2.0 L race engine deck height, How little dare I? Options

[Driver174]

914-4, stock crankshaft, Stock Jugs, H-Beam rods, JE Pistons, 7k redline; how tight can I safely go? I've heard that a deck height of 0.050" is a safe amount. Can I go 0.040" without risking interference issues?

Thanks,
Jim

[Mblizzard]

Others may have more specific information but I would be concerned about interference caused by valve float at 7K especially if you are running stock springs.

You can certainly go through the process of checking the clearance using clay or dial indicator but you also have to account for piston rock and allow additional clearance on the exhaust site for the additional expansion of the exhaust valve due to the heat.

Some of the experts may be abele to give you exact numbers for rules of thumb clearances but if you are running stock springs at 7K rpm then you can be sure that you will get valve float.

[Driver174]

I'm really just concerned with deck clearance; wanting to know how tight I can safely go.

Valves, springs and cam have all been upgraded.

Thanks,

[ChrisFoley]

With CrMo rods and forged pistons you can set the deck to .040" safely.
As tight as .032" is even possible without problems.
The valves should get no closer than .080" to eyebrows cut in the pistons.

[Mblizzard]

I read here somewhere that the deck height requirement was greater than ~0.028 which keeps the piston from hitting the flat portion of the head when the pistons reach max temp.

That seems pretty close to what Chris said.

Attached is a file I got from someone here on calculating shim size and compression ratios that may or may not be useful.


 Compression.xls ( 7.49k ) Number of downloads: 212

[yeahmag]

I’m around 0.038” on stock 2L rods and crank, 96mm pistons, and a 7250 rpm rev limit. I do have valve pockets in the pistons and the valve train to support the rpm.

[ChrisFoley]

I’m around 0.038” on stock 2L rods and crank, 96mm pistons, and a 7250 rpm rev limit. I do have valve pockets in the pistons and the valve train to support the rpm.

That's probably pushing the limit for stock rods.
I wonder what the stretch is above 7000 rpm.

[yeahmag]

Not sure if you remember Evan Fullerton, but he ran stock rods to 8000 RPM! I plan to scope the motor (back side of valves and pistons) here pretty soon, just for fun really. I'll let you know if I see anything.

[brilliantrot]

Not sure if you remember Evan Fullerton, but he ran stock rods to 8000 RPM!

But I have no idea what the deck height measurement was as FAT built the motor (8.6-1 compression on AA 96mm pistons so clearly not pushing any boundaries) and being strictly an AX car it only saw 7500+ for maybe 5 seconds or less a run in 2nd and sometimes 3rd gears. What sorta works for AX, isn't something that will hold together on a track car and it certainly wasn't making power up there, just saved a shift and was faster then banging off the limiter. Truth be told, I just disabled the revlimiter entirely after I got tired of hitting it when it was set to 7500 so I really don't know what it rev'd to as I just held the pedal down until I had to slow down. The tach only is enumerated to 7k so lets just call it high RPM.

I got 2 seasons of AX out of the car before selling it and it still ran great and had maybe 6k street miles on it so it was at least somewhat reliable.

[Driver174]

Thanks for the input guys; I'm feeling better about the .040 clearance now!

[Charles Freeborn]

Ressurecting this thread as I'm unable to run the spreadsheet (for some reason my mac doesn't like the .xls file) to calculate shims / compression ratio change. In short I want to reduce the compression of an engine from 12:1 or thereabouts to closer to 10:1.
Thanks.

[stownsen914]

Ressurecting this thread as I'm unable to run the spreadsheet (for some reason my mac doesn't like the .xls file) to calculate shims / compression ratio change. In short I want to reduce the compression of an engine from 12:1 or thereabouts to closer to 10:1.
Thanks.

I'll be curious what folks say about type 4 motors, but I know for 911s you don't want to have a deck height more than .060". Better to reduce compression in the dome rather than at the perimeter of the piston. Shimming to reduce compression does the latter, of course.