924Board.org

[gla924sem]

My engine rebuilder again feels I have the wrong pistons in my 1982na.
They look exactly like the ones in the Haynes manual

His concern now is......at TDC there is about 3-4mm distance from the face of the block to the edge of the piston.

He contents that the piston should be flush at TDC and wants to shave the block

As far as I know these are the OEM pistons from day one

[Khal]

You might want to get a new engine rebuilder

As far as I know...

[Rasta Monsta]

[fiat22turbo]

perhaps a pic will help:


_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Harm]

+3

Tell him its not a Turbo engine (compression ratio).
I do hope you have Euro valves to go with that (etc.)? Should be a blast then, 125 Euro Hp!
(Let him make sure the valves are clear though… )

@fiat22turbo: Different pistons…
See earlier posts on the matter.
I believe its KS* what you show there: could be distracting to gla924sem's engine rebuilder…?!
Sir gla924sem uses Mahle/stock IIRC?

Regards, Harm.

*KolbenSmit
_________________
Porsche 924 NA 1982 LY7A/A3A3 _ Greater driving pleasure never harmed anyone.

[ideola]

Gary, as we discussed, the NA engine has combustion chamber in the cylinder, not the head. Just tell him to look at the head. It's FLAT. All he has to do is open his eyes and look at the components he's got laid out on his work bench.

This particular builder is NOT a specialist in the 2.0L and is going on what he knows about the 2.5L. There's another thread here somewhere that talks about the precise piston crown-to-deck clearance of the NA setup. Don't have time to dig it out now, but I know the data is here somewhere.

You might consider retrieving your parts and having Livernois Motors (they're much closer to you) complete the short block work and then build the motor yourself. If you need head work, I have an excellent resource who does it for a living for Roush, and works on the side for me. It will end up costing you much less than what they've quoted you, and you'll probably end up with better results and less consternation...which you've already had your fair share of on this particular build.

If you want introductions to my Livernois and Roush connections, let me know, happy to share.
_________________
erstwhile owner of just about every 924 variant ever made

[924RACR]

Actually, the guy building Gary's motor build Chris Marsh's motor, IIRC, so yes, they do know (or at least used to! LOL) how to put the motor together.

Then again, Gary's got different pistons than Chris; this may be the source of the confusion.

Gary, got your email, but I am very busy these days. What CR are you using again?

But yes, they shouldn't be flush...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[924RACR]

Sorry, this isn't quite the photo you might be looking for, and they're certainly different pistons (these are JE's), but here's a peek inside my motor...


_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[gla924sem]

Dan.........Am a little too far into this with the current builder to swap horses in mid stream.

He has the head done and is working on the short block awaiting the crank scrapper.

When I first took the engine to them he questioned the pistons then. After much back and forth I convinced him they were correct. They were not in the engine at the time.

Now he's building up the short block and installed #1 piston. At TDC he says its about 3mm short of of the deck and with the head gasket it will be 4mm short screwing up the CR and loss of HP. He wants to shave the deck. Told him to go no further till I get this resolved.

Have contacted two PCA 924 advisors I have gone to over the years. One has responded and in his opinion these are the correct pistions.

The engine was last rebuilt in 1995 when I restored the 924 from a basket case. Per the original owner......no engine rebuild work was ever performed. Based on that....... the pistons should be the OEM's that came from the factory. Between the street.....DE's......Club Races.....this engine close to 40,000 miles on it before it dropped the rod.

[ideola]

DO NOT deck the block!

All you have to do is tell them to LOOK AT THE BOTTOM OF THE HEAD. If the pistons were supposed to be flush with the deck, where would the combustion chamber be??? The NA head is FLAT, so the combustion chamber MUST be in the cylinder.

According to this thread, which I promised I would find for you, the Euro-spec 9.3:1 pistons are recessed by 4.7mm from the deck. So if you only have 3mm, your CR is going to be higher than stock already.

If the pistons are original 81-82 US-spec, stock CR is 9.0:1.

I'm aware that Mazuro built Chris's motor, but after my experience with them this past month, and especially now with Gary's situation, I have very little confidence in them with the 2.0L motor. I will never understand people who make assumptions that what they know about one platform applies uniformly to every other platform. And then argue with you about it even when you provide empirical data.
_________________
erstwhile owner of just about every 924 variant ever made

[gla924sem]

Dan;

Thanks for the link. Found my answer on the first page....6th entry

Sent the link to the engine builder to review and respond.

[ideola]

Gary, hope you don't mind me posting my email to you for the general benefit of the rest of the crowd.

Here is the tool I was thinking of. I don't know how this widget does the underlying math, but I presume it to be accurate. Plugging in OEM specs turns out a CR of 9.0:1

Here are the numbers I used:
HEAD GASKET
Gasket diameter = 8.8cm (OEM spec)
Gasket thickness = .130cm (OEM spec = .051in)
Volume (calculated) = 7.91cc

CYLINDER SWEPT VOLUME
Cylinder bore diameter = 8.65cm (OEM spec)
Crankshaft stroke = 8.44cm (OEM spec)
Volume (calculated) = 496.18cc (agrees with data on 924board.org)

PISTON DECK HEIGHT VOLUME
Diameter = 8.65cm (OEM spec)
Thickness = .47cm (based on data from 924board.org; this is crown-to-deck distance)
Volume (calculated) = 27.63cc (agrees with data on 924board.org)

Piston Dome Volume = -26.24cc (negative because the piston is DISHED not DOMED; data from 924board.org)
Combustion Chamber Volume = 0cc (because the head is FLAT)
Number of Cylinders = 4

Compression Ratio (calculated): 9.0:1
Engine Capacity (calculated): 1984.7cc

The numbers don't lie. Something is not right in Mazuro's numbers. No way those pistons are wrong, and no way they're only 7:1CR. If the crown-to-deck height has already been reduced to 3mm through skimming the block, the CR jumps up to 10.6:1. Not necessarily a bad thing, but you better run premium fuel.
_________________
erstwhile owner of just about every 924 variant ever made

[ideola]

Here's another tool I've used in the past. Plugged the same numbers in (well, after converting from cm to mm), and got the same results.
http://www.csgnetwork.com/compcalc.html
_________________
erstwhile owner of just about every 924 variant ever made

[Joes924Racer]

The only down-side to decking the block is youll have higher compressiion
though you wont get the right tension on the timing belt .
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Khal]