How is the correct port timing achieved?

[GGRider01]

Is this based solely on the piston at BDC being flush with the bottom of the exhaust port? Or is there something else to this?

I'm looking to get more top and mid power. When I rebuilt the bike I just slapped a gasket down and thought everything would be OK. Lesson learned. It'll grunt its way everywhere, but signs off extremely early.

 

[gasgasman]

There are some good videos on youtube.

Here's one..

https://www.youtube.com/watch?v=ha7IErp65Yg

 

[GGRider01]

I didn't sit through it all, but it seemed like he was talking about modifying the ports.

 

[Jakobi]

Read up!

http://www.macdizzy.com/cyl_primer.htm

 

[(F5)]

Arrrrrrrrrrrr
Arrrrrrrrrrrrrrr
Arrttrrrrrrrrrrrrrrrrghhhhh.

Why?

Why do people keep wobbling on aabout the bottom of the friggn port??!?!

 

[Jakobi]

Arrrrrrrrrrrr
Arrrrrrrrrrrrrrr
Arrttrrrrrrrrrrrrrrrrghhhhh.

Why?

Why do people keep wobbling on aabout the bottom of the friggn port??!?!

Reference point. Nothing more, nothing less. The thing is, no ones actually documented the port layouts across the years, and I'm sure there's been changes in the port height of the exh and especially the transfers.

Thinking about pulling the old 250 with the S3 cyl and head and case matching it before dropping it in my new(er) chassis.

What are your thoughts F5. Use epoxy to close the transfers on the cyl to match the cases, or machine the cases out? I like the former.

 

[Neil E.]

Use epoxy to close the transfers on the cyl to match the cases, or machine the cases out? I like the former.

Most will say get out the die grinder and open the cases to match. This assumes it's just a matching job, not a full on port reshaping. Time to buy a flow bench?

 

[(F5)]

Its a bit of a guess Jake, you'd generally choose wider swooping but you can alter the primary compression quite a bit. In a full house engine that's likely good, but in a low output engine like a dirtbike that could be a disadvantage as the pipe is compromised so as to provide a super widepower spread we love in the dirt.

Flow benches have been proven to be totally pointless for power production on a 2stroke. And fairly misleading on a 4 stroke. Big flow numbers trick you into thinking that's the best way to get the most power

 

[Jakobi]

Cheers

I'll start up a thread if/when I get around to it. Have a mate dropping off an old KYB shock this week too. Going to do some comparisons between the Ohlins 888 on the 13 and maybe look at running KYB front and rear.

From memory I think there's a lot of difference between the cases and the cylinder and opening the cases would leave a pretty narrow gasket surface.

Anywho, back on topic. I posted in the other thread too, but if any comparative measurements are needed I have an OEM 10 300cc cylinder sitting in a box I'd be happy to measure. Could probably do a port map and post a pic with some measurements on it.

 

[(F5)]

After last weekend in the slop and hills the last thing I'd notice is an increase in top end power.

If riding area is open the op should try look for a different pipe.

 

[GGRider01]

Thanks for the offer Jake but I finally got a Wild 300 CDI in the mail tonight.. and I'm blown away. I was running a KTM CDI, and my original 2000 CDI, both were falling flat on their face at over 5-6k RPM. Original CDI I'm convinced was on its way out, the KTM surely had some timing issues, it was relatively new.

This thing wants to pull.. and pull and pull.

For now, the results are quite good. Not the skyward rocket I dreamt it'd be but I'm content to ride it as is until the snow starts to fall. Compares to the '03 CR250 in power delivery.

I've never modified the cylinder on a 2 stroke, I'd probably ship it to Eric Gorr for a complete rebuild for peace of mind, and whatever power delivery I desire. We'll see what the future brings for this old thing. Might be in the parts section as I yearn for a plated Husqvarna or Husaberg.


Also, I've been wanting a more mid/top end pipe but they're difficult to locate for the 2000.

