What is the best way to reduce the compression ratio from 12.0 to 1 down to say 10.5 or 11 to 1 on an L-88 engine? It is a closed chamber engine and I had considered installing a set of open chamber heads. Will this work, and if so, what compression ratio will this yield? Thanks, Rex #8089
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Re: L-88 compression reduction
if you go with 119 cc heads the CR will be 10.6:1. and it will make more HP than with closed chamber heads with a higher CR. we picked up 30 HP when we went from closed chambered heads to open chambered without changing pistons because of better breathing. -
Re: L-88 compression reduction
Thanks Clem, are you referring to the 074 casting heads? Thanks, RexComment
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Re: L-88 compression reduction
that is correct,074 aluminum heads. i do not know if they are still available.Comment
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Re: L-88 compression reduction
clem and Rex-----
The '074' heads are LONG gone. However, there are many other 118-119 cc open chamber and semi-open chamber aluminum heads available.In Appreciation of John HinckleyComment
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check dynamic compression ratio
If you're going to bother to tear your engine apart to lower the compression, I would go to the KB-silvolite.com web site and use their dynamic compression ratio calculator first to verify you'll get the desired result before you bother. I currently have a 427 with the TRW closed chamber L88 pistons and the factory open chamber heads and I can guarantee you that my engine requires about a 50/50 mix of avgas to run well. I'll also bet the static compression ratio with the original 12.5:1 pistons will be well over 11:1 with the open chamber heads as the LS6 11:1 piston made 10.3:1 with the open chamber heads.
Good luck,
MarkComment
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Re: check dynamic compression ratio
with a 119 cc head,44 cc piston dome volume,4.25 bore,3.76 stroke,.025 deck and .045 head gasket you should be close to 10.6 unless your block has been decked to zero deck height. this is not even allowing for the volume lost between the piston diameter and the cylinder above the top ring. BBC have a problem with detonation because of the large bore diameter. make sure you remove all sharp edges from the piston tops and the cylinder head chambersComment
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PS
any good cylinder shop can open the chambers to larger than 119 very easily if you want to lower the CR even moreComment
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should be "cylinder head shop" in the above post *NM*
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Re: check dynamic compression ratio
The most common Fel-Pro head gaskets are 0.039 and 0.041 inches thick. The special order gasket is available in 0.051 and is super expensive. If you used the 0.039 gasket and had a deck clearance of 0.010 you would get over 11:1.
Sorry, I don't mean to get into a p***ing contest, just wanted to warn you to check what you really have in your motor so you can determine what the end result will be before you've rebuilt and installed your motor and figure it out the hard way. That was exactly how my brother and I ended up with our L88 closed chamber piston w/open chamber head motor that the machine shop promised would run on pump gas! After months of tuning, adjusting spark curves, etc - it NEVER ran well on pump gas and all the machinist would say was "oh well!"
That was over 20 years ago. Never used that shop again, always double checked everything since then. The calculator is free to use on the WWW.KB-silvolite.com web site. Don't be as stupid as I once was.
Good luck,
MarkComment
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dynamic CR
with a L-88 cam the dynamic CR should be close to 8:1 and should be OK on 93 octane. the big problem with a L-88 is because the cam timing allows dilution of the intake charge by exhaust gas when used with closed exhaust and this will cause you tuning problems. like i posted before any good head shop can open the chambers to what ever you think is necessaryComment
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