Stepped cyl port into pipe

There's a discussion going on on some FB group about stepped cylinder exhaust mounting onto the pipe. Supposedly it increases torque, usually used with engines with aux exhaust ports.

The theory is that the flat edges create more or a spray nozzle effect, mix out and back in to the jug with more force.

I'm my experience with adding aux ports, there is definitely some low end negative changes that happen when you add aux ports and then match the header to the ex port output perfectly.

I find this really interesting and will mod one of my cylinders with braze to reproduce these steps and see what the butt dyno tells me

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Re: Stepped cyl port into pipe

Dirty30 Dillon /

I'd be interested to see if the lost charge atomization created by intentionally eddying the flow really help with the low end charge, versus losing uninterrupted flow of gas out the pipe.

Re: Stepped cyl port into pipe

If you are on 2stroke research and development most guys that port that step out get a quick lesson as to why the step is there. Something something return wave. If you get a good comprehension on it let me know what is good about it. I've not put much thought into it.

And most of my "race" scooty cylinders ha ve that step cast in. If that means anything.

I would think being so close to the boundary wall layer where the flow is already low on exit having it would not affect exit flow that much

Re: Stepped cyl port into pipe

It doesn't seem like a flow change as much as a pressure change.

More stuffing I bet.

Re: Stepped cyl port into pipe

its pretty common on like nascar style roundy round cars, especially dirt track with a restrictor plate. i'll see if i can find a good article i read on it, basically it stops the reversion pulse you get from the end of the tailpipe

if you have a multi-cylinder engine with a collector, the exhaust pressure wave goes down the tube to the collector but when it hits the end a reversion pulse comes back, those four stroke motors are turning like 6000 rpm maybe so you're talking 1/2 that for it being a four stroke and the tuned length gets pretty long, so the reversion pulse is in theory coming back while the exhaust valve is still open.

the stepped port like that stops the reversion pulse or at least significantly reduces it so you don't get back flow in the exhaust port. this also is the same reason you hear about four strokes liking 'back pressure'... basically if you don't have a blower or something a 20 inch whatever zoomie or open header is gonna send back one motherfucker of a reversion pulse. if your intake port pressure is already on the low side (i.e. restrictor plate)

as far as how it would work with an expansion chamber, i dunno, seems like most high performance two stroke cylinders i've had apart, like factory dirtbikes and snowmobiles and stuff, have a pretty smooth transition for whatever that is worth.

Re: Stepped cyl port into pipe

I think you're looking at it from the wrong angle.

When aux ports are cut in, the cavernous mess in there fucks up your lows. So to mitigate that, less exhaust "tube" can keep pressures up as the wave returns, like a spray bottle back and forth into the cyl.

When one has a giant exhaust into a giant header pipe it's just not gonna have great speeds with the reduced pressure of big piping and giant ports.

Idk, just my take

Re: Stepped cyl port into pipe

Overpriced Parts /

When you have 10 to 15 year-old bikes that start and take you 20 to 30 miles in every direction and iand run for hours let me know, r Hard to start

but being a musician you rock! You are talented!

I tried a few years ago to see you in Akron but I couldn’t make it there in time, keep posting because some of these belt drive moped people counting on you! I like gear drive chain drive or nothing,

either chain drive or gear drive everything else fail oall four seasons!

Re: Stepped cyl port into pipe

Loving the look of that, reminds me of some of the RD air cooled cyclinders.

Let us know how it goes on my butt dyno!

Re: Stepped cyl port into pipe

Are you saying I don't have those reliable bikes riffroff? Shame on you.

I'll race your old ass for a thousand miles for pinks and we'll see if all your big reliability talk holds up.

Jbot vs riffroff 2021

Re: Stepped cyl port into pipe

Kevin Bishop /

I saw that on my DS7 cylinder when I broke it down. I thought it was a casting mistake! Interesting. Yamaha did so many changes to the RD porting over the years it should have been called the RnD.

Re: Stepped cyl port into pipe

The ds7 step is just how the gasket seats on there, if there is any step other than that it would have been somehow missed in it’s finishing. This is only useful with aux exhaust ports and they never had 3 exhaust ports on the rd series bikes. Rd means race developed, it is essentially what you say, r and d. It being a culmination of the 1960s-early 70s yamaha road racing efforts, but de tuned for reliability, consistent power delivery and noise compliance of course. Jbot, when are you gonna make a power valve already? (edited)

Re: Stepped cyl port into pipe

Overpriced Parts /

> JBOT Admin Wrote:

> -------------------------------------------------------

> Are you saying I don't have those reliable bikes riffroff? Shame on you.

>

> I'll race your old ass for a thousand miles for pinks and we'll see if

> all your big reliability talk holds up.

>

> Jbot vs riffroff 2021

Don’t need pinks in my state, i’m sorry I don’t think you could leave a bike alone for 10 years let alone 10 minutes I don’t like belt drive bikes other then stock looking derbis either

I like stock lookin big bikes that last for decades sfter built in all weather only checking ring gap, spark plug and other simple maintenance,

Anyway it sounds like you need to make a power valve like Daniel says,

That is about the best way to accomplish what you want to accomplish and is proven to work reliably

This guy in video does a good explanation of it,

Only problem is it’s made into the cylinder and that’s where it needs to be

My friend had my brother both had CR 250 Rs that were absolutely scary! Way more scarier than H1or H2 Kawasaki triples I rode, they made instant power, The front wheels pulled up on every gear no matter how forward you put your body!

