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Old 11-24-2006, 06:49 PM
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350 chevy compression ratios

When looking in the magazines and on the net you get a lot of variances on the listed compression ratios. I am looking for a combination for around 10.5-11:1 compression for my 355. I have 75cc combustion chambers, (they have been milled just enough for a true mating surface. I have not done any work to the block yet but I intend on having it zero decked. I know I have to run a dome piston but how much. From what i can gather on the info I have seen I need around a .275 dome. Is that correct or has someone got a known setup for that kind of compression with a 75 cc head.

Thanks Dennis

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Old 11-24-2006, 07:31 PM
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here ya go http://www.bgsoflex.com/cr.html
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Old 11-25-2006, 12:24 AM
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This is just my opinion so take it or leave it...

If you're going to the extent of building a whopper engine with that much compression, the least efficient way to do it is with large chambers and domed pistons. Domes kill flame front speeds and require more ignition lead which further increases octane requirements and reduces power potential.

64cc chambers are not only more suited to higher CRs for that reason, but they are also easier to find with performance flow numbers. I strongly suggest a smaller chamber head. It sounds like you're trying to max out compression with what you have, but SBC heads are so cheap these days that finding a 64cc chamber performance head is like falling off a log... its easy.

355s are at about 10.5:1 with 64cc heads and flat-tops with valve reliefs.

Last edited by curtis73; 11-25-2006 at 12:31 AM.
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Old 11-25-2006, 02:06 PM
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I would like to get different heads, but the heads I have are already ported and polished, they have screw is studs and roller rockers along with 2.02 and 1.60 valves. I have $150 into these heads. All this work was done at a very good machine shop the porting is top notch. The previous customer had the heads done adn then decided to go with an aftermarket head. I bought them from the machine shop. Thats why I was looking into the dome pistons.
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Old 11-25-2006, 02:19 PM
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Even with your 150 bucks in those heads you�re still loosing. I don�t know which type of heads they are but to me 75cc + iron heads = smog heads. I dare say that even a good port job in those are no match to a couple of modern heads. And you don�t even have to spend thounsands. Dart iron heads is a bargin and will send most 350 well past 400 hp with the right stuff.
Having dome pistons is not an advantage, only a (un)necessary evil.
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Old 11-25-2006, 02:39 PM
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When we are caught up in the engine shop and machine shop we build some performance street engines and we have had very good results using the Vortec heads and we have seen better results using the older 906 casting as the have a bigger intake runner then the 062 Mexican version.

The last 355 we bullt was with light weight componets and 11:01 compression and a 750 carb and hydraulic cam and it made 460 on the torque and 432 on the horse and was very impressive on the engine dyno.

And last years we did a 383 for a customer same cam 10:5 and AFR"s and it made 501 on the torque and 455 on the horse and we have built the same combo using a set of we prepared Vortec heads and we have seen the same results.

And the Vortecs having a fast burn chamber on the dyno we only had to run 30 degrees timing for the best peek torque and HP.

Here is a link on the 383 we built a years ago.http://www.chevelles.com/forums/show...highlight=DYNO
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Old 11-25-2006, 07:45 PM
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There not smog heads, should have put that in first post. there 1971 lt1 heads. 487 castings. There also set up for .575 lift with crane springs and retainers. I have heard the vortec heads are good but I can't beleive they will out run what I have. If the vortec heads are that much better how muh more money in machine work would I have to make them handle .500+ lift and get them to the status my current heads are at.
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Old 11-25-2006, 08:19 PM
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Quote:
Originally Posted by 62CHEVYC10
There not smog heads, should have put that in first post. there 1971 lt1 heads. 487 castings. There also set up for .575 lift with crane springs and retainers. I have heard the vortec heads are good but I can't beleive they will out run what I have. If the vortec heads are that much better how muh more money in machine work would I have to make them handle .500+ lift and get them to the status my current heads are at.
The Vortecs have a higher intake and exhaust port a fast burn combustion chamber and have harden seats where your heads don't. Believe me the Vortecs are a much better head then something that was built 36 years ago.

I may have a set that came off a circle track engine that ran a solid flat tappet cam guide plates and 7/16 screw in studs and a 60CC combustion chamber for around 6 to 800 dollars as I will check with the customer to see what he wants to do with them.

When the Vortecs came out they out performed the Bowtie heads and I doubt that those 487 heads were even close to the Bowtie heads.

