What Inlet Length?

[alync406 4]

I have just got some throttle bodies made up for my 1.6 gti.They are motorbike throttle bodies.I was wondering what inlet length would be best/which length of trumpets I will need?.I have adapters to suit jenvey trumpets 42mm id.The bodies and the inlet together are about 200mm long.I am hoping to have good torque,should I go for the longest trumpets possible? Also where is the best place to buy trumpets?

[welshpug 1,661]

how long to the valve? 200mm sounds a bit short, I think Sandy works towards 350mm+ depending on the engine tune.

[alync406 4]

Well it is 200mm to the mounting face to the manifold so not much more to the valve.I would need very long trumpets then to get 350mm + ,would there even be room for that length?

Here is some pics of the bodies,they are definatly about 200mm long - the bodies themselves are fairly short.

And the adapters to suit jenvey trumpets.

 

I dont know if there are any trumpets long enough to get 350mm+,the longest jenvey ones I seen are 100mm and they are carbon fibre and very expensive.Any ideas?

 

Thanks,

Anthony

[DrSarty 90 1 Cars]

As Welshpug mentioned, Sandy recommends about 320mm - from outer lip of trumpet to back of valve - as a starting point. In fact I seem to remember him saying 327mm.

 

And *warning*: silicon joiners on bodies, even if they are the proper bike body ones are not recommended. I say this from experience and others will confirm, that the bodies go out of balance easily.

 

They work on bikes because everything's OE and made to perfection. But custom manifolds, unless expertly built won't typically have the correct flanges and lips for the joiners to mount to, and also might not be as accurately spaced.

 

A full, metal and mechanically joined solution is best.

 

I hope it all goes well for you though.

[alync406 4]

I have been doing a good bit of searching and I can only really find info on 16 valve engines.I have seen Sandy saying that he has used inlet tract lengths from something like 350-430mm with good results. I have seen somewhere that on the 8 valve it is 95mm to the valve,so if I fit 100mm trumpets I will get 395mm,would this be the best way to go if the trumpets will fit in or does the tract length not have to be as long on a 8 valve?

 

@DrSarty I know what you are saying about the silicon joiners I would rather that they were all metal but I am going to see how I get on for now anyway,Hopefully they work OK anyways.

[Cameron 16]

It depends where you want your peak power to be, the shorter you make your inlet the higher the peak power RPM will be. I'm doing a similar thing to you and will be using spacers to adjust the distance from bellmouth to valve from 350mm to 385mm or thereabouts, so that it can be tuned on the rollers. I'm aiming for peak power at 6750rpm, which (if I remember it right..) gives a trumpet length of about 375mm.

[Baz 421]

It depends where you want your peak power to be, the shorter you make your inlet the higher the peak power RPM will be. I'm doing a similar thing to you and will be using spacers to adjust the distance from bellmouth to valve from 350mm to 385mm or thereabouts, so that it can be tuned on the rollers. I'm aiming for peak power at 6750rpm, which (if I remember it right..) gives a trumpet length of about 375mm.

 

For a 16v though?

[alync406 4]

For a 16v though?

 

 

I would be expecting it would be different for a 16v to a 8v?

I want to have good midrange torque,that is the most important for me really,peak power at approx 6500rpm or so would be ok I wont be revving it past 7500rpm anyway.

[Cameron 16]

Doesn't make any difference how many valves you have, just depends on where you want your peak power to be. I think 6250rpm (probably more suited to an 8V) was about 390mm, don't have the numbers with me though so I can't tell you for sure. But yeah 6750rpm peak power was for a 16V that likes to rev higher if that's what you meant.