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Old 05-14-2012, 08:07 PM   #1
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compression question

whats the advantage of having higher or lower compression? i know ive heard somewhere(i could be wrong) but you needed something like 9:1 compression to run forced induction, i currently am running 12.5:1 compression according to the guy that built the motor. is that good or bad or doesnt make a difference. and if its true that youll need something like 9:1 compression for forced induction how do you change it? and one last question that doesnt have much to do with compression but more so forced induction, i also read somewhere that b303 cams arent the greatest pick for forced induction or something like that. can anyone clearify that if im wrong on any of this

---------- Post added at 09:07 PM ---------- Previous post was at 09:02 PM ----------

and i also read just now that you cant run 12.5:1 compression with pump gas dont know if thats true or not but the guy swears its 12.5:1 and it runs on pump gas
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Old 05-14-2012, 08:33 PM   #2
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if you have a high compression ratio it is most likely because you have heads with tiny combustion chambers either because of shaved heads or domed pistons etc.

12.5 is pretty high to run FI on. the recommended is about 9:1. but you maybe able to get away with more with a lower boost setup.

now there maybe a couple things you can do. these might seem like a pita, but if you are going to properly prep your block. you will be inside the motor anyways.
you can change the pistons to dish type.
you can run a little thicker head gasket. but i dont think you can get one big enough to drop your compression that much, or would want to run one that thick.
or you can get heads with bigger combustion chambers.

now i am not to sure on being able to run on pump gas or not. i suppose if you were really conservative on your timing it may be possible. but i cant be certain on this. as long as there wasnt any detonation/pinging noises i dont think i would worry to much. you can check your timing for chits and see what its set at. and there maybe other factors on this topic.
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Old 05-14-2012, 08:42 PM   #3
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Re: compression question

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Originally Posted by dreamstang View Post
if you have a high compression ratio it is most likely because you have heads with tiny combustion chambers either because of shaved heads or domed pistons etc.

12.5 is pretty high to run FI on. the recommended is about 9:1. but you maybe able to get away with more with a lower boost setup.

now there maybe a couple things you can do. these might seem like a pita, but if you are going to properly prep your block. you will be inside the motor anyways.
you can change the pistons to dish type.
you can run a little thicker head gasket. but i dont think you can get one big enough to drop your compression that much, or would want to run one that thick.
or you can get heads with bigger combustion chambers.

now i am not to sure on being able to run on pump gas or not. i suppose if you were really conservative on your timing it may be possible. but i cant be certain on this. as long as there wasnt any detonation/pinging noises i dont think i would worry to much. you can check your timing for chits and see what its set at. and there maybe other factors on this topic.
im running a carb setup not efi idk if that changes anything and when the guy first built the motor he had a procharger on it idk if he messed with compression after that or not and and how do i check mmy timing for chits? i aslo have a gear drive and no chain
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Old 05-14-2012, 08:46 PM   #4
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none of that stuff you mentioned affects compression ratio. i cant be certain but i suppose if the bottom end is built near bullet proof it maybe possible to run that high of com ratio with FI.

i meant check timing for *****s. you need a timing light. and it is very easy. this wont help in finding your compression ratio or anything. it just may give a clue as to thats why you dont need higher octane fuel. which may or not even be neccesary.
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Old 05-14-2012, 08:50 PM   #5
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Re: compression question

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Originally Posted by dreamstang View Post
none of that stuff you mentioned affects compression ratio. i cant be certain but i suppose if the bottom end is built near bullet proof it maybe possible to run that high of com ratio with FI.

i meant check timing for *****s. you need a timing light. and it is very easy. this wont help in finding your compression ratio or anything. it just may give a clue as to thats why you dont need higher octane fuel. which may or not even be neccesary.
ok wait whats FI lol
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Old 05-14-2012, 08:50 PM   #6
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forced induction lol
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Old 05-14-2012, 08:51 PM   #7
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Re: compression question

I don't think you are running 12.5:1 compression on pump gas.

