Back on topic on my WOT sometimes detonation in warm weather issue -
Summit provides a pretty succinct recommendation for quench:
https://help.summitracing.com/app/answers/detail/a_id/4941/~/engine-quench
How is it measured?
Quench is a calculated dimension. It uses the following formula:
Quench =
Deck Clearance +
Compressed Head Gasket Thickness
How does it affect performance?
The right amount of quench can promote fast, complete burn of the air/fuel mixture. It creates turbulence, which forces the air/fuel mixture toward the spark plug. This reduces the possibility of
Detonation. It also leads to cleaner emissions.
What should it be?
Quench should be as tight as possible, without the piston contacting the head. Recommended quench depends on your max rpm and the type of connecting rods you use.
Max RPM Connecting Rod Type Quench Range
6,000 or less Steel 0.035 - 0.045 in.
6,000 + Steel 0.038 - 0.043 in.
Any Aluminum 0.050 - 0.065 in.
Notes
- Aluminum rods get longer as they get hotter.
- Increased quench will prevent the piston from contacting the head.
- Don't run more than 0.060 in. quench trying to lower Compression Ratio.
- This will slow the combustion process and could cause Detonation.
So right now my pistons are .015" in the hole and I've got a .027" thick gasket, so I'm at .042" quench. If I pulled my current head gaskets and went from .027" to .030" to get to the max recommended quench of .045" for my setup (only an increase of .003") do you think I would notice a difference in WOT detonation resistance since I likely have some carbon buildup? My original compression ratio calculation of 10.2:1 was assuming a .030" head gasket is used to make .045" quench, but as stated, the aluminum heads currently have .027" gaskets, so the compression ratio is slightly higher with that difference of -.003". Also, is this .045" max quench number for my Steel Connecting rod motor with cam that maxes out at 5800 RPM a hard and fast number? What happens if I go up to .050" total by using a .035" gasket? Do you think chances of detonation could remain since now I'm in the non-optimum quench zone?
For reference, with my old iron heads and head gaskets, the engine builder had measured and calculated my compression ratio to be 9.7:1 and it pinged in warm weather at WOT with total timing at 32.
OK, so I also dug up my original engine info and plugged in all of the values into an online compression ratio calculator (can we trust these calculated ratios as reliable?) Can someone please verify the values shown below are correct for a 451 (400 block) B motor? This is what I have from my notes. Pistons are JE flat tops. The gasket bore diameter I got off of Cometic's info on Summit.
I used this online calculator:
https://uempistons.com/p-27-compression-ratio-calculator.html
.030" over CYLINDER BORE SIZE = 4.370
PISTON STROKE LENGTH = 3.75
HEAD GASKET BORE DIAM = 4.410
COMPRESSED HEAD GASKET THICKNESS = .027"
COMBUSTION CHAMBER VOLUME = 84 cc
PISTON DOME VOLUME (Negative) = 6
PISTON DECK CLEARANCE (Negative) = .015
This is giving me a calculated compression ratio of 10.18:1.
Plugging in a .030" gasket thickness gives a ratio of 10.1:1.
I found the old email from the shop who did these Eddy RPM heads and here is what he originally said:
According to my calculations, assuming the heads are actually 84cc, I get 9.99cr for a .030 gasket, and 9.82 for a .040 gasket.
Why the difference? Not sure how he calculated...
THREAD REVIVAL!!
Kern, my motor is a low deck 400 B block 451 mild stroker (using 400 rods) with automatic, 3.55 gears, 28" tall rear tires, aluminum Eddy RPM 84cc closed chamber heads and flat top pistons with cylinders .030" over. Calculated compression ratio is 10.2:1. When John and I put my new heads on, I knew we were going to bump up the compression to around 10.0: 1 or so. Going for optimum quench volume, it was recommended to use a .030" thick Cometic gasket, putting it at 10.2:1 which the Eddy heads should make tolerable on 91 octane. They didn't have those in stock so we went with .027" thick Cometics, the respected mopar head machine shop guru telling me .003" won't make much difference in compression ratio. Here's the thing, current timing was done on a dyno with carb jetting and my initial/total timing is now set at 17/32. Any more than that and it will ping. I had pinging with the old 906 heads too (but without carb jetting) and hoped the aluminum heads would solve things. It did help quite a bit, but trouble-free only for a short time. I had no pinging at first. At the Sacramento track 2 years ago, night time 90 degree outside temp, full throttle ping unless I splashed it with a few gallons of 110 octane mixed with the crappy California 91 (too much ethanol in that fuel). I ran it again at Willow Springs the year after and in the morning cool air the engine was so happy. By later in the day, even with 5 gallons of 110 mixed with 10 of 91, the motor pinged at WOT. Some have told me it could be due to carbon build up increasing compression ratio (already? with only a few thousand miles on the motor?). I don't drive the car that often, but would like to get this sorted.
Taking from your experience, but keeping in mind I want to stay in the optimum quench zone, should I try a thicker head gasket? Say .040"? How did you determine your .075" thick head gasket thickness? That sounds pretty thick.