Only 7.4:1 compression?
 

After ccing my heads and piston, I found I only have 7.4:1 compression on my 3.4 930 (with 98mm Mahle pistons). I'm adding EFI and twin plug and now I'm thinking I should have gotten new higher compression pistons. I'm planning to have over 500hp with possibly more in the future. My question is: should I go for more compression for low end power or stick with what I have for more top end power and cooler track running?

David,

It depends of course, on what you want. For me 500 HP would be enough and I like the idea of not having to worry about detanation so much. You'll be able to run higher boost with that compression ratio. You also have great long lasting Mahle pistons. If you went with another brand for higher compression you might not get as much life out of them.

-Andy

Everyone using mahle seems to be compression challenged these days, but I still think they're the p/c to run. I wonder whether our stock sets were really 7.0:1? I'm about to do the same rebuild/conversion David. As a daily driver I would like to have more compression, like 8:1. and .7 bar. Between efi and twin-plugs I've heard that 8.5:1 is doable with boost levels up to 1 bar. For track running you always could mix some 100 oct. to run a bit more boost. It's very frustrating, with so many modded 930's you'd think there would be some bullet-proof recipes. 7.4 certainly will be a very safe bet just with efi.

im in the same boat. i bought eric hoods (strocherracing) 8.5-1 je's and don't know what to do! i was going to shim to get perhaps 7.5-1 but now im thinking that perhaps 8-1 would be nice with less boost for more bottom end. what i need is a 930 with a built 7.5-1 engine and a 930 with a built 8-1 engine to try them both out! then i can decide!

Andy

I suggest that you measure C/R in the same way 125shifter did...quoted numbers often differ from reality, sometimes a lot.

8.5:1 isn't that high, compared to modern turbocharged cars.

After checking a few books I have on the subject, I've decided to stick with what I've got. I'm running 1 bar boost now and I will probably go bit higher when I get a full bay intercooler, so the 7.4:1 looks like the right way to go.

David, how's everything coming together? You have any pics for us to show the progress?

I agree, but as we've noted before, combustion chamber shape/dynamics and cooling characteristics are different from modern turbocharged cars too!

I had the stroke dimension wrong. I actually have 7.24:1 compression. EBS Racing has a set of 98mm JE pistons with a 13.2cc dome (mine's 7cc) that will give me 7.7:1 compression so I'm getting new pistons :o .

Steeve, I have the bottom end together, but I've been trying to sort out the top end for last couple of weeks.

I rebuilt my stock 87 930 with 98mm mahle cylinders and RUF 7.5 to 1 pistons. The higher compression and the high displacement can easily be felt. There is a noticeable difference in the low end torque.:D
On the track there is a lot more acceleration coming out of the corners. I am running a max of .9 on the boost. My car also has a ring and pinon installed, but it has been on the car for years. The only drawback is the car runs a little hotter than it use to. I've never seen the temp gauge in the red, but on 95 deg days I have to come off the track before the end of a 20 min session.

Are you measuring by doing math or by filling the combustion chamber with oil? Or combination of both?

7.7:1 will work fine with EFI even @ high boost so I think you might a right decision, replacing the pistons with higher C/R ones.

I calculated the dome displacement first (I actually had to brush up on my calculus) and I got 5.2cc, then I measured it with colored alcohol (I like it better than oil) and got 7.0cc. Even before I subtracted out the valve pockets the calculated volume was too small, so I'd be skeptical about calculated volumes unless they're off the original piston design drawings.

I'm probably over analyzing this issue, but I'm still having trouble coming to a conclusion. I've been calling around trying to find a higher compression Mahle pistons and I can't find any. One shop I talked to said they'd rather have the lower compression and stay with the Mahle's I have rather than go with a JE. EBS on the other hand says the only problem they've seen with the newer JE pistons is if a motor's been assembled wrong. They also said I could run 0.002" clearance with the JE's with no problem. Anyone know of problems with the JE's?

Steve@rennsport where are you?

David, Steve is someone you should talk to. He has a contrary opinion to Stephen(IA) on JE's, though I was a bit heartened to hear Stephen's support of JE's as it gives me some other options to consider on my rebuild. JE is also local to me. Decisions, decisions....

No more decisions to make. I sent my cylinders to EBS to be honed out 0.001" (they were just recoated) since my pistons are 0.001" smaller than the JE pistons.

I would strongly suggest doing the following:

1. With the engine on an engine stand, set number one at TDC with both valves closed.
2. Using a mixture of 50/50 water antifreeze and an accurate syrnge, with the spark plug removed and the opening pointed up, fill the cylinder up just until it reaches the bottom of the spark plug hole, keeping track of how much you use. THis is the EXACT compression volume of your engine, no if , ands or buts about it. NO calculations required.

Next compute your compression ratio which is

measured compression volume plus displacement per cyl divided by measured compression volume.

remember displacement per cylinder is bore times stroke.


ITs that simple and very very very accurate if done this way. Repeat for each cylinder. All should be well within one cc of each other. I cannot think of another more accurate way to measure compression ratio.

sorry to drag up such an old post but i dont get your calculation process Jack. ive measured my compression volume which is 74ml.

my bore is 97mm and my stroke is 74.4mm

could you run through your calculation process for me using these figures? im just getting figures that i can't make any sense of. im sure its something that im doing wrong though.....


thanks

I work that out at 7.43:1

Radius = Diameter / 2 = 4.85cm

Area = pi*4.85 squared = 73.89 cm

Total volume = 73.89 * stroke (in cm) (7.44) = 549.8 ml

Ratio = 549.8 / 74.4 = 7.4297

Steve

interesting result steve. is the last line of the working correct? ratio=549.8/74.4? should that be 74? (ie my measured compression volume) ive since rechecked my volume as my comp should be nearer 8(ish)-1. the pistons are 8.5-1 je's with a .5mm shim. i got 66.75ml and 68mm. im assuming the 74ml measurement was wrong and the difference between the next two was down to me.

If I had it to do all over again, I would go with 8.5:1 JE pistons rather than 8.0:1. Since my car is 90% street (remaining DE's), there is no problem running 8.5 CR with a good EFI system at 1 bar.

And, I would run JE's any day over Mahle's. I recently went through an engine rebuild due to detonation and the pistons were in perfect shape - the Mahle cylinders were toast, split and had holes. This might not be a fair or real good example for most, but the price difference and flexibility in size and CR make up for it.