Swapped from ITB trumpets to a box, now my revs drop at 4000rpm and then climb again?

[WHITCHY]

I just bought a box for my quads to replace my trumpets so it doesn't look so obvious to mr plod if I get pulled over, but now when I have my foot flat to the floor as soon as the revs hit 4k they go backwards for a few hundred rpm and then continue climbing again?
Can anyone tell me what the hell is happening and why? Is there any way of fixing it? I really like the sound it is making now and it looks alot neater in my engine bay, so I would like to keep the box!
Thanks in advance for any help and suggestions!
Cheers.
-Whitchy-

[Hiro]

I'd be guessing that you've created a hole in the torque curve, probably due to Helmholtz resonance in the intake box. This always exists, the trick is to get it out of the usable rev-range. For that, I can't help

[abently]

Make a cover for the Trumpets so that they remain un-sited but still perform close to normal. I.e a Rectangular Box with the bottom removed.

// Or just paint the Trumpets black.

[The Real Roadrunner]

have you had the car tuned by a profesional tuner yet?
changing lengths of ram tubes/trumpets can have a big affect on tune/performance, and putting a box over it can make it worse if you dont have the right standoff length. this should be equal to the od of the trumpets your using or more if it will fit.
cheers
linden

[Enchanter]

I agree with all the above ideas regarding resonance tuning etc.

If you keep the trumpets on (which I would recomend) and there is enough space around the trumpets within the box then you should be fine. If you cant have both the trumpets and the box then i would say ditch the box for one that does fit them.

[DaveW]

have you had the car tuned by a profesional tuner yet?
changing lengths of ram tubes/trumpets can have a big affect on tune/performance, and putting a box over it can make it worse if you dont have the right standoff length. this should be equal to the od of the trumpets your using or more if it will fit.
cheers
linden

what is standoff? im debating over a box or the trumpets i have at the moment... does standoff refer to the distance between the tip of the trumpet and the oppopsing face of the box?




if that makes any sense....

[The Real Roadrunner]

what is standoff? im debating over a box or the trumpets i have at the moment... does standoff refer to the distance between the tip of the trumpet and the oppopsing face of the box?
if that makes any sense....

that is correct.

[ReQuieM]

OD of the trumpet being the OD of the flaired end or the tube? as in a 45mm trumpet has a 88mm flair, 45 or 88? Preffernce would be 88+ I understand but will 45-88 be ok i guess is what I'm asking?

Also whould the effect be as noticable on a forced induction engine? OB the airbox being a plenum instead!

[celicapain]

nope. boosted engines dont care as much as n/a engines.

[Enchanter]

With a boosted engine I would tune the intakes for off boost effectivness because as celicapain has stated on boost it wont really do much.

[ReQuieM]

Yeah I was thinking of running as long runners as I could nicely fit but was kind of interested in the gap from end of trumpet to top of plenum chamber. IIRC the plenum roof reflects the puls yeah? or is it the same as an open trumpt that has the pulse reflected from the top of the trumpt.

The other thing I was thinking about was the best trumpt design for flow. There is a chart kickin around that shows a trumpt that bends all the way round to touch the wall behind it promoted the best flow. some thing else I was thinking aobut. could be of use to the NA's as well.

Sorry for the OT!

[Enchanter]

I would think the only reason the longer bent trumpet worked so well was that it was the correct length for the harmonics rather than the fact it was bent. Id say a straight trumpet/runner is the best if space allows.