Ralph:
Sent you a PM.
J.P.

Ralph:
PM waits for you.
J.P.

This evening Steve and I got to work a couple of more hours with the rain pounding outside. I got the new oil pressure relief guide and installed the vertical assembly in the case. One less thing to worry about.

Then it was time to install the ARP head studs into the case. After Steve took a die and cleaned the threads in the case (there was a ton of gunk in there), he turned me loose with a little Loctite 262 and said get going. With the threads chased, the studs went in beautiful. Here is a pic of the head studs installed:

http://forums.pelicanparts.com/uploa...1098155601.jpg

Measured that they were in far enough but not too far and then double checked by loosely fitting a cylinder and head with head stud nut and washer. Looks good to go.

While I was doing the head studs, Steve was beginning to figure the compression ratio by cc'ing one of the twin-plug heads.

http://forums.pelicanparts.com/uploa...1098155895.jpg

Head CC is 91.8ml. Steve used a concoction of WD-40 and oil. Thick but not too thick that it takes forever to fill the combustion chamber of the head (and later piston/cylinder).

Time to test fit a piston/cylinder to check deck height as well as get the measurement when the piston is dropped enough that the dome is below the top of the cylinder. Since all I had were .25mm base gaskets in my gasket set, a single .25mm gasket is what we started with. Deck height is not where we want it to be at .5mm. Going to have to get a thicker base gasket(s) to get the deck height up to 1mm where Steve wants it.

Here's a pic of the piston/cylinder temporarily installed:

http://forums.pelicanparts.com/uploa...1098156510.jpg

Here's a pic of Steve filling up the cylinder:

http://forums.pelicanparts.com/uploa...1098156661.jpg

The cylinder CC is 71ml. We can start to plug the numbers into the formula. Not the result I was expecting, it looks like the measured CR is going to be 9.6:1, alot lower then the claimed 10.3:1 of this piston and cylinder set.:(

Steve had the same problem with his 3.4L motor. He ended up with 9.2:1 initially (same 10.3:1 was quoted) but took some more material off the heads until he settled for 9.4:1. He also says that it is common for the CR to be alot lower then advertised. I can't speak for other's Porsche motors, but the water-cooled VW motors are notorius for having inflated CR's. The Mk2 GTI's 1.8L were rated 10:1 from the factory and were more like 9.2:1. Same with Mk 3 GTI 2.0L's, stated CR was 10.5:1 but in reality more like 9.5:1.

Anyone else measure a different CR (lower or higher) then what the piston/cylinder set was rated on their 911 motor? Anyone know if US spec Carrera's are actually 9.5:1 and not lower? I would speculate that they are.

Have a couple of choices to make. Can send the heads back to Ollie's to take some more material off the heads or can also let them play with the rods a bit to increase the CR. Since my motor does not have a knock sensor, will run in a warm climate and has to drink 91 octane, I think that I may leave it alone unless you guys can convince me otherwise. Anyone?

Ralph

I'd run more comp. since it is twin-plugged. Comp. = power. The "safe" guideline is 10.5:1 for twin-plugs. But ask around in SoCal -- your gas may be different there.

In fact, I know it is different -- let me mod. that stmt: the gas may sufficiently different there to make you run a different CR.

Wow, great hands on pictures. Keep 'em coming.

I have heard that the 'lower than stated compression ratio' is pretty common with Porsche factory engines. My 993 engine is 11.3:1 - it is probably more like 10.5:1.

Ralph,
You can't mess w/ the rods to increase CR if you've already hit the limit on deck height. (If your deck height was too much, you could lengthen the rod by boring the small end off center). I have a similar motor but w/ an SC crank and J&Es. I think I measured my twin plug 3.2L heads about the same as you. I will check tonight and post. My J&Es came out very close to the 10.5:1 I specd. I gave them my head volume though. I distincly remember the EBS guy thinking my heads were a little "large" on the volume side compared to a regular 3.2 head. The twin plug dimple, 100mm chamfer, and any valve job will yeild more volume. (FWIW, Having the carrera valves sit just 1mm further down in the seats will yield 2.7cc more!) Mine have had at least one valve job.

SMD

Yes, you're right. These are off the shelf Mahle units, not custom made JE's so I think I may be stuck. The 9.6:1 is even going to come down a bit more with a thicker base gasket, I'll run the numbers again and double check.

Looking at the heads closely, I can see that Ollie's already has removed some material, I don't want to remove too much. Thanks for the input and I would be interested in seeing what your head volume was (and others).

Regards,

Ralph

Ralph,
looking through my notes, my heads came out to 92cc. My J&E domes were 43cc, and I ran the clearance right at 1mm which gave me a CR of ~10.7:1. I remember the tighest spot was NOT at the very edge, but just on the inside of the chamfer of the 95mm part.

FWIW, at these settings a 1cc head or piston change makes a 0.17:1 CR change. A 0.1mm clearance change makes a 0.13:1 CR change.

SMD

Are these Mahle 'Sport' RSR type pistons, designed to accomodate high lift/overlap cams (and milder cams, of course)?

Hi Ralph:
From what I've been reading; are you stuck with a 9.5:1 comp. ratio for your engine?
Regards,
J.P.

Maybe I missed this but what head studs are they?

SMD, thanks for posting your info. My 9.6:1 ratio is with a .5mm deck height (using a .25mm base gasket) which obviously won't work. Got a set of .5mm base gaskets from Andial this afternoon (ouch! pricey!) to help get the deck height closer to 1mm. Dieter at Andial recommends a deck height of .8mm to 1.2mm. I will be somewhere near the tighter clearance but since I am using relatively docile cams I don't think (or is it hope?) it will pose a big problem.

