Triumph Vitesse GT6 6-3-1 Manifold & System

[Mark B]

Hi all

After making a 6-3-1 manifold and system for my Vitesse from hand bent lengths of tube, I am starting on another for my GT6 that will eventually get a 2.5 fitted. I originally intended to make three whilst I was in the swing of making the first a year or so ago but got sidetracked on other projects. The first system has equal length primaries and long secondaries joining up to a merge collector then a single back box. One of the difficulties, along with many others, is getting the round tube squared off, inserting into the flange in exactly the correct position at the correct angle. A couple of mm out at the head means 10+mm out at the end of the primary where it's intersecting with the other pipe. Trying to keep primary lengths the same, missing all the obstacles and getting them to thread between the chassis and sump is challenging.

After I finish the one for the Gt6 I was going to try a slightly larger pipe for the primary and was considering keeping it round and welding it to the outside of the flange instead of squaring it off and inserting it in the flange. 

I can't think why it should have any effect on flow.

What's your thoughts?

Below is the first system fitted to My Vitesse.

Mark

[JohnD]

That is impressive, Mark!    You're most welcome to the board, and with skills like this, doubly so!

I can't offer an opinion, but know that many manifold builders include an  'anti-reversion step' in the design.    If your primaries were round right up the the flange, then this might have a similar effect, especially if the ID was more than the diagonal of the rectangular port.   It's not even a subject that a flow bench can illuminate, as it's dependent on the pulsed nature of exhaust emission.   Lots of hits on Google about "Anti-reversion step" but none if you add GT6, so you're on your own!

No less contentious is the absolute length of primary.     GT recommends 17" (0.43meters), but other work would indicate that this will optimise as 6000rpm, so shorter will be better.  See pic.

John

 

[Algy]

Hi,

Looks like you have made a good job.

On the point of square to round. A sudden change in cross section will cause a distinct slowing of the exhaust gas at that point, this will seriously affect the exhaust scavenge that you have worked so hard to improve by equal length primaries.

I match my exhaust manifolds to the head on all the performance engines I work on. Some inlet manifolds can benefit in having a small step of 0.5mm evenly around them at the head interface to help tabulate the atomised fuel to encourage vaporisation. (Triumph 6 cylinders are one.)

[Mark B]

Thanks for the replies. Something else i noticed. I had some exhaust flanges cut out of 10mm mild steel, and made sure that the drawing was as accurate as I could. The position of the stud hole is crucial as it positions the flange over the exhaust port. When offering up the flanges to different heads and different ports on the same head, there is a deffinate descrepecy in how it marries up to the port or not, so either my drawing was off or there is variations in the castings, core shift or location of the stud, not sure.  The closer I look the more different the the shape of the exhaust ports appear to me, when compared to each other.

As the engine I am using is just being used as a jig, I will just tweak the flanges to suit, so should then fit any straight six engine. When  I decide on the head I will be using on yhe 2.5 engine I will grind out the ports to  match the flanges on the next manifold.

Mark

[Algy]

These engines come from the 60’s, well before CAM and so the heads are all different.

Most tubular exhaust manifolds are made with the ports with a slightly larger cross section area, they really work when the head is being ported, that way the head ports can be matched to the manifold.

I use Perspex to do the matching (see the two attached photos).

I first pick up the studs on the Perspex and then use those to align the Perspex on the manifold and use a rotary tool to get the patter off the manifold.

I then transfer this to the head and mark it out as you can see in the second photo.

You would need to use a suitable rotary grinding tool to then dress the head back as smoothly as you can back into the port at least as far as the valve guide. It doesn’t have to be polished just fairly smooth.

Of course, you would do this work on the head with it removed and with the valves out. Without this work more than half the work you have done will be lost by the bad match on the ports and the gas flow in the port.

[Mark B]

Thanks for that, and posting the photos, you have confirmed what I suspected,  and  given me the insentive to modify the 2.5 head ports to the flanges on the next manifold.  For this manifold i have tweaked the inside of the flanges, it's only a mm or two on a couple of ports to allow for the squared off primary as it inserts into the flange to align with the port.

Mark

 

[Mark B]

Hi all

Got side tracked, but over the past few months I've been finishing off the manifold I started a while back.

John sparked my interest when he mentioned Anti-reversion. I had vaguely heard about this in the past so did a little resesrch.  I decided to have a go, and add an anti-reversion chamber in the system. I had seen that some add a single chamber after the collector, where others had been added to each primary. I thought I would add one to each primary. I intend to fuel inject and use a different cam, and thought antireversion chambers would be beneficial.

The manifold was virtually finished when I decided to cut up the primaries and add anti-reversion chambers. This ended up being a remake, and not just a matter of splicing in the chambers as initially thought. Additional clearance was needed which threw out the spacing and angles. 

Reversion chambers have different size ends for the outside and inside diameter of the primaries, and hopefully shaped in a way to catch and trap any exhaust gases going the wrong way.

I need to clean up the welds, and add a pipe to the collector.

Not sure how effective this is going to be and won't know for a year or so as I am planning on using the Vitesse as much as possible this year.

Mark

 

[JohnD]

Mark,

Your second pic is difficult to interpret, from angle but mainly because there are FOUR outlets!

A 6-3-4 manifold?!?

JOhn

[Mark B]

Hi John

No it's a 6-3-1. The chambers on 1 & 6 are further up the primaries.

Mark