downfalls to low compression?
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downfalls to low compression? I am thinking about going with a 8:1 compression forged bottom end motor, but I don't know what the downfalls are. Can anyone tell me of any potential pitfalls to look out for, or what to expect? I figure the car will be a little "soggy" on the bottom end, but once the boost kicks in, it should be fun to say the least.
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From: Just outside of Palm Springs Ca.
That is prety much it, just don't go too low or you might have a real dog. I think you might be better off with like 8.5-8.9:1 as long you don't plan on going really high with the boost like 20 psi or anything.
-Bryan
-Bryan
A drop in a point of compression is reportedly a 3% drop in horsepower. So calculate a 6% drop.
My car is somewhere between 8:1 and 8.25:1 and it seems to drive okay. I running a big cam with low compression and cold plugs so I clear it out if I idle for a long time or right after a burnout.
My car is somewhere between 8:1 and 8.25:1 and it seems to drive okay. I running a big cam with low compression and cold plugs so I clear it out if I idle for a long time or right after a burnout.
I love how everyone says it will be a dog 
It's a 5.7 liter motor. It's pretty hard to make it a dog down low. I use to drive a 2.2 liter turbo car that had a 7.8:1 compression and even it drove fine.
It's a 5.7 liter motor. It's pretty hard to make it a dog down low. I use to drive a 2.2 liter turbo car that had a 7.8:1 compression and even it drove fine. 
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Originally Posted by Zombie
I love how everyone says it will be a dog It's a 5.7 liter motor. It's pretty hard to make it a dog down low. I use to drive a 2.2 liter turbo car that had a 7.8:1 compression and even it drove fine.
It's a 5.7 liter motor. It's pretty hard to make it a dog down low. I use to drive a 2.2 liter turbo car that had a 7.8:1 compression and even it drove fine.
He didn't believe me till I showed him the spec sheet for when I had the motor built. Trending Topics
Originally Posted by eviltwins
Exactly. I have an idiot inlaw who's building up a blown Camaro at 9.5:1 compression, and he started spouting off about how shitty any car would drive with less than 9:1 compression last night. I just ignore him, and let him take my car for a spin after dinner. "Wow, that car drives great, with awesome throttle response? It's gotta be like 10:1 compression, right?" so I say "No, it's 8.15:1, you're just an idiot" He didn't believe me till I showed him the spec sheet for when I had the motor built.
He didn't believe me till I showed him the spec sheet for when I had the motor built.LS1 Tech Stories
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From: Hazard Co. Maryland
Originally Posted by Lasershop
Idiot in laws...... Wife says I gotta' love em'. I have learned to mimic that with pity. When your kids grow up and get married the list is ever growing. Idiots.
I call them the outlaws.
As far as being able to still maintain some good gas mileage (primarily long cruse highway, in the 26~27mpg area with a 6spd and have all the things trimmed out), what's probably the best way to obtain that? 8.5:1, 9:1? Obviously that's on 93 or 93/meth. Also, what about running on ethonal? I know from reading a bit on that subject, you need bigger fuel injectors and pumps, so would it be more bennificial to run high compression ratio too since E85 usually has an octane rating of abour 105~106?
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From: Wallingford, CT
The big pitfall is being able to run so much boost. Its going to break everything else in the car!
As far as driveability, it will depend more on your tune and your set up than the CR. I am shooting for 8.2:1. The car should be ready very shortly.
If you are in NY, drive over to CT, and I will show you.
As far as driveability, it will depend more on your tune and your set up than the CR. I am shooting for 8.2:1. The car should be ready very shortly.
If you are in NY, drive over to CT, and I will show you.
I can share the info I learned and gathered from the Stage 2 Buick Grand National guys that run 76mm and 88mm+ sized turbos on 274ci engines with heads that flow 330-380cfm.
With 8:1 they can run more boost; however the turbo spool time especially on 88mm+ sized turbos at the line is way longer. With 9:1 the car when off boost is more responsive (meaning they are giving up 30-50hp from the point in compression) and spools much much faster at the line.
It is true you won't be able to run as much boost on the same octane.... (I've never heard any number associated with it), but if you really want the maximum potential from any turbo setup you need to run leaded race gas 116+ with increased timing. The engine will make plenty of power. I think a good compromise would be 8:5:1.
When a 76mm turbo is rated at 1000hp+ and with 346ci of exhaust flow to spool it... I would imagine the compression won't be a factor; however going to an 88/94/100mm setup you are really going to have to look at all the factors to make it work right.
With 8:1 they can run more boost; however the turbo spool time especially on 88mm+ sized turbos at the line is way longer. With 9:1 the car when off boost is more responsive (meaning they are giving up 30-50hp from the point in compression) and spools much much faster at the line.
It is true you won't be able to run as much boost on the same octane.... (I've never heard any number associated with it), but if you really want the maximum potential from any turbo setup you need to run leaded race gas 116+ with increased timing. The engine will make plenty of power. I think a good compromise would be 8:5:1.
When a 76mm turbo is rated at 1000hp+ and with 346ci of exhaust flow to spool it... I would imagine the compression won't be a factor; however going to an 88/94/100mm setup you are really going to have to look at all the factors to make it work right.
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Originally Posted by CHRISPY
8:1 is fine. Let the compressor flow the air. 15 pounds boost intercooled is easily possible on pump gas with 8:1.
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http://forums.evans-tuning.com/viewtopic.php?t=1612
Compression has nothing to do with how a turbo spools. If you run lower compression you run more timing off boost to make up for it, it's a pretty simple concept.
Compression has nothing to do with how a turbo spools. If you run lower compression you run more timing off boost to make up for it, it's a pretty simple concept.
Yall sure that ya dont lose all the much poser from dropping compression? I just changes to 6.0 heads with a MLS. I read around that that makes the car go from 10-1 CR down to liek 9.2-1 CR. And i can definatly feel less power. I even put a bigger cam in at the same time. It really doesnt feel nearly the same at the same boost. The cam has no overlap so that aint the problem. I have yet to tunbe it for the cam or add any timing so i expected it to be slower but it feels alot slower. GL man
John
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You added a bigger cam, lower compression, didn't tune for it, and are suprised that it feels worse driving around? Go read that post on Evans-Tuning, one full point in compression needed 7 degrees more timing to bring the motor back up to MBTT, obviously you're going to lose some low end power if you put in a big cam, lower compression, and don't do anything to compensate.
With my drop from 9:1 to 8:1 I had to make 1 more psi of boost to make up for the power lost. With a 348ci V8 engine I am not concerned about having a lazy engine with low compression. It's not a 183ci Supra engine or something or a GN/LC2 engine. Really can't compare the them at all.
