Better to lower CR with pistons or heads?

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Feb 5, 2005 | 12:50 AM

flynbludream

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From: Albuquerque

Better to lower CR with pistons or heads?

I can't seem to get a straight answer about this so hopefully someone on here can tell me. Is it better to setup your CR for FI by going with dished pistons or to use bigger CC heads?

I am looking at going with a stock cubed forged shortblock right around 9:1 compression. Will more than likely be buying the AP shortblock. The two ways to go are with LQ9 or AFR big CC heads which will put me around 9:2 or so for CR or to go with the 205 AFR heads and use dished pistons to lower the CR.

Can anyone explain which would be the better choice and why? I am looking for maybe 750-800 rwhp on a twin turbo setup max with 98% of the driving done around 500-600 hp level. Not trying to set any records here. TIA

Feb 5, 2005 | 05:00 AM

n2oblkz

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From: Versailles, IN

If you are wanting to use existing heads, you can just replace the pistons with a good forged D shaped blower piston since you are doing a motor buildup. If you are going to do heads also, just pick the pistons and heads that match each other. I just purchased some Diamond D-shaped blower pistons for my FI buildup and I have bare AFR heads. I used Diamond's site to match their blower piston with the correct -cc dish to achieve the desired compression ration with a chart showing the cylinder head combustion chamber CC's. Most piston sites have these charts so that you can choose a piston to match the cylinder heads that you have or are going to purchase.

Feb 5, 2005 | 05:22 AM

n2oblkz

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From: Versailles, IN

Below are some examples of what I am talking about from Daimond's site. The first link is the table that I used to pick my pistons. I am going to have my heads finished with a 64cc combustion chamber and with the -22cc piston, it will give me the 8.4 to 1 compression ratio. The second chart shows their LS1 dished piston and if you know what CC heads that you have now, you can choose what piston would give you the desired compression ratio that you want to achieve or if you are doing new pistons and heads, you can use these charts to give you an idea of what type of piston/head combo to use. Keep in mind that you can always contact the piston manufacturers and have a custom piston made if you do not see your desired compression ratio. I think Diamond will make custom sets for about $30.00 more per set. JE and other sites also have similar type charts to help you choose your setup. Or, you can get some advice from an engine builder who has experience in building force inducted LS1's.

http://diamondracing.net/excel/gm/sb...shapeddish.htm

http://diamondracing.net/excel/gm/ls1strtstripdish.htm

Feb 5, 2005 | 08:08 PM

flynbludream

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From: Albuquerque

Thanks for the reply, but I am trying to find out if one way is better than the other. I have no heads for right now so I am trying to figure out should I go with small chamber heads and drop the CR with pistons or should I go with a smaller dish and bigger chamber heads.

Is one way better than the other or does it matter at all? Are there any issues with screwing up the quench area or anything else?