Compression ratio for 91 octane

donraye · #1 (**permalink**) 01-14-2009, 08:34 PM

Thanks to all for your valuable input. I also ran this question on the FordMuscle.com tech forum and got some good advice there as well. Taking in the consensus view of those that had experience in this are I have made my choice which was somewhat limited as i had to get the piston that worked with the crank, cam, rods I alreadt had. etc.
I decided on Mahle froged flat tops with 6.6cc valve reliefs and all edges are factory radiused for detonation control.
This choice would give me 11.1:1 with the usual .040 gasket thickness, but I have decded to run with a copper gasket and to increase the compressed thickness to .064. This will bring the compression down and put me in the 10.6:1 range at which I feel much more comfortable.
Thanks to all for this discussion and your valued input.
I'll let you all know how it came out after the build and dyno tuning, etc.
Don

vettestr · #2 (**permalink**) 01-14-2009, 09:38 PM

Hey Don,
In the end you have to do what you feel good about and I appreciate that fact. Just a couple of items to throw into the mix for you. I ran copper gaskets for years on my supercharged engines but have been using Cometic layered gaskets with great luck and highly suggest you check them out. They will supply about any compressed thickness you need. I run them in an alum BBC with 11.72 compression on pump gas and 39 degrees total timing. I have a knock sensor to be sure but no pre-ignition problems at all.

donraye · #3 (**permalink**) 01-14-2009, 11:24 PM

Quote:

Originally Posted by vettestr

Hey Don,
In the end you have to do what you feel good about and I appreciate that fact. Just a couple of items to throw into the mix for you. I ran copper gaskets for years on my supercharged engines but have been using Cometic layered gaskets with great luck and highly suggest you check them out. They will supply about any compressed thickness you need. I run them in an alum BBC with 11.72 compression on pump gas and 39 degrees total timing. I have a knock sensor to be sure but no pre-ignition problems at all.

Interesting you mention this as I was actually going to use a thicker .064 head gasket to lower the compression down to around 10.6:1 but now I've been told that a thicker gasket might actually increase the chances of detonation as the engine tolerates detonation best in the .035 to .045 deck range. so the .040 gasket would be the best choice with my 0 decked block.
This is new information to me but the source sounded very knowledgeable and experienced so now I am probably sticking with the .040 gasket as originally planed or perhaps go to the cometic as you mentioned. but I think their gasket are .050 unless ordered custom. I'll check into that.
I was hioping to run one of the self ealing type gaskets that do not require any RTV or other goop, but now I have to re-think the whole deal.
does the Cometic require silicone around the water ports? What did you not like about the copper gaskets?

olddog · #4 (**permalink**) 01-15-2009, 01:40 PM

Quote:

Originally Posted by donraye

Interesting you mention this as I was actually going to use a thicker .064 head gasket to lower the compression down to around 10.6:1 but now I've been told that a thicker gasket might actually increase the chances of detonation as the engine tolerates detonation best in the .035 to .045 deck range. so the .040 gasket would be the best choice with my 0 decked block.
This is new information to me but the source sounded very knowledgeable and experienced so now I am probably sticking with the .040 gasket as originally planed or perhaps go to the cometic as you mentioned. but I think their gasket are .050 unless ordered custom. I'll check into that.
I was hioping to run one of the self ealing type gaskets that do not require any RTV or other goop, but now I have to re-think the whole deal.
does the Cometic require silicone around the water ports? What did you not like about the copper gaskets?

This is the squish/quench theory. There are quite a few believers out there that if you get the edges of the piston extreamily close to the head, that this squishes the air/fuel charge into the center at high velocities. All this turbulance mixes things up better. It is supposed to eliminate hot spots, as well. Therefore reduces tendency to detonate. They claim the piston to head clearance must be very tight for this to work. I think there is something to all this, but I do not understand it enough to know exactly what is needed or not.

vettestr · #5 (**permalink**) 01-15-2009, 02:36 PM

I used the copper head gaskets for years on blown engines because they were O-ringed and frequently taken apart. I would reuse the copper gaskets many times and they worked well from that application but also demanded higher maintenance or steps. Most of these engines were dry blocks but I have used copper with coolant in other applications too. You touched on one of my biggest gripes with copper and sealing of any coolant passage.

On an engine with coolant or a wet block with copper head gaskets chasing coolant leaks was just part of the joy of copper. Iron blocks with aluminum heads tend to scrub or work the gasket because of different expansion rates. Cometic gaskets don't need special sealers around coolant passages and they recommend no additional sealers at all be used. After a few heat cycles no leaks on my engines have shown up and they do not need to be re-torqued like copper. You can and I have re-used the Cometic gaskets with great results.

In terms of ideal piston to head clearance I like a minimum of .045" with steel rods anyway. I was trying to drop C/R without replacing the pistons so increased the piston to head as far as I could without trashing my valve train geometry. I was not deciding what parts to purchase but how far I could go or had too go so I could run pump gas on an existing engine.

donraye · #6 (**permalink**) 01-22-2009, 08:25 AM

Just so all who have been following this thread know waht happened, I have just oredered the parts and made my piston choice.
I' decided to go with a set of JE pistons that will give me 10.27:1 with a .039 head gasket.
These are unfortunately very expensive pistons at around $760+ a set as they are made to order but they are the ones that will give me the ratio I wanted so I felt that it was worth the price of admission since the other options were not good ones.
My other choices were to either put me in the 11.1:1 or the 9.3:1 ratios, and neither of those were what were wanted. One was to high for my comfort (and had a very high dymanic compression with the 6.20" rod length and my cam) and the other one at 9.3:1 was just plain "wimpy" in my opinion. So... it seemed a no brainer to go with the JE #194951 17cc inverted dome and have the correct 10.3ish compression ratio for what I wanted from this motor.
Thanks to all for your valuable input!