as.racing wrote:olefud wrote:I’m not sure there is an issue. Trumpets, I would guess, are primarily to fix the distance from the intake valve to the intake opening for pressure wave tuning. Thus if the manifold runners are longer then you’ll need only shorter trumpets. Of course if your manifold has a plenum rather than individual runners it won’t make any difference since the tuned length will be to the plenum.
If you’re just trying to smoothly feed air relatively short parabolic trumpets should work just fine.
My thinking is though, that a longer manifold will give the fuel more time to mix, hence the placement of the injectors ....
agreed, in principle the aim should be the optimal overall inlet system length for charge(ie length pre-trumpet is tradeable with trumpet length)
but with carbs the reverse airflow inevitable with a sporty cam at lower rpm will be carburetted
unless this air/fuel mix is contained for re-ingestion (ie by use of a suitably long trumpet) consistent carburation is impossible
when British motorcycles (all pushrod valves) went sporty in the late 50s trumpets were introduced for this reason
one had 135 deg of valve overlap and the flow reversion was conspicuous
presumably the Japanese carb mouths and airbox/plenum and ohc have together dealt with this (is this all transferrable ?)
I guess the carbs will have complex mouths unsuited to adding trumpets ?
mixing time is good for economy and emissions, but not necessarily good for the track
ideally for power the cylinder should ingest fuel as tiny droplets (ie not vapourised)
any vapourisation before valve closure displaces some induction air, 'stealing' some of your 1900 cc (this is partly why DI is good)
non-direct injection will often give less max power than good carbs for this reason