Crossover exhaust for a GTD

Eric Johnson · 06-22-08, 01:36 PM

Hi,

I have a late GTD and am considering a crossover bundle of snakes exhaust to replace to GTD sourced non crossover unit.

Why did GTD go this route with the exhaust system? Any clearance issues with a crossover system? The GTD units are very well executed and very nicely constructed, but I'm looking for THAT SOUND

Where can I get a nice ceramic coated crossover system in the US?

Thanks!

Mark Pickford · 06-22-08, 02:06 PM

I would ask Fran Hall, RCR.

Brian · 06-22-08, 04:48 PM

I got mine used from the UK, and that's pretty much how I built my whole car, used stuff from the UK. I would suppose at this point that the GTD club guys can source just about anything required if you can' get it over here. I have attached a pic of my crossover and there is plenty of clearance.

I know one thing though, the CAV exhaust will not fit as I purched a used one a while back and it contacted the rear clip.

Brian

Eric Johnson · 06-22-08, 05:51 PM

Brian,

Thanks. Scott Calabro and Tornado can help me out I believe, but thought if I can get some units here , it would be better with the exchange rate and all.

By the way your car is looking great. That baby really shines!

40 Bud · 06-23-08, 12:03 AM

For sur the CAV would have fitting problems. If you look at a CAV compared to a GTD there is a big difference in the height of the rear deck panel surrounding the carbs. It is at least an inch higher on the CAV. Tornado,GTD and RF should all be pretty similar.

Malcolm · 06-23-08, 04:09 AM

GTD did end up supplying a cross over system. Why they didn't do this from the start s a mystery but perhaps because it was a lot cheaper to make and supply. I think there was a price difference of about £400 between the two types of system. The GTD supplied cross over exhaust rifles the gases in each collector. The original 40's I beleive didn't do this. There is a distinctive different sound between a GTD non cross over system, a GTD cross over and non rifling cross over. You pay your money and takes your choice!

Martin Weigold · 06-26-08, 04:45 PM

Hi Malcolm

I am looking at having a new crossover system built for 6GTD when my Gurney Weslake engine arrives from Jay Cushman. Can you explain for the non-technical types like me, what you mean by a rifle crossover system, as I am keen for the original GT40 noise.

Thanks, Martin

Malcolm · 06-27-08, 04:42 AM

When a bullet is fired the barrel of the gun is rifled to spin the bullet for various reasons. The GTD design was meant to spiral the exhaust gases. To achieve this the crossover plays along with the firing order of the engine so that in each 4 to 1 collector the order of a pulse of exhaust gas arriving is in sequence, say starting at top right pipe 1st, bottom right 2nd, bottom left 3rd and then top left last. There are no double pulses going to one collector whilst the other stands idle for a pulse nor do the pulses jump diagonally across a collector ie top right 1st, bottom left 2nd etc as each of these situations would stop the spiral effect. Whether this is more efficient or does what it is hoped to do, I do not know but to a layman like me it sounds reasonable. Consistantly equal back pressures on each silencer and all that stuff.

With your new engine coming, from what I hear it will sound great whatever you choose to do! To really get the original 40 noise first things first, scrap your silencers! Adrian Newey got his pipes from a superb exhaust maker about 35 minutes from me (towards Heathrow), not cheap but fantastically made (out of inconel). If you need the new engine running in, just let me know.....

Martin Weigold · 06-27-08, 09:11 AM

Malcolm

I would be very interested to get the details of the guys who made Newey's exhaust. I am planning to have a set of straight pipes made as well as silencers. I am particulaly concerned about ensuring that it can get to <105dB with silencers, as my existing engine only just clears that threshold

Regards, Martin

Malcolm · 06-27-08, 12:18 PM

I will look up details and get them to you.

Cliff Beer · 06-28-08, 02:57 AM

Seems to me that the "rifling" notion depends on exactly equal length pipes (not easy to achieve). In other words, assuming equal flow rates across all pipes, if the pipes are not of equal length, then the timing of the arrival of pulse at the collectors would be off, and likely out-of-sequence, no?

Malcolm · 06-28-08, 04:30 AM

Interesting point Cliff. I would agree to some degree for sure if pipes were not identical length but compared to sequencing of pulses where no attempt is made to get a rifling order imposed on the gases it still seems a sound principle. In addition the issue is potentially overcome by getting individual pipe lengths so that the pulses arrive at the collector at the correct time. In theory this should be acheivable without exactly equal pipe lengths so long as the frequency of the pulses is considered so a shorter pipe is "correctly" shorter so that a pulse within it arrives at the right time after the pulse before to continue the rifling objective. The back pressure would be different of course in this scenario I guess. I hope that makes some kind of sense!

However I would be very surprised indeed if GTD actually paid attention to anything other than the concept of trying to get close to equal pipe lengths and then added rifling on top. Without computer simulations (I saw some F1 simulations at Ricardo Engineering a while back with the GTD club visit) to acheive perfection is nigh on impossible. At our level of driving we (being Joe Average owner) most likely would only appreciate a sound difference between systems as opposed to a noticeable road driving power difference. But it is fun to try and acheive more where we can.