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Is 12.5:1 compression ratio too high for a 3.0 engine street car?

Looking at a '78 SC that has some 12.5:1 pistons among other modifications. From everything I've been reading, that appears to be more of a racing ratio and requires racing gas. Can anyone confirm that this can be run on the street with street gas?

Perhaps there's other mods that allow this ratio to work on a street car. Here are the other mods:

Twin plugged 3.0 engine
JB racing twin plug distributor
C2 964 Cams
Mahle pistons 12.5:1 compression
ARP bolts and fasteners
Titanium retainers
BS valve springs
930 tensioner with race locks
Two stand alone MSD ignition systems with adjustable rev control
Clewett plug wires
Turbo valve covers drilled for lower spark plugs
Stock CIS

Would this be a streetable 911 SC?
Old 05-16-2018, 04:17 PM
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Remember our cars are aircooled and therefore thermally loaded and challenged. Add a older combustion chamber on top of this without full race cams to bleed off dynamic compression and I'd expect it to need 100 octane especially in moderate climates. You'd need gear, pull timing, and rich'n the mixture to offset detonation.

Short answer NOT IDEAL and would likely be doggy even if made to work.

I don't really care for combo of 3.0 heads with mild cam and high compression breathing through stock CIS. BUT i'm no CIS expert
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Old 05-16-2018, 04:37 PM
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EFI and E85 would be quickest fix if you love all other factors of the car.
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Old 05-16-2018, 04:38 PM
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Quote:
Originally Posted by sadcaper View Post
Twin plugged 3.0 engine
Talk about your ignition timing.
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Old 05-16-2018, 04:46 PM
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SC engines, compression and timing.............

Quote:
Originally Posted by sadcaper View Post
Looking at a '78 SC that has some 12.5:1 pistons among other modifications. From everything I've been reading, that appears to be more of a racing ratio and requires racing gas. Can anyone confirm that this can be run on the street with street gas?

Perhaps there's other mods that allow this ratio to work on a street car. Here are the other mods:

Twin plugged 3.0 engine
JB racing twin plug distributor
C2 964 Cams
Mahle pistons 12.5:1 compression
ARP bolts and fasteners
Titanium retainers
BS valve springs
930 tensioner with race locks
Two stand alone MSD ignition systems with adjustable rev control
Clewett plug wires
Turbo valve covers drilled for lower spark plugs
Stock CIS

Would this be a streetable 911 SC?
About 5 years ago, I built a 3.3SS twin plug engine for my street SC. I also used Mahle pistons and 964 cams, but at about 11:1 compression. I used the Electromotive XDi crankfire ignition that has fully adjustable timing via 4 setting knobs on the control box. My engine runs just fine using 93 octane pump gas with 8 degrees initial timing and 20 degrees additional coming in at 3000 rpms. Total advance is 28 degrees. One thing I would want to know is if the Mahle pistons are of the sharply crowned type or the relatively flat type crown. The high crowned pistons would be more likely to keep the ignition flame front from spreading evenly over the combustion chamber, making the absolute timing much more critical. One of the mods that I made to the CIS on my engine was to add a push switch to the throttle linkage that allows the cold start valve to operate at full throttle. This makes the fuel mixture richer while under full load. The idea here is to help avoid detonation and/or a burned valve from a lean condition. Hope this helps.
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Old 05-16-2018, 05:28 PM
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Quote:
Originally Posted by fred cook View Post
About 5 years ago, I built a 3.3SS twin plug engine for my street SC. I also used Mahle pistons and 964 cams, but at about 11:1 compression. I used the Electromotive XDi crankfire ignition that has fully adjustable timing via 4 setting knobs on the control box. My engine runs just fine using 93 octane pump gas with 8 degrees initial timing and 20 degrees additional coming in at 3000 rpms. Total advance is 28 degrees. One thing I would want to know is if the Mahle pistons are of the sharply crowned type or the relatively flat type crown. The high crowned pistons would be more likely to keep the ignition flame front from spreading evenly over the combustion chamber, making the absolute timing much more critical. One of the mods that I made to the CIS on my engine was to add a push switch to the throttle linkage that allows the cold start valve to operate at full throttle. This makes the fuel mixture richer while under full load. The idea here is to help avoid detonation and/or a burned valve from a lean condition. Hope this helps.
I am not sure what piston style was used. The current owner says it runs just fine on 91 octane gas. Says he hasnít had any issues running on street gas. Would the dual MSD ignition system be retarding the timing to prevent ignition perhaps similar to the electromotive system?

Last edited by sadcaper; 05-16-2018 at 05:55 PM..
Old 05-16-2018, 05:47 PM
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12:1 is far higher than pump gas can handle. Retarded ignition is not the answer. It can help prevent detonation, but at the cost of performance. My stock 9.3:1 3.0 can handle 6 degrees advance beyond factory spec. 20/21 cams. Runs pretty good.
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Old 05-16-2018, 07:13 PM
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If it were me, I’d consider modifying the engine to lower the compression ratio. If you had high lift cam with a lot of overlap like a race cam GE60 on up, then that high compression would make more sense. For 964 cams I don’t think you’re benefiting from it. You might get same performance at a lower ratio with that cam using pump gas. I wouldn’t risk the possible detonation.
Old 05-16-2018, 07:34 PM
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Are you really sure it's 12.5 to 1?

It's the dynamic compression ratio that matters anyway so you need to keep any discussion of static compression with that in mind.
Old 05-17-2018, 01:23 AM
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Quote:
Originally Posted by Peter M View Post
Are you really sure it's 12.5 to 1?

