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Old 07-02-2003, 11:24 PM   #1 (permalink)
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Compression Ratio for Supercharger?

As you may have already read elsewhere, I'm building a 331. I have a set of forged pistons that bring the compression ratio to around 10.1:1. I have another set of forged dished pistons that take the compression ratio back to around stock, 9.1:1. I would rather have the lower compression ratio for forced induction, but I'm wondering why I built a 331 then? Wasn't I doing it to build compression?

It seems silly... build up compression by building a stroker, then take it away by installing dished pistons ?

Maybe I'm missing something? Thanks for any info or opinions...

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Old 07-03-2003, 01:50 AM   #2 (permalink)
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Compression Ratio for Supercharger?

Its not the compression you gain. A 331 stroke is LONGER thus pulling the piston down further which allows more fuel/air into the engine. Kinda like a supercharger....the more air/fuel you can get in there the more power.
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Old 07-03-2003, 09:40 AM   #3 (permalink)
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Compression Ratio for Supercharger?

So I would still gain power by lowering the compression ratio? Or should I keep it around 10:1?
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Old 07-03-2003, 11:11 AM   #4 (permalink)
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Compression Ratio for Supercharger?

You asked for opinions? This should start the discussion and get alternative points of view going!

Rule of thumb: The higher the static compression, the lower ultimate safe boost.

Unless you are using some kind of sophisticated fuel, spark management (computer control)...an effective static compression of 8:1 to approximately 9.5:1 is probably the best compromise for a reliable, street-level boosted engine. 10:1 is a little on the high side, if you restrict boost below approximately 6-8 lbs, it probably will be O.K. The problem with boosted engines is greed, when you see what 6-8 lbs does to your power; you WILL turn up the boost! The stock, thin-wall, 5.0 liter block will crack under a blower, around 500 hp or higher, the question is only how long before it happens. You can go a little higher with an intercooler or aluminum heads or water/alcohol, NOS injection (it cools the mixture, making it denser, more stable). If the compression is much lower, the engine will be a "dog" when off boost. Much higher and you start running into spontaneous detonation. "There is a reason that diesel engines don't need spark plugs... high static compression creates enough heat to ignite the fuel without an induced spark".

Note: Your mileage will vary. If you have access to racing gas, high octane, you can raise the boost even more. If you can get an A-4/R-4 block...you can safely go above 800 hp. The new Dart, sportsman blocks are a lesser alternative. Some people swear by the thicker ribs of the "Mexican" blocks. However, some authorities explain that the "Mexican" block is thicker, only because the foundry produced inconsistent metal, too much porosity, so the engineers decided to compensate by making the blocks thicker. If everything is up to spec, you get a really good block. If not, it is no better than any other stock block.
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Old 07-03-2003, 11:30 AM   #5 (permalink)
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Compression Ratio for Supercharger?

Maybe I should have added that the only supercharger I can afford is the 5 psi standard output. Those are "only" $1800. Anything with more boost is over 3 grand.

[ This Message was edited by: 89Trooper on 7/3/03 10:36pm ]
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Old 07-03-2003, 01:08 PM   #6 (permalink)
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Compression Ratio for Supercharger?

Build the 331 with 9:1 compression. It will be a great foundation. And it leaves a margin for later upgrades...such as bigger blower..
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Old 07-03-2003, 03:14 PM   #7 (permalink)
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Compression Ratio for Supercharger?

IMHO I would go with the 10:1 and save up for the S-trim blower. The A-trim will never satisfy your needs, at least not in the long run and the long run is all we care about. With higher compression a turbo would be much better. An intercooler is a must regardless which way you go. I'm currently mocking up a twin turbo setup to upgrade my 80 hatch which isn't even running yet

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Old 07-03-2003, 04:02 PM   #8 (permalink)
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Compression Ratio for Supercharger?

What's the difference between the A-Trim and S-Trim? Just the psi?
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Old 07-03-2003, 04:26 PM   #9 (permalink)
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Compression Ratio for Supercharger?

Impeller and it's designed efficiency range and rpm. The A-trim is rated at 4-5 psi while the S-trim is 9-20psi. The pressure isn't the whole story it's the Volume of air the impeller moves is the hp secret. A full Vortech S-trim kit can be had for about $2900. An intercooled ATI Procharger(14psi up to 20+) goes for about $3100-3200.
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Old 07-03-2003, 04:38 PM   #10 (permalink)
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Compression Ratio for Supercharger?

If I get S-Trim, shouldn't I definitely go for the 9.1:1 compression ratio?
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Old 07-03-2003, 10:37 PM   #11 (permalink)
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Compression Ratio for Supercharger?

Or build your very own custom twin turbo for around $1000-1500. with 9:1 compression and 10-12 psi it should make an easy 450 HP with very little tuning. Beowolf hit the nail right on the head: Boost is addictive, once you find out what 5 psi will do and get used to it, its so easy to turn it up that you'll do it in a heartbeat (REALLY easy with wastegated turbo(s)). I have the broken junk parts to prove how addictive it is

oh by the way, an early 302 block only will take around 600 HP with a main girdle and about 15-18 psi boost. Broke one right down the center

[ This Message was edited by: mavman on 7/4/03 9:39am ]
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Old 07-03-2003, 11:24 PM   #12 (permalink)
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Compression Ratio for Supercharger?

If tuned properly by someone knowledgeable about SC engines, 10:1 is not a problem. 9:1 is simply more forgiving to a bad tune, it has a higher fudge factor.
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Old 07-04-2003, 05:48 PM   #13 (permalink)
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Compression Ratio for Supercharger?

Thanks for all the points of view. I think I will go with the dished pistons and give myself a strong, reliable 9.1:1 engine. It will never see over 5-6 psi, but I am still more comfortable with the lower compression ratio. Thanks again!

[ This Message was edited by: 89Trooper on 7/5/03 4:50am ]
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Old 07-05-2003, 04:14 AM   #14 (permalink)
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Compression Ratio for Supercharger?

one question you asked was if you are going to supercharge the engine, why did you build the 331? answer more low end torque thn the 302 will produce. also since the engine wont see more the 5-6 psi(i really doubt that myself as the others are right, once you try boost, you want more!!) then buikd the engine with the 10:1 pistons. however if a bigger blower, or more boost, or you doubt your tuning skills, then build the 9:1 engine.
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Old 07-05-2003, 11:11 AM   #15 (permalink)
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Compression Ratio for Supercharger?

Just when I think I have my mind made up . Well, there's not much tuning to do on a fuel injected car. I can adjust fuel pressure and maybe the throttle position sensor a tad, but that's it. Timing is no big deal... set initial then keep advancing until you hear detonation, then turn it back a degree or two. There's nothing to really be bad at. When I get used to the 5-6 psi, I'll just change the rear end (still a 3.08 )... that's a lot cheaper than more boost!

[ This Message was edited by: 89Trooper on 7/5/03 10:11pm ]

[ This Message was edited by: 89Trooper on 7/6/03 12:24am ]
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