Does anyone know where I can find the exhaust duration in degrees for a 1976 DT 400 C?

Toying with the idea of making my own expansion chamber - done a few in the past, and exhaust duration is one of the key factors I need.

I have all the other data that I need, once I have the exhaust duration I can get an idea of what sort of size chamber I am looking at - it would be an underslung design which suits me fine as I dont go off road.

Just a thought at the moment - may well come to nothing.

Can anyone help?

Thanks.

Measure it?

After so many years, it may be "ported" already.

cliff

Cliff

To be honest I dont want to pull the motor apart to measure it, I was hoping someone would know the figure so that I could put the number into my calculations so that I could see what sort of dimensions the chamber would work out at.

I dont think it has been ported as it is very original, but you are right only way to check is pull it apart, but since it is running fine I probably wont bother until I need to.

I'd probably do something dopey like mark the flywheel with chalk, then use a 20 thou feeler blade up the port & turn it down & up until it hits the blade again then chalk mark it again & go--Hmmm--exactly 168, or something lol. I have got a degree wheel--somewhere. Was right into it miniature road racing & building chambers, a lifetime ago. Let's know how it goes, when it happens.

Yes as RT325 said you can get the answer by pulling exhaust and ignition cover... 

Or if it's easier, pop the head and measure how far below TDC the port opens and calculate the duration. 

I haven't seen a port map for the later models but that would get you the answer too if someone has one.

Just googling & doodling away & this cropped up. Not sure if its relevant or standard porting but 'is' DT400. Says 86mm so 1mm oversize. This is from post #51 i think. From page 3 so not sure whats on ealier pages but hit page 4 & shows the finished hydroformed chamber. Have fun!!.

photos.google.com/search/_tra_/photo/AF1...S2kfJ5mBw1zVymuSMmGz

www.advrider.com/f/threads/yamaha-dt-400...acker.1067666/page-3

Great find RT325!  I was guessing 178 on duration but it's actually a little less than that.

And yes, he does say it was bored 1mm over. 

So it looks like 176 degrees, but given that it is fairly easy to check I may just do it myself anyway.

It looks like if the motor is left as stock - i.e. no porting, peak power still at 5,500 RPM you end up with a total length - downpipe to end of chamber exit pipe of roughly 1.8 metres, to end up with a shorter pipe you would need peak power to be higher up the rev range, which woudl mean porting.

I have not had a look to see whether 1.8 meters can be accomodated on the bike, but I think it would probably look out of place.

Apart from the sound there looks to be nothing gained by making an expansion chamber for a stock motor - happy to be corrected but that is my thinking at present, having said that if 1.8 metres can be accomodated I might just give it a go for the hell of it.

Need to wrap that head pipe around the front of the engine to get the overall length right. Only way to do it is turn out to the left after the frame rails, bend it around 180 degrees underneath and go back across and turn back upward toward the stock routing. If you go under there's no way to snake the head pipe around, you would have to add length in the back and there's just not enough room. Suzuki tried with their TM400 and even with the design compromises that sort of pipe would never fit a DT.

It's a lot of work but the results are outstanding. I paid almost $600 for the pipe on mine and that was a steal for the effort that goes into it and the effect it has on power delivery. The long pipe can use longer, shallower tapers for a broad powerband. With a proper pipe, the motor will respond very well to a big carb, maybe a larger reed block and some port work. Doesn't have to be a top end screamer with no driveability, a long pipe and appropriate porting will make gobs of power from under 3000 RPM until the stock redline.

A screamer is not what I am after, I was hoping to keep thing pretty stock.

There are people here that know a lot more than I do about expansion chambers I profess to know very little.

Having said that using guidance from the Gordon Jennings handbook I have I have made a couple of expansion pipes for MZs - they are only low reving engines, but the results have been pretty good, bottom end power staying pretty much as stock, but the motor revs much more freely and feels a lot livlier.

I will probably put this idea on the back burner for now, but it will always be there in the back of my mind.