And the 487-X heads are a little bit better then the standard 487 casting heads.
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Old 11-25-2006, 08:31 PM
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If you listen to CNC and take his advice, you will see better performance and be happier with the Vortec heads than the older 487 castings. The port flow is much better and the combustion chamber is a fast burn configuration. It's 1970's technology vs. 1990's technology. Run the Vortec heads with a flat top 2 valve relief piston, shoot for a zero deck height and you will have the compression you are looking for and a much more efficient combination. I use Vortec heads on many street performance and some bracket engines.
The only modifications required to the heads to accommodate the desired valve lift is some valve guide work for guide to retainer clearance and the correct valve springs.

Barry
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Old 11-25-2006, 08:40 PM
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Even with a zero deck shortblock and 64cc heads you would need some kind of Dome to get a true 11:1 with valve notches in the pistons.Since you dont own pistons nad have a set of heads that you really want to use,there is nothing wrong with adding a little dome.I have done a bunch of 355's with the old 2304 TRW pistons over the years.With your heads and a zero deck..040" thick gasket they will give you about 9.9:1,which is better then the 8.8:1 that a 4 relief flat top would give you.It wont be 11:1,but you can still make fair power with it.You could run a cam with about 250 degrees duration at .050" and the engine would be slightly down on compression but it wouldnt hurt you by more then 5 horsepower or so compared to having 11:1.At 8.8:1 such a combination would be hard to keep clear at idle and would see a bit too soft at low rpm cruise speeds.In the future you could add a set of 64cc heads to this combo and have about 10.8:1,but I wouldnt bother doing it until you are ready to step up to something with signifigantly more flow.A set of AFR or Canfield 195 cc heads with the 250 degree flat tappet cam I mentioned and a port matched Victor Jr would be a stout combination.

Good luck.
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Old 11-26-2006, 01:08 PM
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It is possible to reach 10.5 with the combination CNC described above. It is always better to run a small combustion chamber with a flat top piston to get the compression you need as long as the chamber size doesn't shroud the valves. Check the link below and you will see 10.5 is indeed possible with out a dome with a 60cc chamber.

Barry

http://www.race-cars.net/calculators...alculator.html
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Old 11-26-2006, 03:05 PM
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The engine these heads was on was in a class where they were only allowed 11:01 max. compression and they ran 2 valve relief pistons.

Some one else bought these heads as they thought it was a good deal with the titanium retainers.
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Old 11-26-2006, 04:29 PM
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Quote:
Originally Posted by topfuel
It is always better to run a small combustion chamber with a flat top piston to get the compression you need as long as the chamber size doesn't shroud the valves.
I completely agree. Back in the day, the Hemi was the paradigm of power, but it required tons of ignition lead and lots of octane. Why, big chambers, big domes. It will always be beneficial to make the compression you want with as close to flat tops as you can. There are several reasons in my opinion. First, efficient quench is almost always more possible with flat tops. Quench will allow leaner burns, better efficiency (both in terms of thermally releasing the energy in the fuel and mileage), and require less ignition lead. That means that (to an extent) you need less octane to run the engine. My guess is that if you went Vortec at that 10.5:1 you could pretty easily get by with pump gas if you put some time into tuning it. Flat tops also don't slow down the flame front like domes further requiring more ignition lead.

The LT-1 heads are (no offense intended) really not performance by any means. The 350 LT-1s n 1971 were rated at 330 gross HP which translates to about 280 by today's standards, and to get that it relied on 11:1 compression and a solid lifter lumpy cam. Today on vortec heads you can get that much (literally) on 9.5:1 and a 191/196 cam. Running 11:1 on old-school LT-1 heads will require 104 octane minimum, maybe more. Technology is your friend.
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Old 11-27-2006, 12:05 PM
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I might get some vortec heads in the future, but for this engine i am going to run the heads I have. I am sure there capable of 400hp because the engine they were on made 435hp on the dyno. I saw the dyno sheet and talked to the machine shop that built the engine. It was a .060 327 with some insane dome pistons, a huge comp cams solid lifter circle track grind. The engine these heads were built for was a dirt track car powerplant. I think the machine shop said it was 13.5:1.


Back to the original question, does anyone know a true figure to compression ratios I am after with what size dome piston. I have a comp magnum 292 cam. A holley single plan intake, and a 750 holley. carb. This engine is a low buck build for my 62 pickup. I am not concerned about idle, or low end power. I have a muncie four speed in the truck already and I have manual brakes. i just would like a solid answer on the compression ratios.

Thanks Dennis
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Old 11-27-2006, 02:14 PM
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Quote:
Originally Posted by nitro_baller4692

This guy answered your question in the 2nd post. Use the link to a compression ratio calc.
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