Thread: compression ratio for iron heads

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Hi everyone, i talked to an engine builder who said that he refused to build any motor with iron heads with a CR higher 9:1 . He said he has seen to many motors blown up trying to run a high CR with iron heads, he said that detonation becomes a serious problem past A 9:1 CR.
what is your thougths and input guys?

how about 14.9 ?? i ran this on the street .it may be he dont want to see it detonat if some one will not run good gas . i do not like buliding more then 10to1 for the street .but i run alot lot more but i do alot of work to the dome of the piston and the head chamber

Thats your static compression ratio right ?

You must be using a wide cam to get your true dynamic compression ratio.

Tech1 has explained several times in detail how to use the "quench" idea to raise C.R. and avoid detonation. I wish I knew about that before I had my engine built but I did stop at 9:1 with iron heads for the reason you mention. As I understand it you need to deck the block because the factory tolerance leaves about 0.025" deck height and then you need to use thin head gaskets to achieve about 0.040" between the bottom of the head and the top of the piston. The idea behind this is that the bottom of the head surface actually benefits from a little cooling but more importantly the gas is "squished" severely against the step in the head and this induces more turbulence in the mixture which suppresses detonation. Here is another explanation:

http://www.speedomotive.com/Building%20Tips.htm

Apparently the quench clearance has to be between 0.035" and 0.060" and once the clearance gets more than 0.060" a lower compression ratio actually increases detonation! Of course less than 0.035" clearance leads to the possibility that slop in the piston pin and rod bearings may eventually develop and lead to destructive contact between the piston and the bottom of the head and the eyebrows in flat top pistons better do their job for valve clearance, but Tech1 has explained this procedure several time on this Forum. Even so I would think 10:1 might still be the limit with 93 octane. OF course if you use aircraft fuel for special events higher C.R. may work OK but for the street you have to figure what you can do with only 93 octane. Another option is to buy 50 gallon drums of toluene from a chemical supplier! I am hoping I can use 87 or 89 octane with iron 882 heads at a C.R. of about 9:1 due to flat top pistons and resurfacing to get 73 cc valve pockets instead of the stock 76 cc pockets. Yet another option is to use iron heads with 64 cc valve pockets and dished pistons, and then there is the possiblility of using aluminum heads with which 10:1 has been reported many times.

Don Shillady
Retired Scientist/teen rodder

Great response Don

alot of big block have 0.040 heads gaskets so they need to get close to o deck this would been done on any engine that is built rigth the so if it is a big bore and a hi rpm engine you may want to run down in the hole but not to much all my engine i have built i run them very close to o deck or down in the hole but not to much .i have a lot of stuff out there that has not come a part. like 8000 rpms in a engines wth 4.500 to 4.600 bores some thing to think about this were the fire ring set on the head gaskets you do not want it falling in the bore or way to big way back on the deck of the block you can play with toluene BUT buy the race fuel then you know what you have . if you want 11to5 or more run as much cr as you can the price will be the same for a 11 to5 or 14to1 . and intake timming has alot to do with cp my 14to8 had 285 psi and was on 108 lsa roller cam and rpm like a chain saw

I'm running 9.25 on the street in a truck on 89 octane without a hint of detonation, even going over steep grades. If I were to pull a heavy load over that steep grade, I would run premium, but I don't believe I would have to. I certainly feel the truck is capable of doing it without any detonation issues.