down on power

down on power

Author
Discussion

wedged up

Original Poster:

353 posts

93 months

Tuesday 25th April 2017
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I just changed my plenum and fitted the carbon trumpets along with some water hoses . the car is not starting as well as it used to and seems down on power . Any ideas ?

ElvisWedgeman

2,714 posts

164 months

Tuesday 25th April 2017
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Air leak?

Tony. TCB.

mrzigazaga

18,534 posts

164 months

Tuesday 25th April 2017
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mmmm...Could be an air leak....Unless due to the shorter trumpets there is more air so a tad more fuel pressure might be needed...scratchchin

ed_crouch

1,169 posts

241 months

Friday 28th April 2017
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If the new trumpets are shorter, you'll lose torque but should gain some top end, assuming the valve gear is taught and light enough and the heads are capable of flowing the extra volume without choking.

It's a function of the vibro-acoustics of the plenum system. If you lengthen the inlet runner, the dynamic pressure wave caused by stagnation pressure from the inlet valve closing on the flow last time round bounces back up the runner, off the plenum top and back down the runner again, and that process takes longer the longer the runner since acoustic velocity is reasonably constant in there. Maximum VE is achieved by having the inlet valve open whilst that pressure wave shoves more air in. Cam and inlet timing is all about matching things that take a set number of crank degrees to occur with those that take a set number of milliseconds to happen.

My old GSX400T (bike) ran like crap without the air box, better with it on.

Consider plenum mods carefully...

You'll get it to work, but you might need a session on the rollers and to tweak other things so you can get the top end.

If your new trumpets are longer, no head flowing required AFAIK, but the potentially increased VE at low RPM would need more fuel. I'd stay away from just winding the reg up as that will give you more fuel all over the rev range, and you want to adjust the SHAPE of the fuel curve, not just move it up and down.


Wedg1e

26,760 posts

264 months

Friday 28th April 2017
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GSX400T, that takes me back 😁
B667MHN was mine...

ed_crouch

1,169 posts

241 months

Sunday 30th April 2017
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Good old bike actually. At about 6kRPM it transitions from a plucky sort of bugger-bugger-bugger-bugger noise and starts howling all the way to 10k. 45bhp is plenty with those brakes and that suspension!!! It's kinda golf GTI fast off the line.

Before I replaced the ruined airbox-to-carb trumpet seals which were leaking like seives, 4-6k used to be a black hole of torqueless woe.