Low compression

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Re: Low compression

#16

Post by FlatHeadSix » Mon May 07, 2012 2:10 am

Kit
I think what Cotten is trying to say is that a re-engineered radical cam changes everything. A conventional 4-stroke with a stock cam has a definite compression stroke, both valves are completely closed, and you you should get a predictable amount of compression. When you install a cam that has overlap it means exactly that, there is an overlap of a certain number of degrees, one valve is not completely closed before the other valve starts to open. BOTH valves are open, pretty hard to build compression during that part of the engine's rotation. Performance of an engine with an "overlap" cam is directly proportional to RPM, the faster it revs, the better it runs. Unless you have documented specs telling you what kind of compression numbers you should expect at kicking RPM, you are completely on your own.

Yes, it is still a 4-stroke engine and all the rules apply, all you need to make it run are fuel, air, compression and spark. But if you are going to try to "improve" on what the factory installed, as the old saying goes; "you pays your money, you takes your chance". The factory spent a lot of time and money engineering that stuff to begin with, for the most part it all worked, the bikes started and ran reliably.

mike



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Re: Low compression

#17

Post by Huck » Mon May 07, 2012 3:08 am

kitabel wrote:I suggest a little remedial study on the 4-stroke cycle.
Wow! Ever wonder why a race motor won't idle? Because they can't, too much overlap. Over laping what? Um?

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Re: Low compression

#18

Post by Panacea » Mon May 07, 2012 6:57 am

I'm no expert but I believe the "overlap" occurs when the intake valve begins to open as the exhaust is still exiting the cylinder, which takes advantage of the exhaust flow to Pull the intake charge before the piston begins going down. On the compression stroke, normally the intake and exhaust valves would not affect the compression since they would both be closed. From what I understand some racier cam profiles leave the intake open even past the point where the piston begins it's upward compression stroke, as some cylinder filling can still take place. Here's where you need RPM for this to benefit performance. While kick starting or compression testing you would see lower compression with one of these cams....Mike

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Re: Low compression

#19

Post by 58flh » Mon May 07, 2012 12:44 pm

AND then ANTI-REVERSION!---joins the game :lol: ----Richie

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Re: Low compression

#20

Post by Cotten » Mon May 07, 2012 7:25 pm

Sorry I bluffed you out so easily, Kitabel.

I could use refreshing on many subjects.
The latest was simple algebra just to get back at geometry before the trig. Forget the Calc. I didn't understand that when I passed it. (And that's been a coupla decades,.......or more.)

But my foot still remembers the easy starts of the bodacious genuine Sifton 'Cobra', and how the 4 1/2" stroke appreciated it.
The same motor with even an FLH cam's duration would make most 70kg operators bounce upon the kickarm as if it were spring loaded with no bottom.

So it really isn't all about racing either.
But as Huck mentioned about racer's idling, those radicals even needed them roller thingies to get them up to starting "compression"!

Fuel efficiency, or low end torque, were rarely issues.

....Cotten

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Re: Low compression

#21

Post by Huck » Mon May 07, 2012 7:44 pm

And don't forget in order to get the most out of that full boogie cam you need to bump up the compression to make up for the bleed, its all made to work to together.

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Re: Low compression

#22

Post by Cotten » Mon May 07, 2012 7:51 pm

Huck wrote:And don't forget in order to get the most out of that full boogie cam you need to bump up the compression to make up for the bleed, its all made to work to together.
And by that, you mean squeezing the combustion chamber volume and piston crown, right?

....Cotten

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Re: Low compression

#23

Post by Dave_R » Tue May 08, 2012 1:45 am

Usually the more overlap a cam has, you need to compensate the loss of cylinder pressure by going to a higher compression ratio piston to take advantage of what you are trying to do. Otherwise, the result will be dissapointing and people usually blame the cam.

But, going the other way, using a high compression piston with a low overlap cam can result in cylinder pressures too high for available fuel. Usually destroys things, and again will not run well. Not to mention a lot of pre-detonation.

This is where the piston design that matches a cam can result in a sweet runner!

For a street engine running on 87 octane, a static cylinder pressure around 80psi would run well. Any higher and it become hard to get the ignition timing right without pre-detonation. Unless you want to run higher octane fuel.

So yes indeed overlap affects cycinder pressure!

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Re: Low compression

#24

Post by Huck » Tue May 08, 2012 2:47 am

Cotton,

Yes! and then after all that messin with your hotrod pan a 400 cc Japazuki blows your doors off.

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Re: Low compression

#25

Post by kitabel » Tue May 08, 2012 5:21 am

Right, you're just too smart for me.

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Re: Low compression

#26

Post by steinauge » Fri May 11, 2012 1:15 pm

I believe Cotten meant to say "duration".Most long duration aftermarket cams also have a lot of overlap and a lot of duration certainly affects cylinder pressure at cranking speed.

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Re: Low compression

#27

Post by mbskeam » Sat May 12, 2012 3:31 pm

dont forget intake valve close time, this can change the amount of static compression you get.
latter closing means there is less cyl vol to compress for a given stroke (valve being open as piston is rising)
aint this fun.....

yes I'm still alive..... :wink:

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Re: Low compression

#28

Post by ingram » Sun May 13, 2012 12:10 am

Having worked for a camshaft company over 30 years I felt the need to post this.

Everyone knows the 4 cycles of an internal combustion engine…(1)intake, (2)compression, (3)power, and (4)exhaust. Now picture what the intake and exhaust valves and the piston are doing during these cycles.

(1)On the intake cycle the intake valve will open before the piston is at the top of the stroke and will close after the piston is at the bottom of the stroke.

(2) The compression cycle starts when the intake valve closes. Remember the piston has already started moving up the cylinder when the intake valve closes.
The cranking pressure of the engine will be determined by the static compression ratio and when the intake valve closes.

(3) During the power cycle the intake and exhaust valves are both closed as the piston moves upward toward the top of the stroke. The air and fuel mixture is ignited and the piston is pushed down by the expanding gases.

(4)On the exhaust cycle the exhaust valve will open before the piston is at the bottom of the stroke and will close after the piston is at the top of the stroke. During this period when the intake valve is opening and the exhaust valve is closing is known as the overlap. Notice this overlap period happens at the end of the exhaust cycle. The piston is not under compression at this time.

Overlap is one of those camshaft terms that gets misunderstood. If you install a camshaft with more duration and overlap then the engine will have less cranking pressure. Even though the camshaft has more overlap the reason the engine has less cranking pressure is because the intake valve is closing later. This creates less volume during the compression cycle(2) as noted above.

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Re: Low compression

#29

Post by hplhd » Sun May 13, 2012 6:39 pm

kitabel, i look forward to and respect what info you put on this forum. with that said allot of sh#t is over my head. explanations on here helps out and i'm not sure where i'd begin to start doing "my homework".
don't get so frustrated with under informed guys like myself.
i ain't taking anything away from whats been posted on this subject but don't take it like your knowledge is being ingnored.

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Re: Low compression

#30

Post by Cotten » Sun May 13, 2012 7:28 pm

See what you started Bondough.

No matter what the technical explanation may be,..

Cannot we all at least agree that some performance cams just don't give a good compression reading when kicked.

It doesn't mean anything is wrong.

....Cotten

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