924Board.org

[Kenodog]

Does someone know off hand roughly the distance from the block deck to the top of the piston rim when its at tdc ?

Thanks,Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[emoore924]

turbo or n/a?

[!tom]

[Kenodog]

Um,there lies the problem. Its a turbo and the stock diameter pistons,86.5mm, but the pistons are reportadly out of an amc gremlin. They are flat tops but only come up to wihin .3" of the block deck. Thing is the owner said it goes like stink so the compression must be good. I took the head off,cleaned the piston and did the measurment then when I put the head back on I found out that the engine is no longer an interferance type motor because the pistons dont come up high enough. Frankly im a bit baffled.
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[leadfoot]

Well it seems obvious the pistons have been changed, iirc the stock cam has about 12.something mm lift and there is approx 7mm of valve recessed into the turbo head and 5mm into the bore depending on gasket crush and cold-hot clearances
When using flatops most people have reported between 5 and 7mm deck clearance. iirc mine ended up as 5.7mm, its on the board somewhere...
The big question was the all important quench debate that you can either live with or use forged dished pistons.
If you're planning on a boosted app then these pistons can work well as a cheap alternative to replacing the stock cast units which from what ive seen seem to fail around the ring lands on a frequent basis. What you would need to trust is the piston wall clearances that were set by the builder as most of the pistons are classed more in the hyperutectic range rather than forged...
All in all if theve done a good job the compression figures should check out ok
Stu
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[WEASEL149]

Well doing a quick calc using CR calculator over on Ideola's Garage...
http://garage.ideola.com/top-DL-CompressionRatioCalc.html

If you have .3" crown to deck height then I'd say your static CR is about 7.9:1 which is fine.

Personally, with flat tops I wouldn't worry about quench/squish as long as your crown to deck height is greater than ~4.57mm, which it is.

That's just going on accepted combustion research. Quench/squish is important if you use the original Porsche piston crown design (or any that use quench area and a dish).

Just my 2 penneth.
_________________
1979 UK 932

[nickthompson]

I wonder if those pistons would work on an n/a... hmmmmm

[Rasta Monsta]

VW Type I pistons will work on an NA won't they? Got to be the cheapest possible option.
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

You need to measure the deck height of the block. This is easy to do, but requires a 12" digital caliper, available here in the states for about $50. Measure from the bearing cap mating surface to the head mating surface. This is the deck height. This figure is usually between 230-232mm depending on the specific block and how many time it's already been skim cut. (Anything less than 230mm and you'll start getting into trouble with the timing belt tensioner).

Once you have this figure, you can plug that into my compression ratio calculator, and then adjust all of the other figures accordingly. Assuming stock rods, the rod length and stroke are givens at 144mm and 84.4mm respectively.

Then you need to measure the compression height of the piston, which can be done with aforementioned digital calipers. The stock wrist pin is 24mm, so measure from the top of the wrist pin to the top of the piston and add 12mm (i.e. the radius of the wrist pin bore, or half of the wrist pin bore), this gives the compression height.

The calculation is basically to add half the stroke (i.e. the distance that the centerline of the rod big end will be from the centerline of the crank while at TDC, which is 84.4mm/2 = 42.2mm) + rod length (i.e. the center-to-center distance from the big end bore to the wrist pin bore, or 144mm) + piston compression height (about 39.3mm on stock NA pistons), and then subtract that sum from the deck height. This yields the deck clearance. Or you can just plug the figures into the calculator and it will tell you what the deck clearance is. CR will depend on other variables like which head you plan to use (NA or Turbo) and gasket thickness.
_________________
erstwhile owner of just about every 924 variant ever made

[Kenodog]

Thanks for all the replies fellas. Since the head is back on and the PO said the car went like stink I think our only option is to just use a manual boost controller to start at 2-3 psi and work my way up from there until the car feels like a relatively stockish 931. When and if she blows we will deal with it then. Its always a crapshoot buying something someone else has built ! Crazy business.
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[nickthompson]

[gegge]

My 924 NA broke down last week. Going to use the shortblock from my 931.

The pistons are forged flattops and rods from BMW M5 (US) which are 142,5mm. Expect CR in the 10,5:1 region. The chambers in the head has been ported and adding some duration will lower the dynamic CR even more.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[nickthompson]

[gegge]

The first generation M3 (E30) with the S14 engine use the same 144mm rod as the second generation M5 (E34) S38 engine. But the US version of that engine had a shorter rod of 142.5mm. The bigend of the BMW rod fit the 924 crank without modifications. Note that there are common M10/M30 bearings and S14/S38 bearings for HD applications. The width of the bigend of the rod isn´t as wide as the 924 rod. You need spacers to guide the rod from the piston wristpin, a common solution in Mercedes and Volvo engines. The smallend of BMW rod use a 22mm wristpin and works fine together with the VW flattop.

I bought forged aftermarket 142,5mm rods for my 931 engine because I was afraid that the CR would end up to high. Instead of 8.7:1 it became 7.9:1(IIRC), perfect for high boost and pumpgas. Since I am converting the 931 to E85 the CR turned out way too low instead. Will use 144mm rods and skim the deck in order to get at least 9:1.

I am going to use that shortblock in my 924 hoping for ~10.5:1.

There are lot of cars with 48mm journals; Honda, Subaru, Nissan and M-B. Common performance rods in US are Chevy SB with Honda journals 1.885". The problem is still the big end width - too narrow.

EDIT: Could be that the early S38 3.6 used 144mm rods and the later S38 3.8 used 142.5mm rods due to 2mm longer stroke.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs