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.

Your comp ratio is 8.4:1 given your numbers of 549.8 and 74.4.

THe compression ratio is the displacement Plus the comp vol all divided by the comp vol.

Thats (549.8 + 74.4)/74.4= 8.4:1

Think about it. At bdc you have the total displacement of the cylinder PLUS the compression volume all filled. At tdc you just have the compression volume. Thats why you must add the compression volume before dividing. Not only that, thats how its supposed to be calculated.

Hi Mike:

LOL,...I'm reading this thread sitting on my hands with a sock in my mouth,...:) :) :) :)

Each to his/her own.

http://www.pelicanparts.com/support/smileys/paddel.gif I know, I know, it's just that I'd like to get a true 8:1 with my EFI conversion and I've always heard that the 3.4 Mahles run below their stated 7.5:1. I'd like to also run a 3.6 crank for a little extra displacement. What about coating the skirts for more wear protection especially from stroking?

BTW, missed seeing you at GAF this year!

I'd agree that its very tough to get that with the Mahles, but there are FAR more factors at work here,....:)

You are correct in that Mahle's spec CR is always higher than reality, but in an air-cooled, turbocharged engine with only 4 studs per cylinder, thats not a bad thing.

Years ago, one of the monthly performance trade magazines I receive published a matrix chart showing static CR against boost pressures to show effective CR. Quite enlightening, to say the very least. This chart was done by one of the blower manufacturers to show what happens when one runs a high CR with modest boost (and don't forget that this was aimed at water-cooled engines with MUCH stiffer, better supported decks and heads).

As the old saying goes,.....Less is More,....:)

Make HP with airflow and a good turbocharger, not simply piling on boost & CR.

I wish I could have attended GAF, but we were really busy. Hopefully, we'll go next year as long as we aren't backed up.

Jack, please ignore my lack of basic maths for a moment. how do i arrive at 549.8 as my displacement? my bore is 97mm and my stroke is 74.4mm. 97x74.4 is 7216.8 right?

Andy look at my working out for the volume that is correct, id just overlooked what snowman pointed out that you need to add the compression volume onto the stroke volume for final cylinder volume. As for the 74.4 bit Id just copied it down wrong as you pointed out it should have been 74 been your compression volume.

Volume of cylinder = area x height (stoke length)

Area of circle = Pi (3.141592654) * radius squared

your radius is half you bore diameter = 9.7 / 2 = 4.85

so volume = (3.141592654 x( 4.85 x 4.85)) x 7.44 = 549.8

Notice I changed the numbers from mm to cm this gives the final result in cm cubed which is equivalent to ml as this is what you are measuring you final compression volume in.


Steve

So if as you say you think you compression volume is nearer 68, then the total volume would be 617.8 this would equate to a ratio of nearly 9.1:1. However if you was right in the first place with the 74ml for compression volume it would work out at 8.43:1. Just curious what method are you using to measure you compression volume?

thanks steve. flippin maths!

I completely agree with this statement if the engine is going to be run at high rpm and high boost most of its life, i.e. track conditions.

However, for most of us who live on the streets, the higher CR works very well around town. With proper hardware and heat/engine management (dry lube skirts, ceramic coat domes, polish combustion chamber, proper timing and AFR's, etc), 1 bar at 8.5:1 is completely within reason and rewarding around town (especially if you are running a 930 gearbox).

Since I have EFI, I will back off the timing for DE's, even with the 8.0:1CR.

steve im using snowmans method of 50/50 water antifreeze injected and measured while piston is at tdc.

Why water/antifreeze?

Antifreeze normally has anti corosion agents in it I would presume thats Y????

I used #1 fuel oil, it worked well in my burette when I checked the cc of my 912 engine, also use a dial indicator to set tdc, not the pulley mark.

I would guess because antifreeze has wetting properties?

wetting and colour properties i think. its a bit of a mission trying to spot the exact point that the fluid touches the underside of the spark plug hole. the colour certainly helped me

Andy is right on, color and wetting. Any liquid could be used as it will not be there long. Oil, eg tranny fluid, is good but what a mess when you dump it back out.

I also use a syrnge, usually a 50 cc one. They are very inexpensive, accurate and in most states readily available at feed shops or out of some hobby stores or scientific supply catalogs. some come with blunted needles, some don't.

Steve and Andy,

I think you two are not talking about the same set of volumes....

Steve, I'm not following how you arrived at your conclusion without taking into account three separate volumes, piston dome, swept volume, and head volume. It appears you used only two, swept volume and head volume. But maybe I'm just not understanding it right.

I have posted a method on my website that remove some of the confusion. Check it out if you like. Let me know if there are any questions.

<http://members.rennlist.com/jimwms/Tech/comp_ratio.html>

Piston dome, head volume and all the little dimples in top of the piston and head are included when you measure it with the engine assembled, the piston at TDC, valves closed, sparkplug removed. The engine is placed on an engine stand, spark plug opening up and a fluid introduced into the cylinder untill it reaches the bottom of the spark plug hole. Some agitation may be required to remove any trapped air. This is the most accurate way of measuring compression I know of as you cannot overlook anything, everything is accounted for, everything is MEASURED, except the final calculation and swept volume. The number obtained by this direct measurement is V1 in your example. This method dosen't miss important things , for example the compressed head gasket thickness. Other sources of errors accounted for include differences in rod lengths, differences in compression heights of pistons, differences in crankshaft throws, differences in valve protrusion, differences in piston tops. Finally one LESS measurement is involved, reducing the possibility of error.

Please note that this is a FINAL compression check. The method you have cited would be used while the engine is apart. Its always useful to know where you will end up, and then to prove it when you get there. I have found there is often as much as a 1 cc difference in the two methods due to accumulative errors.

This was helpful to me: http://www.csgnetwork.com/compcalc.html

If you measure it like Jack does and I did you only need to know a few things to get your CR, bore 86mm stroke 74mm and Combustion Chamber Volume In CCs 51cc you come up with 9.428 just that easy aint it!

Jack,

No doubt the method you describe is as about as accurate as you can get, if everything works as planned. The only thing not accounted for with the method I describe is the compressed head gasket. Most 911 head gaskets I have seen that have been compressed (removed from an engine that had them compressed) were flattened out even with the top of the channel they were seated in, so I can't imagine a compressed head gasket introducing much of an error. Everything else you mention is taken into account.

The only unkown with filling the combustion chamber, head on the engine, piston at TDC, engine on a stand, sparkplug hole oriented up, is whether all the air bubbles are actually gone, as one can't see inside. Even in a controlled situation,like in measuring the head volume on a bench top, where you can see through the plexiglas cover, and have a way to level it, you may have to do some slight movement to get out the last of the air.

All things considered, I guess one should use the method they are comfortable with.