Tractorsport Flowbench Forum Archive

Posted: **Sat Oct 10, 2009 5:47 pm**

Here are somethings to think about when trying to visualize in your head what is happening when flowing a head:

1. At 6000 rpm your piston is at top dead center 100 times per second. Yes that is per second and that is only at a "safe" 6000 rpm

2. At 6000 rpm each one of your valves opens 50 times a second.

3. At 6000 rpm your spark plug fires 50 times per second.

4. At 6000 rpm a mixture of air/fuel flows through your intake system 50 times per second.

5. At 6000 rpm your cylinder exhausts 50 times per second.

Whew! Are we sure flow benches are useful? ;-)

Is 28" H2O test pressure enough? Makes my head hurt and makes me doubt myself sometimes.

John

Posted: **Mon Oct 12, 2009 4:47 pm**

6000RPM is just where things BEGIN to get interesting with motorcycle engines :p

Rather drives home the importance of the "time" value in time/area equations

Posted: **Mon Oct 12, 2009 9:16 pm**

Kind of adds a whole new light to "Lazy Port"

Rick

Posted: **Tue Oct 13, 2009 3:36 pm**

What fascinates me even more, is that if one COMPLETE revolution is over so very quickly, valve timing events can be sensitive to only a few degrees of one complete revolution.

Posted: **Tue Oct 13, 2009 3:44 pm**

also Tony, how important is a perfect leak down then? This has been batted around in our circle also.

Also what we have found is a smaller cam seems to be more sensative to timing than a bigger one.

John

Posted: **Wed Oct 14, 2009 2:37 am**

I am not sure that a leak down test actually means much, apart from comparing cylinder to cylinder.

Every leak down tester that I know of uses a 40 thou (1mm) orifice. and works like a mini orifice plate flow bench. But so very much depends upon what you are testing.
A certain "volume" of leakage flow, will be indicated.
But that reading could be excellent for a really large capacity engine, but totally unacceptable for a very small engine.

So your leak down tester indicates 95% (of something) ??
That could either be very good or truly horrible, depending on what you are testing.

Just like numbers on a compression gauge, the numbers don't mean much in themselves. But cylinder to cylinder differences can be a useful indicator of trouble in one cylinder.

Posted: **Wed Oct 14, 2009 10:00 am**

Very well said Tony! Good points made. Thanks.

John

Posted: **Mon Oct 26, 2009 6:22 pm**

Posted: **Mon Oct 26, 2009 10:47 pm**

FPV, the static (still) compression is set by your combustion chamber volume but the cranking compression (using compression guage) is controlled by your camshaft. when you change cam with different timing, your cranking compression will normally change also. The intake closing will control this factor. Larry C, my motorcycle engines are just about ending at 6000.LOL Jim

Posted: **Mon Oct 26, 2009 11:04 pm**

As far as the leak down and compr. test go, I always compared cyl. to cyl. pressures with a compression test but used the leak down when I knew there was a problem internally. With a leak down test, air will blow into the bottom end-block or crancase if the rings are leaking. But out thru the intake or exhaust port with a bent or just leaky valve. Easy way to find a problem. Hope this will help with your question. Jim

Posted: **Tue Oct 27, 2009 12:16 am**

One more reason why the actual ### don't matter but the comparison does.

Altitude! You car will run faster at sea level then it would at say 4400 feet. and the compression gauge will read less compression at 4400 feet.
The eng is still in the same condition but the gauge reads lower at higher altitude.
My compression gauge went from a 165 psi at 4400 feet, to 140 psi a 7000 feet, that i live at now!
The only difference was the drive up here.

Posted: **Tue Oct 27, 2009 2:36 am**

Posted: **Tue Oct 27, 2009 12:48 pm**

Good morning guys.It's morning here anyway.To me, the cranking compression is extremely important. I compare it to taking the temperature of a human. I ALWAYS check compression BEFORE tearing an engine down. I also always record it on a fresh engine. That way I know if there is a problem before teardown.Now i'm talking performance engines.Even if I didn't build it I still check to see if problem. On my performance stuff, I shoot for 170-180 lbs.cranking. This can vary with cam timing and lobe ctrs. All this doesn't mean much if your heads don't flow well. Static compression and running compression are defenately two different things.You need that good cylinder fill from good airflow to make that killer horsepower. Just my opinion. Hope it helps you. I hope Bruce isn't going crazy about us talking compression on an airflow section. I just added to what you guys were talking about. Skol, Jim

Posted: **Tue Oct 27, 2009 6:41 pm**

Leak down.

My leak down tester that I use is set up very simply with a regulator a valve and two gauges one clockwise normal scale one counter clockwise reverse scale. It functions like this connect to air then I set the main (clockwise gauge to 100psi) then connect to cylinder hose with the piston at TDC-PS I open the valve the second gauge will come around toward 0 if all is ok the final reading say 3 to 10 equates to cylinder % leakage.

What does this mean, well on a BT Harley if I do a top end and just fire the motor etc. and have the owner do a standard break in at 500 miles the motor will normally read 6 to 9% leakage HOT. If I do a bench Heat cycle which starts at 15 seconds and over 12 steps ends at 2 minutes run and cool over the better part of a day after 500 miles these motors will leak 3% hot.

This has been proven time and time again and the tool shows it along with the dyno.

Food for thought.

Rick

PS. if you can find a good way to hold the crank you can learn a lot about the motors wear by leaking every 5 degrees through the power stroke.

Posted: **Tue Oct 27, 2009 8:25 pm**

Thanks Rick. Good info. Jim

Tractorsport Flowbench Forum Archive

Food for Thought for flow testing