X compression ratio increase = X HP increase
 

Yesterday I talked to the machinist who is completing the work on my top end and we got talking compression ratios, etc. I have an 82 SC with a Porsche rated 9.3:1 CR. He explained to me the Porsche rated CR's are actually .25-.50 less than stated, so my 9.3:1 CR is actually around 8.9:1 due to CIS piston dome size, valve cut out, etc. So I asked him about the JE 9.8:1 pistons, and he said these are true 9.8:1 pistons. This would be close to a whole CR point increase which would be a 'very nice upgrade and would complement the porting and new cams nicely'.

My question is, what would be the equivalent HP (approx.) increase if I performed this mod. I have the Alusil cylinders which would need to be Nikasil replated to work with the JE pistons....a $1900 total cost for both P/C's. If I'm going this far, why not a 3.2 P/C upgrade with the 9.8:1 CR. Would this work well with CIS? Just want some input and suggestions.

Thanks...

I've been over the numbers on about 30-40 different 911 configurations. I doubt that you'll see much of a HP increase with a bump in the CR. HP is more dependant on the amount of air consumed by the motor and is thus impacted more by cams, porting and the resulting rev's. A higher CR generally improves the thermal efficiency of a motor and so should increase the torque. Looking at torque (BMEP) charts for a number of configurations though, there wasn't a drastic change in the BMEP between a 2.0E or S and a 2.4E or S even though the CR dropped significantly in both cases. My conclusion was that in the normal (8-9:1) range of CR's, that a 911 motor is relatively insensitive.

It can also help fill in the off-cam running of an engine in cases such as the S where the cams' large overlap reduce the static CR significantly. So the peak HP and torque #'s won't change, but the usable torque range will be extended at least in lower rev's. In an air restricted CIS engine, I could imagine it helping some, but more in the mid to higher rev's when the air meter is restricting airflow.

Someone on this BBS made a good analogy to an engine's output being like a bike rider. Torque is analogous to how hard the rider can push on the pedals. HP is how fast the rider can make the move the bike by pumping hard AND fast.

Increasing the CR should help the torque, but have a much smaller impact on the overall HP.

Thanks John, that makes sense. Don't know if the added torque is worth $1900 though. If I had a decent estimate of HP/torque increase, that would help my decision.

Its about 10% per compression point. One of the most dependable and quick ways to more performance.

Jack,

I had a feeling you'd chime in. Thanks;)

So, I'm looking at a 15-20 HP increase. What about torque?

Should be the same.

Here's the data that I have which suggests a somewhat mixed impact. In general it appears that a higher CR helps at lower rev's, especially when "off-cam". This may also appear to apply once the engine has peaked.

Once the engine comes "on-cam", it doesn't appear to offer any benefits, or maybe a better way to say it is that other factors such as combustion chamber design and the resulting combustion efficency, etc. have a larger affect and the overall affects of CR are negligible within the 8.0:1 - 9.9:1 range.

http://forums.pelicanparts.com/uploads/BMEP vs CR.jpg

A couple of notes:
A) Note that in '70 Porsche changed the design of the combustion chamber in the head which by most accounts improved combustion.
B) Unfortunately I don't have complete data for the '69S, so I didn't include it here.
C) The '72 and '73 E camshaft had the same profile as the earlier E camshaft, but with a wider lobe angle which reduced the overlap. The '72E also appears to have been mildly port constricted at peak RPM's compared to most 911 motors. Draw your own conclusions.

So is adding CR going to be like adding cc's? I don't think so.
Can it make the engine stronger, at least at certain points in the rev range? Certainly.

John,

Thanks for the tips. Both Walt (engine builder) and EBS mentioned that an added CR point would create more torque throughout the RPM range, espically noticable down low....where its needed most. Walt gave me a call yesterday and said the machine work is complete, so I'm at the point now where I need to decide to have the Alusil's shipped back home or have them shipped to EBS for the Nikasil replating and the 9.8:1 JE's.

If I do decide to replate (I think by USChrome), has anyone heard any negative comments about its durability compared to Mahle's Nikasil process/durability?

You can't go wrong with higher compression, unless you run into detonation. Otherwise its all a big plus.