily delay ignition with 2.8 degrees with a switch in the DME; but am not sure whether this is sufficient to run 95 fuel instead of 98.

[snbush67]

Have you considered machining your heads for dual plugs. Or, there is a set in the classifieds already set up.

Shane

[88-diamondblue]

Increasing the deck height is not a good way to drop compression. Possibilty for detonation increases with too much deck height. Easiest way for the Carrera setup is to have SteveW do a custom chip for you based on what fuel you are going to use. No guesing about what setting to use and safe optimum performance from your 3.2.

[lindemans]

I thought detonation risk is associated with too early ignition and too high compression only; certainly not with LOW compression ... ?

Steve confirmed the car should run fine on regular 95 gas instead of the 98 when all is well configured.
In fact I tried and I know it runs fine in normal use; wouldn't like a high rpm / full throttle detonation bang though so I am very cautious here...

[longhornchris04]

lindemans - Changing the deck height modifies the combustion geometry for the worse... hence the increased risk of detonation despite the lower compression. It is the cheapest option to actually change the ratio.

Still, I'd suggest chipping it to get the right curve.

[lindemans]

Great insights guys, thanks.
I will keep the design in place and don't mess with compression ratio then.

I wasn't sure whether the 10.3 ratio could ever work with lower octane fuel, but if it can reliably be covered by a modified DME mapping I'll go for that one.

My existing DME has a switch to delay ignition by 2.8 degrees; deliberately put in place for markets with lower quality of fuel; maybe that one will sufficiently do the trick. I will touch base with Steve W for sopme expertise on that.

[911st]

Late to the party but I think 911chips.com has the chip tunning that should work ok (less timing and richer AFR I suspect).

Another way to lower "effective" compression is with cams and cam timing.

For example a stock Carrera cam has about 130 deg of compression stroke. A C2 cam has about 124 deg of compression stroke to TDC. You might even be able to advance the C2 cam a several deg's more to shorten the compression stroke further.

Net effect should be more power in the upper rpm's and up to about a .3 or more of a point lower effective compression ratio for a little more margin.

Not an expert so this is just a thought.

[kenikh]

Dialing out timing is not ideal, but will work on DME motors. Another thought is to drill for twin plugs, clearance the plug hole like the stock one, then close the hole with a fastener flush to the head. That will rebate a cc or two, drop compression a bit and leave you ready for modifications in the future. DME high compression pistons can also accomodate about 1mm for a valve pocket to clear high lift cams, so you could take a little material off the piston to decrease compression even more. With 964 cams, you'd be right in the sweet spot of compression without crappy timing.

[JFairman]

At upper rpms the 964 cams with their little bit higher valve lift and longer duration and later closing intake valve during the beginning of the compression stroke comes into it's own because of the inertia of the high speed intake airflow which has a ram effect that continues during the beginning of the compression stroke while the late closing intake valve is still open a little but closing.
More volume of air and fuel air goes into the cylinders at high rpms with longer duration and later timed cams.

Anotherwords more air and hopefully the right mixture of fuel is entering the cylinders at high rpms with 964 cams so the compression is a little higher than SC cams at high rpms (thats why they have a higher rpm powerband and make more horsepower) and can detonte because if that, while there is more valve overlap and less compression and lower efficiancy at low rpms as Keith said.

[911st]

Interesting.

Jim,

Thank you for getting me thinking.

Just spit balling further:

Risk of detonation I believe comes with higher cylinder pressure which has a direct correlation to TQ.

Thus, if the cam change dose not increse TQ peak, it would probably not be increasing the risk.

Both cams are very well developed and are not going to leave much on the table and looking at the shorter compression stroke of the C2 we might be seeing why it works well at 13/1 CR (there are a host of other reasions).

Unless there is something else to improve breathing like an exhaust improvement, I suspect we would not see an increase in Peak TQ or cylinder pressure with such a change. We would probably just see Peak TQ move to a higher RPM because of the different cam timing.

In theory we might even see peak TQ and cylinder go down with a cam change just a hair. But in this case it would probably be a very small change and there are other variables involved.

What I am thinking is we would probably see TQ build a couple hundred RPM later in the lower rpm range and just see it peak and fall off later a couple hundered RPM later with no peak increase. This will however make for more HP.

Again, I do not know if this strategy is valid but I first heard of in in the 70's when a lot of the motor's were high compression and our fuel quality took its first hit.

I would probably just rely on Steve Wong at 911chips.com to do a chip that will keep the motor safe.

[kenikh]

What about additional overlap and RPM increasing VE at speed, generating more power as a result?

[911st]

Kenik,

Interesting question and I still have a lot to learn.

In general with 'optimized' motors, all other things being equal, VE tends to fall with increases in rpm due to other factors such as friction, windage, a shorter power stroke to capture combustion energy and other variables that work against us. This is just a genneral principle.

However increased frequency of the power strokes with greater rpm can more than offset these factors and increase peak HP.

Thus, if we had two optimized motors. On might make say 200# at 5000rpm and 207hp at 6000rpm. Another might make 190# at 7000 rpm and make 262hp at 8000rpm.

What a good tuner should do is increase the VE in other ways along the desired operating RPM and tune any bump in VE from intake and exhaust reversion or scavaging to happen where he wants it.