Compression Question
Thread Starter
Burning Brakes
Joined: Jun 2005
Posts: 754
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From: Tamarac Florida
Compression Question
I getting ready to purchase new heads for my 89 L98. Am going to go with AFR 195's with 74cc cumbustion chamber. Will these raise or lower compression on a stock block? Or will compression stay the same?
Thanks in advance,
JB
Thanks in advance,
JB
Race Director
Joined: Jun 1999
Posts: 14,924
Likes: 9
From: Merritt Ils Fl
Rethink the 74 cc heads. You will have a serious drop in compression ratio.
Went to a CR calculator; assuming standard SBC bore and stroke with a 6.6 valve relief,.041 crush height on the head gasket and a deck height of .028 your 58 cc heads will give you a CR of 10.09 : 1. Change the combustion chamber to 74 cc and you get 8.56 : 1. That is a big drop. My knee jerk reaction is you will loose horsepower even with the better heads.
My guesstamates for valve relief,gasket thickness and deck height may not be exact but are close enough.
Went to a CR calculator; assuming standard SBC bore and stroke with a 6.6 valve relief,.041 crush height on the head gasket and a deck height of .028 your 58 cc heads will give you a CR of 10.09 : 1. Change the combustion chamber to 74 cc and you get 8.56 : 1. That is a big drop. My knee jerk reaction is you will loose horsepower even with the better heads.
My guesstamates for valve relief,gasket thickness and deck height may not be exact but are close enough.
Last edited by Muffin; Oct 26, 2006 at 12:52 PM.
Thread Starter
Burning Brakes
Joined: Jun 2005
Posts: 754
Likes: 0
From: Tamarac Florida
Alright, so what do I do? I ruined one of the spark plug holes on my 113's while doing a tune up. I can still get the plug in there but there is a slight pisst pisst sound coming out of it now.
I figured I would just do a top end change now and get everything running nice again with some more power, but if I do this change and lose compression, will I essentially lose power?
As I said before, I was looking at the AFR 195cc heads with the 74cc chambers. And the cam is going to be the LPE 219. AFR's website said the the 64cc chammber heads would have to machined for dome pistons. Does the L98 have dome or dish pistons? Would the 64cc be better?
Sorry for all the questions here but I'm in a real jam and not sure what to do?
So what is the best course of action here?
I figured I would just do a top end change now and get everything running nice again with some more power, but if I do this change and lose compression, will I essentially lose power?
As I said before, I was looking at the AFR 195cc heads with the 74cc chambers. And the cam is going to be the LPE 219. AFR's website said the the 64cc chammber heads would have to machined for dome pistons. Does the L98 have dome or dish pistons? Would the 64cc be better?
Sorry for all the questions here but I'm in a real jam and not sure what to do?
So what is the best course of action here?
Burning Brakes
Joined: May 2006
Posts: 857
Likes: 1
From: Morrisonville Illinios
It is not a tough job and you can get a kit to repair at just about any auto parts store. Just keep alot of grease on everything so you will keep the metal fines trapped and keep them out of the cyclinder.wro87
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Your present heads have 58 cc combustion chambers. You can work backward, knowing your original compression ratio, to figure the total volume above the piston. Then simply add (or subtract if checking out smaller combustion chambers) the difference in the combustion chamber ccs and re-figure the compression ratio. It works for any sized combustion chambers. You can even check out different gasket thicknesses and deck heights, too.
RACE ON!!!
RACE ON!!!
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Thread Starter
Burning Brakes
Joined: Jun 2005
Posts: 754
Likes: 0
From: Tamarac Florida
Thanks to all for the replys. I have alot to think about here, but will probably go the helicoil route for the time being as PeteK suggested.
So after drudging this a up, whats the point of going to better flowing bigger heads if your going to lose horsepower with a loss of compression? Do you have to do more than heads and a cam to get the most out of it? Other than the obvious intake and exhaust.
I thought changing the heads and cam would really wake the engine up, not put it too sleep.
Forgot to mention, the current stock compression ratio for the 89 L98 is 9.5 to 1, based on that what kind of ratio should I be aiming for?
So after drudging this a up, whats the point of going to better flowing bigger heads if your going to lose horsepower with a loss of compression? Do you have to do more than heads and a cam to get the most out of it? Other than the obvious intake and exhaust.
I thought changing the heads and cam would really wake the engine up, not put it too sleep.
Forgot to mention, the current stock compression ratio for the 89 L98 is 9.5 to 1, based on that what kind of ratio should I be aiming for?
Last edited by JBI; Oct 26, 2006 at 09:39 PM.
Race Director
Joined: Jun 1999
Posts: 14,924
Likes: 9
From: Merritt Ils Fl
http://wallaceracing.com/Calculators.htm
This is a wonderful site with about a ton of automotive math dumbed down so even an old soldier can use it. Play around with some numbers and you will see where power comes from.
This is a wonderful site with about a ton of automotive math dumbed down so even an old soldier can use it. Play around with some numbers and you will see where power comes from.
Thread Starter
Burning Brakes
Joined: Jun 2005
Posts: 754
Likes: 0
From: Tamarac Florida
Originally Posted by Muffin
http://wallaceracing.com/Calculators.htm
This is a wonderful site with about a ton of automotive math dumbed down so even an old soldier can use it. Play around with some numbers and you will see where power comes from.
This is a wonderful site with about a ton of automotive math dumbed down so even an old soldier can use it. Play around with some numbers and you will see where power comes from.
. I'll check it out tomorrow, need to get off the computer for the night. Good night all and thanks again for the help.
Drifting
Joined: Oct 2006
Posts: 1,554
Likes: 13
From: Romeoville IL
If you are simply looking to fix your current head, I suggest using a teme-sert instead of a heli-coil, better product in my opinion. Heli-coils are great for holes that bottom out, but it's a pain to break the tab at the bottom off cleanly for fixing a spark plug hole, and it's still a coil. This is a full threaded insert.
http://www.timesert.com/
http://www.timesert.com/
Melting Slicks
Joined: Jul 2004
Posts: 2,695
Likes: 0
From: Chicago Illinois
Originally Posted by JBI
Thanks for the link. I need things "dumbed down" . I'll check it out tomorrow, need to get off the computer for the night.
Good night all and thanks again for the help.
. I'll check it out tomorrow, need to get off the computer for the night. Good night all and thanks again for the help.
