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Engine - General, Exhaust & Induction Topics about the general engine items, exhaust system discussed here & Improving the intake tract - air filter to intake valve.

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Old 04-19-2011, 05:40 AM   #1
2.5 or 2.2 NA - raising compression ratio  
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What's the best way to raise the compression ratio on a TBI 2.5 or 2.2?

I want to convert one to propane, and so want a CR of about 11 to 11.5 to 1, to take advantage of the high octane rating of propane.

Thanks!
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Old 04-19-2011, 05:52 AM   #2
Re: 2.5 or 2.2 NA - raising compression ratio  
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really the best way would be to buy pistons but having the head milled would bump it up also
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Old 04-19-2011, 06:07 AM   #3
Re: 2.5 or 2.2 NA - raising compression ratio  
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The BEST and cheapest way is to mill the head. However you run into a problem when you mill too much that the timing belt becomes to long. I have answers for that. I just got done having my head milled .180" which gives me 14.3:1 CR. .120" off the head will put you @ 11.5:1. For my engines I use a mid 80's Volvo timing belt. it's 123 teeth instead of 124 teeth and has all the same pitch and size requirements. I also have to advance the cam to compensate for the milling. 5-6 advance is what is needed. Easy to accomplish with a Fidanza adjustable cam sprocket for under $100.

The only way to increase compression with new pistons is custom made dome pistons. That'll run you over $500. Milling the head should run under $60, HG $20, cam sprocket $100. total cost under $200.
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Old 04-19-2011, 08:13 AM   #4
Re: 2.5 or 2.2 NA - raising compression ratio  
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deck the block also. I had my head shaved .10, and my block decked .20, with new flat top pistons
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Old 04-19-2011, 08:59 AM   #5
Re: 2.5 or 2.2 NA - raising compression ratio  
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I wish I could find the pictures of when I had my head milled but I just used the stock pulley and belt.

I cut the A/C mount in half (or you could find a non-A/C bracket) either way. Then I cut and channeled the motor mount for the belt to run through and made a bracket with a pulley that was in line with the timing belt,(in place of the top portion of the A/C bracket) pushing the timing belt into/through the motor mount to take up the slack.

For the pulley I just went down to AZ and sifted through allot of boxes till I found that I thought would work and BAM!! it was done.

After it was all said and done and the head had been CC'd my guy at the shop told me that I had somewhere around 11.6:1, then I took it a step farther and put in a Comp-cam master cam with 280 duration and OMFG it was a beast. I could chirp third with 17x7 yoko's on the front, and they say that the NA has no power...
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Old 04-19-2011, 12:38 PM   #6
Re: 2.5 or 2.2 NA - raising compression ratio  
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Yeah and I had a guy on another site telling me that raising the compression was a waste of time and wasn't going to do anything for me.....LOL. I agree milling it is the easiest fastest cheapest way to get you up on compression, everything else will certainly cost more money and is more work. While the heads off a cleanup work around the valves will also help keep detonation down by removing sharp edges and you can just plain run more compression with the aluminum head to begin with.

If you really wanted to be risky I believe you can get a 0.027 thick Cometic gasket but as we all know the thinner the gasket there are risks associated with their sealing ability.

I thought I read someplace milling beyond the 0.120 caused the heads to crack so 0.120 was the upper limit for milling?
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Old 04-20-2011, 10:01 PM   #7
Re: 2.5 or 2.2 NA - raising compression ratio  
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I have a head with over .180" off (287) no problems with cracking. 14.7:1 CR. 30cc combustion chamber volume. Not realist for daily street driving. .080"-.100" off the head will put the man at 11-11.5:1 CR he seeks. However a shorter timing belt or other alternatives will be necessary along with advancing the cam.
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Old 02-13-2013, 02:12 AM   #8
Re: 2.5 or 2.2 NA - raising compression ratio  
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Quote:
Originally Posted by idkanything View Post
The BEST and cheapest way is to mill the head. However you run into a problem when you mill too much that the timing belt becomes to long. I have answers for that. I just got done having my head milled .180" which gives me 14.3:1 CR. .120" off the head will put you @ 11.5:1. For my engines I use a mid 80's Volvo timing belt. it's 123 teeth instead of 124 teeth and has all the same pitch and size requirements. I also have to advance the cam to compensate for the milling. 5-6 advance is what is needed. Easy to accomplish with a Fidanza adjustable cam sprocket for under $100.

The only way to increase compression with new pistons is custom made dome pistons. That'll run you over $500. Milling the head should run under $60, HG $20, cam sprocket $100. total cost under $200.
Does anybody know what model Volvo that idkanything was referring to here for the shorter timing belt that can be used on our 2.2/2.5 8v's? I am going to see tomorrow if the p4529499 belt is still available from Mopar but I have a feeling it will be NS1.
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Old 02-13-2013, 03:03 AM   #9
Re: 2.5 or 2.2 NA - raising compression ratio  
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The Pope has really helped outThe Pope has really helped out
BTW keith black still has a higher compression piston for the 2.2.

KB Pistons for Automotive Applications
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Old 02-13-2013, 04:36 AM   #10
Re: 2.5 or 2.2 NA - raising compression ratio  
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Here is all the timing belt info you need. No need to try to hunt down the Mopar Performance belts. http://www.turbododge.com/forums/f4/...ming-belt.html
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Old 02-13-2013, 09:46 AM   #11
Re: 2.5 or 2.2 NA - raising compression ratio  
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shave the head, deck the block, bore it over, flat top pistons, water/methanol injection....
i have a set of brand new 0.060 over keith black silv-o-lite pistons and rings for an old 2.2 non-common block if your interested in em pm me!
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Old 02-13-2013, 07:19 PM   #12
Re: 2.5 or 2.2 NA - raising compression ratio  
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Chris65 thanks for the link! Got the part number I need!
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Old 02-13-2013, 10:22 PM   #13
Re: 2.5 or 2.2 NA - raising compression ratio  
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Chris65 thanks for the link! Got the part number I need!
You're welcome.
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Old 02-14-2013, 03:25 AM   #14
Re: 2.5 or 2.2 NA - raising compression ratio  
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What is the maximum milling one should do to a swirl head on a common block 2.5 TBI engine? Goal would be to still use the stock type starter and battery. And not require higher than 92 pump. Just interested in maybe playing with higher compression but not interested in sacrificing reliability. According to some of the above statements, milling .080"-.100" can get the engine in the 11.5:1 CR range. I believe these statements were referring to G heads only. I could get my hands on the 123T Dayco belt and I already have offset cam keys. Any thoughts? Anybody done a similar procedure on a swirl 2.5? Gains worth the trouble?
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Old 02-14-2013, 10:31 PM   #15
Re: 2.5 or 2.2 NA - raising compression ratio  
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125 HP would be cool. I am looking into picking up a beater for a commuter car, 2.5 TBI/Auto. I won't just pull the head to perform milling, I am not going to put a lot of money into it. But, if it needs a HG, which most vehicles I get do in the first little bit of owning them, I would consider doing the milling work.
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