Dynamic Compression Ratio

[Pork Chops]

I’m preparing for a turbo specific build on a 3.2. There may be several topics I’d like feedback on so you may see me pop up a few times, and to keep the discussion on track I’ll try and keep it narrow.

It’s full EFI. It’ll be tuned to run on 98 RON fuel and E85 via a flex fuel set up. I’m considering compression ratio and in the reading I’ve done over the last few days it seems that cam choice is a large determinant of dynamic comp ratio.

I’ve been considering the 964 cam and 8:1 static and boost of anywhere from 15 to 20 psi.

I’d love some feedback on what dynamic ratio would result and given the fuel types I’ve mentioned how appropriate it may be. If you’d like to suggest a better cam then please do so BUT please ensure you relate it to the dynamic comp ratio which is the heart of what I’m trying to explore.

Many thanks.

[Tippy]

I run 93 octane with 7:5:1 up to 1.5 bar (22PSI) no problem with both SC/Carrera 3.2 cams and the now 993SS cams.

Been inside the motor twice and no signs of detonation and I have ran aggressive timing.

If I were running E85 and could pick the compression ratio, I would be at 9.0:1 at a minimum IMO for off boost low-end grunt and torque even at those boost levels.

[Ollies930]

There are too many determining factors to worry about dynamic compression ratio. Giving 15-20 lbs boost does not tell that much if the turbo is not specified. Are you running twin plug? How high are you willing to turn the engine? How long do you expect to stay on boost. Better approach would be to tell us what hp you are looking for.
As for fuel choices, E85 will take you to some very high hp levels, whereas 98RON will probably limit you to around 600 rear wheel, depending on intercooler and general tune. Fixating on Dynamic CR will not do you any favors.
In general, an 8-1 compression ratio is a good compression ratio for an air cooled 911 turbo and the 964 cam is a good choice if you want power to come in "relatively" lower in rpm range. If you want to spin a little higher, Dougherty's 993SS cam will work well. Then there is the GT2 Evo cam, little rough on bottom, but works well at higher rpms. You can also alter this to some degree by messing with cam C/L timing.
But if you want to know the numbers, 8-1 with 964 cam set at 113* intake C/L is 6.93, 2* advanced is 7.02 and 2* retarded is 6.86.

[Pork Chops]

Originally Posted by Ollies930

There are too many determining factors to worry about dynamic compression ratio. Giving 15-20 lbs boost does not tell that much if the turbo is not specified. Are you running twin plug? How high are you willing to turn the engine? How long do you expect to stay on boost. Better approach would be to tell us what hp you are looking for.
As for fuel choices, E85 will take you to some very high hp levels, whereas 98RON will probably limit you to around 600 rear wheel, depending on intercooler and general tune. Fixating on Dynamic CR will not do you any favors.
In general, an 8-1 compression ratio is a good compression ratio for an air cooled 911 turbo and the 964 cam is a good choice if you want power to come in "relatively" lower in rpm range. If you want to spin a little higher, Dougherty's 993SS cam will work well. Then there is the GT2 Evo cam, little rough on bottom, but works well at higher rpms. You can also alter this to some degree by messing with cam C/L timing.
But if you want to know the numbers, 8-1 with 964 cam set at 113* intake C/L is 6.93, 2* advanced is 7.02 and 2* retarded is 6.86.

Turbo is GT3582 A/R 0.82. Power is whatever I can get (500-600fwhp maybe) with the 964 cam (based on your description of how it behaves). Redline will be set somewhere about 6,800 I'd think. I'm assuming if I rev it any higher I'll need a cam to justify it and I'll lose too much low torque with a more aggresive profile. Higher revs = new rods and probably invites other reliability issues too. It's not a track car so driveability is a concern. It's a flex fuel set up so needs to run adequately on 98 and E85 which probably means I'll be leaving something on the table compared to a dedicated E85 tune - I can live with that. I really don't want to twin plug it as it's a PITA and $$.

Thanks for those numbers. Could you clarify for me what C/L is? If we adopt as 7.0 as the DCR, is this considered low or high in this application?

[Tippy]

I hit the 6750 rev limiter with ease with 993SS cams.

The stock Carrera 3.2 cams could not even get there under load. Out of breath by 6300 IIRC.