The power valve lets the bike make power from the get go, if moped cylinder kit manufacturers made cylinders that had power valves you could probably use the oil injector pump that comes on some bikes to actuated it or just use manual cable with the lever on handle bars,

You would be capable of making power on a huge RPM range if you had a power valve and then having a adjustable rear exhaust chamber that would slide forward a inch or two after powervalve was fully opened,

You’re crafty so take a cylinder drill out the inside of a cylinder, put a valve inside and while your at it prototype sliding chamber, post pictures when you’re done, Thanks!

Re: Stepped cyl port into pipe

Maybe I'll do it.

I'd need to see one in person someday.

Re: Stepped cyl port into pipe

On most they kind of have a cam like thingy where it twists the valve shaft to open at a specific rpm. Some are electronic and are controlled via the cdi box and some a governed by flyweights, you could also do manual I guess, but it might be a little harder to control and the added complexity of riding it might be too much for the human brain.

Re: Stepped cyl port into pipe

Overpriced Parts /

> JBOT Admin Wrote:

> -------------------------------------------------------

> Maybe I'll do it.

>

> I'd need to see one in person someday.

Stock cr 250R with power valve geared correctly is a 10 second bike, quick shift at the next gear to bring the front end down and being only a four-speed and close ratio within mili seconds the front and came back up I never was scared in my life the 5-6 times I rode these bikes in my life! And I rode 360s Husks in hair scrambles back in the day and got finisher pins, Though I didn’t finish in the top three I finished ninth place and they were nothing like these wicked Honda 250cc powervalve bikes!

Re: Stepped cyl port into pipe

Uh, a 10 second trap is like 200 KPH at the end of the 1/4. No way a bike rated at 120kph top speed is doing a 10 anything no matter how much gear you put in it.

Most 200+ HP street bikes are low 11's or high 10's

Me thinks a 50 HP CR250 would be in the 13's. maybe high 12 if you got the stink on ya.

Re: Stepped cyl port into pipe

jbot what tools are you using for your auxiliaries? I feel like I gotta open up the tract and flange a fair bit to be able to get the bit deep enough at a good angle for shaping. angled exhaust flanges are kinda dumb

I needa find some longer bits, im just using hardware store Dremel tungsten carbides. it seems like the ebay/amazon diamond and carbide ones dull or get bent easily

Re: Stepped cyl port into pipe

Just gotta buy 5 sets of eBay ones.

Re: Stepped cyl port into pipe

Yeah long shaft carbide eBay blue packs like $8 or something.

But I use a drill bit usually.

Just aim well and take your time.

Then after the drilled hole is cut, nibble away with the carbide cutters

I've only fucked it up once, (cue Jackers) but the cyl still ripped. Did one aux port and then made the 2 ports like an off center bridge..

Re: Stepped cyl port into pipe

> Joel P Wrote:

> -------------------------------------------------------

> Uh, a 10 second trap is like 200 KPH at the end of the 1/4. No way a

> bike rated at 120kph top speed is doing a 10 anything no matter how much

> gear you put in it.

>

> Most 200+ HP street bikes are low 11's or high 10's

>

> Me thinks a 50 HP CR250 would be in the 13's. maybe high 12 if you got

> the stink on ya.

Seriously? Dam that’s actually really sad it takes that much hp to only be in the low 11s.

R u sure?

Re: Stepped cyl port into pipe

http://www.dragtimes.com/Yamaha-YZF-Timeslip-9916.html

Some guys with the new R1M broke into the 9's with one in Canada.

But for the most part I stand by what I said. Most liter bike crotch rockets are 10-11s bikes.

Re: Stepped cyl port into pipe

so I talked with one of the old school engine builders at work (a performance auto cyl head manufacturer) and he said yeah, all the performance 4t setups have a step on the exhaust. He mentioned something about is accelerating the flow out of the port, acting like a a diffuser.

However, I did some work designing the intake and exhaust ports using cfd and different exhaust tube header size (lots of automotive guys bolt up round headers to a square port) and increasing the header size had a negative effect on the flow, past a certain size. I think it's ultimately based on the pressure difference/ vena contracta effect.

I think in 2t, if a step in the exhaust helps, the port timing is incorrect.

Re: Stepped cyl port into pipe

> JBOT Admin Wrote:

> -------------------------------------------------------

> Yeah long shaft carbide eBay blue packs like $8 or something.

>

> But I use a drill bit usually.

>

> Just aim well and take your time.

>

> Then after the drilled hole is cut, nibble away with the carbide cutters

>

> I've only fucked it up once, (cue Jackers) but the cyl still ripped.

> Did one aux port and then made the 2 ports like an off center bridge..

I followed this process. Was awesome 10/10 would recommend. That blue pact of carbides is the best porting bits.

Re: Stepped cyl port into pipe

But be careful with some shitty plated cyls (tomos)

The plating likes to pop off if you push the bit too much once it gets to the cyl wall. Best thing to do is use the ball cutter in the pack to cut out the aux ports plating before punching thru with the bit

Re: Stepped cyl port into pipe

Get the diamond dental burrs for plated cylinders. Much less chipping.

You can still use carbide burrs on all the aluminum stuff.

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