D ports and jetting

Schwany · Sep 17, 2026

worntorn said:
I tried a complete original Dunstall 2 into 1 into 2 system with Dunstall megaphones on the 850. According to the Dunstall ads, that system will make the bike 1 full second quicker in the quarter mile.
It slowed my 850 down on dyno hill. 111 km at top was reduced to 107, and that was after several main jet changes to optimize jetting.
It was louder but not faster, unfortunately.

Glen

Given the design of that Dunstall pipe, your result is not surprising. If you were closer, I'd get a kick out of testing the Rattler on Dyno Hill. Good thing it's not closer. I might embarrass myself, or set a new record. lol

Schwany · Sep 17, 2026

Fast Eddie said:
I’ll add my random thoughts to the randomness…

Stock headers, without balance pipe, into non-muted peashooters work VERY WELL on Commandos. You gotta do well to improve on this.

2:1 pipes are a minefield of variables. It’s much easier to get wrong than right. And what works well with one cam may well not with another.

The Maney system is excellent. Steve spent many, many hours testing many, many variables on the dyno and he got it RIGHT. One of the big keys to its success is the 2:1 manifold design, so that is well worth studying.

Interestingly for a ‘race’ system, Steve’s pipe actually gives a much bigger mid range boost than it does a top end boost.

But how much of a boost is very dependent on other factors, in particular the cam. Steve’s pipe was designed for Steve’s cam, which is close to a 4S and has relatively long duration. Put a short duration cam into the same bike and the benefit of his 2:1 pipe reduce somewhat. I know from experience that they work very well with the JS #1 cam.

Lastly, with regards to jetting, I have found that the Maney pipe requires significant jet changes in order to realise the benefits, they actually require weaker settings than a 2:2. I have swapped peashooters for a Maney pipe on the dyno and seen zero improvement until we corrected the fuelling, then BOOM !!

You should try the JSM D-Port sleeves and see how they work with your final tune on one of your bikes with a Maney pipe. I'd be interested to hear if they are plug and play improvements with any of your toys.

Fast Eddie · Sep 17, 2026

Schwany said:
You should try the JSM D-Port sleeves and see how they work with your final tune on one of your bikes with a Maney pipe. I'd be interested to hear if they are plug and play improvements with any of your toys.

I have a Maney stage 3 head so they wouldn’t be appropriate and wouldn’t even fit !

Schwany · Sep 17, 2026

Fast Eddie said:
I have a Maney stage 3 head so they wouldn’t be appropriate and wouldn’t even fit !

So your exhaust side ports are enlarged on the Maney head, or it already has welded up D-ports? I know the intake side is larger.

maaseyracer · Sep 17, 2026

Torontonian said:
I will now be refitting both D Ports using hi temp silicone to the underside of both D port floors , after a good carbon cleaning out . As instructed .

The fit is based on an 850 mk3 head and 750 head I have that has a fairly rough finish in the port. Every port is different, and not all 3D prints will be exactly the same. On the exhaust floor, a gentle tweak of the tip in a loose vise to fit your exact port might not be the worst strategy, gentle is the key here you are only looking for 1mm of movement if there is not tension on the tip, while the top of the "flat washer part is in the last thread. Once installed with tenson I can turn the cylinder head upside down without these falling out.

TBW · Sep 17, 2026

Torontonian said:
I will now be refitting both D Ports using hi temp silicone to the underside of both D port floors , after a good carbon cleaning out . As instructed .

Will be interested to hear your opinion. FWIW I’ve used JB Welds’ red hi temp sealant with good results. Similar to RTV106 in aerospace world.

TBW · Sep 17, 2026

worntorn said:
I tried a complete original Dunstall 2 into 1 into 2 system with Dunstall megaphones on the 850. According to the Dunstall ads, that system will make the bike 1 full second quicker in the quarter mile.
It slowed my 850 down on dyno hill. 111 km at top was reduced to 107, and that was after several main jet changes to optimize jetting.
It was louder but not faster, unfortunately.

