Let's take a look at the factory exhaust parts first. Below is the original V6 downpipe set - N/A cars were first off the production line. Problem with a transverse Vee engine is the pipework from the rear cylinders. With the 2GR rotated downwards/rearwards, that downpipe runs under the engine/transmission unit and then has to do a 180deg turn to merge with the front downpipe. You really need the front and rear cylinders to have the same pipework lengths/back-pressure to deal with. BUT, this early set had a difference of 9" (228mm) between rear and front downpipe lengths. Not good.
Lotus/Janspeed soon wised up though and they managed to pick up some extra mm's on the front downpipe so that both cylinder banks had the same exhaust back-pressure to deal with.
At 51mm diameter (2") it's not ideal for a 3.5Ltr N/A V6 putting out some 240bhp. For a 350bhp (or more) Supercharged install, it's hopeless! Look at that disaster of a "merge" collector. Hold the two downpipes together, fill in the gap with some squirt-gun welding (MIG), then use a cheap pressing to cover up your crimes. I've seen leaks from there several times.
If you're looking for more power, you replace that mess with the 2bular 54mm diameter design below. I took the time to build a little cutting guide for the 2-1 merge - below.
For a SUPERCHARGED 3.5Ltr V6 you need a 57mm bore for your downpipe set. Like the one below! Only 2bular offer these diameters . Only 2bular differentiate between N/A exhausts and supercharged exhausts.
Again, I took the time to build a cutting guide for the bigger 2-1 merge for the supercharged installs. Moving up two bore sizes on the factory pipe needs superior pipe-bending and tooling. You need a nice round, smooth radius to keep the gas-speed high and turbulence free. No other manufacturer in the Lotus aftermarket builds anything like the 57mm downpipe set shown below. Everything sealed using flanges and gaskets and a 76mm diameter flexy section to cope with heat expansion and engine movement. NOTE! - you NEVER use a hanger bracket immediately after a flexy section - that is a stupid thing to do.
Looking around, all I see are squashed bits of tin for merges, wrong diameters and lengths on tight-radius bends all giving different back-pressures between front and rear cylinder heads. A prime example, shown below, being the BOE downpipe set for their miraculous V6 header which works with the 270bhp N/A motor AND the 400bhp supercharged install. Apart from using too small a bore, Mr Verhaege's fabricator built an extremely tight 180deg bend into the rear downpipe. Compound that disaster by using different lengths to those pipes and finish them in a 2.5"diameter pipe to a 2.5" system. That bore is ok for an N/A motor - no use for a 3.5Ltr 400bhp supercharged V6 though. At least he ditched the stupid springs holding it all together. Still using a 600cpsi ceramic catalyser though - but that's because he doesn't want to spend money on a quality HJS HD catalyser.
One misguided individual even had that downpipe expensively wrapped - just to emphasise a fool and his money are soon parted
Tight, Tight TIGHT! Strangle me why don't you?!
I was intrigued to see the latest 60mm Exige 380/Evora 400 downpipe design. Alongside the 57mm 2bular design. Their long flexy with accompanying hanger bracket isn't the best idea - too much flex and nowhere to go. 2bular short flexy and no bracket - perfect. Just enough flex and no restriction on that flex!
Squashed tin and a leaking squirt-gun weld anyone?
Bore increased from 51mm to 60mm! The final bend into the system and the system itself (when the exhaust valve is "open") up to 76mm. Why? Each new V6 model that appears apparently has a power increase to accompany the nomenclature. Thus an Evora 400 has a claimed 400bhp and the Exige GT430 a claimed 430bhp. Those nasty little UTC's in the factory log-manifolds are under more and more stress. But Lotus can't remove them. Solution? Reduce the "back-pressure" in the previous smaller-bore downpipes and 63.5mm system. Unfortunately, you lose a lot of boost when you go up several sizes in exhaust bore - hence a power loss when the 76mm exhaust valve is opened.
But an even bigger problem was the NOISE issue. Those small-bore downpipes and their 2-1 merge collector were GREAT at noise-cancelling. I was amazed at being able to run a 5"diameter system on an Evora S during R&D testing. No way could I do that on the 4-cylinder 2ZZ installs! However, the 60mm downpipes and a truly abysmal T-junction for a collector plus the 76mm system on the Evora/Exige 4** cars meant noise-cancellation wasn't happening any more. The EU drive-by test became a serious problem. Solution? On the early cars the by-pass pipe on the exhaust valve was 54mm diameter. On the later cars with big-bore-everything, they had to fit a smaller, 48mm by-pass pipe to pass that drive-by test! Think about that for a minute (but no longer than that). A 3.5Ltr supercharged V6 is breathing through a pipe diameter that I last used over 30 years ago on a Morris Minor.
Consequences - there's always consequences. When I tested my 70mm bore Track system against the 76mm factory system, the GAIN was 5bhp. Not much but it proved that the smaller-bore increased the boost and didn't lose power. More gains would have been realised once the ECU adjusted for that increased boost but it was just a snap-shot test to prove that 70mm was a better bore than 76mm.
If you're looking for maximum power from the supercharged V6 2GR motor, you need the combination of 2bular 3-1, 48mm stepped manifolds (headers!), into 57mm downpipes then into a 70mm exhaust system. Anything else is wasting your hard-earned cash!
It absolutely baffles me why an Evora/Exige 4** series owner would fit a restrictive(quiet) exhaust system which drops power by at least 20bhp and re-introduces the dangerous back-pressure.
I know UK track noise-limits are strict but strangling your motor to help meet them is just insane.
Having mentioned manifolds (headers!) let's take a look at the factory offerings.
Not the best for performance - despite a TLF forum moderator's claims. Lotus HAVE to fit these manifolds but I bet they'd rather not. Never listen to a forum moderator - they know nothing about everything. Especially when they preface a statement with "the factory told me" - then you KNOW they're an imbecile. That's you 'Bravo 73'.
Dyno graph above from Komo-Tec showing gains on a UK Exige 380 after fitting the 2bular EPK (Engine Protection Kit). You REALLY have to get rid of those restrictive factory manifolds! Look at that mountain of TORQUE - power is as anticipated - but the torque comes from the 2bular stepped manifolds (headers). I'm surprised no-one has copied my designs yet - oh wait! ;-)