 

[motopsycho87]

Personally I think aligning the piston at BDC is very worthwhile, I rang a bunch of tuners in the UK they all asked if I'd done that first and said it makes the best difference. If the piston dwells beneath the port opening too long apparently an awful lot of turbulence occurs, hurting power all across the rev range. You might move power down, but you'll have less of it also.

If you look at other 2 stroke tuning guides, you'll find people are getting insane power boosts by raising their exhaust ports 0.5mm, i had to raise everything over 1mm on mine, frankly I love it! Still linear, but pulls everywhere!

 

[(F5)]

Turbulence eh? That sounds bad.

In the final years of the Aprilia 125GP bikes they were experimenting with raising the bottom of the port so the piston dropped below it , - and found more power. These were the highest output bikes yet made. Smaller ports, more power.
Not the full story but a case study into the folly of obvious tuning steps (of which Ican pick up a book I used to consider the reference and debunk).

 

[GGRider01]

Personally I think aligning the piston at BDC is very worthwhile, I rang a bunch of tuners in the UK they all asked if I'd done that first and said it makes the best difference. If the piston dwells beneath the port opening too long apparently an awful lot of turbulence occurs, hurting power all across the rev range. You might move power down, but you'll have less of it also.

If you look at other 2 stroke tuning guides, you'll find people are getting insane power boosts by raising their exhaust ports 0.5mm, i had to raise everything over 1mm on mine, frankly I love it! Still linear, but pulls everywhere!

If the piston dwells below the opening it creates an issue.. but what are the affects of having it about .7mm above the bottom of the port at BDC?

I've got a good CDI in the bike and it seems better (finally) than I received it. Might hold off on this step until snow falls.

 

[(F5)]

I give up.

 

[motopsycho87]

Turbulence eh? That sounds bad.

In the final years of the Aprilia 125GP bikes they were experimenting with raising the bottom of the port so the piston dropped below it , - and found more power. These were the highest output bikes yet made. Smaller ports, more power.
Not the full story but a case study into the folly of obvious tuning steps (of which Ican pick up a book I used to consider the reference and debunk).

I'd like to see it. But by the sound of what you're saying it's a case of raise the bottom of the port, but retain the height from TDC?

And don't give up! I've got all the 2 stroke books aswell, on my 8th year of Mechanical study, and frankly, love this stuff. You can't tell me that to a certain degree raising the exhaust port doesn't benefit top end power?

 

[motopsycho87]

If the piston dwells below the opening it creates an issue.. but what are the affects of having it about .7mm above the bottom of the port at BDC?

I've got a good CDI in the bike and it seems better (finally) than I received it. Might hold off on this step until snow falls.

In short, the lower the barrel, the torqier the bike, personally I think with a degree of overlap you are adding turbulence, and reducing the time ports are open, less flow less time = less power in my opinion. Though I'd like to look into what F5 has on this actually increasing power...

 

[motopsycho87]

Eric Gorr seems to like doing the same thing on most YZ's also.

 

[(F5)]

So much of what Jennings and Bell have written has been debunked. Not because it was all intrinsically wrong, but times have changed. The biggest change has been the effects of blowdown. The ability of the cylinder to evacuate the expanding gas before the transfers open.
And that's the problem. As the transfers open the cylinder pressure is still above that of the primary (crankcase) compression pressure and the still expanding burnt gas is forced down the transfers when they open as the gapping exhaust still hasn't lowered the pressure enough.

Eventually the pressure equalises and the flow, aided by the first returning part of the headerwave ccreates max flow in the transfers about bottom dead centre.
So the best thing to do is evacuate as much pressure a soon as possible. Opening the ex early helps for sure but tthat'sa twin edged sword. Having eenormous auxiliary ports to the sides of the ex port helps hugely, and this is whatthe old school tuners didn't have to their favour.

They also didn'thave gglorious power valves
.
Incidentally raising the barrel reduces the blowdown as it percentage wise increases the transfer timing slightly more than the ex.

 

[motopsycho87]

Simple, but makes sense, though I don't understand how raising the barrel changes the ratio of durations, both the transfers and the exhaust bottoms are at the same point?