Port timing

I think I have asked this question before, but I don't remember the answer.

It seems to me that if you raise the exhaust port timing to let more out, then it seems logical that you would also raise the intake timing to let more in.

I built six cylinder Mercury outboards back in the early to mid 70's and that was what we did. If you let more out, then you had to let more in to push more out.

I would think that if you were going to make your exhaust timing 180 degrees, then the intake timing should be 157 to 160 degrees instead of the 153 that comes from the factory.

When you lower the cylinder (to raise compression) you automatically increase intake port timing. In most cases it puts you pretty close to where you want to be if using a .004" cylinder base gasket.

Steve,

The trouble with making greater intake timing specs becomes a case of WHAT HAPPENS TOO POWER BAND & Torque.

Our little Zenoah based engines have such a BROAD rpm range they operate under, along with having a pretty large carb for the displacement they are.
The length of the intake tract has been established to give a good broad powerband withing the rpm range engine runs with the timing specs used.

Because of the LARGE carb and intake tract size, increases in the intake duration start to cause excessive fuel spit back at lower operating speeds making the engine run sloppy and inconsistent.

* If you INCREASED the intake tract length, you can get away with more radical intake duration specs. ( Or use a smaller carb )

Scott

what happens if you raise transfer timing to make up the difference when you lower the cylinder,or is it better lower for building crankcase pressure.

50 stowe wrote:

what happens if you raise transfer timing to make up the difference when you lower the cylinder,or is it better lower for building crankcase pressure.

The greater blow down spec are beneficial when we are increasing the transfer window area by eyebrowing the ports.

if you increase the "Area" and also increase the "Duration" port is open, the velocity of gas's moving threw the port is greatly reduced.

SO it is better to eyebrow the transfer insted of raising it.

50 stowe wrote:

SO it is better to eyebrow the transfer insted of raising it.

Absolutely !

Raising transfers is still a VERY viable way to keep the stock tranfer timing specs when lowering the jug for a tighter squish clearance.

Trouble comes from upsetting the symmetrical flow pattern on all 4 transfer windows.
Crisscross or high low flow errors will kill the power band and peak power quicker than most other porting mods done incorrectly.

* This why it is BEST to leave transfer window roofs alone !! What we are loosing in open duration is more than made up for with greater area from eyebrowing instead.
Add higher exhaust port timing while suffering from decreased compression ratio, the increased "Blow Down" helps get some of the lower/mid torque back :woohoo:

I allways wonderd why nobody raised them, Nice explanation. Where would you recomend timing numbers be for the intake,exhaust,transfers be for a cat,mono and hydro.