924Board.org

[Kevin D]

I have a 924 with 40DCOE webers and a Bursch? header. I also have a 931 head, pistons and rods sitting in the garage. According to some other posts, these might be a good combo. Is there another string you can point me to that would detail this out? How much (if any) work needs to be done to the 931 head before swapping. I guess the better question would be what would be nice to do... cam, porting, shaved deck, 931 pistons? My final goal is not ambitious. Just a more "spirited" daily driver. Track days aren't in my near future.
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'88 924S 3L 6sp RedRat
'92 Duc 900ss
'02 Duc 998 Carbon
'72 RD 350

[Raceboy]

Just using 931 pistons with NA head will give you higher comp. ratio (see other threads), but 931 head is better flowing. To use 931 head and get some CR suitable for NA you need to shave off the block A LOT (how much? it depends of the 931 block year), which is bad, use 933 pistons (unrealistic), or get custom pistons (also bad).
931 head has different bolt pattern for intake manifold so your carb setup is not bolt on to this.
If yur 931 block is 1982 or newer, you could rebuild it, shave the block and use 931 head, if it's pre-1982, then just rebuild the block and use the NA head with it.
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'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[CBass]

Just to throw some basic numbers around, this isn't going to be 100% accurate due to variables such as dish volume and chamber volume, but it'll be useful enough for estimation.

Using the '80 931 head with the '80 931 pistons, you'll get a 7.5:1 compression ratio. Awesome if you want to supercharge it, there are a ton of threads going detailing that.

We'll assume that since the displacement of the engine is 1984cc, that 1984 / 4 = 496, the displacement of one cylinder.

496 / 7.5 = 66cc, the displacement of the cylinder at top dead center, or full compression. IIRC, when I measured it the chamber in teh head displaced around 23cc. Not sure about the dish, but for the sake of argument we'll say around 15cc, I can verfiy this late if need be. So we'll call it an even 40cc for the chamber and the dish, leaving us with 26cc between the deck and the piston top, which we can now consider to be a cylinder with a flat top and bottom.

This is the effective area you can mill at, and maybe you can take a bit off of the head as well.

The bore is around 86mm, so to get the area of the bore circle, we square the radius and multiply it by Pi.

4.3 * 4.3 * 3.14 = 54cm2.

The formula for the volume of a cylinder is base area * height, so we can use the formula to find the height like this. Height = volume / base area.

26 / 54 = .48, or 4.8mm, which converts to be 0.190"

Start shaving the block, and you'll get the following numbers

At 1mm shaved, 54 * .38 = 20.5, plus the 40cc for dish/chamber = 60.5cc.

496 / 60.5 = 8.2, or 8.2:1 compression ratio.

at 2mm shaved, 54 * .28 = 15 + 40 = 55, 496 / 55 = 9.01, or 9:1.

at 3mm shaved, 54 * .18 = 9.7 + 40 = 49.7, 496 / 49.7 = 9.97 or 10:1.

at 4mm shaved, 54 * .8 = 4.3 + 40 = 44.3, 496 / 44.3 = 11.2:1

That math isn't 100% accurate, I have no idea what the dish volume really is, just throwing some ideas around of how much you'd have to shave to get any meaningful compression out of it.

You might have to find a new timing belt tensioner as well to take up the extra slack, since taking 4mm off the block will shorten the distance the timing belt has to travel by 8mm, and tensioner might not be able to take up the extra slack. The tensioner in my Rabbit GTI looks quite a bit bigger, so maybe another VAG tensioner would fit.

With a wild camshaft fitted, there's no telling what kind of interference you might run into, and whether or not you'll need valve reliefs cut. I'm not ready to do that kind of math right now
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'81 931 in various states of assembly

[Kevin D]

Thanks for taking the time on the detailed response. I was hoping there was an easy way to make use of the larger valves. It looks like the place to start is my math skills!
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'88 924S 3L 6sp RedRat
'92 Duc 900ss
'02 Duc 998 Carbon
'72 RD 350

[Peter_in_AU]

CBass, it's great to have someone who can do the math. I just crank up my old Dyno2000 and get that to work it out for me.

How reasonably, morally, certain are you of the 23cc volume figure for a 931 head chamber?

I've got john_h motivated to have his mechanic friend investigate the idea of using Mitsubishi 4G63 rods to stroke the 924 out to 2200cc. Doing this would require different pistons, of course, but I've worked out a list of about 8 OEM pistons that will give compression ratios of between 8:1 and 10:1 using a 924 head.

I should redo the calculations with the 931 head volume and see what compression ratios come out. There were a couple of OEM pistons that gave ratios up around 11:1 on a 924 head that might work with a 931 head.
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1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[Andrew NZ]

Peter, I'd be very interested in this combo if you can get it to work. Please keep us informed of progress.
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Andrew
1977 RX924 race car
12a bridgeport supercharged
www.race4-dcup.co.nz

[Peter_in_AU]

Do a search for posts on VW flat-top pistons for the air-cooled beetle engine or posts by Racing. Racing uses a set in his turbo and had to use a very thick head gasket to get the compression ratio down to a turbo-friendly level. As I recall, with a standard head gasket and 931 head the VW pistons gave a good compression ratio for a NA.

Forged VW pistons are quite cheap. I got a set off US ebay and had them shipped to Sydney for a total cost of about AUS$240.
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1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[Joes924Racer]

years ago guys... when things were new I chatted with cbass
daily for awhile about things.. I cam to find he was good at math
& other things too. .. I would like to hear more on stroking
with diff. rods Ive wanted a higher cp for awhile & plan on later
pistons with smaller bowl. Decking the block could give me as c
lose to 10-1 as i want to be say maybe 3/4 of the way..racings done
alot of innovative performance stuff for sure.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[CBass]

[john h]

[Peter_in_AU]

[CBass]

Boy do I feel embarrassed... To double check, I looked up my post from when I measured it, and it turns out it's 21.5cc Not enough of a difference to significant affect compression, but good to know. Sorry all
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'81 931 in various states of assembly

[Lizard]

also cbass in regards to the use of the 931 gasket, the 924 headgasket has been superceeded to the 931 part, so they all should be the same now.
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3 928s,

[CBass]

Yeah, there are still a few floating around though... I bet the felpro gasket I'm running now is on borrowed time. Another reason why I'm contemplating a new build with lower compression and EFI/nitrous.
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'81 931 in various states of assembly