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hmm I just had a one year old set of 40ies rebuilt. I was looking at how the last person had the linkage hooked up. I also just had the cannon intakes welded up and redrilled to be straight. What intkaes do you have? why are you getting rid of the 40ies? Are you not going to run duals anyone?

They finished my engine today. Compression is 220 so I think it is about 11 to one. It seems I used 30 over 305 pistons.
 

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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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I didn't, they did... I just stood there grinned foolishly and nodded

but I will find out how the number came to be tommorrow. I didn't grin so much when they said i most likely will have to use 104 or some other additive because of our 92/3 low octane here in Mo.

Funny the guy at advance auto parts said the same thing when I told him the compression was about 220 he said 10 and a half to 11 also just with that number. This is the kind of question that bob could answer with his eyes closed.

I think I will need one of those houseing with my cannon intakes...
 

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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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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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40's to 45's

I look at it this way:

Going from a 32/36 to a 38 DGAS is a big differance. Especially if the motor is built up. I remember I had to go back to the 32/36 to pass emissions (I had a 38 DGAS on my 2.2) and there was a world of difference in power loss. And I really noticed it when I passed emissions and went back to the 38's. Remember the 32/36 is a single barrel that only opens the second 36 barrel about halfway thru the throttle travel. A 38 is a two barrel design that feeds all 4 cylinders at the same time.

Let's look at some numbers:

On average, a 32/36 is providing 17mm of "air/fuel" flow at a time. And thats assuming both are open.

A 38 DGAS is providing about 19mm of "air/fuel" flow all the time to 4 cylinders.

(38 x 2 barrels / by 4 cylinders.)

This gives us a difference or increase of 2mm per cylinder.

Dual sidedrafts provide 4 barrels to 4 cylinders. This is an amazing 40mm per barrel! An increase of 21mm over a 38 DGAS!

Going to 45's increase this further to 26mm over a 38 DGAS or 5mm over dual 40's.

Were talking a 5mm increase. Remember is was only 2mm that made a huge difference over a 32/36 and 38 DGAS.

So the answer to your question is: It will make a big difference in performance.

(anyone wanna back me up on this. This is probably a crude way to look at from an expert engine builders standpoint. And I'm no engineer either)

And besides. The 45's I bought are brand new. So they look prettier too. :D


-k
 

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What you say is true as far as carbs go. But remember, after the carbs are the manifolds, the port runners, the valves, etc. Consider that a 45 mm side draft that has to go through a 40mm intake valve... and if the valve is only 1/2 open (as it is twice as much as it is full open), then the velocity through the side draft is really low. Also, the plenum of a stock manifold has an effect vs a single runner of side drafts.

So, I guess what I am really saying is that bigger is not necessarily better. It's the total set up that counts.

Oh yea... pretty new carbs are always good
 

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yea, good point.

Man...I have a feeling the carbs alone will give me hours of time wasting enjoyment. But then I've never really learned much from doing it right the first time. That's whats so much fun about the unknown.

:rolleyes:

-k
 

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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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