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Old 02-01-2004, 08:13 PM
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how to keep low compression

i am just wondering, if i wanted to build a motor to have about 500 hp, but still have low compression, is it possible? the reason i am interested in this is because soon i will beable to rebuild my pontiac 400 and once i get it done i want it to have a low enough compression so that i can either throw a super charger or a turbo on it. so any and all opinions are wanted. thanks for the help

daniel

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Old 02-01-2004, 08:58 PM
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to get 500 horsepower with compression low enough to run a blower isn`t quite possible, the engine has to ingest a lot of fuel to make that kind of power along with good sized ports with a big cam, with low compression it`ll be a lazy non throttle responsive turd, especially since the bigger cam size dictates the need for more compression since it`s lost on the overlap cycle.
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Old 02-01-2004, 09:01 PM
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He is probably right. Your best bet would be to build it how it is supposed to be N/A right now, and when it comes time to build a blower motor, buy a new set of pistons, a new cam, and have a new set of berrings put into the motor. 500 hp is in no way impossible on 10:1 compression with the right parts, but it would be a feat on 7.5:1.

Adam
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Old 02-02-2004, 07:29 AM
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so what your saying is just build the motor to how i want it now, and then when it comes time to charging it, swap out the pistons and cam and bearings? what kind of pistons would yall recommend for low compression? and for the cam does it need to have a lot of duration with a small lope, or what? would yall mind explaining that to me? thanks so much

daniel
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Old 02-02-2004, 01:07 PM
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I say to change the berrings just to be safe, and so that you know that you have a good set. I think you would be disapointed with the motor if you built it at 7.5-8:1 compression with no supercharger. I would just build it at 10:1 for now, and when you have saved up for a supercharger, rebuild the motor for a supercharged application. Pretty much all that you will have to change is the pistons and the cam. You can probably sell your old pistons. As far as the cam goes, If you look in the comp cams catalog, they have cams made for supercharged applications. The only other think that I would think about is the block. I saw a person who had a supercharged motor with a stock block, they ended up cracking the block in between the cylinders. That motor was pump gas, but it was making in excess of 800 hp. I dont know if that person just had a bad block, or what, i just thught you would like to know that. If you build a 406 with AFR heads, with a 2500-6500 powerband, you can still get 550 hp out of that motor without a supercharger. I think that would be plenty for someone starting out. Then once you get used to that much hp, you could put a blower on it. I would go with a set of JE dished blower pistons when you go to that.


Adam
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Old 02-02-2004, 01:10 PM
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You'll never push 500 HP out of a pontiac 400SB. Atleast not for long.

EDIT: Take that back, just did some research and pontiac even had a 350 BB, weird.
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Old 02-02-2004, 01:16 PM
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Daniel,

check out my gallery photos, I have worked with 6-71 blowers alot. You really need to give us some more imformation and what your doing and what kind of blower you want to run. A paxton and roots style are two different blowers and need different setup's. Although alot of the theory is the same with super charging there are differences with compression, cams, pistons, ect.. Blower type, size, and make all play a role in how to set it up. For basic street guidlines on a 6-71 not a centrifical blower, you are probably going to want something around 112 or 114 lobe center, and somewhere around 8:1, normaly you dont need alot of lift with a blower cam, and I have even used a good RV cam with good results. I think most people tend to over cam with a blower. One important aspect is getting your exhaust out. A duel pattern cam favoring the exhaust usualy will have better results then a single pattern.
If you are going to run a 6-71, you can use charts to work the equation backwards, for example, with running on a 1:1 blower ratio you can estimate you want an over all 10lbs boost at 14:1 compression at WOT, then use charts to estimate a 8.3:1 compression piston, if you want an over all 13:1comp at WOT it would use a 7.9:1 comp piston.. Try to come close to the overall compression you want at WOT then you can later change drive pullys to lower or raise your blower ratio. These figuers are not actual and only used for an example, you can get the charts online from dyers, bds, hamptons, ect...
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Old 02-02-2004, 01:23 PM
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oops, I thought you were building a 400 chevy. I didnt read very closely, sorry. I dont know a whole lot about pontiac motors. AFR doesnt make a pontiac head, so you cant use them.

