Mahle PS84-001 Compression Ratio

Stretch32 · 10-25-2021, 02:07 AM

I’m helping to assemble a 2.2S and a Mahle 84mm piston / cylinder kit (PS84-001) is being used. The kit is advertised as having parts that will build a 9.8:1 CR engine with a 66mm crank by nearly every vender that sells it. The exception is EBS which shows it as 9.3:1 on a 66mm crank and 9.8:1 on a 70.4mm crank.

The issue is, Mahle themselves say this kit will only produce an 8.5:1 CR on a 66mm stroke based on the numbers they publish for the kits specifications.

I know there are differences that can increase and decrease compression (head volume and deck height) but 8.5:1 is well below the factory claimed 9.8:1 that s 2:2S should have. Also, even with differences in head volume and deck height, there’s a significant gap between what Mahle says and what the retailers are saying this kit will produce.

Am I missing something? If anyone has used this kit in a stock 2.2S build what CR did you get?

Thanks in advance for the help.

Stretch

356RS · 356RS's Garage

Can you provide a photo of the piston dome?

Stretch32 · 09:34 AM

I’ll try and get a picture today.

I will say, the dome / piston top of the new Mahle 2.2S pistons LOOK identical to the original 84mm 2.2S piston we pulled out of the engine.

The pistons appear to be identical from the top ring up with the only differences being the new pistons have shorter skirts and are lighter.

Henry Schmidt · Henry Schmidt's Garage

A picture would be helpful.
That said: here's a little history.
The original piston specs in early Porsche 911 engines all seem to over rate the compression. That amount seems to be about .5 points of compression.
If you calculate the compression abased on zero deck height the numbers seem to be accurate. In discussions with a few builders familiar with these engine, the consensus seems to be that engineers hate ambiguity. The flat six engine has deck height that varies from engine to engine given the crazy tolerance numbers early Porsches use. If you calculate the compression at zero deck, the numbers are constant with advertised values.
Now, when you built the engine you're left with a dilemma: Believe the numbers and run it or measure the numbers for yourself.
Our experience when measuring the numbers are that a 2.2S pistons (9.8:1) actually measure at 9.3:1 with .040" deck. This variance seems consistent across the board.
When using other manufacturers aftermarket pistons, this strange variable is absent. They build .040" deck into their compression calculations.

A good way to verify this compression inconsistency is to look at an old rebuild upgrade. In the past aftermarket pistons were marginal at best. To make improved power using stock pistons it was common to use a 2.2S [9.8:1) piston on a 2.4 crank. The calculated compression based on the published 2,2S [9.8:1] number should give you 10.3:1 compression. These engine were a common upgrade and yet if was rare to seeing detonation issues. 10.3:1 compression on a single plug 911 on street gas couldn't avoid this issue and yet, no detonation.

Stretch32 · 10-25-2021, 03:40 PM

Pictures of the new Mahle pistons in question. I don’t have a picture of the original S pistons that were removed right now. As mentioned, the new pistons are identical from the top ring up to the old pistons.

Jeff Alton · 10-25-2021, 06:45 PM

Henry is correct, Mahle does over estimate CR on assembled engine vs their theoretical CR. Replacement piston sets and OE supplier as well (at least on the aircooled engines)

If you are at all concerned, measure it. It really is not that hard. Buy a CC Burette and measure. Do the math and see what your actual CR will be. It looks like you already have the pistons, so go and measure!

You may be able to get ahold of the spec sheet for those pistons which may already tell you the dome volume.

Cheers

Stretch32 · 01:56 AM

Mahle lists the crown / dome volume as 27.2cc on their spec sheet page for Porsche P/C kits. This is the number they’re using to compute the CR along with the 1mm deck height as Henry mentions.

We are going to compute the CR on this engine and plan to cc the heads but, unless something is way off, I don’t think we’ll see 9.8:1.

As a side note, Mahle lists 70.5cc as the head volume which I assume is the average volume of an uncut head. Any idea what the normal (if that’s possible to say) volume of a head is after it’s been rebuilt / cut?

dannobee · 10-26-2021, 03:58 AM

"Guessing" on combustion chamber volume is a fool's errand. As Jeff said, get a burette and a flat piece of plexiglass and measure.

And to properly measure the piston dome volume, put it in a cylinder and set it so that it's down a known amount, like 1.00", then get out your burette and plexiglass and compute the difference of actual volume to the computed volume of that diameter cylinder 1.00" down. The difference is the actual dome volume.

Stretch32 · 08:50 AM

We aren’t guessing on combustion chamber size. We are going to cc the heads.

My original question was simply why there was so large a disparity between the advertised CR from vendors and what Mahle publishes.

As a follow on, I was curious if anybody had built a 2.2S with this kit. If they had I was curious if they were seeing 9.8:1 or closer to Mahle published numbers.

More to follow once we’ve cc’d the heads but thanks for the help so far.

