Exhaust power paradox
Discussion
One for the engineers. Simon will probably know.
My Exige S was a standard touring pack car. Not a particularly strong engine, just an average one. It felt wheezy and out of breath at high revs, but presumably delivered the 210 bhp or whatever they're meant to make.
I've had a 2bular exhaust fitted, no other powertrain mods. It's the biggest one Jim makes, to keep noise as low as possible to keep me on track days. The car now feels a hell of a lot more eager, the wheeze and 'strain' at high revs is gone - it just wants to spin all the way round, and seat-of-the-pants feels significantly faster. Not just me, but Parky @ Lipscomb had the same conclusion - he said it felt as quick as his 240 PP car, and he's got a lot more experience than me
Now I know that exhausts on the Toyota Lotuses aren't meant to make much difference to the power. But worse than this, on a supercharged engine, Jim's style of exhaust surely would *reduce* power? The Toyota engine switches cams at high revs, as we all know. On the nasp engines, a high-lift, long duration and high-overlap cam gives nice loads of power at high revs (but is undriveable at low revs because of the overlap, hence the mild 'slow speed' cam). The 2bular zorst is a free-flow design (no baffles) with a big can full of sound absorbing mat. This should surely have much less back pressure than a baffled exhaust, leading to the 'hot' cam being able to work better, enhancing the duration and overlap?
However, IIRC, long-duration big-overlap cams are the last thing you want on a supercharged engine. With a blower pushing air into the cylinders, any overlap will just result in air / mixture being blown right through the cylinder and down the exhaust without getting burnt. I was always told that for top power in blown engines, keep overlap to a minimum so the blower can force as much air into the cylinder as possible when the inlet valve opens (if the exhaust is open at the same time, no additional compression will take place in the cylinder and the work done by the supercharger will be wasted).
Now, unless Lotus have put entirely different camshafts in the supercharged version of the Exige, the 'hot' cam will be sub-optimal for a supercharged motor, no? The popular Katana conversion doesn't change cams, and that must have the same problems?
In the face of this evidence, it's likely that my 'knowledge' is faulty - what's special about the 'hot' cams in the Toyota engine that make them work well with superchargers? Obviously with direct injection you won't suffer the 'unburnt mixture pushed through the engine into the exhaust' effect with a supercharger and cam overlap... but you'll still waste some of the work as all the blower is doing is pushing fresh air through the system.
Is it just that the Toyota 'hot' cam doesn't actually overlap at all, and the only 'hot' thing about it is that it has higher lift than the 'mild' one?
My Exige S was a standard touring pack car. Not a particularly strong engine, just an average one. It felt wheezy and out of breath at high revs, but presumably delivered the 210 bhp or whatever they're meant to make.
I've had a 2bular exhaust fitted, no other powertrain mods. It's the biggest one Jim makes, to keep noise as low as possible to keep me on track days. The car now feels a hell of a lot more eager, the wheeze and 'strain' at high revs is gone - it just wants to spin all the way round, and seat-of-the-pants feels significantly faster. Not just me, but Parky @ Lipscomb had the same conclusion - he said it felt as quick as his 240 PP car, and he's got a lot more experience than me
Now I know that exhausts on the Toyota Lotuses aren't meant to make much difference to the power. But worse than this, on a supercharged engine, Jim's style of exhaust surely would *reduce* power? The Toyota engine switches cams at high revs, as we all know. On the nasp engines, a high-lift, long duration and high-overlap cam gives nice loads of power at high revs (but is undriveable at low revs because of the overlap, hence the mild 'slow speed' cam). The 2bular zorst is a free-flow design (no baffles) with a big can full of sound absorbing mat. This should surely have much less back pressure than a baffled exhaust, leading to the 'hot' cam being able to work better, enhancing the duration and overlap?
However, IIRC, long-duration big-overlap cams are the last thing you want on a supercharged engine. With a blower pushing air into the cylinders, any overlap will just result in air / mixture being blown right through the cylinder and down the exhaust without getting burnt. I was always told that for top power in blown engines, keep overlap to a minimum so the blower can force as much air into the cylinder as possible when the inlet valve opens (if the exhaust is open at the same time, no additional compression will take place in the cylinder and the work done by the supercharger will be wasted).
Now, unless Lotus have put entirely different camshafts in the supercharged version of the Exige, the 'hot' cam will be sub-optimal for a supercharged motor, no? The popular Katana conversion doesn't change cams, and that must have the same problems?
In the face of this evidence, it's likely that my 'knowledge' is faulty - what's special about the 'hot' cams in the Toyota engine that make them work well with superchargers? Obviously with direct injection you won't suffer the 'unburnt mixture pushed through the engine into the exhaust' effect with a supercharger and cam overlap... but you'll still waste some of the work as all the blower is doing is pushing fresh air through the system.
Is it just that the Toyota 'hot' cam doesn't actually overlap at all, and the only 'hot' thing about it is that it has higher lift than the 'mild' one?
WOW!
why too deep!
long and short of it is *any* back-pressure in the exhaust system is a bad thing.
std ones are built to pass drive-by noise etc, this has a bad effect, higher up the rev/power range, the more the problem.
as for blowing the fuel/air though the engine with too much overlap, yes this can happen, but you need some serious boost to get to this point, and you can easily control this with cam timing in the map.
Simples!
why too deep!
long and short of it is *any* back-pressure in the exhaust system is a bad thing.
std ones are built to pass drive-by noise etc, this has a bad effect, higher up the rev/power range, the more the problem.
as for blowing the fuel/air though the engine with too much overlap, yes this can happen, but you need some serious boost to get to this point, and you can easily control this with cam timing in the map.
Simples!
Edited by Scuffers on Monday 28th September 21:25
My 8"x24 2Bular with a sports cat gave good gains with a custom ECU map on a stock Katana install. According to Jim you need a free flowing exhaust on forced induction engine, something which the hairdryer stock exhausts don't do on the stock Elises/Exiges. A decent manifold will also give good gains from the dyno's I've seen.
Scuffers said:
WOW!
why too deep!
long and short of it is *any* back-pressure in the exhaust system is a bad thing.
std ones are built to pass drive-by noise etc, this has a bad effect, higher up the rev/power range, the more the problem.
as for blowing the fuel/air though the engine with too much overlap, yes this can happen, but you need some serious boost to get to this point, and you can easily control this with cam timing in the map.
Simples!
Wot 'e said why too deep!
long and short of it is *any* back-pressure in the exhaust system is a bad thing.
std ones are built to pass drive-by noise etc, this has a bad effect, higher up the rev/power range, the more the problem.
as for blowing the fuel/air though the engine with too much overlap, yes this can happen, but you need some serious boost to get to this point, and you can easily control this with cam timing in the map.
Simples!
Edited by Scuffers on Monday 28th September 21:25
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