My mods are in my sig. The 2 changes are I am now running Jesel SS's undercut for my springs, which are stock PP Gold's but shimmed .060", and I also have Ferrea hollow shaft lightweight intake valves. This is what I'm planning:

DCR is now 8.62:1. I want to bump it to 8.82. My deck height is -.004" (pistons out .004"). I am running .040" Cometics which gives me a quench height of .036".

What I'm thinking is to pulling the heads and replacing my Comp 850's with the Cadillac lifters and when I replace the heads, put on a Cometic .036". This will reduce my quench height to .032" and bump my DCR to 8.72:1. Then I'm going to advance my cam from +0 to +1 which will bump my DCR to 8.82. I can get 93 octane all day so I'm not worried about ping and don't get any now with 28 degrees of timing.

I don't care about the expense of the gaskets and I'm already changing the lifters so what I need are opinions on the .036" gaskets. PTV is no issue either as I have flycut the pistons. Is .032" quench height to tight? I am rev limited to 6800rpm's.

 

.032 is extremely tight and youre pistons better be very tight in the cylinder bores or else the rocking of a piston at TDC can cause the piston to crash into the valve. How tight are the pistons in the bore? I dont see internal motor mods inj your sig so i am assuming your bottom end is stock, if thats the case I would surely not go .032 quench because the pistons are probably too loose.

Read this http://www.nitrophotos.com/240z/nitrous.html

 

I would have no worries other than the quench. As Brad stated that's quite tight @ 0.032".

 

Yes, it's a stock bottom end. I'll have the heads off so I can have them do a clean up of .006". Without advancing the cam I'll be at 8.82... And I'll use my new .040" Cometics.

 

.032 isnt absurdly tight, though i wouldnt really push any further. Im surprised you get no ping. You shoudl be right on teh edge even for 93. Bumping to 8.8 DCR might push you over and youll have to take out a bit of timing. Might be worth it, might not. Could always try the toluene/xylene mixed with gas.

 

Or you could just advance the cam 2 degrees and have 8.82 DCR

 

So, in your opinion it would be ok, clearance wise, to use a .036" gasket? I already have a case of Torco in the basement.
As far as ping, maybe the valve reliefs are keeping the DCR a little lower than I'm calculating?

 

I thought about that, but it used to be +2. Maybe I'll put the gaskets on and leave the cam straight up.

 

You said youre pistons are flycut, so P to V isnt an issue. People routinely run .035 quench on the street without issues. Some even run .030. Race cars can even see .025 quench. If youre confident on your .004 out of hole measurement for the pistons, then id feel comfortable with .032.

With the smaller duration advancing it might limit how high you can take it. Installing the cam straight up and raising the compression is there the power is at, however its not easily reversable if you went too far. If youre not opposed to losing some timing or having to mix in a hint of race gas, id go for it.

 

If I were you I wouldnt dare go .032 quench with a stock bottom end. Now if you get forged bottom end with tight clearanced pistons and rods, then i say yeah no problem, but the stock bottom end parts are not most presice parts out there and have some slop in them.

It might be Ok but it might not, especially if those weak stock rod bolts stretch a little bit.

 

just mill the heads some,use the .040 cometics and be done with it.

 

I agree with the above.

What's the risk / benefit?
How much HP gain for the slight increase in DCR and Risk of Detonation ?
Street driven? Mileage?

 

Well the .032 quench would bump DCR up slightly, but the real advantage of it is the great amount of turbulance that would be caused in the chamber, which would make it LESS prone to detonation as well as giving it better heat dynamics and creating a extremely quick flame front.

The downside is that it would be so close on a stock bottom end that some bolt stretch or just the piston rocking in the bore could cause a crash.

 

Thats how I feel. The tighter quench will help the motor to actually run without pinging on 8.8x dcr.

However if the rods stretch youll probably have plenty of other issues like spun rod bearings. IMO, I dont think a motor will run very long with stretched bolts regardless of the quench.

 

I would be most concerned with the piston rocking in the bore and bumping the head because remember a stock bottom end piston will come out of the hole a slight bit.

 

Not to mention the factory deck are usually not perfectly square which could mean the pistons are .004 out of the hole on one end or on one side and maybe .006-.008 out on the other end or other side. Have you checked all of them, or at least one on each end and on both sides?

 

Yes I did when I had the heads off last time. #1 was .004", #2 was .005", #7 was .004", #8 was .005". I didn't measure the center holes though.

 

That was my main reason for going with the tighter quench.
I checked my sig and saw I don't have in there that I changed my rod bolts to ARP Pro's about 9k miles ago. I'm not sure if that makes a difference due to rod stretch.

 

I put the out of hole measurements above. When I measured the piston deck height I did see rocking and took my measurements from the centerline of the piston. I also remember measuring the rock, unfortunately I don't remember those numbers .
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I'm also concerned about a piston to head crash. That's one of the reason's for this thread. I needed some opinions as to whether I'd definitely be ok, or I'm playing with fire, so to speak.