Crankcase compression??

[roadkill636]

Has anyone actually measured crankcase compression?
The amount of air between on the inside of the piston to the reeds.
I'm curious of what it would be.
I'm relating it to boost such as turbo boost or blower boost. Because from the combustion chamber point of view all it knows is that air-fuel is being forced in .
So what kind of PSI do ya think is in the crankcase of various motas? And it would be neet to know before and after numbers of the same mota with installing intake stuffers.

[KIRCHNER]

I Dont think there is any real compression, just air flow. at high rpm the reeds stay basically open,once the column of intake air gains momentum. oil injection works using case pressure, just a few lbs. i could be wrong...

[David - WI]

Good luck!

https://www.screamandfly.com/showthr...on-for-experts

[H2OPERF]

I put a gage with a checkvalve on 1 cylinder of a 225 merc promax and it read about 4-5 at idle and 7 when you reved it on the hose #s were quite abit lower on a 3l omc.

[roadkill636]

H20... where were you able to hook up a guage?
And with a 7 peddle front. Do you think the compression could get to 6 or 7 on a 5 peddle front?

[90VIPER]

Maybe read pressure at the oil injection pulse fitting that usually just gets blocked off?

[H2OPERF]

Really long time ago but i think i used the low pressure diaphragm fuel pump fitting on both engines , the promax was a 99 so it had 5 petal cages.. Its a factor of case volume, if your asking what the difference in the 7/5 petal front volumes with the cages in is i have no idea, im sure the merc gurus can chime in. I mainly did it to compare merc to omc to see how much less the omc had with the much bigger bottom end volume. My test was certainly crude... the check valve spring and hose volume would all play in the numbers... I did read there is a point of diminishing returns but i dont think you could get to that point on an outboard, I rebuilt a few Honda cr500 engines and the crank to cases are as tight as it gets with just enough room for the rod to run between the crank cheeks and thats it...

[Chaz]

Unless this is some type of supercharger, then it can only put out what it's being powered by ...
So ifin ya gotz 5 psi of fuel pressure, chances iz purty good ya gotz 5 psi of vacuum runnin it from the other side ..



Chaz = thinkin that they figged out that they dont need the crank swingin thru the bottom of the grand canyon ... they used that volume at the mouth of the transfers and up thru the funnels .. where it would do some serious good ..

[Euroski]

Chaz... can't have "

5 psi of vacuum", vacuum is negative pressure.

[SS Minnow]

Actually, you can. A few ways to measure vacuum and psi is one of them.

https://www.setra.com/blog/vacuum-pr...you-measure-it

[Motv18]

No positive pressure is generated. Map sensor always reads less then 14.7psi.

if any is created it’s only from piston tdc to intake open.

their is a fan effect and centrifugal mass acceleration.

[H2OPERF]

Map is in front of the reeds... like chaz said u dont get something for nuthin..

[Instigator]

Part of what I have so much fun with inside a 3.0 OMC looper.

Some nylon plugs, a pint of epoxy, some flow work��

[H2OPERF]

A pint that's it

[Instigator]

Your comment about s CR500 was good.

Look at a good, tight 2 stroke, then look at our junk ��

A pint that's it