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Engine Twist On Downshift 

 

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 Post subject: Engine Twist On Downshift
Posted: Thu May 13, 2010 11:19 am 
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OK, I should give a little backround so you know what I'm rambling on about.

In September last year I had the extractors fitted. It drove around in forward gear fine but in reverse the left extractor would bang on either the sway bar/crossmember if you gave it anymore than 5% throttle. This obviously because there is a opposite force on the motor causing it to twist in the opposite direction to what it will in forward gear. The exhaust guy told me it's because my mounts had sagged and to have them replaced.

When I initially had them replaced they lifted the motor a good 1-2" further up and all was good. It wasn't until 7000km later that my brand new mount snapped and I had learnt the reason that it had lifted it up so high(and broken) is that the useless mechanic actually put the incorrect mounts in (6cyl mounts). Needless to say I now have brand new V8 mounts in and all is good except it is banging in reverse like it was originally (which would imply there was nothing wrong with the original mounts in the first place, but thats another story).

What I am getting to is I am collecting parts for a Manual conversion and I was thinking, will downshifting/engine braking replicate the same opposite force that driving in reverse does, and therefor cause the extractor to bang?

I can deal with it banging in reverse since it only does it if you give it throttle and besides, who drives around in reverse. However if it is going to continually bang during downshifts it's going to cause me no-end of headaches.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 13, 2010 3:36 pm 
Getting Side Ways
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Sounds like your Extractor pipes run out too far. What brand extractors? The pipes should clear everything in the engine bay by a fair whack.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 13, 2010 4:11 pm 
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Does it do it when you downshift in the auto? It's the same type of load...

But yes, as above - there should be a tonne of clearance, might be worth fixing the issue rather than working around it.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Mon May 17, 2010 1:03 pm 
Tyre Shredder
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Thanks for the responses.

s-tranzor wrote:
Sounds like your Extractor pipes run out too far. What brand extractors? The pipes should clear everything in the engine bay by a fair whack.


As per signature they are Pacemaker 4001's (Tri-Y's).

Your probably right, for the money I paid I guess I should expect it to work properly. I'll get the exhaust guy to deal with it. I'm guessing it a production fault/defect with the pipe.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Mon May 17, 2010 1:12 pm 
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and yes, if it touches in reverse with the auto it will touch on down shift in the manual

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 20, 2010 4:58 pm 
Getting Side Ways
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Hang on - I could be wrong but your basic idea seems wrong to me - you're saying that the torque reaction between reverse and first is opposite - maybe my brains on hold but whether the car's in reverse or drive the engines turning in exactly the same direction !

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 20, 2010 5:46 pm 
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It is to do with what direction the tailshaft load is.
Forward it turns 1 way.
Reverse it turns the other way.
Same forces under power and decel.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 20, 2010 5:48 pm 
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engine revolves the same way but torque angle reverses from driveshaft back through the power train. Now I think about it, if there is enough reverse torque through the powertrain to result in this I'd be checking gearbox mounting aswell as engine mounts.

 

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 Post subject: Re: Engine Twist On Downshift
Posted: Thu May 20, 2010 6:19 pm 
Getting Side Ways
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yeh, had similar thoughts - I mean i know that obviously the tailshaft is going in opposite direction between reverse and fwd but I discounted much of a torque effect - if there was a dodgy engine or g box mount might be different...

 

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