62T port timing

[Dustin]

I have a 96 Superjet and upon putting in new crank seals and top end, I realized the ski is a 5mm stroker (73 mm stroke) with 62T cylinders and trenched cases. I had just ported my exhaust and intake ports from Yamanubes template. Had a protec head on which had 150-160 psi. Had to take 0.0125” out between the head and cylinder as there was some warpage. When at BDC the edge of the piston is pretty much perfectly at the bottom of the exhaust port. The top of the piston protrudes 0.263” above the cylinder deck when at TDC. I was planning on putting a girdled ADA head on with 35cc domes for approx 185 psi, but I would now require me to add a 0.080” spacer plate and 0.040” base gasket, which would raise my porting by 0.120” (3.048mm) to get the proper squish (0.045”). Now would raising the porting just over 3mm really badly affect my bottom end? Also my timing would remain the same as TDC would still be the same correct? I’m looking for primarily freestyle riding.

My other option is to just machine the protec head for the proper squish which should leave my porting but then I don’t have the added insurance of the girdled head.

Or if possible get the domes machines 0.120” up but that seems like a lot!

Picture of piston at TDC. Keep in mind this is my first time working on a 2 stroke so please bear with me.

 

[Half flip95]

Get a set of domes machined. I had a +6mm stroke that ran a .020" base gasket and the rest out the top of the cylinder. I was in a similar situation to you. Jetworks set it up for me and Art had said that a +6mm was the largest you could get away doing this with

 

[Dustin]

Get a set of domes machined. I had a +6mm stroke that ran a .020" base gasket and the rest out the top of the cylinder. I was in a similar situation to you. Jetworks set it up for me and Art had said that a +6mm was the largest you could get away doing this with

Awesome thanks! I’ll give them a call!

 

[Storbeck]

By my *rough* calculations, a stock 62t cylinder has port timing of about 182/119 exhaust/transfer

If you put a stroker crank in that with no change in cylinder height, your piston would stick out the top a little over 2mm (the change in stroke makes the piston go up 2.5mm further, but the piston is a little below the top of the cylinder at TDC in stock form). This port timing would come out to to roughly 184/126 exhaust/transfer. You'll have super low blowdown timing of only 29 degrees, I believe that's less blowdown timing than a stock 61x cylinder, and it's kind of an awkward combination of high ish (still not really that high, but higher than your stock cylinder was) exhaust port timing was but less blowdown.

If you space the cylinder up 2.5mm you'd be at 192/135 exhaust/transfer, which is probably WAY more transfer timing than you'd ever want. And a super funky combination of high exhaust port with very little blowdown.

It's a thing that I think most people don't understand with stroker cranks, they affect the transfer port timing way more than they affect the exhaust.

Choosing from those options I would leave the cylinder low and cut the heads. I'd be tempted to port the exhaust up a degree or two to like 186 or 187, which is roughly stock duration on a 760 cylinder, but I personally have zero experience with what that combination would do.

A pro builder would (should) have better suggestions.

The one thing I would say with a lot of confidence is I would not raise the cylinder up.

 

[fastgtfairlane]

The blowdown timing plays a huge role in how and when the power comes in. Unfortunately the yamaha castings are very thin in the transfer port area so raising the transfers much to get the blowdown timing back up isn't really an option..

 

[Storbeck]

Raising the transfers would decrease blowdown, wrong direction.

 

[Dustin]

By my *rough* calculations, a stock 62t cylinder has port timing of about 182/119 exhaust/transfer

If you put a stroker crank in that with no change in cylinder height, your piston would stick out the top a little over 2mm (the change in stroke makes the piston go up 2.5mm further, but the piston is a little below the top of the cylinder at TDC in stock form). This port timing would come out to to roughly 184/126 exhaust/transfer. You'll have super low blowdown timing of only 29 degrees, I believe that's less blowdown timing than a stock 61x cylinder, and it's kind of an awkward combination of high ish (still not really that high, but higher than your stock cylinder was) exhaust port timing was but less blowdown.

If you space the cylinder up 2.5mm you'd be at 192/135 exhaust/transfer, which is probably WAY more transfer timing than you'd ever want. And a super funky combination of high exhaust port with very little blowdown.

It's a thing that I think most people don't understand with stroker cranks, they affect the transfer port timing way more than they affect the exhaust.

Choosing from those options I would leave the cylinder low and cut the heads. I'd be tempted to port the exhaust up a degree or two to like 186 or 187, which is roughly stock duration on a 760 cylinder, but I personally have zero experience with what that combination would do.

A pro builder would (should) have better suggestions.

The one thing I would say with a lot of confidence is I would not raise the cylinder up.

