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How COMPRESSION affects JETTING....

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Big topic... 

Can't type it all right now but didn't want to forget the thread for discussion...

You guys can start without me...

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 High compression affects braking on a 230f. The more compression, the easier the bike slows down.

Side effects are larger smiles when on the throttle.

 Another side effect:

High compression affects exaust noise on a 230f. The higher the compression, the louder the “Pop” from the exaust, not as noticeable with a quiet exaust, but if loud already, it will be even more loud.

 

 

 

 

 

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Ok then...

Always need a good illustration/ analogy for 3D modeling inside my head...

So we will go with the syringe model for our cylinder/piston/ combustion chamber model...

Another term I'm fond of using is "capacitance"... As in a volume of air having a more "sponge" or "spring like" characteristic to it as volume of substance...  Air does behave much like a fluid for a lot of it's physical properties, but NOT when it comes to compressing it's volume... Fluids don't compress at all... That's why you can use them for hydraulic systems, for example braking systems... Fluid will transfer all the force/volume displaced from one part of a closed system to the other part of the system with no loss of force...

Now... What happens when you have air introduced into the fluid in the brake line??  Spongey... A spring like disconnect... Capacitance added to the system... The air will absorb some of the force as it compresses it's volume, before it will transfer pressure to the fluid on the other side of it....

And in a fluid syphoning system... Air will require much more vacuum force/volume to be applied , as it's capacitance absorbs force as the air expands and becomes less dense...before it will pull hard enough on the liquid column behind it to move it... 

Our engines are basically air pumps.....

This info doesn't apply so much for fuel injection , but it does to carburetors ....

Carburetors are pressure differential metering/ mixing devices.... High airflow velocity through them creates the vacuum that the fuel circuits respond to with fuel delivery.... More vacuum signal will pull more fuel through the same size jet....

 

Now ... Back to the syringe model of an engine....

Let's think of 2 different syringe plunger insertion depths to make a model with low compression and high compression.... High compression one would have the plunger rubber go within a few millimeters of the end of the syringe... Low compression one has the plunger stopping 10mm from the end of the syringe....

You can now think about the difference in capacitance the 2 syringes gave in them...  There will be more spring like action to the one with a bigger volume of air ..  more downward movement will have to happen to equal the vacuum that the smaller volume/ higher compression can create with less movement....

 

A higher compression engine creates a stronger vacuum signal to the carburetor circuits than does a lower compression engine... The higher compression engine has to deal with less volume and spring like capacitance before it's plunger motion creates a stronger vacuum through an intake port ...

Another benefit of high compression happens during the magic portion of cam timing  known at the " overlap " portion .... Piston being close to the top of the bore ... Intake valve just beginning to open as exhaust valve is finishing closing.... An effective exhaust system is capable of making a decent amount of suction/ vacuum/ negative pressure to be felt through the exhaust port.... While both valves are open during overlap, that negative pressure will help to vacuum out the last of the exhaust gasses from the combustion chamber.... AND start velocity/ momentum/ cylinder filling from the intake port..  even before the piston begins it's downward intake stroke....  Higher compression/ smaller combustion chamber helps with higher vacuum and velocity being transferred through from exhaust port ...through combustion chamber.....to intake port ... With less capacitance...

For those reasons.... Higher compression engines deliver stronger vacuum signals to the carb circuits... And pull more fuel through the same size jet... Requiring a smaller jet than lower compression setup....

 

That's it in a nutshell...

 

Edited by mixxer
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Nice write-up.

How effective would a higher compression piston be with say a quiet stock exhaust?  Would too much restriction force the charge back out the intake valve? 

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Nice write-up.
How effective would a higher compression piston be with say a quiet stock exhaust?  Would too much restriction force the charge back out the intake valve? 


With More compression:

Stock bore, stock muffler?
1)
Very effective, impressive results, many smiles.

Again, stock bore stock muffler?

2) Zero,nada, not going to happen

I have ran up-to 13.5-1 with 262cc with stock Honda canister muffler.

But using one of these:

IMG_0849.JPG
(Photo is not my Honda OEM canister muffler but the power ring is similar)
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3 hours ago, firsthere said:

Nice write-up.

How effective would a higher compression piston be with say a quiet stock exhaust?  Would too much restriction force the charge back out the intake valve? 

Thank you..

AdnohGuy covered the answer completely....

You aren't going to run into anything negative with adding compression to the 230....

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   That's why diesels also get better fuel efficiency than gasoline engines. Higher compression extracts more energy from the combustion cycle. The only downside is the extra heat pumped into the various parts touched by the combustion event. Add in an efficient chamber that minimizes heat soak and it's a win/win. Or throw in the towel and resort to water cooling, which violates the whole purpose of all the fun we're having with these 230/200 motors.

   By the way Adno, what'r you doing to deal with the extra heat and increased octane requirements of your sweet, high compression, builds? Race gas? Premium oil?

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I think Guy said he had easy access to 100 octane "at the pump" somewhere near him...

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   That's why diesels also get better fuel efficiency than gasoline engines. Higher compression extracts more energy from the combustion cycle. The only downside is the extra heat pumped into the various parts touched by the combustion event. Add in an efficient chamber that minimizes heat soak and it's a win/win. Or throw in the towel and resort to water cooling, which violates the whole purpose of all the fun we're having with these 230/200 motors.

   By the way Adno, what'r you doing to deal with the extra heat and increased octane requirements of your sweet, high compression, builds? Race gas? Premium oil?

 

Good old Sunoco 100 Octane California street legal unleaded race fuel right out of the pump.

Just pull in the station and fill up your jug, tank, car or truck.

 

$9.00 gallon out of the pump

Or $55.00 for a 5 gallon pre filled can.

 

IMG_0437.JPG.454f4b679839840d17dd0185680c3f79.JPG

 

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7 hours ago, adnohguy said:

 

Good old Sunoco 100 Octane California street legal unleaded race fuel right out of the pump.

Just pull in the station and fill up your jug, tank, car or truck.

 

$9.00 gallon out of the pump

Or $55.00 for a 5 gallon pre filled can.

 

IMG_0437.JPG.454f4b679839840d17dd0185680c3f79.JPG

 

Jealousy overwhelms me....

Color me grreen with envy...

It's just not fair :(

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   The Shell station 4 miles from house has 110 leaded race gas (I'm currently using it 50/50 with 93 octane, no ethanol, in my XR)

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3 hours ago, Doogee57 said:

(I'm currently using it 50/50 with 93 octane, no ethanol, in my XR)

   The reason for the extra octane need is a Wiseco 12:1 piston with stock XR cam. It can get a little pingee if I flog it (which is of course precisely what I do), especially on hot days. I keep thinking I should probably get a "bigger" cam but it keeps getting pushed down my priority list.

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