J.P., the problem is even 9.6:1 is now optimistic. I ran the numbers again, and with a .75 deck height, the CR is 9.13:1. Now can you guys see why I am crying in my beer?

Kobalt, yes these are the carb/mfi type "hemi" piston rather then the stock wedge dome type. Their design lends itself better to twin-plugging then the wedge dome type which is also one of the reasons (others as well) why they are rated with a higher CR. I also spoke with Pete in parts at Andial while paying for my base gaskets and he says they don't sell many of the 98's or 100's of this type with the 23mm pins anymore but has heard through the grapevine that this is a problem with these particular sets. Steve's motor confirms this, as his 3.4L uses the same piston carb/mfi type and he ended up at 9.2:1 before milling the heads to end up with 9.4:1. The compression ratio of both of these sets is not the claimed 10.3:1! I guess I am learning the hard way.:(

If I look on the bright side, my motor with twin-plugs will run on water, right? Joking, only joking.

The Motronic wedge dome 98's and 100's are apparently very close if not spot on to the claimed CR of 9.8:1. Who would have figured that these pistons would actually have a higher CR than the hemi style???

Bill, head studs are ARP. You know something that may be of use? I am getting desperate.

Steve suggested and I decided to have the heads milled. This evening, I disassembled them (yes, I labeled the heads, valves, springs, retainers and shims where they came from) and tomorrow my buddy Eric at PMNA will remove .015" from them to try and get back some of the compression. Steve says .020" can be safely removed but it is better to be able to remove more material if required but you can't put it back if you go to far!

For reference, here is the PMNA way of calculating the compression ratio:

1) Figure area of bore and divide by 1000
2) Measure volume of cylinder head. This is "Head CC" (A)
3) Write down final deck height
4) Drop piston enough to get dome below top of cylinder and write down the distance
5) Add item 3 to item 4
6) CC the cylinder and write down this number
7) Multiply item 5 by item 1
8) Subract item 6 from item 7. This is "Dome CC" (B)
9) Multiply item 1 by the crank stroke. This is "Swept CC" (C)
10) Multiply item 1 by item 3. This is the "Deck CC" (D)
11) Compression Ratio:

C + A + D - B divided by A + D - B

Will get the heads back in a day or two and then check everything again. In the meantime, this is my motor hibernating:

http://forums.pelicanparts.com/uploa...1098246435.jpg

Word to the wise: Don't assume the CR is what is claimed and don't buy the carb/mfi Mahle sport 98's or 100's for the Carrera if you want to have a high compression ratio.

Ralph

Craig, based on the above, what is your guess now?SmileWavy

Ralph

Thanks, Ralph. This line of pistons definitely look different depending on the B&S dimensions plus head cc they are designed for.

I'm sorry to say I don't have any groundbreaking ideas for you. Unfortunately you're not the first to find a mahle set measuring out to below CR spec, which you already knew.

I recalculated - my guess is now.........255 rwhp......my 'official' (final) guess. Gonna be one hell of a motor no matter the compression!

Way to go !

Hell of the project.................. Great R&D:)

Just shim that bad boy up and turbo charge it with that compression ratio - then you really will end up with a beast!

If not then at least you will not be sweating it if you get some crummy gas...

Guys keep them coming... I am learning so much from these threads.
I am still on the fence on rebuild vs 3.6 Vario swap. I just bought Wayne's and BA's book on the 911 engine. But this forum is incredible!

Craig, will you be my dyno operator?:D I think you are way optimistic with your revised quote but I thank you for your positive outlook! I am going to venture a guess in the 235 rwhp range.

Not much more of note to report. The heads will be cut down tomorrow and measured to make sure they are even in an attempt to try and get back some of the compression. Steve will then CC a head or two and I will plug the value into our formula. Steve is optimistic we can get back to 9.5:1 which is the stock 3.2L US spec CR but short of the 10.3 "advertising" of the Mahle set. We will also have to check the piston to valve clearance with the Web 20/21 cams that just arrived.

I had a nice talk with Dieter on the Andial side after dropping the heads off to my buddy at PMNA across the courtyard. He said that Andial NEVER, EVER builds a pre-964/993 street motor with more than 10:1 CR. He doesn't feel that a higher ratio is compatible with 91 octane. He also has told Mahle countless times that their "CR claims" are inaccurate on some of their sets but it has apparently fallen on deaf ears. He is positive though that the new 10.7:1 98's for SC & Carrera race applications that are in their new Excellence ad are indeed accurate because he did the development work on them.

He also says not too worry about the lower compression, I am giving up at most 5 horsepower. He says I should be able to get away with more ignition advance to help compensate and Mr. Wong can help me there.

Didn't do much tonight, just installed the oil thermostat I finally got as well as checked the ring gap on the pistons and all are between .010" and .012". Here is a pic of a 100mm piston with a cylinder that has been "mooned" by Ollie's:

http://forums.pelicanparts.com/uploa...1098419292.jpg

Also took apart the chain tensioners, idler sprockets, idler arms, tensioner sprockets, etc for cleaning. The sprockets should all be looked at for wear, but don't necessarily need replacing. Mine look fine and will be reused, saving some money (about $150) on the rebuild. YRMV.

img]http://forums.pelicanparts.com/uploads4/IMG_08581098419496.jpg[/img]

Will update again when we have the CR finalized and we can proceed further.

Ralph

Sorry, here is the pic:

http://forums.pelicanparts.com/uploa...1098419848.jpg

Have a good weekend all!

Ralph