Glen

So I also have Dunstall 2-1-2 …gonna keep it until engine break-in completed then adjust n test one thing at a time

Fast Eddie · Sep 18, 2026

Schwany said:
So your exhaust side ports are enlarged on the Maney head, or it already has welded up D-ports? I know the intake side is larger.

I don’t think the ex ports are deliberately enlarged, but they’ve had work, enough for the inserts to not fit for sure. No, they are not D ports. Actually the 2:1 manifold is designed to provide some anti reversion IIRC.

Schwany · Sep 18, 2026

Fast Eddie said:
I don’t think the ex ports are deliberately enlarged, but they’ve had work, enough for the inserts to not fit for sure. No, they are not D ports. Actually the 2:1 manifold is designed to provide some anti reversion IIRC.

In that case the sleeves wouldn't do anything other than mess up your tune more than likely. Some reversion relief is my random point about my exhaust, which does the same. It is one of the characteristic of 2 into 1 exhaust systems if some time is put into making them work better. I did not make the headers, collector, or megaphone. I did however shorten the headers, reshape the inside of the collector some, extend the exit between the collector and megaphone, and make my own baffle. It took me a couple of months to figure it all out via seat of the pants trial and error. I did not get advice from the internet, just some old geezer with a proddy at a bike show said make your pipe longer. I guess I got lucky. I'm not Maney. However my first name is Steve, and I excel at random jibber jabber.

maaseyracer · Sep 18, 2026

Reversion happens regardless of how we try and mitigate it. There is efficiency where we try and mitigate it. The most efficient mitigation is as close to the valve as possible, this is why modern engines now use port shape to maximize speed and keep the ports moved upwards to maintain the direction of the flow. The speed of the exhaust does not allow for gasses to flow back as easily.

Merge collectors aid as well. The issue with a merge collector to manage reversion is that they are down stream and not as efficient as a port shape or something like an anti reversion chamber in the entrance of the pipe, as there is more area for gasses to flow backwards.

The merge collector is still mitigating what the port shape is missing, The issue then becomes the merge collector can become a bottle neck at higher RPMs, which is the discussion most 2 into 1 vs individual pipes usually goes. I have played a lot with various exhaust side anti reversion on various British twins. I do not believe in a single solution, and often there are more mitigating factors. Duration and advance of your cam is a big one as well as your valve job. Longer duration cams have more issues with reversion. On my single carb Triumph the reversion is so bad with the longer duration cam the carb slide would not settle back down. My MK3 with the original Amals did a small bit of the same when decelerating down to a stop.

acotrel · Sep 18, 2026

If what you are talking about are these: - https://jsmotorsport.com/product-category/ports/ . I would certainly use them. However, I would not have a clue about what 'reversion' might be. I suggest that when a motor is running, we have a sound wave. It is not so much about gas flow. Sound does not like going around corners. If I alter the exhaust system on my motorcycle, I rejet the carbs systematically. I do not usually ask for advice from other people and fit the jets they recommend. The only time I have ever done that was when building a T250 Suzuki, and jetted it for methanol. You cannot drill a smaller hole in a needle jet in a Mikuni carburetor unless you buy a new jet. In Amal carbs, the needle jets can be made from brass hex. The Mikuni jets are usually very complex, and probably expensive. Petrol in different places have different compositions, so using another person's recommendations, might not achieve the desired result. Many years ago, I met 3 friends who were returning from a blast across Fishermans' Bend airstrip. One was being towed. He had an immaculate 500cc Clubman's Velocette. They had fitted a megaphone to it without changing the jetting - it immediately burned a piston.

hoffman900 · 2026-09-27T07:42:06-0600

jseng1 said:
The main change with the ex port floors is that they have an anti reversion effect. because of the step at the bottom of the port where it meets the ex pipe. Outbound exhaust gas have enough momentum to blast past the step smoothly, but low-energy return pulses at the bottom of the port hit the wall of the step and break up instead of re-entering the port and contaminating the combustion chamber. I don't know for sure if this has enough effect to require rejetting or not. In a highly tuned engine I think it would.

Hybridracer - the earlier twin carb XR750 had round ports. The cobra head ports didn't come in untill the mid/late 1980s and gave a signifitant HP increase. Now you see that shape in nascar motors.

