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Discussion Starter · #1 ·
I am going down the same road as Eric with a Holley 174 blower. Am I better off with higher compression ration (9:1) and lower (6 lbs) boost or lower compression ratio (8:1?) and higher (12 lbs) boost?

Thanks
Arch
 

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It is generally better to have lower compression and higher boost. There is more volume in a larger combustion chamber to fill with boost so in effect more opportunity for power production. 8 or 8.5:1 compression seems to work well with boost. Good Luck with it!

Mike
 

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Discussion Starter · #4 ·
Thanks, Sounds like lower compression higher boost for performance and th 9:1 and 6 lbs for reliability. I think I am leaning towards the reliability side since these cars are so light in the first place it doesn't take much to make them scream.
Arch
 

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You did not mention what heads you are using. Alum heads will handle higher compression than iron by about one digit.

Frank n Sons.
 

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Discussion Starter · #6 ·
Frank n Sons
I am planning on using AFR 185's. This seems to be the head of choice. The 205 seems like overkill and the 165 underkill.
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Arch
 

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Discussion Starter · #8 ·
Knight,
Sorry! Since I am starting from scatch here, do you think the 165 would be a better choice?
Thanks,
Arch
 

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just built a normally asperated 306 with 185s for a customer. no problem with low end torque. with a blower i woudn't have a problem with 205s.if it were mine i'd build it with 205s , 8.5:1 , 0 deck the pistons (critical to prevent detonaton) and boost the h*** out of it .these cars rarely see more then 5000 miles a year. properly built you should see at least 10 seasons before re-freshening the engine.if you're gonna go go all out!
 

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Discussion Starter · #10 ·
jwinwin,
I like your style. I hadn't thought of it that way. With all forged bottom end and girdle, I guess it would all hold together. Hmmm.
Thanks,
Arch
 

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"9:1 and 6 lbs for reliability"

Huh I guess Subaru builds their turbos for power instead of reliability since the new Legacy Turbo has 8.2:1 cr and 13.5psi from factory.

205s are fine with a 331ci and boost but I would go with 185s myself. 165s are better suited to a 302ci NA motor.

Go with more boost if you can
 

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The follwing is courtesy of induction motorsports. http://www.inductionmotorsports.com/

People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.
 

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With a positive displacement blower there's really no downside to dropping the compression. As for reliability, you guys aren't giving these motors enough credit. I've run power adders on my stock 5.0 blocks for the last 8 years without a problem. I currently have a turbo running 18psi with a ~7.8:1 setup.
 

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3070 has 12lbs of boost with stock compression (whatever that is ...like 9.0:1?). Got 10k miles on it now and we twist it's tail all the time. We have the ignition set at 12 degrees btc and do have some boost retard set in. Burns pump 93 gas, no pinging under any conditions. Runs like a scalded whore. If you think 6lbs of boost is neat, try 12.

Frank n Sons.
 

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Higher static compression will provide you with a bit more torque (not needed with these cars), where lower static compression allows more boost (as mentioned above).

Higher static compression also gets the turbo spooled a bit quicker in those applications.

I agree with most of the others, in that you should go with 8:1 or so static compression and boost up to 10-12 psi. Above 12 psi and you may want to consider the use of an intercooler due higher intake air temps associated with compressing air.

I run 18lbs of boost (pump gas) with my Stealth, with stock twin intercoolers, and a home built water injection setup. Damn fast for a 4000lb car.
 

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Static compression on the Stealth is 8:1 BTW ;)
 

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Discussion Starter · #19 ·
Notch – Thanks, I agree that the 185 may be the better head in this situation.
Mike N – Great information! Clearly less static compression and more boost appears to make sense.
Znaut – As I understand it with the Holley 174 there is no spool up lag and very little boost “leakage” so it is essentially instant on boost. Thanks for the reminder and info on your current setup.
Fank n Sons – What heads are you running? Thanks for the info, I am looking forward to “twisthin its tail”.

Ozona – Your input is always welcome, thanks
Jeremy – Looks like the consensus is with you – 8:1 and 12 pounds of boost.

So to recap, looks like AFR 185’s, 8:1 compression and starting out at 12 pounds of boost. I do plan to use the MSD ignition module with the boost timing master and a knock sensor.

Thanks All,
Arch
 

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Arch - we have Edlebrock alum heads. They are great but I believe the AFR's are a bit better.

Frank n Sons.
 
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