Tag Archives: Dobsy

Reverse engineering

Moving directly on from the last post at the end of January, I thought I’d share a little progress on the exhaust front.
Although I’ve been completely absorbed in doing engines for other people, I’ve been wanting to find some time to go back to basics and map the Gibsons made exhaust that I’ve been using on the Scott racer. Now this isn’t an easy thing to do completely accurately as the exhaust is shaped to wind its way through the frame, but I think I made a reasonable job of measuring it out.
I drew it up with CAD, but it’s actually easier to see on sketch up.

model of current Gibsons pipe

I know you can’t see all the detail but it’s interesting to see the lengths involved and tapers used. A couple of things are interesting, the stinger ID works out to be around 54% of the ID of the start of the header pipe. Now I don’t know much about expansion pipes except what I’ve read and been told, but the stinger ID is a critical dimension and very small dimensional changes can make noticeable differences in the effect of an exhaust. If too small a stinger, excess pressure can cause overheating in the engine. Now back at Cadwell in 2015 I was told that as a rule the percentage difference between the stinger and the header should be around 60%, which would make it ~29.8mm ID. That’s 2.8mm difference, which is not even that small. Also, there’s another thing: this is a 2 into 1 pipe, although I’ve only drawn a single pipe in the example. I’ve a Scott engine behind me on the bench with a similar exhaust duration as mine; mine is around 160° duration and the one behind me is around 157°. If this bike had a pipe for each cylinder, then it would have ~203° of crank rotation from the closure of the exhaust port to the re-opening on the next cycle. Since this is a 2 into 1 pipe, the exhaust has ~23° of crank rotation from the closure of one exhaust to the opening of the other. That’s around 11% of the time at any equivalent RPM or even if there’s a mathematical reason why that’s not completely true, it’s a lot less time to do the same job.
Given that the 60% guideline for the stinger is supposed to be based on a one pipe to one cylinder design, am I to extrapolate the time difference to apply to the cross sectional area of the stinger? Again, I’m sure the maths are well above my pay grade for that, but what’s obvious is that it’s about time and area, and most bikes rev faster than a Scott.
A quick googling leads to me think that an early 70’s TZ 250 would rev to 10,500 rpm, for example. Given that the tuning data from Bell would have been used on actual racing machines in that era, then it’s fair to assume that this was representative of the kind of engine speed that the 60% Stinger ID to Header ID rule of thumb, was worked out for. For sure the exhaust durations are different too, but I need to look at this to make a guess at what the stinger ID should be.
Also, having been able to establish the design and measurements of the Gibson pipe, I found that it’s actually made for an engine that revs quite a bit higher than mine.. like my dad’s! (although it’s even on the high side for that. Working backwards, the dimension between the piston face and the mid point in the deflector cone is 1244.5mm, which according to Bell makes it tuned for a whisker under 5500 rpm. Now Roger’s engine, with his substantially balanced four bearing crank will spin up past 5500rpm, but that’s not a place he likes to ride it anyway. Dobsy used to rev it hard, but even my dads engine is getting a little out of breath at that point.. just a limit to what the ports can flow, even with the differences on his engine. My calculations would suggest that I need a tuned length of around 1530mm for 4450rpm. 5000rpm is about right for max revs, but as I’ve said before, I just don’t think it can breath properly up there. Thats 285mm longer, not insubstantial in terms of getting it to fit on the bike. We might be looking at the muffler the other side… I know that’s been done.

So I bought Bell’s two stroke tuning book as well as John Robinson’s to go with my copy of Jennings and have been working through some possible re-designs. I purchased a TIG welder a few weeks ago and so that’s another step in the right direction. I think hydroforming would be the best way of testing some designs.. the most fun you can have with a pressure washer as far as I can see!