Adam
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Old 02-02-2004, 01:26 PM
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Check and see if the pontiac has 2 types of heads, open and closed chamber.. It could be possible to maybe use an 10:1 with close chambers, then have anouther set of open chamber heads that might drop it to 8.5 or so.. I dont know pontiac just trying to offer anouther idea so you would have to change pistons
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Old 02-02-2004, 01:40 PM
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I gather that you have 670 heads, and will want to retain them. They are a good head, but have 72 cc chambers which will net you about 10.6:1 actual compression if rebuilt with normal .030 over flat top pistons. Pontiac street engines have a hard time with anything over 9.5 compression on pump gas, and it's a loosing endeavor to keep retarding your timing to get rid of pre-ignition. It gets hot where I live, and I actually prefer staying at a flat 9:1 for the Summer months with the 91 octane available. Easiest thing to do is use TRW forged pistons and have a dish milled in the piston by your machine shop. It doesn't take much, but somewhere in the vicinity of a 20 cc dish will keep you around 9.2:1 compression. The TRW's have a lot of meat, and can take the milling without a problem. You could go with the dished pistons and stock heads, and then when you added the blower, throw on a set of large open chamber heads to drop your compression ratio to where you need it for the blower.
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Old 02-02-2004, 01:58 PM
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this probably sounds bad, but i havent done much research on what type of blower or anything like that. i was just curious to whether or not making that much hp on a low compression motor was possible and what i would have to do to make it blown. i have really been leaning towards a turbo though, i have read many threads in hear and then i was shown this website http://www.highperformancepontiac.co...rbo/index.html

and i was really surprised to see how little he spent and how much he got out of it. are there any differences for building a motor for a supercharger compared to a turbo? and if so what are they? thanks for the help, i had no idea that i would get this much feed back!

daniel
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Old 02-02-2004, 03:20 PM
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Yes there is a difference I have never done a turbo but i do know they have different cams made for turbos.
One thing also that i like about superchargers other then the looks a big fat 6-71 sitting on top, is i would run mine on the street with 6% underdriven, very drivable, and still worked on 93 octane, then at the track in 10min i could switch pulleys are be at 6%overdriven with the 114octane race fuel.
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Old 02-08-2004, 03:19 AM
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Quote:
Originally posted by 87442lover
You'll never push 500 HP out of a pontiac 400SB. Atleast not for long.

EDIT: Take that back, just did some research and pontiac even had a 350 BB, weird.

Pontiac engines are neither big block or small block they used the same external dimensions for all their engines.

Last edited by Robinson Robin; 02-08-2004 at 03:46 PM.
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Old 02-08-2004, 07:44 AM
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You could get big power out of a low compression engine, just trick it into thinking it's a high compression engine. Look at Nascar 9:1 engines, they scream.

The SD 455s were low compression and some of the fastest stock musclecars every built.

Maximize your camshaft timing to suit your compression ratio, contact a knowledgeable cam tech and tell him how much power you want with the fuel you intend to run.

Cylinder head flow and exhaust flow will be paramount to making it work. All of the little details will need to be in place, but who knows? You might get close.

There is no rule that says you have to run 12:1 compression to get big power. If a 350 small block chevy can make 425 horsepower on pump fuel, then a 400 Pontiac can top it, that I am sure of.
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Old 02-08-2004, 07:10 PM
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Quote:
Originally posted by 87442lover
You'll never push 500 HP out of a pontiac 400SB. Atleast not for long.

EDIT: Take that back, just did some research and pontiac even had a 350 BB, weird.
They weren't really a big block. They were more of a one size fits all block.
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