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Geddy T · Joined: 10 Nov 2002 Posts: 149 Location: Bellingham, WA

what's so horrible about an interference engine? If you're careful, it shouldn't be a problem. Most hi-pro cars are, anyway. Besides, cutting valve reliefs in the pistons can lead to hot spots that make your new high CR problematic.

924 turbo · Posted: Sun Dec 08, 2002 9:57 pm Post subject:

An interference engine wouldn't be problematic unless you were the kind of owner who seriously considers decking his block and lightening his flywheel and at the same time replaces his timing belt tensioner (a $15 part brand new) with a used one from a junkyard.

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Jon Furst

CBass · Joined: 03 Nov 2002 Posts: 2807 Location: Vancouver, Canada

Nothing wrong with an interference engine!

Wait for a 931 head on ebay, and then get a custom set of pistons made... port the 931 head, and have 10:1 compression. Then all you need is a good cam, like one of those Integral cams.

Geddy T · Joined: 10 Nov 2002 Posts: 149 Location: Bellingham, WA

Maybe off the topic, but I'd like to see someone convert to a chain timing system instead of a belt.

Lizard · Posted: Tue Dec 10, 2002 1:57 am Post subject:

geddy I have thougt of it but what I found was the chain would have to be so long it would make it more problematic
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3 928s,

Joes924Racer · Posted: Tue Dec 10, 2002 11:45 am Post subject:

naa you could design and build a back and front cover for it..
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

just curious, but why would you want to use a chain? they've always been a PITA on every car i've had that uses one. the belt is easy.

-nick

Joes924Racer · Posted: Tue Dec 10, 2002 3:59 pm Post subject:

i didnt someone else suggestecd it im not that silly ...the belts fine really.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

Hey Joe,
I just put the head back on my motor after checking valve clearance and it is fine. It has a good .100" clearance with a .460" lift reground cam. This is after taking .050" off the deck to up compression and quench. I also think you may be a little off on your compression math (or I may be

) but according to my computations you should end up with a full point more decking .050" (or 2mm).

Todd

Joes924Racer · Posted: Fri Sep 12, 2003 6:49 am Post subject:

endwrench hes a pioneer. of the 924 world
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

Roger · Joined: 06 Jan 2003 Posts: 1235 Location: Cordova, TN

From what I have been told a turbo head will not improve performance in an N/A motor. Since forced induction motors dont use the flow characteristics that N/A motors do. Also cutting reliefs into the pistons would lower the compression.

Why cut the block to fit the pistons then cut the pistons to fit the head. I would seriously go with the higher CR pistons, and not mess with the all the machine shop work.
_________________
1981 924 NA

Some people are like Slinkies. Not really good for anything, but you
still can't help but smile when you see one tumble down the stairs.

Joes924Racer · Posted: Fri Sep 12, 2003 1:18 pm Post subject:

Seems like a inexspensive way to get 10 -1 cp and
its not all that unheard of.

_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

I have to mention something here. It seems that people are "assuming" if you put a turbo head on an NA motor it will automaticly become an interference engine. I don't beleive this is so. The valve reliefs in the NA pistons are cut to within .050" of the edge so I don't believe this would be an interference point and the Turbo valves are actually recessed into the head so are actually farther away from the piston when fully open. This leads me to beleive the interference culprit in a turbo motor is the piston hieght itself. This would make sense in order to create some turbulance in the combustion chamber of the turbo head. I don't know this for sure as I have never seen a turbo piston in comparison to a NA piston.

Todd

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

i've been thinking about this a little since eturbo is planning on using the 931 head on his na. this is going to be a general "using the 931 head on an na" post:

the 931 pistons come up level with the block height at the top of their travel. the na pistons leave some room (obviously, or else there would be no combustion chamber with the na). so the wrist pin is located further down on the turbo piston which brings the top of the piston further up inside the cylinder.

my 8.5:1 turbo pistons are dished, as are the 9.3:1 na pistons i've got. the dish on the turbo pistons is certainly larger than the na.

so you need to make up the cc's in the turbo head by decking the block by that same amount to get the same compression as the na head. off the top of my head, that seems like a lot! there is a fair bit of combustion space in the 931 head.

here's the simple calc:

area of the cylinder: 87mm bore (or 8.7cm to keep it in cc's), 4.3cm radius = 58 square cm's area.

what times this area = say 30cc (rough estimate of the 931 head combustion chamber).

30/58 = 0.52cm = 5.2mm does this seem like a lot to anyone else?!

of course, we also need to know the difference in the dish cc's between the 9.3:1 na pistons and the 8.5:1 turbo pistons. there might be as much as a 10cc difference. if that's the case then we're looking at decking 3.4mm. still a fair amount, but not too bad.

the timing can be made up using an offset cam key- simple enough. i think the real thing to watch out for is how much can you take off the block before the na pistons poke out the top!? as long as the volume between the top of the na pistons and the surface of the block is less than the volume in the 931 head then we're safe. i'd be willing to bet that this is the case.

so we're going to have to wait until we get the cc's of the head and piston dish and the distance the na piston crown is from the block surface. then we can figure how much the block needs to be decked.

maybe this is why the DP cars used "funky looking pistons". i imagine they are likely domed to make up the extra volume of the 931 head.

i think someone in the performance forum was asking the real gain using the 931 head. honestly, there's no doubt that the 931 head will make more power than the na head. turbo or not, the valves are bigger and the ports are designed better. if you believe that the na motor is limited by the head then what about the 931 head wouldn't make more power?

the worry was that the turbo head will only come alive in the upper revs. that would be true if only the valves were enlarged which gives more volume of air in the engine, but the shape of the ports will improve the velocity of the air too. this will give you a torque boost.

the DP cars likely make all their power only in the upper rpm's because they use a cam that moves the powerband up there. it's a racecar, that's where you want it.

ps- decking the block and using na pistons WILL create an interference engine. no doubt about it. you will need less combustion chamber volume than the turbo which is already an intereference engine.

-nick

Joes924Racer · Posted: Sat Sep 13, 2003 10:36 am Post subject:

Well thats all great though what about the 80 pistons there 9.0 to 1 and have a little smaller dish than the 9.5 though bigger then the 8.5 to 1 pistons and as far as turning it into a interference your wrong unless Endwrench in victor montana is tryin to blow one by us thats not nice and your not doin that right Todd[ also known as endwrench, He said he successfully has removed 2 mm from the block and still has the proper
clearence even if he fudged on that he still claims that its not a interference motor so whats up..

Endwrench.. lead us into the light what dya say. I think his answer is he didnt use a turbo head.

Joes924Racer
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.