924Board.org

[Luvmy80-924]

Is the 1980 924 Turbo engine a Interference Engine. Or if the engine was idleing and the t-belt broke, would it bend valves.
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1980 924 N/A Everyday Driver
1986 930
And now a 1980 931

[Nein37]

Yes and yes.
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1981 931 CGT replica, OEM CGT intercooler, .8 BAR WG spring, GTS Headlights, Innovative Wide band A/F, A/C delete, 16" Fuchs, Weltmeister 200lb lowering springs, Bilstein HDs front, Koni Sport rears.

[morghen]

[Luvmy80-924]

I sort of knew the answer but figured I would ask anyway. I was looking at a 80 turbo and the belt was broken, of course the guy said the valves were not bent. So we agreed on a price just over scrap weight and I picked it up. Removed the head last night and of course the valves are bent. Anybody have a source for valves, should I replace the guides also? And is there anything else I should give some attention to while the head is off,cam, followers? I didn't see any signs of oil in the exhaust or the turbo and the head was acually really clean. The car has 113,000 miles on it.

I have my N/A running fantastic and I'm sure I'll love the turbo when I get it back together. Like the old saying goes "There's no replacement for Displacement", well, I disagree it's called a TURBO.
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1980 924 N/A Everyday Driver
1986 930
And now a 1980 931

[fiat22turbo]

Ebsracing.com and garage.ideola.com
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Rasta Monsta]

I've got some spares. Which valves, and how many?
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

Yes, you should replace the guides too.

Ironically, I was just updating my website this weekend, and have finally added the SI Valve OEM replacement kits to my site. Pricing and info is here:
http://garage.ideola.com/prod-Kits.html#head

The kits include 4 exhaust valves, 4 intake valves, 8 bronze guides, and 8 valve stem seals. Valves are stainless, and cut to OEM specs (for the 931: intake = 40mm OD x 133mm L, exhaust = 36mm OD x 133mm L). Full kit is $270 plus shipping. Or you can consider a big valve kit for between $365-$445 depending on options.

PM or email if you want pricing on individual components. However, it would be best to replace the valve train as a complete kit.
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erstwhile owner of just about every 924 variant ever made

[ideola]

I also have several sets of used valves here:
http://garage.ideola.com/used-Engine.html?albumid=5319733430965308993&photoid=5356631299955622434

But you should still replace the guides, as the hammered valves are quite likely to have loosened the guides in their bores. You do not want to put the head back together with loose guides, trust me.
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erstwhile owner of just about every 924 variant ever made

[pmcaya2]

Interesting thread - the 924 N/A is a zero clearance non interference engine - you can't crash valves and pistons. So what made the 924 turbo (931) an interference engine? Must have changed the valve design for the turbo? And the 944 is an interference engine. Aside from the valve damage, was there any damage or hits on the pistons? - Peter

[Luvmy80-924]

Really, I thought the N/A was interference also. I do know the psitons are different in the turbo. I read somewhere the only thing that is the same between the N/A and turbo is the block, crank and cam, but that could be wrong. I know the head is different, spark plugs are on the same side of the injectors, the N/A is not. It looks like to me that 3 or 4 exhaust valves bent, no major marks on the pistons, really no marks at all. Something must have fell into # 2 cyclinder when the guy or his mech. was working on it and marked up the head a little bit but nothing major. I still have to order some parts to get it back together but my N/A went down this week so I have to get that one running again being my daily driver.
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1980 924 N/A Everyday Driver
1986 930
And now a 1980 931

[pmcaya2]

I wanted to check my statement that the 924 N/A was not an interference engine- although it was on a list of interference engines, the following post on germanautoforums seems to verify my post

" Default RE: 1980 924 Turbo (931) - Interference engine??
I know the 2.0 NA engine was a non interference engine, but for some reason it is sticking in my mind that the 931 was indeed an interference engine. I can do some research and let you know for sure."
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Doc Wilen
PCA Instructor
A few toys.

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The difference is possibly the head design.
It's great that you have a 924 N/A daily driver - My 1982 has 194,000 miles and likes to be driven, although it hibernates in winter. Good luck on the turbo! - Peter

[fiat22turbo]

All Newbies Read: http://www.924.org/contents.htm Should help clear up a few things, then read through some of the info in the 931 forums.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ideola]

The NA pistons and 931 pistons have some notable differences, even though they share the same block, rods, crank and wrist pins:

• At TDC, the NA pistons sit approximately 6mm BELOW the deck surface
  
• By contrast, at TDC, the 931 pistons sit FLUSH with the deck.
  
• NA pistons have valve reliefs cut into them.
  
• 931 pistons do not have valve reliefs.
  
• NA pistons are dished, with approximately 26cc of volume in the dish, plus or minus a few cc's depending on model year and market (i.e. ROW NA pistons are higher compression and have a shallower dish, but in all other respects are identical to US NA pistons).
  
• 931 pistons are dished, with anywhere from ~48cc to ~37cc of volume depending on model year and market. Shallower dish creates higher CR.
  
• Due to the flat (heron) NA head, ALL of the combustion chamber on an NA is comprised of the ~6mm of volume in the cylinder itself + the shallow piston dish + valve reliefs in the piston, with ZERO volume in the head itself.
  
• By contrast, the 931 has a combustion chamber that is made up of a the dish in the piston + the combustion chamber in the head, with ZERO volume in the cylinder itself (piston sits flush with the deck at TDC) and NO valve reliefs.
  
• OEM 931 pistons have a quench ring cast into the crown of the piston. If a piston reaches TDC and the valves have not closed, it is the quench ring on the 931 piston that interferes with the valves.
  
• By contrast, the valve reliefs on the NA prevent interference (for example, such as would be caused by a failed timing belt or valve float due to over-revving).
  
• The quench ring is not essential on the NA given the relatively low compression ratio.
  
• By contrast, the quench ring on the 931 is essential to help minimize the chances of pre-ignition and detonation.

Here is a photo that illustrates a stock 931 slug next to a custom Diamond slug. Note the evidence of valve interference on the OEM slug. Also note the difference in the quench ring thickness, and the depth and shape of the dish.


Here is a photo that shows a custom NA slug on the left designed for an NA head with the big valve kit, and an OEM US-spec NA slug on the right. Notice the shallower dish on the custom slug (setup for ~10.5:1 CR) compared to the dish on the OEM slug (OEM US-spec late model 9.0:1 CR).


To sum it up, all NA OEM configurations are NON-INTERFERENCE; all 931 OEM configurations ARE interference. Hopefully, these pix and explanations help illustrate why.
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erstwhile owner of just about every 924 variant ever made