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Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

So I am looking at 88mm VW flat tops but am a bit concerned with how far down the block they sit at tdc. Do they offer enough quench area for a turbo car and what is the usual compression with them ?

Answers from someone actually running them is much appreciated.

Leigh
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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

gegge · Joined: 27 Jul 2007 Posts: 1124 Location: Sweden

I run 87mm VW in my block and do intend to build a second engine with a 88mm set gathering dust on the shelf. Both sets are "forged".

They have the same piston height measured from pin to crown. IIRC some 5mm down from block deck at TDC with 144mm rods.

87mm gave close to 9:1 with a skimmed block and thinner MLS gasket. 88mm is easy to calculate, my guestimate is 9:1 on a stock 931.
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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

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

If you can measure the distance from the center of the wrist pin to the top of the piston crown, you can plug that number into the "Compression Height" field in my Compression Ratio Calculator and figure out exactly what you're dealing with. http://garage.ideola.com/top-DL-CompressionRatioCalc.html

Just use "0" for "Volume in Dish". It would be best to know the exact deck height of your block (measured from the main bearing cap mating surface to the deck), but you can use somewhere between 231-232mm as a good guessing point if you don't know for sure. The other key variables will be the volume in head (19cc or so is typical for 931 head depending on how often it's been surfaced), and the head gasket (.046" is stock thickness, but I have a picklist of the standard Cometic thicknesses as well).
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erstwhile owner of just about every 924 variant ever made

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Just playing around with the numbers a bit, if gegge's figure of 5mm below deck is accurate, that would yield somewhere around a 41mm compression height on the pistons, and would result in a CR of 10.3:1 with everything else being "stock" dimensions.

5mm is less than ideal quench (<1mm is ideal), and the 10+:1 CR will limit how much boost you can run.
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erstwhile owner of just about every 924 variant ever made

gegge · Joined: 27 Jul 2007 Posts: 1124 Location: Sweden

VW flat tops have 39,5mm as compression height. In charged applications doesn't quench matter as much as in natural aspired engines. The boost will cause turbolence instead. Yes, less than 1mm would be ideal, but more than 3mm does work fine. Just like stock 924.
_________________
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

leadfoot · Posted: Sat Apr 05, 2014 6:22 pm Post subject:

Leigh,
These are hyperutectic pistons,
Sit 6mm down in the bore and will handle easy 1 bar boost, compression/ quench are a mute point... Depending on cam may end up non interference
Makes for a perfect cheaper solution.
All documented in my first supercharger build as well as a few topics from nick,
They need bronze bushings from EBS or you can buy my rods which have these along with ARP bolts, make me an offer...
I also used the stock head gasket on that build.
Also still have my old forged JE's too,
Cheers stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

So with a compression height of 39.5mm, dish volume of 0cc, bore of 88mm, deck height of 232mm, and head chamber volume of 19cc, you end up with exactly 9.0:1 CR and displacement of 2053cc. This puts the deck clearance at 6.30mm.

If you shave the block by 1mm, CR goes up to 9.8:1 and deck clearance reduces down to 5.30mm.

http://garage.ideola.com/top-DL-CompressionRatioCalc.html
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erstwhile owner of just about every 924 variant ever made

leadfoot · Posted: Sun Apr 06, 2014 10:24 am Post subject:

Yeah IIRC i calc'd around 8.7:1, including gasket area and gap around the crown, this may have been at 21cc head volume,
Stu
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

9:1 is a bit much for me. I am having some material taken from the head , a few cc's, and I will probably get a thicker cometic gasket. Looking to have 8.5:1 maximum. Just dropped the head off today for pressure testjng and will go from there. Going to order the pistons on Monday. New cam, big valve kit and buckets are ordered. Turbo is going this week for rebuild. Block and crank are also going this week. WUR and FCU were sent to Special T yesterday for overhaul. Just need to source the air to water ic and thats about it, off to the races.
_________________
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

MikeDanger · Joined: 21 Nov 2002 Posts: 774 Location: Denver

Hey If you might be interested i have a .080" thick SCE custom made Copper headgasket, I was about to list for $100
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It was either this or a giant box of legos

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

Thanks for the offer Mike but I actually have a Cometic from a parts lot I bought.

Cheers,
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