Thanks for the info! When you referral to degrees, what is that referencing to? Like 0 degrees would be the crankshaft or piston position at what position? Sorry I’m not too familiar but trying to learn. I did measure about 39 mm from top of exhaust port to the top of my cylinders. I’m not even sure if the sleeves are stock 62T 701 or not. They were 81mm bore originally.

I’m away from home for a week but once I’m back I’m going to measure all the ports, cylinder total height etc.

 

[Storbeck]

Degrees of crank revolution that the port is open.

So for example an exhaust duration of 180 degrees (to make the numbers easy) would open 90 degrees before bdc, and close 90 degrees after bdc, for a total open duration of 180 degrees.

120 degree transfer would open 60 degrees before bdc and close 60 degrees after bdc (or 120 degrees after tdc and 120 degrees before tdc if that is easier to picture) for total duration of 120 degrees of crankshaft rotation.

Blowdown is the number of degrees of rotation that the exhaust port is open before the transfer port opens. So if you had 180 degrees of exhaust duration and 120 degrees of transfer duration, the exhaust would open 90 degrees before bdc, then the transfers would open at 60 degrees before bdc, 30 degrees between the two, you have 30 degrees of blowdown.

 

[Dustin]

Degrees of crank revolution that the port is open.

So for example an exhaust duration of 180 degrees (to make the numbers easy) would open 90 degrees before bdc, and close 90 degrees after bdc, for a total open duration of 180 degrees.

120 degree transfer would open 60 degrees before bdc and close 60 degrees after bdc (or 120 degrees after tdc and 120 degrees before tdc if that is easier to picture) for total duration of 120 degrees of crankshaft rotation.

Blowdown is the number of degrees of rotation that the exhaust port is open before the transfer port opens. So if you had 180 degrees of exhaust duration and 120 degrees of transfer duration, the exhaust would open 90 degrees before bdc, then the transfers would open at 60 degrees before bdc, 30 degrees between the two, you have 30 degrees of blowdown.

Awesome! Thanks for the explanation! I bought a degree wheel I’m going to put on the crank, and I’ll check the timing for everything with the stock (0.020”) gasket. I may put some shims in and check at different cylinder heights. I understand the durations could change a bit with the cylinder going up as the speed of the piston is slower at each end of the stroke.

 

[Dustin]

Uhmm...Port Duration is measured in Degrees of Crankshaft Revolution. It does not change with RPMs or Piston Speed.

RPMs and Piston Speed does affect the amount of Time the Ports remain open and closed.

So, at higher RPMs and Piston Speeds, more Port Height and/or Width is needed. AKA more Port Time-Area.

Read about Port Time-Area, not the same as Duration.

"Time area
Two-stroke port duration is often expressed as a function of time/area. This integrates the continually changing open port area with the duration. Wider ports increase time/area without increasing duration while higher ports increase both."

Thanks for the info. Ya i guess I’m not right with the terminology, I just figure if I put a spacer plate below my cylinders , the piston will be travelling at a faster speed through the exhaust lessening the time as the piston when near the end of the stroke is deaccerating/accelerating. I’ll read up on it. As this is a 5mm stroker what I’m curious with once I get all the port measurements in degrees and also the physical measurements in reference to cylinder deck height, I gotta figure out how to interpret them to figure out where the best place for the cylinder ports to be. Right now I have a 0.020” base gasket in and the one that was in originally may have been 0.060”. I want to make sure I get all the measurements right before I figure I have to get some larger domes machined for the piston protruding above the cylinders. I emailed Art at Jetworks with no reply yet, but I figure he’s going to need all that info.

I live up in Canada and there aren’t any good engine builders around here that I know of, so I figure my best best is to get the data and order custom domes from a builder in the states who knows what they’re doing and can interpret the data I give them. Plus I get to learn a bit more.

 

[loictahiti]

Interrested

 

[Dustin]

Alright I checked the port timing with different base gaskets (shims). This is what I’ve got. To get the proper squish with my current domes id need 0.120” as per my calculations. What are your thoughts on how it’d be for bottom end with that setup. Ideally it looks like base gasket of 0.020” would be best but I don’t think I can machine my domes that much

 

[Storbeck]

The opinion of a random guy on the internet: I would keep it as low as possible. Even if you wanted to run higher timing and you ported the exhaust port up, I don't think there is any situation where you want more than 126 degrees of transfer timing unless you're building a very high rpm motor.

 

[Dustin]

The opinion of a random guy on the internet: I would keep it as low as possible. Even if you wanted to run higher timing and you ported the exhaust port up, I don't think there is any situation where you want more than 126 degrees of transfer timing unless you're building a very high rpm motor.

Thanks for your input! Looking at these domes I may just be able to get them machined to fit the pistons. Good thing my father in-law is a machinist.