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[If it's got an engine]

Are there any differences between Turbo and NA shortblocks? Are they interchangeable, if there are differences?

I have a 1984 924 with what I think is a broken rod, though I haven't disassembled it yet. I've found a 924 turbo engine cheap. Could I mix and match parts and make a good NA engine from the two? if there's lower compression on the Turbo, is it from deeper dished pistons, or a different combustion chamber, or a combination of the two?

Thanks in advance!
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Automotive commentary . .
www.ifitsgotanengine.com

[PORSCHEV]

It's a combination of lower compression pistons a dished head. No go for you unless you are running boost of some kind.
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1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

[If it's got an engine]

Any idea what compression I'd end up with if I used the NA head on the turbo shortblock? Will it even fit?

Thanks again.
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Automotive commentary . .
www.ifitsgotanengine.com

[Fifty50Plus]

If the NA pistons are okay, you can swap them onto the turbo rods/crank/block combo. But get the block honed by a good machine shop and install new rings and bearings. Bolt on the NA head, oil filter piece, pan, flyheel/clutch pack and ancillary front of the engine pieces and you are golden.
BOL
Chuck
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

[ideola]

As Chuck and Porschev stated...in terms of the short block only, the difference is limited to the pistons. So you can take any set of NA pistons, bolt 'em up to the rods and the rest of a 931 short block, and you should be good to go.

S1 931s had a CR of 7.5:1, S2 had CR of 8.0:1, Euro-spec had CR of 8.5:1. So combining with an NA head is not going to result in anything pleasant to drive.
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erstwhile owner of just about every 924 variant ever made

[!tom]

I thought a N/A head and turbo pistons had a very high CR, too high to be practical.
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78 924 NA
5-lug

[Joes924Racer]

The oppsosite Tom, lower cp. with the NA head.\ cause its flat no dish.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[PORSCHEV]

I am mixed up now I know for certain the turbo head is dished so if you compare the NA head to the Turbo you take a big jump in compression with the NA because there is relief with the dish of the turbo.

But Tom! might be right. IIRC I was looking at a set of turbo pistons with the NA head. If I can recall I didn't go that route because it would become an interference engine and I didn't want that!

Someone set it straight. Please............
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1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.

1978-#53 "D" track racer.

[peterld]

The N/A head is flat - the combustion chamber is effectively in the piston.
The Turbo head has a slight combustion chamber, as well as using dished pistons to achieve an overall combustion chamber.
Thus, forgetting for the moment about valve interference, if you put a turbo head on an N/A block the compression would be lower than with the N/A head.
Both blocks are identical in all ways - same forged rods and crank - just the pistons are different, and several small drillings in sump, etc. Very early N/A cranks ran different width bearing sizes.
Now to compound matters, the Euro/ROW N/A had different pistons to the US version - these were somewhat 'lumpy' with valve cutouts to still make the motor non-interference.
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80/81 932/8 ROW

[If it's got an engine]

Thanks guys! I think you've answered my question enough that I'm not going to buy the turbo engine.

However, I wonder. . .
I read somewhere (here, I think) that the turbo head flows better than the NA head. I wonder what compression ratio you'd end up with if you used Euro pistons (which I think I have in my '84) with a Turbo head. You'd give up some CR with the turbo head, but might make up for it with better breathing.

Hmmmm.
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Automotive commentary . .
www.ifitsgotanengine.com

[ideola]

[!tom]

[unheardofinstruments]

I also found an affordable turbo block ostensibly with low k’s and have rebuilt a head to na specs and then not needed it because I found another recently rebuilt head to use on my na 81 model restoration. However I have an sc14 supercharger and some extra bits already so could do a boosted engine with the mysterious lower cr number the combination of turbo pistons and na head will produce so maybe one day we can get the measurement. Having read several threads on supercharging an na I don’t recall anyone using a thicker head gasket on an na engine under boost but maybe the boost levels were conservative enough, I would think somebody did maybe on a turbo conversion with big boost.
I did buy the thickest copper head gasket cosmetic made when the plan was to use the na engine. The boost that requires to make it run would probably would also require a racing clutch and break stuff though.
If there is a compression reading is there a rule of thumb for how much boost makes up for how much lower compression?
I am guessing the turbo model would be the first guidepost and slightly higher compression than that means the same boost makes more power but needs a little more retarding of the timing on the dyno later? Yes efi conversion is on the cards if I were to boost an engine and an intercooler would also help.
I have a larger throttle body, injectors, dodge neon fuel rail, oil cooler, microsquirt, blow off valve, O2 wideband sensor and bung and adjustable fuel pressure regulator so it seems like it should happen to a cheap bare block and an old stripped martini shell I have lying in the grass...

[safe]

You don't need to worry about lowering the compression if you have EFI and an intercooler. Just boost it.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe

[Beartooth]

I'd agree with safe: you don't need to lower compression much if at all to boost an NA engine. Of course, I'm thinking reasonable boost levels (say 10 PSI / .7 bar). I don't know what compression using a NA head on top of turbo pistons would net, but I'm certain it would be much lower than you'd ever want. It would run terrible until you got some boost into it, to the point you'd probably hate it even if it did allow you to make more power, and could possibly make it more likely to detonate. The design of the combustion chamber (that includes the piston and head) on most turbo motors of this era is centered around minimizing the possibility of knock and detonation, and making wholesale changes to that (even when significantly lowering compression) can have unpredictable results. Back in the 70s, when US automakers had to lower compression for emissions reasons, the large, open-chamber heads they used would sometimes knock and detonate more easily with the same piston because of the "quench" effect of smaller chambers.

Modern engines have a lot of ability to save themselves from bad gas and such, so they can push safety margins. Without that protection, manufacturers had to design the engine to survive a few bad days at least. I'm fairly certain the 931 didn't need such low compression under optimum circumstances (especially the S1 at 7.5:1) - they would have held up fine and made significantly more power with another full point of compression if good, high-octane gas was guaranteed. The bean counters aren't going to approve it unless you can at least put a tank of cheap gas in it, drive it like you stole it, and not have it blow up though. That means that for the likes of us, there's more power available running good gas and an optimum state of tune, especially with updated fuel injection and modern ignition with knock protection (that's not an absolute "need" though: you can do more with a bone stock motor too if you're careful and know what you're doing). Going back to the question of combining a NA head and turbo pistons, I wouldn't say never, but I'd say the compression would be impractically low. You'd have to find the NA head volume (I found one post saying 22cc on the turbo head, FWIW) and turbo piston volume; worst case, if you have the head and bottom end at hand, you could measure it yourself pretty easily. I'd guess the combination of NA head and turbo piston would fall in the 6-6.5:1 - that's lawn mower territory.
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1980 931 diamond in the rough