On the car stuff we prefer to keep this transition seamless and use a anti-reversion type devices later in the exhaust (such as a "Fueling" chamber or a double step with tubing overlap into the next primary step). This way you can preserve any and all velocity that you can. During blowdown the port isn't saturated and the mass is hugging the outside, but port depending, on the pumping side, the port becomes more "full" because the velocity is much less. The best "reversion" device is to preserve as much as the exhaust energy as you can - you can't recover it. Steps, etc at the port transition is the worst spot to do it. As large of bend radii as can reasonably fit is crucial as well.

Also, a popular mod with hemi chambers in top level NHRA Super Stock and other road race car stuff is to shrink and sink the exhaust valve some. This helps hurt low lift flow and helps remove some of the exhaust port line-of-sight from the intake valve - and helping reduce over scavenge. Also, even if the flowbench doesn't suggest it (and the flowbench is notoriously unreliable for the exhaust port, unless you can flow at 40" H20 +, and that's still mostly a separation thing), less valve timing on the exhaust side helps. You need to preserve the higher lift area if you can (lift at BDC on the exhaust stroke is a good measure), but if you can reduce the .050" down to the seat, that's usually better.

- this is a good example of what is going on in an exhaust port (you can slow things down) from a mass flow standpoint.

hoffman900 · 2026-09-27T08:23:09-0600

To expand on the hemi point: When I say “over-scavenging”, I mean fresh mixture leaving through the exhaust during overlap, which is after the job of clearing residual exhaust gas has been done. Scavenging itself is desirable. The question for anyone that plays with engines, is how much of the incoming charge stays in the cylinder when the exhaust valve closes.

A wide-angle, two-valve hemi makes that tradeoff sensitive. With both valves open near TDC, the intake and exhaust can have a direct path across the chamber. The piston dome, valve placement, port direction and the pressure wave arriving at the exhaust all affect the actual path. If the exhaust is pulling strongly at that moment, improving its low-lift flow or holding the exhaust valve open longer may remove more residual gas—but it may also pull some of the fresh charge straight through. This is a pretty common phenomenon with hemi's and it shows right up in the BSFC curve or with some crazy EGT numbers. That is why a smaller or slightly sunk exhaust valve, or less exhaust timing near the seat, can sometimes help an engine even when the exhaust flowbench number goes down. It is a combination, not a rule for every hemi, but happens more often than not.

While I did poo-poo the flowbench, it is still valuable. It tells us about the port and valve’s flow capacity at a given lift and pressure difference, and it can expose a bad turn or separation. What it does not reproduce is the running engine’s changing valve lift, moving piston, hot blowdown flow, returning exhaust waves, or simultaneous flow through both valves. Flowing at 40 inches of water may make some port behavior easier to see, but it does not turn a steady test into an overlap test.

So my advice, is that I would be cautious about calling either the biggest exhaust flow number or the strongest anti-reversion step a win by itself. The result to watch is trapped charge and torque across the operating range, with the intake, cam timing and complete exhaust system in place - which on a bike, it's easy to feel in the butt dyno.

TBW · 2026-09-27T15:41:12-0600

hoffman900 said:
To expand on the hemi point: When I say “over-scavenging”, I mean fresh mixture leaving through the exhaust during overlap, which is after the job of clearing residual exhaust gas has been done. Scavenging itself is desirable. The question for anyone that plays with engines, is how much of the incoming charge stays in the cylinder when the exhaust valve closes.

A wide-angle, two-valve hemi makes that tradeoff sensitive. With both valves open near TDC, the intake and exhaust can have a direct path across the chamber. The piston dome, valve placement, port direction and the pressure wave arriving at the exhaust all affect the actual path. If the exhaust is pulling strongly at that moment, improving its low-lift flow or holding the exhaust valve open longer may remove more residual gas—but it may also pull some of the fresh charge straight through. This is a pretty common phenomenon with hemi's and it shows right up in the BSFC curve or with some crazy EGT numbers. That is why a smaller or slightly sunk exhaust valve, or less exhaust timing near the seat, can sometimes help an engine even when the exhaust flowbench number goes down. It is a combination, not a rule for every hemi, but happens more often than not.

