ah yes, that thing!!
The question that springs to mind is..
..and yes why bother???
It won't work.
There's a simple explanation, which is called rudely..
...well, go buy a cross flow ford...at least you get a good reliable engine and easily up to 140bhp on a 1300, (more with a BDG or even a TC...) take it to bits and study the grossly over simple engine and wonder how it works so well.....with a sintered crank and rods and ridiculously heavy pistons?
Why do 4-1 manifolds grossly and annoyingly work on crossflows and NOT on Triumphs, and perhaps, also, why do crossflow cams give such universally lousy results (with just a few exceptions) on Triumphs when they work great on Crossflows?
The simple answer is,- The Triumph head doesn't work, burn, breathe or even remotely work like a X-flow Ford.
Of course loads of people stupidly like to try to increase the lift with those lousy roller rockers, lighten the valve gear like crazy, and who knows what else....!
I never did any of those things, didn't need to.
Never did those things either on F Fords so what exactly is supposed to be the point of 1.65:1 rockers on a Triumph?
For a start it grossly modifies the profile of the camshaft, and because it's a "rocker" does it in a completely NON linear way...
(A rocker always draws an arc, it's NOT a simpler lever like on the Pinto.
So you then have a totally unknown cam driving totally unknown or probably far from optimal acceleration in the valve train, and overlap which has nothing in common with the original cam design, and nothing in common with the flow characteristics of a crossflow Ford.
So, -Exit TH3, or whatever you name it, they're all cheapo Xflow Ford copies, some others I saw were from a Norton bike I believe....another fatal error...old brit bikes usually have hemi heads.....hemi heads have NOTHING in common with Triumph heads....and overlap on hemis is limited to prevent backflow....etc etc etc
Even on mild cams, a F Ford 1600 gives 100-120+bhp.
Even a reasonably WELL tuned 1500 Spitfire is struggling to get close to this, and the torque characteristics are simply dreadful in comparison despite the long stroke, tall block and nice long rods....
I mean when the Ford gets going from about 3000rpm, with a decent cam and head it goes like an ABSOLUTE rocket from 5000 on, as you would expect with monster valve area, (even with piddling little short rods and low down gudgeon pins), that is with even a "244" GOOD strong power up to 7500 with no worries about blowing up !!!!
What's the secret?
Even the pre-xflow 1500 Ford head (which is similar to the Triumph head in many ways) breathes a TON better than the Triumph one.
(I've seen some of those old Lotus7 heads doing 160bhp+ on a 1550 with 30mm (yes 30mm chokes!!)
They all have one thing in common, excellent breathing and the X-flow has superb valving and brilliant combustion.
So, here's the point and it's highly contraversial.
You can't get an exhaust manifold to resonate properly unless you get the engine to produce power and torque.
This is called the virtuous curve. When the engine breathes deeper, the exhaust resonates better, and hey presto the engine breathes even deeper...and so it goes on, more power, until the different bits reach equilibrium!
The only thing you can do with the Spitfire engine sensibly then is to try to get the head to breathe better (almost hopeless with the angle and shapes of the exhaust tract), and as the small valve area, and bad chamber shape, is the primary reason for the poor torque and low efficiency at medium to high revs, (shrouded as it is by the deep chamber), then extra power can only come from better valve shapes, and a shallower chamber.
This of course is what makes the 1300 work so much better, and produce good torque at 5000rpm. It's the same for the GT6 engine EXACTLY.
Increasing the lift above 0.440 doesn't appear to make any difference whatever, even with the shallow chamber, so you are stuck with a 4-2-1 manifold as the only measure to get back the lost torque above 6000rpm, and forget wasting the energy lifting the valves to 0.500+, it's NOT a mini engine and it doesn't work like one!