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i know its a little off subject but since it was brought up, how exactly do you figure you compression ratio off of compression numbers?
 

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greensmurf20 said:
i know its a little off subject but since it was brought up, how exactly do you figure you compression ratio off of compression numbers?
You can't!!! Everything from the intake to the exhaust system and most things in between will all affect the cranking pressure. The cam especially WILL affect this.

People tend to gloss over the affects VE of the system has on the cranking compression number. A high static compression(compression ratio) engine which is tuned for high RPMs can have a LOWER cranking compression than a low static compression motor which is tuned for low RPMs. If you were able to crank the motor over at the RPM which provided peak VE, then things would be different...

-Travis
 

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Travis,

I ALMOST understand what you are saying!

This past weekend, I pulled the plugs on my new 2.0, just to check the color since I changed the jets, (up to 150's now). While they were out, I ran a compression check just for grins. With all 4 plugs out and throttle wide open, it had 150# even on all 4. Now this is with 11:1 Venolia pistons, but a hot cam. I only have 125 miles on it, so the rings may not be fully seated yet.

So if I understand you correctly, I'd have a much higher compression pressure (read on the gauge, not the ratio), if the starter could spin the engine up to ~4000 RPM, where the cam really comes on.

James
 

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I also have the venolia 11:1 pistons, with my cam I get 180# across all 4. With a mild cam I'd be more like the 220 above they said.

I accidently got surprised at a light, didn't get a chance to rev the motor before taking off... OMG there is no power to get the car rolling. It got to 3500 and took off but getting from 1000-2500 took forever!
 

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I hope that i don't have 11 to one because of the gas. The cam I have is 486 lift 236 dur @50 302 adv and 108 sep. I hope that that will bring things down enough to be able to run super and not play chemist.

thanks for the imformation Travis.

For street and local club events applications.


just how much differnce is there between the 45 and 40ies from a performence standpoint? C
 

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Static compression is what folks refer to when they talk about when they say "compression". This is calculated by determining the total volume in a cylinder (including cylinder head and gasket)with the piston at bottom dead center. Then dividing that number by the volume left in the in the cylinder/gasket/head with the piston at top dead center.

Here's how the cam affects dynamic compression. If you had an imiginary cam with very small intake duration, the intake valve would close with the piston at bottom dead center... just as the piston started its compression stroke. This would result in the same thing as the static compression number.

A real cam has longer duration and the valve closes ABDC... after bottom dead center. In other words, the valve closes after the piston has begun it's compression stroke. The wilder the cam, the more duration, the later the intake valve closes. As long as the intake valve is open, the piston is not compressing... thus a lower dynamic compression.

Now, you may ask... why not close the valve earlier to get higher compression?. Well, simply put the valve needs to stay open so that the cylinder can become better filled with fuel/air. Even though the piston is starting to move up fuel/air is still comming in through the valve. So we give up a little dynamic compression to better fill the cylinder.

This is one example of why the components of an engine need to be "engineered" to work together. You can't just throw a wild cam at a low compression engine and expect it to work correctly. Cam, carbs, valves, inlet tract, and compression all have to be designed together if you want everything to work right.
 

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Hey Keith,

How about saving TGSI Bob's compression explaination somewhere?
Great explainaton Bob!
Jc
 

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There are some "on line compression calculators" to determine static compression. If you do some searching you should be able to find one.

I should have added something in the previous post. As Travis said, dynamic compression also has to do with voluumetric efficiency (VE)... VE is a percentage of how full your cylinder is with a fresh fuel/air mixture. For instance, if your engine AT A PARTICULAR RPM... say 2500 RPM has a VE of 55% (55% of full capability), then it will have one compression. Now, let's say that this is a full tilt race cam (and induction system), and at 7000 RPM the VE is 110% (more than the displacement of the engine), then the compression will be different... and the plot thickens

The 11:1 compression engine using this cam won't detonate... nor get out of it's own way.. at 2500 RPM on 93 octane fuel. The same engine will probably detonate severely at 7000 RPM on 93 octane... and probably do very bad/expensive things to the pistons & rings.

So, in the final analysis dynamic compression is a combination of the VE at a specific RPM and when the intake valve closes.
 

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Discussion Starter · #9 ·
sorry to get this thread started on another subject, keith and gary are gonna beat me down with a 3 wood golf club. thanks for the info tho, and travis, that was a great explination. made alot of sence and got me thinking about alot of things.
 

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Anyone knows where this thread was split from?
Some very good info and I'm always looking for more.
 

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Found it Tech Reference Library\Engine FAQS
The static compression is one thing but a running compression test (while not perfect) can help better reflect the effects of the camshaft.
Thats one reason I'm so interested with a mass air flow sensor.
In my humble opinion V.E. is king of tuning.
 
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