While I did poo-poo the flowbench, it is still valuable. It tells us about the port and valve’s flow capacity at a given lift and pressure difference, and it can expose a bad turn or separation. What it does not reproduce is the running engine’s changing valve lift, moving piston, hot blowdown flow, returning exhaust waves, or simultaneous flow through both valves. Flowing at 40 inches of water may make some port behavior easier to see, but it does not turn a steady test into an overlap test.

So my advice, is that I would be cautious about calling either the biggest exhaust flow number or the strongest anti-reversion step a win by itself. The result to watch is trapped charge and torque across the operating range, with the intake, cam timing and complete exhaust system in place - which on a bike, it's easy to feel in the butt dyno.

Mate, sounds complex n it’s gonna take me a while to digest

…do like the last sentence tho’

Schwany · 2026-09-28T09:07:47-0600

hoffman900 said:
So my advice, is that I would be cautious about calling either the biggest exhaust flow number or the strongest anti-reversion step a win by itself. The result to watch is trapped charge and torque across the operating range, with the intake, cam timing and complete exhaust system in place - which on a bike, it's easy to feel in the butt dyno.

I'm a firm believer in the butt dyno. It's definitely easy for me to tell when going in the right or wrong direction with everything involved in a tune using a well educated butt dyno. Nice thing about using a butt dyno is I have mine with me all the time, and don't have to make an appointment to use it.

EstuaryBoy · 2026-09-28T09:13:39-0600

Schwany said:
I'm a firm believer in the butt dyno. It's definitely easy for me to tell when going in the right or wrong direction with everything involved in a tune using a well educated butt dyno. Nice thing about using a butt dyno is I have mine with me all the time, and don't have to make an appointment to use it.

Can I offer two variants (and I'm sure there's more!).. With a single 't' there's the: It feels quicker, 'but' truth be told, it ain't dyno... (Fuelled by wishful thinking and 'I spent a bloody fortune, it MUST be so')

Or the more common: He's obviously talking out of his 'butt', dyno.....

Schwany · 2026-09-28T11:31:58-0600

EstuaryBoy said:
Can I offer two variants (and I'm sure there's more!).. With a single 't' there's the: It feels quicker, 'but' truth be told, it ain't dyno... (Fuelled by wishful thinking and 'I spent a bloody fortune, it MUST be so')

Or the more common: He's obviously talking out of his 'butt', dyno.....

That's what I'm talking about.

You win todays parroted wisdom

award. More thumbs up are coming your way soon.

It took a lot more effort on my part than bolting on performance parts to make my Norton work well. Fueled by years of tuning outside the OEM stock box on 2 and 4 stroke motorcycles. Tech babble in forums isn't going to make an engine run better, only the person building the bike can make it happen. Just saying.

Anyone in the Seattle area that wants to show me what real stock Norton 750 engine power is like after getting their bike dyno tuned is welcome to stop by and show me. What would be really special is a 750 dyno tuned after getting the D-port inserts installed.

mdt-son · 2026-09-28T15:14:08-0600

Schwany said:
Given the design of that Dunstall pipe, your result is not surprising.

Isn't it? What makes you think the design is disadvantageous?

- Knut

Schwany · 2026-09-28T16:37:30-0600

mdt-son said:
Isn't it? What makes you think the design is disadvantageous?

- Knut

It doesn't matter what I say. I'll get a spanking for not towing the line.

Center section where the headers go into 1 pipe is too long and small in diameter before it goes back into too pipes. I think the stock cams in Nortons seem to work better with two pipes going all the way to the peashooter silencer exits. Glen got reduced performance using it. I've never touched one, and wouldn't. I've only looked at one in an image. Anyway, question Glen. He was not impressed.

I did not follow through here to see if anybody actually made one of those 2 into 1 into 2 Dunstall pipes make more power than a stock setup on a stock 850?, or whatever Glen was riding. Product advertising does not count as a reliable source to me.

D ports and jetting

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