What would approximate compression ratio be in S4 engine if it uses 944 Turbo pistons with cuts made for valves (if they're even needed). Anyone done the math?

 

On the low side of 7.5:1. You do not want to run that on the street as it will be a dog NA.

 

So low? This would be SC application but I was expection it to be around 8.0:1 at least. 951 has combustion chamber mainly in piston top then and not on head like in S4? Any easy way to get CR little above 8.0?

 

I would say between 8.5 and 9 to 1. You cant go much over 500hp anyway since there are no drive transaxle upgrades. Properly designed and an efficient setup with an intercooler at 12 psi and you should be making between 500 and 600 hp. 12psi should be no problem at 9 CR, as long as you run aftermarket FI with timing control, like a tec III.

 

Oh sorry, I did not understand you were looking for a #. If you know the piston dish cc, chamber cc, stroke, cyl diam, and clearance between the piston top and deck you can figure it out here...


http://www.csgnetwork.com/compcalc.html

 

Taking numbers from here and here it seems CR is:

619.7 (cylinder) + 23 (piston) + 41.6 (head) + 8.3 (gasket) = 692,6 (BDC)
23 (piston) + 41.6 (head) + 8.3 (gasket) = 72.9 cc's (TDC)
692,6 / 72.9 = 9,50:1

Can't be that simple calculation and so far from 7.5:1, can it? What's wrong in those numbers? 928 and 951 bore & stroke is same. Both can use same conrod casting so it's lenght should be same. Only variable I can think of is piston's height from wrist pin to top and even it should not affect CR if I understand correctly. Just looking for ball park figure and 9+ is something I wasn't expecting.

Is 9,5:1 right because 951 head is 60 cc's and piston cc is in fact smaller than in S4 stock version?

 

Isnt the 951 piston dished. If so I would think the volume would be higher. You might want to check your #'s. All things being equal, if the 951 pistons have less vol then you should get a higher CR ??

 

I recall there being some other problems with the 951 piston's use. I can't recall specifics though. And if 951 pistons are out of an engine, they are "USED" as in U S E D badly. On average. Alot of wear whenever I have searched.

 

From what I remember, using the 928 5.0L stock crank and rods, at TDC, the crown of the 951 piston is 2mm below the deck.

As for choosing a static compression ratio, first determine how much horsepower your want. From there you can calculate the amount of boost required to hit the target. With target boost known, calculate the max safe compression ratio.

The bottom line, run as much compression ratio as possible. Your 'reward' will be crisp performance off-boost, which will be 99% of your driving time. In other words, don't run low compression just to run low compression (..like 7.5:1); you'll hate the car if you did.

 

I've got a set of four 951 pistons. If I get a chance, I'll take a burret to them.

 

Thats what it was Lag - the compression height is wrong.

 

The numbers I have on piston dish volumes:

S4: 24 cc
951: 25.6 cc

It'll be good to see if the 951 values are right.

 

Same calculation with new numbers:

619.7 (cylinder) + 25.6 (piston) + 15.7 (compression height) + 8.3 (gasket) + 41.6 (head)= 710.9 (BDC)
25.6 (piston) + 15.7 (compression height) + 8.3 (gasket) + 41.6 (head) = 91.2 (TDC)
710.9 / 91.2 = 7.79:1

There's one thing I do not get. When/if 951 piston compression height is 2mm different shouldn't it be also 2mm below deck in 951 block? Bore and stroke are same as 928 S4. Unless 951 block has shorter deck height?

If only variable in 951 is larger (60 cc?) head these numbers give strange CR:

619.7 (cylinder) + 25.6 (piston) + 15.7 (compression height) + 8.3 (gasket) + 60 (head) = 729.3 (BDC)
25.6 (piston) + 15.7 (compression height) + 8.3 (gasket) + 60 (head) = 109.6 (TDC)
729.3 / 109.6 = 6.65:1

Leaving out compression height do not result correct 951 CR either:

619.7 (cylinder) + 25.6 (piston) + 8.3 (gasket) + 60 (head) = 713.6 (BDC)
25.6 (piston) + 8.3 (gasket) + 60 (head) = 93.9 (TDC)
713.6 / 93.9 = 7.60:1

What's wrong in these calculations?

 

I did this calculation many times, you get about 7.5: 1 on a early 32v head IF the 951 piston's wrist pin was at the same location, but it's not. Your compressio ratio would be in the low 6's if you used either rod. You'd have to use a 2mm longer rod or stroke the crank 2mm to bridge the difference.

 

Mark,

By early 32V head do you mean US '85-86 engine heads? Their shape and cc size is totally different than S4 heads. Not sure if it makes things better or worse though.

What we are looking for is to use 951 pistons and as much S4 parts as possible to get CR to lets say 8.5:1. What besided stock S4 stuff is needed and why? Longer connecting rods is only thing?