Henry Schmidt · Henry Schmidt's Garage

Using the numbers you supplied plus adjusting the chamber for machine work, your compression calculates out to 8.65:1.

ed mayo · 10-26-2021, 02:42 PM

In reference to your quest for numbers here is what I got using the 2.2 S pistons on a 70.4 crank. Cylinder head volumes on average were 68.5. Piston dome volumes were on average about 23.7. I keep my piston to head clearance very conservative at about .055". Using a 1.40 base gasket I could have had about 9.7 to 1 on average. I used a 1.58 base shim to arrive at an average C.R. of about 9.5 to 1. Swept volume was 389.4 cc. By mixing heads and pistons I got 9.65 to 9.46. Since these are 2.2 S pistons on a 70.4 crank I had to trim the corners of the skirts for counter weight clearance. Hope this helps.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by ed mayo

In reference to your quest for numbers here is what I got using the 2.2 S pistons on a 70.4 crank. Cylinder head volumes on average were 68.5. Piston dome volumes were on average about 23.7. I keep my piston to head clearance very conservative at about .055". Using a 1.40 base gasket I could have had about 9.7 to 1 on average. I used a 1.58 base shim to arrive at an average C.R. of about 9.5 to 1. Swept volume was 389.4 cc. By mixing heads and pistons I got 9.65 to 9.46. Since these are 2.2 S pistons on a 70.4 crank I had to trim the corners of the skirts for counter weight clearance. Hope this helps.

Hummm
Using your numbers the calculated compression is 8.75:1

second calculator

To get 9.7:1 with a 1.4mm base gasket your dome volume would have to be 31cc.

ed mayo · 10-26-2021, 05:20 PM

Henry, if the BDC volume is 434.6 divided by TDC volume of 45.2 that comes out to 9.61. Did I miss the simple division?

dannobee · 10-27-2021, 10:23 AM

Hmm... 84mm bore, 70.4mm stroke

390.1cc swept, 68.5cc head, -23.7cc dome, 5.5cc (1mm deck clearance) =

440.4cc bdc, 50.3cc tdc = 8.755:1 cr.

Stretch32 · 10-27-2021, 11:09 AM

So we cc’d the heads and they average about 67cc.

With that volume, an 84mm x 66mm bore / stroke and the posted Mahle numbers (dome volume) I’m getting a CR of 9.06:1.

Interesting.

ed mayo · 10-27-2021, 02:52 PM

I think what you're missing is that you're adding deck clearance twice. When I cc the piston dome its done during trial assembly with piston installed at TDC in the cylinder with necessary base shims installed. I don't add it separately. So if you don't add in the already accounted for 5.5 cc you get 9.70 C.R.

Stretch32 · 10-27-2021, 03:45 PM

Ed,

Where does the 5.5cc’s come into play?

I was using a CR calculator (Summit Racing website) so I might be failing to account for something.

ed mayo · 10-27-2021, 08:52 PM

That is a volume resulting from what is called deck clearance or piston to head clearance. some do the math calculation to determine the volume, I do it at the same time as measuring piston dome volume.

Stretch32 · 04:20 PM

Looking at this again and using Mahle's CR calculator tool, I can only arrive at a 9.07:1 CR with my components. I am not using a 70.4mm but rather a 66mm crank so my numbers should be lower.

I think, at the end of the day, it still appears a Mahle 2.2S P+C kit will not produce a 9.8:1 CR with the 66mm crank using effectively stock 2.2S components or unless serious machine work is involved.

My next question becomes, if we are leaving anywhere from .5 to .8 of a CR point on the table (maybe more), what sort of HP are we loosing. My rough guess is 10-15HP but that's not based on anything other than a guess.

My second question is, if Mahle P+Cs were not putting out an actual 9.8:1 but more probably somewhere at or slightly below 9.5:1 then were the engines still making 180HP as claimed? I think the answer is yes they were making 180HP and still doing it at a lower CR than 9.8:1.

Final question, has anybody built a 2.2S using JE 9.5:1 or 10.5:1 CR pistons and if so, what sort of power did you get on a dyno? My thought with these is that one could set a slightly higher deck height than 1mm (for the 10.5:1) or if the heads measure below 68cc's (use the 9.5:1) to arrive at between 9:8:1 and 10:1 CR to get the most out of a 2.2 "S" engine and still be able to run single plug with pump gas.

Trackrash · Trackrash's Garage

Personally HBTDT, I would want a true 9.5 to 9.8 CR since you are already in there and are looking for a performance motor. My last motor had 8.4-1 CR. It ran fine AND I could run regular gas.

The two negatives were poor low end performance, meaning little power below 4K rpm and obviously I was leaving around 10 HP on the table with my 2,5 motor.

According to my calculations if you flycut the heads and leave out the base gasket you could get over 9-1 CR. Apparently zero deck clearance will result in 9.8-1 CR. Just my two cents, for what it is worth.