does porting have any effect on compression... less or more..
figurin no since there is no change the size of the chamber.. is this a correct assumption..

 

Ports that raise the closing edge, thereby extending the duration farther into the stroke will lower compression readings. Even the stock ports are open part of the way into the compression stroke, and at cranking rpm, there is some charge escaping back into the intake runners during the beginning of the compression stroke. As the RPM's increase, the inertia of the air column overcomes the reversion.

Its actually a common trait among motors in general. The dynamic compression ratio is whats measured as the engine is turning at a speed. Static compression is the volume difference between bdc and tdc. Go down to the dynamic compression ratio section here for more info


The longer the port stays open into the compression stroke, the higher the rpm peak torque will be found at. Turbos kinda muddy this though, as boost may come on before the torque peak. Such porting also makes the powerband fall off less drastically above peak hp.

 

so in essence.. larger the port.. the lower the compression numbers..
if so.. and in consideration of this.. then what are the values to reference against for the various port sizes...
stock ports are deemed acceptable and borderline @ 85psi.. anything less is rebuild time from what ive read..
how much of a difference less is.. lets say for a street port.. minus five.. ten.. or...

 

In the realm of street motors, you arent going to see much difference between a good streetport and stock ports. But how the engine was built will play a big factor in the compression readings, far more than porting in most circumstances. Tight clearances on the seals will read higher on a compression test, and the condition of all the seals and housings can change the readings as well.

Id probably concerned about a motor with less than 110 psi. 85-90 is definitely rebuild time. Theoretical perfect compression should be 132 psi for 9.0 cr rotors, and 143 psi for S5 n/a.

My last heavily ported n/a, with the ports extending farther into the intake stroke than about anything short of a PP, still made 120 psi on the compression test when it was broken in.

There are a lot of variables in play here, so its hard to give any solid numbers or rules. And I wouldnt consider it a hard line that x number is either good or bad. 90 psi is generally accepted as the area where the motor may be finicky starting, and is more prone to flooding. Ive driven on motors with less though. The higher the number gets above that, the more likely the motor is good.

 

Cranking speed also affects the compression reading

 

vacuum yes not compression

 

would differences in types of seal provide any sort of variation in compression...
3mm..2mm.. ALS.. OE.. etc

 

Ive always heard that 3mm doesnt seal as well as 2mm, but I have no firsthand experience. Also, Im not sure it would affect the compression as much as idle vacuum.

Also heard that carbon seals dont seal as well at low rpm, and softer aftermarket seals(hurley, atkins) seal better than the super hard tool steel ones, like the RA super seals. But I think thats more general, and hard to get any real numbers out of.

 

The harder seals will be just fine after a break in period, as the seal wears into shape on the rotor housing the sealability improves. Thats why people sometimes recommend reusing old apex seals if they're in good shape.

Just quickly in regards to compression/vacuum.. The higher the duration the less idle vacuum, advanced port timing=higher compression due to the compression cycle being increased. If you were to retard port timing (not really possible), then the compression would decrease due to the compression cycle being shorter.

 

Porting a rotary retards the port timing, by keeping the ports open longer into the compression stroke. Even stock ports are open well past BDC.

Rotaries are not like piston engines where you can advance or retard the port timing. You can only extent the port to open earlier and/or close later. Overlap is another important factor that contributes to compression, idle quality, and vacuum. But on a rotary, you can only increase overlap, shifting the powerband higher, and reducing compression and idle quality and vacuum