intake trumpet length on 400

[bone]

hi,i have a 400 chimaera,can any one advise me on the measurement to be taken off the bell mouth trumpets once they have been removed from their base as i have been told different figuers some say all 8 should be shortened by the same amount, some say centre 4 different to outer 4,thanks for any advice.

[bone]

@

[Pupp]

Been a while since I did this but think they are two different lengths as stock once out of the base so you'd preserve any differential by taking equal amounts off all (same applies if they are all the same of course). Although they sit at different heights in the plenum, the overall runner lengths to the valves are similar or the same with the trumpets in situ as the tracts in the manifold vary. Worth a search as there was a load of material discussing this although it might have dropped out of the search range by now

[Blues]

You could try a PM to TVRJock. He has done this recently with astonishing results

[TVR-Boy]

I've never done this myself, but I have read up alot. I believe that they should all be equally shortened between 15mm & 25mm, i.e. all of them the same amount, up to you how much.

Thinking logically, if you buy the thicker plenium spacer which raises the plenium a few mm's, then the increased gap between the plenium & top of all the trumpets would be equal.

[bone]

thanks for the advice everyone,let you know how it goes.

[chim]

Hi
Check this site.
http://www.g33.co.uk/tuning.htm

Chim

[BIGRAH]

I took 25mm off all of mine and added the spacer, runs great !

[CHGRIFF]

SC Power offer Alloy Bell mouths with information to shorten the trumpets based upon the installed cam. www.sc-power.co.uk

[blitzracing]

Although you can get technical about tuned lienghts on the inlet tract and the effect on the peak torque area of the powerband, I dont believe it changes things much if it where 15mm or 25mm . What it does do is unshroud the top of the trumpets that are a bit close to the top of the plenum, so it reduces the slight air restriction in the 5400 -6200 rpm range that the origional Range Rover set up does not normally reach and 15mm will do this. Id take the same amount of each trumpet, to keep the inlet tracts the same lienght. You can glue them back in with JB weld.

Mark

[spend]

The trumpet lengths are trimmed to accommodate the gap between the roof of the plenum (which is domed) I believe. The RV8 (1,2,7&8) cross-over inlet manifold runner lengths vary far more than you can adjust by shortening/lengthening trumpets IMO.

Much easier to remove the plenum when the trumpets are shortened (or even removed) though

I am a little surprised that you haven't considered the uneven running caused by the inlet manifold design Mark. This combined with the batched injection makes a complete joke of cc'ing heads IMHO.

Daves' Grandmother

[daz the plumber]

why not go for a blended plenum , don`t think thee that expensive

[blitzracing]

spend said:

The trumpet lengths are trimmed to accommodate the gap between the roof of the plenum (which is domed) I believe. The RV8 (1,2,7&8) cross-over inlet manifold runner lengths vary far more than you can adjust by shortening/lengthening trumpets IMO.

Much easier to remove the plenum when the trumpets are shortened (or even removed) though

I am a little surprised that you haven't considered the uneven running caused by the inlet manifold design Mark. This combined with the batched injection makes a complete joke of cc'ing heads IMHO.

Daves' Grandmother

Now you've got me thinking, Id always assumed the trumpet lengths where different to compensate for the inlet manifold tract length being different on the front and rear cylinders, and Ive never regarded this as more than an easy job that might release an extra hp or two. I would not mix different lengths due to the alteration of peak torque points on different cylinders, and that would be rough running. I also remember in the days of fitting Webers, a minimum ratio of the inlet trumpet diameter to the distance to the airfliter backplate to prevent restriction, much the same as the top of the plenum. Ill see if I can dig it out. One thing I do find odd, is the lambda outputs between left and right bank is different in fixed map mode, so there have to be some odd physical differences somewhere.

Mark

[GreenV8S]

blitzracing said:

Now you've got me thinking, Id always assumed the trumpet lengths where different to compensate for the inlet manifold tract length being different on the front and rear cylinders, and Ive never regarded this as more than an easy job that might release an extra hp or two. I would not mix different lengths due to the alteration of peak torque points on different cylinders, and that would be rough running. I also remember in the days of fitting Webers, a minimum ratio of the inlet trumpet diameter to the distance to the airfliter backplate to prevent restriction, much the same as the top of the plenum. Ill see if I can dig it out. One thing I do find odd, is the lambda outputs between left and right bank is different in fixed map mode, so there have to be some odd physical differences somewhere.

Mark

I too believe the different trumpet heights are to compensate for the different runner lengths in the manifold. I don't know whether the plenum top is actually shaped to provide equal clearance over each trumpet (it doesn't seem curved enough to me to achieve that) but if so I would suspect the plenum was shaped to fit the trumpet heights not the other way round.

I don't think that making all the trumpet heights the same is going to cause or cure any significant problems. We already have massively unequal exhaust primary lengths, there is a lot of interference between adjacent trumpets in the plenum, and the whole thing is a long way from perfect flow distribution. This is not a highly tuned intake that is going to be thrown out of balance by having one intake a few percent longer than another.

Edited by GreenV8S on Friday 5th September 12:26

[blitzracing]

Ill second that!

Mark

[spend]

If you are interested, I measured my inlet manifold, c/h flange to trumpet base flange along the upper and lower inner wall as follows:

Cyl..Upper...Lower
1.....180.....200
2.....160.....180
3.....130.....145
4.....130.....145
5.....130.....145
6.....130.....145
7.....160.....180
8.....180.....200

Its ported a bit differently, but the lengths should be reasonably representative. The trumpet base is an additional @40mm plus the thickness of an insulator if you use one.. Note that its not just the extra length, but also the extra convoluted curves that add to drag in the outer ports (partic 1&8 but also 2&7) to allow them to cross each other.

I can't see that the minor difference in trumpet length would alleviate this imbalance, and as stated previously I am not convinced that the inlet manifold design lends itself to a smooth running engine with balanced air/fuel into each pot? Trimming trumpet lengths just seems to be a P in the ocean, trying to incrementally open up the restricted/longer ports would seem more viable IMHO.

[Pasco]

spend said:

The trumpet lengths are trimmed to accommodate the gap between the roof of the plenum (which is domed) I believe. The RV8 (1,2,7&8) cross-over inlet manifold runner lengths vary far more than you can adjust by shortening/lengthening trumpets IMO.

Much easier to remove the plenum when the trumpets are shortened (or even removed) though

I am a little surprised that you haven't considered the uneven running caused by the inlet manifold design Mark. This combined with the batched injection makes a complete joke of cc'ing heads IMHO.

Daves' Grandmother

I dedicate my 5000 post to the knowledge of Spendy Top Bloke

Cheers buddy for your Help... It is much appreciated Dave thanks mate