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  #31 (permalink)  
Old 12-16-2011, 09:32 PM
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http://www.popularhotrodding.com/tec...ech/index.html

say you had a 425hp engine with 10:1 cr, Then dropped the compression ratio down to 8:1. Then the engine would lose 6.5% power or 27.6hp in this case.

Shortening up the mechanical advance slot and running a lot of initial ignition timing helps low compression engines gain more throttle response. What does your timing curve look like?

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  #32 (permalink)  
Old 12-16-2011, 09:48 PM
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This is part suggestion and part question. If you ran a compression test and it reads less than 140 you probably aren't making good power; too much overlap of valves being open suggesting you either increase compression or go with a smaller cam. What do some of you other friendly car guys think of this diagnostic strategy?
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Old 12-17-2011, 08:48 AM
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Quote:
Originally Posted by 454C10
http://www.popularhotrodding.com/tec...ech/index.html

say you had a 425hp engine with 10:1 cr, Then dropped the compression ratio down to 8:1. Then the engine would lose 6.5% power or 27.6hp in this case.

Shortening up the mechanical advance slot and running a lot of initial ignition timing helps low compression engines gain more throttle response. What does your timing curve look like?
Ok so i see that you are saying that if i simply did the work to increase my CR that i would see a hp gain but would i drop a bit it torque? or would i also see a gain there? I am lookin for High torque with moderate hp
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Old 12-17-2011, 06:50 PM
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yes, torque and horsepower goes up with more compression. But be careful, BBC don't tolerate compression as well as a sbc.

FYI, valve overlap doesn't effect cylinder pressure, as the engine isn't trying to make cylinder pressure during this part of the 4 stroke cycle. The intake closing point is what effects dynamic cylinder pressure (what you would measure on a compressing gage). And a "longer duration cam" will tend to have later intake closing point, the extra overlap is often wrongly accused of reduce pressure as more overlap typically accompanies longer duration cams.

Actually extra overlap can be used to increase torque (as in circle track engines). For example...Two cams with the same duration, but one has a tighter LSA (more overlap), will close the intake sooner in the compression stroke and develop more torque. Advancing the cam will have a similar effect.
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Old 12-17-2011, 10:21 PM
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I stand corrected
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Old 12-18-2011, 04:53 AM
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Cranking PSI isn't a very good indicator of how an engine will perform. Wear and ring/valve sealing aside, a radical cam will tend to pump lower than a stock cam but will have much more top end power. A tactic that has long been used by some engine builders is to change the cam timing in relation to the crankshaft in order to get the highest cranking pressure. This isn't a cure for a poorly selected cam, but could help, regardless. I repeat could help- there are a LOT of things that enter the picture so this is not an absolute by any means.

One reason overlap/LSA is mentioned in regard to cylinder pressure is because the intake closing point is not a spec that is as 'advertised' by the cam makers in their ads or in brief catalog descriptions. But it stands to reason a cam w/long duration at 0.050" is going to have a later intake closing point than a shorter duration cam, all else being equal. Hence the connection between the two.

What a lot of overlap/tight LSA can do that causes a poor idle or "lope", is it tends to decrease the vacuum at idle and can add to reversion. Either/both of these things will cause a poor idle.
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Old 12-18-2011, 10:05 AM
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here is a low compression 454 on the dyno.

http://tcnw.activeboard.com/t3341994...4-build-on-tc/
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Old 12-19-2011, 12:22 PM
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Quote:
Originally Posted by 454C10
FYI, valve overlap doesn't effect cylinder pressure, as the engine isn't trying to make cylinder pressure during this part of the 4 stroke cycle. The intake closing point is what effects dynamic cylinder pressure (what you would measure on a compressing gage). And a "longer duration cam" will tend to have later intake closing point, the extra overlap is often wrongly accused of reduce pressure as more overlap typically accompanies longer duration cams.
^Agree, thanks for posting this.
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