Designing a pipe. Am I doing this right?

[FinnishFish]

73 ts185 with light porting. I know the factory pipe is supposed to be "good", but with the factory ports I can tell the flow is reverting, the transfers are very dark. I calculated the blowdown area to be 21deg, so I think that is ample with a good pipe, I think the stock pipe is actually just very restrictive... It should be since it is also a muffler. I'm just going to chamfer the exhaust and that will raise it just a hair to further increase blowdown, so I began designing a pipe since that is the main issue I think.

I am just using a free software for now, Ezin Hobeki's Pipe designer. Using the Fritz Overmars formula gives a pretty reasonable pipe, Blair was pretty much just spitting out a straight pipe. One thing I am not really sure of is how does it know the width of the exhaust flange? D0 is a few mm too large to connect to the cylinder. Are you just expected to neck up or port match? And should the exhaust port with be measured by arc length or chord length? I am using arc length because I transferred the port to masking tape, but I am assuming it is just asking for the linear width across the port, I did not have anything small enough to measure it with though. I am open to any other pointers as well, this is my first time doing this, just running the numbers though.

[teazer]

A few random thoughts:

Anything from Frits is probably good to use.  Blair, Jennings and AG Bell all have a place to start.

For blowdown, 21 degrees is really short and it that;'s correctly measured, would explain the reverse flow in the transfers.  Blowdown time-area is a better way to calculate blowdown and allows you to take the width of the ports into consideration.  A wide exhaust will blowdown faster than a narrow one.

MOTA has a free pipe calculator.  Bimotion has a great pipe design tool but it's not free.It also has their own design plus a Blair design. 2 stroke Wizard also has a cheap pipe designer but the designs seem to be lacking something. MOTA also has a really neat pipe design optimizing tool.  You can just use the formulae and rules of thumb from any of the authors mentioned.

The data in that screenshot is a really low state of tune and if it's correct, a pipe won't make a huge difference. A modestly tuned motor should have a BMEP closer to say 6-7 and hot street would be say 8, though I aim for 10 and see what's actually possible. The higher the BMEP and HP goal, the more the important the pipe is.

For a relatively low port, low power goal, you can go with parallel header and single stage diffuser and save a whole lot of work without leaving much on the table.

   

[FinnishFish]

Ok, so I definitely want to raise and widen the port? I want to keep it able to run on the stock pipe because I don't have the means to build this right now. I think that will limit how high I can make the exhaust port. I also just don't expect an extreme amount of power out of this bike, credit where its due, its not bad for what it is. If BMEP hits 6 that is probably pretty good.

As for the blowdown... I'll believe you that its short, I think I saw 20deg was good for a chainsaw lol. This early model engine has only one transfer per side. I'm assuming widening the transfers and exhaust both improve blowdown time-area because it should speed up flow out and in? We are aiming for a higher pressure differential of the incoming air and outgoing air, so even things like a bigger carb or more aggressive intake timing help, right?

I plan on bending this entire pipe out in a brake because I dont have a roller, it will be simplified, header will be mostly parallel and I want to keep as much of the diffuser out of the main bend as possible, then I can cover more ground bending a longer cone.

[Dvsrd]

When I read the dimension table, it appears that the belly section diameter is around 150 mm. This is way more than normal. I'd say somewhere between 90 and 110 mm seems more appropriate for a 185 cc single. In AG Bells book, the belly should be around 2.5 times header diameter, max. That would be the header diameter next to the port if using a tapered header.

[FinnishFish]

I will have to try a few other softwares and see what they spit out. The blair formula made the belly like 80mm, I could hardly tell where the cones started and ended. Header diameter is like 40mm.

How do you modify the pipe to go to a ~100mm belly? just guess and taper the cones more, but keep the lengths roughly the same, or keep the tapers and extend the belly or shorten the pipe?

[rodneya]

When I read the dimension table, it appears that the belly section diameter is around 150 mm. This is way more than normal. I'd say somewhere between 90 and 110 mm seems more appropriate for a 185 cc single. In AG Bells book, the belly should be around 2.5 times header diameter, max. That would be the header diameter next to the port if using a tapered header.

The 60's and 70's design formulas like Bells have been left behind a long time ago. Much larger belly diameters are common now as is bigger stingers and stinger length considerations.

[pdxjim]

Jan Theil and Frits Overmars are both pretty active on Facebook.

They are both posting quite often in the "2 stroke research & development" group, and would likely be glad to answer your questions directly.

[FinnishFish]

When I read the dimension table, it appears that the belly section diameter is around 150 mm. This is way more than normal. I'd say somewhere between 90 and 110 mm seems more appropriate for a 185 cc single. In AG Bells book, the belly should be around 2.5 times header diameter, max. That would be the header diameter next to the port if using a tapered header.

The 60's and 70's design formulas like Bells have been left behind a long time ago. Much larger belly diameters are common now as is bigger stingers and stinger length considerations.

Yeah I didn't think 150mm looked super large, I still have to try other software and fix up some numbers, maybe it will get closer to 2.5x header diameter, but I would rather go off the dimensions that are calculated rather than guess.

Jan Theil and Frits Overmars are both pretty active on Facebook.

They are both posting quite often in the "2 stroke research & development" group, and would likely be glad to answer your questions directly.

I would love to talk to them, but I am quite anti-facebook.

[Dvsrd]

When I read the dimension table, it appears that the belly section diameter is around 150 mm. This is way more than normal. I'd say somewhere between 90 and 110 mm seems more appropriate for a 185 cc single. In AG Bells book, the belly should be around 2.5 times header diameter, max. That would be the header diameter next to the port if using a tapered header.

The 60's and 70's design formulas like Bells have been left behind a long time ago. Much larger belly diameters are common now as is bigger stingers and stinger length considerations.

I still believe 150 mm is extreme. What is the largest diameter on a "FMF Fatty" pipe for a 250 MXer? I seriously doubt it is much over 120/127 mm (5 inches). And just the packaging aspect of finding space for a 150 mm belly section would be a serious challenge.

[m in sc]

id widen the transfers TBH, and angle the roof a bit (2 degrees or so up towards the exhaust). prob solve the issue w out making it a lightswitch. I know has nothing to d w the pipe, but im not sure a pipe will 'cure' the transfer port issues.