Even with aluminum heads your timing would have to be so retarded that it would be a slug to keep it from detonating and destroying the engine.

You can sometimes get away with as high as 11:1 with aluminum heads and conservative timing, but 12.5:1 is just too high for pump gas.
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Old 05-14-2012, 08:53 PM   #8
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Quote:
Originally Posted by Trojan Horse
I don't think you are running 12.5:1 compression on pump gas.Even with aluminum heads your timing would have to be so retarded that it would be a slug to keep it from detonating and destroying the engine.

You can sometimes get away with as high as 11:1 with aluminum heads and conservative timing, but 12.5:1 is just too high for pump gas.
i was waiting for you to hop in here. because i wasnt sure or that part.

---------- Post added at 08:53 PM ---------- Previous post was at 08:52 PM ----------

thats the problem with buying a used car. is you never know if the seller is telling the truth.
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Old 05-14-2012, 08:57 PM   #9
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Quote:
Originally Posted by 93_347foxbody
whats the advantage of having higher or lower compression? i know ive heard somewhere(i could be wrong) but you needed something like 9:1 compression to run forced induction, i currently am running 12.5:1 compression according to the guy that built the motor. is that good or bad or doesnt make a difference. and if its true that youll need something like 9:1 compression for forced induction how do you change it? and one last question that doesnt have much to do with compression but more so forced induction, i also read somewhere that b303 cams arent the greatest pick for forced induction or something like that. can anyone clearify that if im wrong on any of this

---------- Post added at 09:07 PM ---------- Previous post was at 09:02 PM ----------

and i also read just now that you cant run 12.5:1 compression with pump gas dont know if thats true or not but the guy swears its 12.5:1 and it runs on pump gas
To answer your question about the bcam it wouldn't be my first choice of FI cam but it works very very well with my combo and is very street able and gives that nice choppy lope.. I wouldn't mind a fcam but honestly this combo is amazing 425rwhp with bcam and AFR 165 heads..
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Old 05-14-2012, 09:01 PM   #10
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Originally Posted by 90GTNovi2000

To answer your question about the bcam it wouldn't be my first choice of FI cam but it works very very well with my combo and is very street able and gives that nice choppy lope.. I wouldn't mind a fcam but honestly this combo is amazing 425rwhp with bcam and AFR 165 heads..
i agree. when i was originally planning to put a s/c on my car i read some people saying not to run a bcam with FI. but i was able to dig up a few people who where running it and were pleased with there results. so i had made the decision at that time to keep that cam. but i ended up not going that route yet. but maybe one day i will have that roots blower i have been dreaming about. :drool:
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Old 05-14-2012, 09:54 PM   #11
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Re: compression question

thanks for clearing this up bc the previous owner insists on 12.5:1 but when i was reading about it everything says its not possible. and whats a good compression for forced induction or a necessary compression
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Old 05-14-2012, 10:05 PM   #12
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Also on that note. I would like to kinda know my compression if possible. My pistons are TRW's L2305 .030 with AFR 165 heads they have been milled 15 thousandths..
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Old 05-15-2012, 06:56 AM   #13
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Re: compression question

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Originally Posted by 93_347foxbody View Post
thanks for clearing this up bc the previous owner insists on 12.5:1 but when i was reading about it everything says its not possible. and whats a good compression for forced induction or a necessary compression
The rule used to be 8 to 8.5:1 compression for forced induction. Nowadays you can get by with higher compression, but note that you may not be able to add as much boost as you could with a lower compression engine. To keep the off boost power up I would start with a 9 - 9.5:1 compression myself.

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Originally Posted by 90GTNovi2000 View Post
Also on that note. I would like to kinda know my compression if possible. My pistons are TRW's L2305 .030 with AFR 165 heads they have been milled 15 thousandths..
Compression is difficult to figure with the engine put together.
If you have all of the information you can use a Compression Calculator.
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