Dunstall 2:1:2 pipe lengths

I'm in the middle of building a BMW road bike and since the engine is not dissimilar to a Commando ( in that it's 360 degree twin, both pistons arriving at TDC together) I am thinking about making a copy of the Dunstall 2:1:2 exhaust for it.

I'm hoping someone has got one that they could measure the header and secondary pipe lengths for me please.

Thanks in advance.

Considering that the three pipes of a 2 into 1 exhaust system are tuned lengths to suit the column of gas within them, the tail pipe must be tuned to vibrate at twice the frequency of one header pipe, so it should be half the length. The most important thing is that it istwice the cross-sectional area. I use skinny header pipes. To get the best out of a 2 into 1 exhaust, you might have to advance your cam timing. My inlet valve opens at about 65 degrees BTDC, the exhaust valve opens at about 94 degrees BBDC. The power band is from about 3000 RPM and straight through the top. The motor pulls like a train. However I use methanol fuel, so heat is not so much of a problem. With thatexhaust opening setting, the paint is burned off to way down the exhaust pipe.

From head flange to end 15-3/8' measuring at the mid point- not outer- not inner

3" length for coupler at 1st Y
mid pipe... from exit of 1st Y, to end inside 2nd Y coupler = 23"
head pipe 1-1/4"
mid pipe 1-1/2"

dynodave said:

From head flange to end 15-3/8' measuring at the mid point- not outer- not inner 3" length for coupler at 1st Y
mid pipe... from exit of 1st Y, to end inside 2nd Y coupler = 23"
head pipe 1-1/4"
mid pipe 1-1/2"

Click to expand...

Brilliant! Thank you.

Interesting how short the headers are. I reckon I can wrap that around a BMW motor. I know that the BMW has a much shorter bore/stroke ratio so it will be interesting to see how well it works, even though it's just a road motor I will take it to the dyno to get some proper figures.

Even if you get it slightly wrong, it will still work. The exhaust system is only really optimised for one particular rev level. Everywhere else it is a compromise. All that happens if it is wrong is the power band might shift slightly. In any case, a 2 into 1 will always be better than separate pipes, as long as you don't restrict it. With it, you can use a much more radical cam and still be able to ride the bike safely.

I tried a Dunstall 2 >1>2 with the decibel silencers on a tuned 650 SS. This required some tube bending as you would imagine. It was much louder than on my Dunstall 810. I guessed it was that the smaller displacement of the 650 could not adequately fill the system. I decided to cut off and close the resulting hole by welding and run it as a 2 into 1. The sound was much more like my old Dunstall 810 and the 650's performance dit not change at all.

When I fitted a 2 into 1 pipe to my Triton, I started cutting back the outlet to the collector and welding on larger diameter tail pipes. I did not regain decent top end power until the tail pipe cross-sectional area was the same as the total of the two header pipes entering the collector. But the overall result was my lap times improved due to the better performance characteristics with the new pipe. The bike was no longer nasty to ride and had much more midrange. The speed down the straights is not a sole determining factor in racing. If you get around the corners faster, you get into the straights faster.

I used 41mm on my BMW racebike on a 2 into 1 and midrange suffered. My German advisor said to use the stock 38mm. Can't recall the header length but think it was around 850mm to the 50mm merger, then 50 mm to 'muffler'.
What was really interesting was the $100 of 38mm headers gave an extra 10 HP across the range and the dip reduced.
Was pretty impressed with 83 BHP with only non stock parts being the cam, ignition and the Dells. More power to be found but alas have now hung up leathers due to a couple of hipops.

Were the header pipes on your BMW of equal length ?

I f

72Combat said:

I used 41mm on my BMW racebike on a 2 into 1 and midrange suffered. My German advisor said to use the stock 38mm. Can't recall the header length but think it was around 850mm to the 50mm merger, then 50 mm to 'muffler'.
What was really interesting was the $100 of 38mm headers gave an extra 10 HP across the range and the dip reduced.
Was pretty impressed with 83 BHP with only non stock parts being the cam, ignition and the Dells. More power to be found but alas have now hung up leathers due to a couple of hipops.

Click to expand...

I found a big hole in the mid range on the three different 2:1 exhausts I've tried. That's part of the reason I have 2:2 on my BMW and Norton. 2:2 is so much easier to design.

My 2 into 1 works perfectly right through the whole rev range and right through the top. But it is too loud. If you don't adjust the valve timing to suit, you might get a hole. The 2 into 1 has a lot more back pressure. I advanced the cam. The exhaust valve now opens at 94 degrees BBDC. I met another guy recently at an historic race meeting who had done the same thing with a Triumph.