Hello all. i am currently modding a set of ss heads for my latest project and have come across a problem where there are may correct answers I am removing a terig system and going wil a silmoto front pipe spag arrangement, however there is a big difference in the diameter of the termig and the sil. do i blend the port to the new pipe or do i step the outlet to aid the reversion of the pipe. there is a lot of talk about a d shaped port but i do not have room for this convenience, my general thought is to blend the port leaving a 2mm step all round to the header, but of course there will be many reasons for not doing this. BTW if this kills the torque at the bottom end i am not bothered, i am trying to get the peak torque to around 7k-7.5k and feel the larger header will help this along. if anyone has any comments or answers it would be a great help.
In all my years of poncing around gas flowing cylinder heads and ports etc, the objective was to remove as many obstacles and imperfections so as to smooth the flow of the gases as quickly as possible away from the head (or, into the head).......if there is a step because of change in diameter, IMO it is better to smooth it out so as to avoid turbulence right where you don't need it. But then...........what do I know? AL
No, new header is 43 id, i feel 50mm is just too big for one of these motors with the limited RPM. Port size std is 35mm so a 4mm step both sides.
Unless your engine is producing monster power (NPI) at 43mm I think I would do as you have suggested and part-blend as mismatch unlikely to affect running that much (MOO) but would be interesting to hear what difference in tractability (not talking about performance increase) is/is not noticeable at very top end. I can't remember what approx average exhaust throat dimension is tolerable (for 450 cc?) but I expect you already have some figures. This thread could be interesting :smile:
Spike, always blend and smooth when it comes to gas flowing as what happens is that the gasses pass unimpeded. if you have a step the gasses become turbulent and can cause a stalling action which is where they shoot off in different directions to the one intended and trip up the main flow. Doesn't matter if that smoothing isn't a long transitional blend - this is down to tuning, but you should aim for no sharp edges as a rule of thumb, like Arquebus said.
Either arris (chamfer) the step and radius the 'corners' that are formed; or try radiusing the step....which will probably be more difficult to do, but less likely to weaken the pipe....As long as the sharp corners are gone and there are no flat bits facing the gases.... If the step is the other way around ie a smaller port than pipe, just smooth the corners.......because the gases will be blasting into a bigger hole....... .......but I bet it doesn't run well with that set-up..... ...the object is to get the gases away as fast as possible (you don't really want back pressure on a four stroke) but by sticking a larger diameter header onto the smaller port, the gases will instantly slow down the moment they leave the port. AL
talk about premonition - we are talking about the last part of the exhaust port as it meets the exhaust header here aren't we?..
current thinking seems to be port smaller than pipe with an anti reversion step at the top of the port regards Steve B
Sorry guys, i was away from the puter for a couple of hours. So Al, i say the same, what do i know? i have been cutting heads for many years now but theories change with tech changes. the theory was to D shape the base of the port and blend the top, but times do change. and of course the theories of reversion are not as clear cut on a desmo as the valve is open more for the same given time as a sprung engine, hence my question. Chris, tractability is not an issue, as neither is max torque, however where that peak torque occurs is important and is the basis for building this engine. as i honestly do not know the answer to what does really work with these two valve engines. as for figures i have some data for exhaust ports but again on motors with springs not desmo so i am in fresh waters for myself here. and yes we are talking about port to header, interestingly no one really goes big on the exhaust port or valve on these. (why?). duc904red, steve, your comment does mirror my first thoughts however i have to take into consideration the thoughts already posted as there are some clever folk on here! my question on your comment is if i were to leave a step how much of a step? the difference in the port to header at the mo is 35mm to a 43mm pipe and this is clearly incorrect, however if i do match directly to the 43mm pipe i then transition from the 39mm exhaust valve pocket (which size i have not looked at closely as i have not cut the port yet) this is quite a radical change in cross sectional area in such a short run???? Looking at later bikes they do use monster headers but with a transition in the size first. Some great answers guys, keep them coming. it is interesting that there are differing thoughts for such a small subject.
a right can of worms! vizard et al say stepped headers only work if something else is wrong l think there may be many answers to this as there are questions currently there seems to be a shift to smallerexhaust valves/ports ,used to be exhaust 80 percent of inlet ,now not so much empathis on this not sure how this affects port transition regards Steve B
depending on thread input will add some more later maybe - was hoping some professionals might chip in. EDIT Is this for 900cc or bigger spike?
This motor is based around 944cc je pistons 94mm, with more headwork than i usually do, i have plumped for a 45mm inlet on these and stuck with std 39ex worling on the .8 ratio. i am however using shorter rods to try and help it rev, this will alter the power on the crank angle hence my interest in shoving the torque peak up the range. As i said its all new to me at the mo, it may work or there may be a big bang, who knows. it will be on 41 fcr''s on modded manifolds and using a modified ignitech with a different trigger system fom the std pickups. This has gone on a while but i just let it drop for a while due to illness, i feel the results may make me more ill!
l beleive the 2v 900 is ultimately restricted by it's inlet ports so l think within reason you can do what ever seems ok to the exhaust big power 2v have major re working of the inlets ,welded re angled ports are you going that far? look at the heads done by CJS andMBP Brad Blacks site is worth a look too regards Steve
Steve, yes I have looked at the heads by these folk and with some parts I agree and others I disagree, I don't want to get into welding the ports as I think IMO its is not necessary if you cut the port slightly different. I have had good results previously with these but only on the long manifold motors, the short manifold motor throws many more problems into your lap so hence the questions to ask a broad spectrum of users, I think there is merit in an enlarged inlet valve but not the exhaust as has been said ideas have changed.
Ihave made a desicion, and cut the heads! First off i have given myself a chouce of step on no step by looking at a spare termig system i have, this uses adaptors that will match the port with a perfect blend if cut at 37mm, the sil however leaves a generous step. so until it is run i will not know the merits of either. i do feel the differences in pipe dia will affect things but i dont think by a great amount. A conundrum that came into conversation the other day was regards the step causing poor midrange, the conundrum being "whats midrange"? In theory if yo took a 1.8 litre car engine with the same config re pistons and rod sizes, this motor in a car would have a max of 6ooorpm so midrange would be 3k, with the ducati engine having the same spec and revving to 9500rpm the midrange is 4500rpm how can the same thing alter the midrange at two differing points? it cannot be down to cam timing as we are just talking about the step on the exhaust port. someone point me in the right direction please!