[Pork Chops]

Originally Posted by Tippy

I hit the 6750 rev limiter with ease with 993SS cams.

The stock Carrera 3.2 cams could not even get there under load. Out of breath by 6300 IIRC.

OK this is good info. To me 6,800 is a low redline (just generally - 8k is far more civilised!) so the last thing I want is a cam falling over at 6,300 or before. How is street drivability on the 993SS?

[Pork Chops]

Originally Posted by Ollies930

There are too many determining factors to worry about dynamic compression ratio. Giving 15-20 lbs boost does not tell that much if the turbo is not specified. Are you running twin plug? How high are you willing to turn the engine? How long do you expect to stay on boost. Better approach would be to tell us what hp you are looking for.
As for fuel choices, E85 will take you to some very high hp levels, whereas 98RON will probably limit you to around 600 rear wheel, depending on intercooler and general tune. Fixating on Dynamic CR will not do you any favors.
In general, an 8-1 compression ratio is a good compression ratio for an air cooled 911 turbo and the 964 cam is a good choice if you want power to come in "relatively" lower in rpm range. If you want to spin a little higher, Dougherty's 993SS cam will work well. Then there is the GT2 Evo cam, little rough on bottom, but works well at higher rpms. You can also alter this to some degree by messing with cam C/L timing.
But if you want to know the numbers, 8-1 with 964 cam set at 113* intake C/L is 6.93, 2* advanced is 7.02 and 2* retarded is 6.86.

I just used this calc Wallace Racing: Dynamic Compression Ratio Calculator and got 6.9. Full inputs and outputs in the attached images.

[Tippy]

Originally Posted by Pork Chops

OK this is good info. To me 6,800 is a low redline (just generally - 8k is far more civilised!) so the last thing I want is a cam falling over at 6,300 or before. How is street drivability on the 993SS?

Honestly, I couldn’t really tell a difference, but I’m G50. A 930 trans would be noticeable for sure

[Pork Chops]

Originally Posted by Tippy

Honestly, I couldn’t really tell a difference, but I’m G50. A 930 trans would be noticeable for sure

OK thanks I’m G50 too. Sounds like both would be similar enough.

[Ollies930]

Pork Chops,
C/L stands for camshaft center line, meaning the 964 typically is installed at 113* intake centerline, but you can advance/retard it by a few degrees as you see fit to help move the power band up/down a bit.
Funny enough, the Wallace Racing Calculator is the one I used myself. But it does not accurately reflect what is going on with the air coming out of the turbo. Density and temperature of the charge will have an effect on the cylinder combustion pressure, which will affect how much extra charge you can stuff in there.

[Pork Chops]

Originally Posted by Ollies930

Pork Chops,
C/L stands for camshaft center line, meaning the 964 typically is installed at 113* intake centerline, but you can advance/retard it by a few degrees as you see fit to help move the power band up/down a bit.
Funny enough, the Wallace Racing Calculator is the one I used myself. But it does not accurately reflect what is going on with the air coming out of the turbo. Density and temperature of the charge will have an effect on the cylinder combustion pressure, which will affect how much extra charge you can stuff in there.

OK thanks for the extra info. Well it's intercooled and I'll be experimenting with ramming air into the engine grill with a sort of ram air scoop. With luck it will be high enough to catch air as I know it's a low pressure zone. Might also look into misting the cooler with the set up controlled by the ECU. Be interesting to log IATs and head temp as I don't overly want to rob Peter to pay Paul.

So for the sake of discussion, why would I not use a static ratio of 8:5 instead of 8.0? That might go to my question I posed earlier about what number is deemd 'ideal' as an dynamic number.

[Ollies930]

There is no ideal, it is a balancing act. A lower compression ratio will allow for more boost and vice versa. You can run 10-1 if you like, but you will not be able to use a lot of boost.

[stownsen914]

Cool project. It might be worthwhile to do a yarn/tuft test to check the airflow in that area, and help in shaping/placing the scoop. Air is relatively unattached in that area on the back of a 911. A low scoop may not get much air. You might be better opening the grill area to the rear more where there is positive pressure. (Note that a scoop would cover this area and could actually restrict flow.) A yarn test would tell you though.