Engine Compression Question? Turbo/NA

[ae1969]

The 86' turbo has a 8 to 1 compression.

ANd the 86 NA has a 9.5 to 1

?
Questions:

1. Is the head of the turbo that much different? I thought they were virtually the same except for some of the coatings on the valves? If someone has a couple of heads laying around, humor me and do a few measurements.........

2. Are the pistons the same size? Height?

3. I want to retrofit the turbo head and the turbo pistons on to my NA. (You are probably figuring out why). Will this drop my compression to the 8:1 range? I am assuming the stroke and bore sizes are the same.

My understanding is that If I swap the head and pistons I will drop the compression of my engine.

Thanks

[Mike B]

Hi Alex....Just got off work so I need a couple hours sleep but I can measure whatever you need...I have a couple 951 heads...Send me an email @ mikeybain@shaw.ca with what you want and I'll give you numbers and photos

[porsche1987_944s]

I think compression was lowered by using dished pistons. Take a look at the pictures in 1.2gees thread "Rebuild update 2 (Pictures)"; you can see that the pistons are dished. With this in mind all you should need is a set of 951 pistons to lower your compression. Ring depths on the 951 pistons might be different to help handle the boost, but that shouldn't matter.

[slevy951]

Originally posted by ae1969
The 86' turbo has a 8 to 1 compression.

ANd the 86 NA has a 9.5 to 1

?
Questions:

1. Is the head of the turbo that much different? I thought they were virtually the same except for some of the coatings on the valves? If someone has a couple of heads laying around, humor me and do a few measurements.........



2. Are the pistons the same size? Height?

3. I want to retrofit the turbo head and the turbo pistons on to my NA. (You are probably figuring out why). Will this drop my compression to the 8:1 range? I am assuming the stroke and bore sizes are the same.

My understanding is that If I swap the head and pistons I will drop the compression of my engine.

Thanks

1) Turbo head uses different valves (exhaust are hollow, sodium filled), and exhaust ports are ceramic lined. I'm not sure if the valves are the same size or not.

2) Yes. But, turbo pistons are dished. Also, turbo pistons are forged instead of cast.

3) Bore and stroke are the same. Changing to turbo piston will lower your compression.

[ae1969]

Thanks for the replies guys. I have a project I am working on (too much time on my hands)

Mike:

If you have a turbo head, I wouldn't mind taking it off your hands? (could you do that for a fellow canuck !!!!

Porsche and Slevy:

1. Dished pistons, that would explain the drop in compression. Ok So I need to get a set of pistons. I will tear the engine down in the fall? I recently took the head off and the chambers looked great no ridges, scores, nothing............as smooth as it gets??? Would I stay with the stock turbo piston size or do you think I may need to move up a size????

2. The block for the NA and Turbo as I understand are identical....unless someone can tell me otherwise?

3. Suggestions for aftermarket knock sensors....

[slevy951]

Alex,

Are you converting? Sounds like your trying to do it right instead of just adding a turbo. Have you thought about water for cooling? The turbo has a significantly different cooling system.

1) Yes, the dish lowers the compression. The size you need depends on what block you have, standard, +1, or +2. I think the 1 or 2 is stamped on top of the piston and by the cylinder on the block. Not sure on this......

2) The left balance shaft cover is tapped for oil feed to the turbo. Block mounting bracket is different for the turbo. Oil pan is tapped for a oil return from the turbo. Other than that, I believe they are identical.

3) To work with your stock DME? No idea.......

HTH

[ae1969]

Slevy:

Yes its a conversion (callaway), I bought the turbo kit from Marksports. I drove with him last year, and the car scared the crap out of me......

1. Pistons: so I will not know the size I need until I strip her down??? so much for trying to buy things ahead...... I am planning on freshening up the entire engine, bearings, seals etc....

2. The callaway turbo system sits on the same side as the exhaust (lots of heat). I am looking at modifying my hood (924 scoop). The callaway turbo does not have the water cooling that the 951 does, so life expectancy will definitely be decreased.

3. I think detonation is and will be my major problem, but we will see how it goes.... water injection??? there has got to be a better way.

4. Clutch is a concern. I have access to a few 924 transmissions and have heard that they are able to handle more stress than the transmission in my car...... (lots of reading ahead). If I were to find an S2 transmission it would be ideal......but those are hard to come by. I will probably stay with my NA clutch until I kill it.

Either way I think it will be a fun project....

Alex

[slevy951]

Sounds like a fun project.......

I've heard the S2 gearbox has a weak R/P. You may want to check for 951 gearboxes..........

[adrial]

Originally posted by slevy951



2) Yes. But, turbo pistons are dished. Also, turbo pistons are forged instead of cast.

They were forged for the 86 turbos, but cast for all other year turbos as far as I know. Or am I thinking of Rods??

[bogomili]

Adrial:

You are thinking of rods.

Alex:

If the stock pistons on the turbo are forged, and I beleive they are, they are uniform from year to year. I know 3.0 pistons from the S2 and 968 are forged so it just makes sense that 951 stockers would be too.

Anybody with first hand experience confirm or deny this?

____________
Matt