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Dynamic vs effective compression
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rogus
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1976 Chevrolet Chevelle

PostPosted: Sun Nov 20, 2005 11:48 pm    Post subject: Dynamic vs effective compression Reply with quote

What is the differrence between dynamic and effective compression ratio's? Which on is more important when it comes to determining how much compression I can create while still using 91 octane?
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jonny_b
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1979 Chevrolet Camaro

PostPosted: Mon Nov 21, 2005 6:48 am    Post subject: Reply with quote

Dynamic compression is what you hear refferd to all of the time. It is the calculated compression ratio, from the swept vol of the piston. It basically is just a number and means nothing.

Effective compression is the actual compression that eninge sees while its running. This a lower number because of cam overlap, leakage past rings, valves, etc...

My stats could be wrong, but I belive to run 91 octane, your effective compression ratio needs to be <9.0

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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Mon Nov 21, 2005 10:38 am    Post subject: Reply with quote

Effective compression ratio is calculated based on supercharged engines.

Dynamic compression calculates the compression ratio based on where the Intake valve closes and calculates the compression ratio from that point.

Lot's of variables come into play which determine how much compression you can run on a set octane fuel.

Is this engine supercharged? What's the compression ratio you're thinking of running?


Last edited by SMOKEmUP on Mon Nov 21, 2005 10:43 am; edited 1 time in total
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jonny_b
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1979 Chevrolet Camaro

PostPosted: Mon Nov 21, 2005 10:42 am    Post subject: Reply with quote

OOPS!

My appoligies, I got my terms screwed up.

Static compression ratio is the calculated by swept vol.

Dynamic is actual running compression

Effective is compression with boost

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rogus
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1976 Chevrolet Chevelle

PostPosted: Mon Nov 21, 2005 11:14 am    Post subject: Reply with quote

No, the car isn't supercharged (wish I could afford it!). My cam has 40 degrees of overlap so I know I am losing some compression with the stock 76cc heads. I want to use a set of 305 HO heads on the 350 motor but wasn't sure if that would put up my compression beyond being able to run on 91 octane. At the moment the engine is pretty much stock.

the only mods on it at the moment is the Power Plus Cyclone intake and Holley 600 cfm carb and a set of shorty headers.
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rogus
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1976 Chevrolet Chevelle

PostPosted: Mon Nov 21, 2005 11:16 am    Post subject: Reply with quote

Am trying to get to around 9.0 to 9.5 compression.
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SMOKEmUP
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PostPosted: Mon Nov 21, 2005 1:54 pm    Post subject: Reply with quote

I don't think swapping 305 heads on a 350 is a good idea. You have soooooo many choices with newer aftermarket designs that flow a ton more air than the 305 heads.

If you're looking for extra compression with little money you can swap head gaskets. Not sure what you're running now but if you're running a 0.039" gasket you can buy a set of GM metal shims that are approximately 0.015", part # 10105117, or 0.022", part # 3830711. Don't quote me on those part numbers that's from some old notes I have. Switching from a 0.039" to a 0.015" will gain approximately 0.5 point of compression.

When swapping head gaskets you need to consider quench height, and piston to valve clearance. These will both need to be checked. Quench height for stock internal shouldn't go less the 0.045" and piston to valve should be about 0.100" on the exhaust valve and 0.075" on the intake.
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The_Raven
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PostPosted: Mon Nov 21, 2005 4:12 pm    Post subject: Reply with quote

Hey smoke...

Why wouldn't one want to swap to the 305 heads, while swapping to those thinner gaskets? The 305 heads, at least the later model heads have smaller chambers that will help bump the SCR.

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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Mon Nov 21, 2005 4:23 pm    Post subject: Reply with quote

The_Raven wrote:
Hey smoke...

Why wouldn't one want to swap to the 305 heads, while swapping to those thinner gaskets? The 305 heads, at least the later model heads have smaller chambers that will help bump the SCR.

I think the 305 heads aren't that great. If he's on a budget save up some money and buy aftermarket heads or maybe vortec's. The flow numbers on the 305's are around 190 cfm @ 0.500" lift if I remember correctly. Vortec's flow about 230 and most other aftermarket heads will flow in the 250 range.

With little money he can swap to a thinner gasket which will bump up the compression a bit and add power.

rogus....what is the cast number on the heads you have now?
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87calais
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PostPosted: Mon Nov 21, 2005 5:48 pm    Post subject: Reply with quote

If I remember right... the 305 heads only have something like a 1.72" intake valve... choke the hell outta anything.... unless you have them machined... but at that point, buying another set of heads isn't to much more.
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jonny_b
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1979 Chevrolet Camaro

PostPosted: Mon Nov 21, 2005 7:37 pm    Post subject: Reply with quote

I only used 305 heads on my 350 because:

1. we had them laying around
2. they flowed better than the stock 79 350 heads
3. i didn't quite have engough money for aftermarket heads

but once I saved up some money I bought a set of Trick Flow's which at the time were under $1,000.

Basically what i'm saying is, if you have the 305 heads laying around I say go for it. but you are going to buy them, you'd be better off putting that money toward some good heads. or a lotto ticket to win money to buy good heads.

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rogus
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1976 Chevrolet Chevelle

PostPosted: Tue Nov 22, 2005 2:42 am    Post subject: Reply with quote

Smoke - I am currently using metal shims. Can't stand normal head gaskets. I believe the metal shime transfer heat better. The heads I am currently running are 882's (1.95 intake, 1.5 Exhaust). I read in a Super Chevy article that GM has come out with a set of their own aftermarket heads that flow better than stock at about $400 a set. Might save for a set of those.
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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Tue Nov 22, 2005 9:55 am    Post subject: Reply with quote

Switching from 76cc to 58cc chamber will jump about 2 points in compression ratio. What compression ratio do you have now?


BTW... Here is the info I have on the 882 heads.
Code:
 
GM 882                     
Cast-Iron                   
                       
Chamber Size     76cc       
Intake Runner     151cc     

Lift            Int     Exh   I/E ratio           
 0.100       70   58   82.9
 0.200       125   108   86.4
 0.300       175   135   77.1
 0.400       204   141   69.1
 0.500       205   142   69.3
 0.600       206   142   68.9
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rogus
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1976 Chevrolet Chevelle

PostPosted: Tue Nov 22, 2005 11:38 am    Post subject: Reply with quote

if my calc's are correct about 9.0 (static) and about 8.0 dynamic (excuse me if I get the terms wrong here)
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rogus
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1976 Chevrolet Chevelle

PostPosted: Tue Nov 22, 2005 11:44 am    Post subject: Reply with quote

I correct myself it's only 8.7 compression ratio.
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