Affects of Exceeding 1.5mm Deck Height?

[kstylianos]

Some background:

Heads were flycut .35mm giving me a 86.8cc on 3 heads and 87.2cc's on the other 3 rather than the stock 90cc's.

I decided to go with JE 9.8:1 P's. CR for these P's are calculated with a 1mm deck height.

I don't want to exceed the 9.8:1 CR since I'm using a single plug w/ CIS. With the heads being flycut, I need a deck height of 1.45mm to get back down to 9.8:1, anything less would bump my CR above where I'd like it. What are the chances that my DH will fall right between 1.45 and 1.50?....very slim.

So.....if I measure my DH at 1.35 (CR 9.92), adding another .25mm shim will bump my DH up to 1.60 but reducing my CR back down to 9.65.

What are the downfalls of exceeding the recommended deck height of 1.50mm? I could send the 9.8:1 JE back and get the 9.5:1 to give me a little more wiggle room.

Probably a bit trivial, but I want to do it right.....

Thanks for any help.

[Wayne 962]

Exceeding the deckheight? Increasing it? There are no downfalls really, except that you are lowering your compression (which is what you want to do).

-Wayne

[kstylianos]

well....the book mentioned a 1.25-1.5 range. I understand why the minimum is a minimum, but don't understand why the 1.50 max. If there is no real problem with increasing the DH past 1.50 (other than decreasing CR), then off I go. Thought it may have something to do with squish, atomization, fuel pooling, etc....

Thanks Wayne.

[ChrisBennet]

A couple of points:
1. Increasing the deck height is going to move the heads, cams etc further away from the crank. I wonder how this will effect the chain slack?
2. Steve Weiner told me once that he likes to keep the deck height at or under .04" (1mm). Something about larger deck heights increasing the chance of detonation in the resulting "pocket" out at the edge of th pistons.
-Chris

[kstylianos]

Originally posted by ChrisBennet
A couple of points:
1. Increasing the deck height is going to move the heads, cams etc further away from the crank. I wonder how this will effect the chain slack?
2. Steve Weiner told me once that he likes to keep the deck height at or under .04" (1mm). Something about larger deck heights increasing the chance of detonation in the resulting "pocket" out at the edge of th pistons.
-Chris

1: I agree here. I'll search the archives re: the ammount of play allowable in the cam chain housings. I don't think I'll have a problem, but I'll take a look.

2: Yup.....seems to correlate with:

http://www.boyleworks.com/ta400/psp/compression.html

"Since it is the close spacing between the piston and cylinder that reduces the prospect of detonation, never add a shim/head gasket, or flat cut the pistons tops to reduce CR. If you have proper quench with 10 to 1 CR, and then reduce the CR to 9.5 by one of these two methods, you will create more ping with the 9.5 CR then you had with the 10 CR. By all means, deck the block first and under all circumstances when building an engine for optimum power output, and then determine what chamber volume will be needed in the heads to arrive at the final CR."

[ChrisBennet]

Despite what the article says, I'd take a little of the top of the piston. I think it is pretty common solution.
-Chris

[kstylianos]

I need to measure my actual DH before I decide to do anything yet, but would like to get the DH down a little, maybe not to 1mm, but I think this is one reason CE and Wayne mention 1.25-1.5. Especially with a higher CR, a smaller DH would be beneficial.

Thanks Chris....

[dtw]

JE should be pretty good to work with about dialing in your compression. My machine shop ordered 10.3:1 pistons for me; when they finally came in the shop CC'd the combustion chamber and they ended up measuring out around 11:1. They called JE and said JE was quite gracious about rushing a turnaround on the pistons in about 4-8 days. Pretty impressive. I can't wait to see these pistons.

On a side note, I guess this is one more data point to support the legend that JEs traditionally come in a point or so over on compression.

[911pcars]

I see some confusion regarding terms:

1. Deck height = distance between top of cylinder and top edge of piston at TDC. Changing this (by modifying the cylinder) affects the cylinder height.

2. Cylinder height = height of cylinder from bottom seating area to top of cylinder. Changing this (by triming the cylinder or adding base gaskets) affects the deck height.

Thus, changing either one could affect the timing chain distance between the cams and crank.

Changing deck height increases or decreases combustion chamber volume which affects the compression ratio. Excessive deck height may create a pocket in the chamber that could induce detonation or, at the very least, uneven combustion. Insufficient deck height decreases the clearance between piston and valves; piston and cylinder head.

Sherwood Lee
http://members.rennlist.org/911pcars

[Wayne 962]

Sorry - I'll be a bit clearer. I don't think that you will have any significant problems with an increased deck height of 1.60. You can't arbitrarily increase the deck height to infinity without encountering the problems talked about previously. However, with 1.60, you should be okay...

-Wayne

[kstylianos]

Thanks for the input all......Just trying to do my homework.

Another couple articles for those that are interested. I found them interesting and made things a bit more clear.....They both look to be geared towards a wedge-shaped combustion chamber w/ flat top pistons, but probably can take home some useful information.


This one gets into specifics of the benefits of a tighter piston to head clearance.

http://www.chevyhiperformance.com/techarticles/94138/

This one gets into preventing pre-ignition....one item is decreasing the DH.

http://www.kitcarmag.com/techarticles/65298/

[Wayne 962]

I haven't read those articles, but I can tell you that the 911 engine is very different than these water cooled ones. Becareful which analogies you pull over from the Chevy world...

-Wayne