Gilly: the idea of changing trumpets on the conditions is a sound idea, however you may find that the engine may well need a retune between changes. However if you have an ecu that accepts twin maps then your well ahead and the power is there for your taking. You could also tune it for some better fuel while your at it too
As for the taper of inlet trupets I am still looking into it, I know it’s a very complex thing and the subject of countless man hours on a dyno for race teams. The theory is that it speeds the air up with a taped inlet, I don’t know myself.
The inlet opening is a little more clear-cut though, a pipe with a flared end will flow 30% more than just a straight pipe or just an opening. This is why anyone who just takes the trumpets off and slaps an airbox on is doing a very bad thing. Going from a flared end to a full curled bellmouth gains another 3% of flow.
The debate is this: have a full bellhouth on the inside of an airbox or have a curved taped inlet in the side wall of an airbox. I have heard various opinions on this and I am staying out of it.
Drift-pig: Brendon what you can is make a set of trumpets that are adjustable without too much effort, well that is assuming my idea would work! What you do is get a flange plate for the throttles (chopped out from the stock airbox is great). Then weld some 2 inch exhaust pipe stubs on it about 20mm long. That will let you slide a piece of stiff rubber hose over each and clamp on, then at the other end have a set of bell-mouths with a short pipe on each at that end again to let you slide a piece of rubber over. Chop some 2 inch rubber pipe to the longest length that you can practially have the trumpets, do this again till you have 4 equal size pieces. Now what you can do is have the rubber lengths acting as a pipe between the flange and the bellmouths. Adjustments can easly be made from here by chopping pieces off at a time.
Oly ae86: assuming that your talking about a 20v then the tuned rpm for your two setups are as follows:
20mm: 9200rpm
70mm: 7500rpm
did you find that you lost any top end with the longer trumpets? because you shouldnt of
Riojin: I don’t think a modified 7afe would like short trumpets, simply because unless some costly internal work is done then you wont be able to rev it hard enough reliably to make use of it. From what I hear peak power with a 4age head is at around 5500rpm and I can only assume that with a high modded 7afe head it would be the same with mild cams. It would be different with more aggresive cams but then theres the rods and crank issue again. For me personally with that engine I would have a resonance at 5000rpm or so.
What you talk about with staggered trumpets its somewhat interesting so let me explain what I make of it: First up the theory is great, instead of a single point in the rev range that all the pipes work in resonance it is spread over the rev range as the shorter pipes resonate at the higher revs and the longer ones at lower revs. Reality is very different, on every engine I have seen with multiple cylinders the load metering with the ecu is done over the whole engine and that’s where the problem is. You see if at for example the longest trumpets start resonating at 4000rpm and the rest are not because the are shorter then it means that those two cylinders are getting more air in them that the rest. This does sound so bad till you consider the way the ecu works. It just measures and works out a total load of an engine, every ecu that I know of assumes that every cylinder of an engine is equal and if it isn’t like out example earlier it would get an average reading of all the air going in. So the cylinders that are sucking a this-proportionate amount of air less compared to the longer trumpets will run richer. Again not so bad but the pipes that are flowing better than the rest will run lean and then is very bad news indeed. So yeah unless it had carbies which may work its just for looks, make sense?