Port Timing

[farmerj]

So I've read the posts on port timing & squish, and want to make sure I have my base gaskets right before sending of my head to RB.

It appears that my piston is 1mm above the bottom of the exhaust port at BDC.

I'm going for as "trials like" and "no stall" a bottom end as possible.

Will I really lose on the low end by making the piston flush at BDC?

Can .5-1mm on the port timing really make a difference?!

So many questions!

Jeff
'05 DE 300

 

[AZRickD]

The subject of port-timing always bedazzles me.

Some 300s could use a little more top end depending on the pipe.

 

[Jakobi]

Jeff. 1mm can make a significant difference. IMO it is more evident when you are simply changing base gaskets without correcting the head as you are also changing the compression ratio, and a high compression ratio will also favour bottom end and kill off top end.

 

[AZRickD]

So, if the piston is about 1mm above the port, adding base gaskets will reduce the compression but allow more revs. (correct?).

Let's say, on the other hand, that I plan on sending the head to RB-Designs for a squish mod, anyway.

So, I stuff in a couple of .5mm gaskets to get the piston flush, then I send the head into Ron to reduce the factory squish from 2.1-ish down to 1.2-ish, with Ron doing his usual combustion chamber work as well (adding volume).

How would one expect that motor to behave?

 

[Jakobi]

Very well yah! Setting the port timing flush favour the engines ability to make peak power. It will soften the lower rev range some though. Correcting the squish will gives gains across the board, and running a decent amount of compression will also give good snap down low.

Have a look at these sims that Dmcca ran on the 300 he worked on.
http://www.gasgasrider.org/forum/showpost.php?p=84668&postcount=69

You can clearly see what happens to top end if you go too far without other porting mods. On the 300 I always ran the stack flush as it still had heaps of power down low and I prefered the softer delivery and more top end. On the 250 I actually set the piston above the ports a bit, however this was using a race ported cylinder so it had already been set to favour top end over bottom. The port timing just brought it back closer to stock config.

 

[GMP]

I think using the piston position @ BDC relative to the exhaust port floor as a parameter is of little value. At this point in time of the port opening, the show is over, that extra fractional mm is neglidgable. Clearing the transfers is what is important, and the slight change in timing as well as compression from dropping the deck height can help low end slightly. At the end of the day they run very well bottom to top with a .5mm gasket, corrected squish, and max compression for fuel used. This is how Ron does it and it works.

BTW, not sure if they all are now but my '12 cylinder was cast by S3 (marked on bottom), and had a nice, smoother port surface compared to my older bikes.

 

[farmerj]

Thanks guys, I was out of town for the weekend.

That gives me something to go on for the next step in the process.

Jeff

 

[farmerj]

I do have one more question -

What is the stock base gasket configuration?

Is it the same for all bikes, do all 300s have the same base gasket configuration?

Thanks,

Jeff

 

[Jakobi]

There is always a little variance from one engine to the next re cases and cylinder heights. I think different years and models have come with different gasket stacks and port heights. My 2010 has 1.3mm worth.