It's the dynamic compression ratio that matters anyway so you need to keep any discussion of static compression with that in mind.
Good point! Frequently see "12.5 to 1" pistons actually measure out below their rated spec. Further, as referenced above, twin-plugging gives one a lot more leeway advance-wise than a single plug setup.

Test drive the car on a hot day and push the engine to detonate (high load/lowish revs/full throttle) next to a jersey wall/under a bridge/etc. to confirm the seller's observation.

Another (potentially important) data point -- how many miles on the engine since the rebuild?

Last edited by darrin; 05-17-2018 at 04:25 AM..
Old 05-17-2018, 04:22 AM
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Quote:
Originally Posted by Peter M View Post
Are you really sure it's 12.5 to 1?

It's the dynamic compression ratio that matters anyway so you need to keep any discussion of static compression with that in mind.
Iím not sure but thatís what the seller is saying. I donít have documentation otherwise. Any way to tell? Compression test or leak down?
Old 05-17-2018, 04:22 AM
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Quote:
Originally Posted by Peter M View Post
Are you really sure it's 12.5 to 1?

It's the dynamic compression ratio that matters anyway so you need to keep any discussion of static compression with that in mind.

Dynamic compression identifies compression at various volumetric efficiencies which are dependent on RPM, and cam/head flow, hence 'dynamic' i.e. altering. Static compression level is eventually realized when VE reaches 100%, which is dependent on head flow and cam selection. Peak VE is also when peak engine torque is made. A well-built and flowed race engine often reaches 105-110%+ VE meaning static compression is being exceeded. So if this particular engine in question eventually reaches even 90% VE (thus 11.25:1 max compression) then premium pump fuel still sounds like it's not quite enough to handle things without detonating. With the engine in question still running CIS, the engine might just be ok, but it's not a project I would want to take on just to find out. If it were ever converted to carbs or ITB EFI, VE would undoubtedly push VE too high. In the end, not enough info is known to really be able to effectively say as we don't know head flow, stock CIS flow, thermal efficiency of the head etc. Stick with what's known for certain
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Old 05-17-2018, 05:08 AM
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Another (potentially important) data point -- how many miles on the engine since the rebuild?
About 30k on engine/tranny since rebuild.
Old 05-17-2018, 05:28 AM
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About 30k on engine/tranny since rebuild.
interesting, unless the seller is lying about it running fine on 91 octane (where is car currently located? as running at a high altitude can reduce octane requirements -- e.g. 91 octane at 8,000 feet is going to have better anti-knock properties than 91 octane at sea level), it would seem that 30k is a lot of time for the engine to grenade . . .
Old 05-17-2018, 05:31 AM
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Originally Posted by darrin View Post
interesting, unless the seller is lying about it running fine on 91 octane (where is car currently located? as running at a high altitude can reduce octane requirements -- e.g. 91 octane at 8,000 feet is going to have better anti-knock properties than 91 octane at sea level), it would seem that 30k is a lot of time for the engine to grenade . . .
Well, he does live in Denver...but the previous owner before him lived in Chicago.

Would running 87 gas resolve any potential issue? Or are you losing power then by doing that?
Old 05-17-2018, 05:35 AM
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the octane rating in gas is a measure of its antiknock capacities. So, the higher the octane, the better able to resist knock. E.G., 100 octane racing gas is needed by very high compression engines and an engine that runs OK on 91 octane gas would have a much higher probability of knocking with 87 octane gas. The issue here is that modern cars have knock sensors that will automagically retard timing (and reduce power) when knock is sensed, while older (e.g. pre 964) engines do not. Another problem is that these cars' engines are already pretty loud and can significantly knock without being heard while driving.
Old 05-17-2018, 05:41 AM
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Originally Posted by darrin View Post
the octane rating in gas is a measure of its antiknock capacities. So, the higher the octane, the better able to resist knock. E.G., 100 octane racing gas is needed by very high compression engines and an engine that runs OK on 91 octane gas would have a much higher probability of knocking with 87 octane gas. The issue here is that modern cars have knock sensors that will automagically retard timing (and reduce power) when knock is sensed, while older (e.g. pre 964) engines do not. Another problem is that these cars' engines are already pretty loud and can significantly knock without being heard while driving.
Ah sure, I had it the wrong way. So closer to sea level requires higher octane rating than higher altitude location to prevent knocking.
Old 05-17-2018, 05:44 AM
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Trying to determine if this car is even worth getting a PPI done or if the engine modifications are too extreme/unknown and not worth the effort. It's out of state so can't test it personally before committing.

I don't want a project car. :-)
Old 05-17-2018, 05:59 AM
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sadcaper, I sent you a private message
Old 05-17-2018, 06:01 AM
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Quote:
Originally Posted by sadcaper View Post
Trying to determine if this car is even worth getting a PPI done or if the engine modifications are too extreme/unknown and not worth the effort. It's out of state so can't test it personally before committing.

I don't want a project car. :-)

In another thread, you were hesitant to spending $300 for a PPI and then really hesitant to spend another $200 for a leak down test on a $75,000 asking price Carrera RS clone.

Realize, these old air cooled 911's were dubbed "Hobby Cars" more than 20 years ago. Let that sink in. They are old cars. They all will require some work beyond scheduled maintenance. Repairing them yourself is part of the hobby. That's a pretty good reason for this outfit that sells parts to have a message board where us hobbyists gather from across the globe for help and advice.

Murphy's Law tells us "what can go wrong will go wrong". You do realize that the English translation of "Stuttgart" is "Murphysboro", don't you?

Last edited by SCadaddle; 05-17-2018 at 07:59 AM..
Old 05-17-2018, 07:54 AM
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