Home away from home
From Melbourne Australia (and likely under the car)
Very general comment - running a quick ET/good mph in teh 1/4mile is about the only place you'd ever want a 'loose' exhaust spec on a turbo. Even for actual drag racing you'd still want it on the tight side. THe difference is that the 'clocks' only start timing once the car moves forward and breaks/leaves the starting beams, so you can 'technically' run an absolute dynamite time and mph but sit at the start spooling the damn thing up as long as is necessary before launch. in a drag race whoever gets across the line first (assuming they don't red light either at the start, or break out and run quicker than the nominated time in bracket racing I suppose) wins.
So in that (actual racing) scenario, you need quick predictable spoolup. If you look at some of the videos out there of some cars that are effectively the flagship of this or that tuning mob, some of them sit at the start line for what seems like ages. Don't get me wrong, i have massive respect for the power levels and performances of these vehicles and the sheer time and money they have sunk into getting every little bit of potential out of them, but such a combination is, in fairness, less optimal on a streeter. In fact on a heads up scenario (like at traffic lights - and I only use this hypothetically, I'm not encouraging anyone out there to race on the street) - or overtaking on the highway - where you need good power instantly, the tigher exhaust spec, quicker spooling combo will show its superiority over the laggier setup even if the latter makes a bit more peak power.
I'm curious what sort of power level you are chasing? I'm assuming more than 400bhp but don't know for sure. This next idea will be a little hard to precisely put into words, so if it comes across sounding dodgy, never mind. And that idea is this: I'm absolutely NOT suggesting you run a restrictor plate at all, and I _do_ concede that they effectively have to do a number of things to try and tweak and optimise the setup to work in conjunction with the restrictor plates, so you can't just do 'precisely the same combo' and remove the restrictor and then have it optimal - as without the restrictor you'll find more power with a slightly different combo. BUT having said that, if you take a look at some of the IPRA videos on youtube (not just datto 1200 ones) you'll see that, sure, they run out of puff by what - 6000rpm. Certainly they could go quicker without the restrictor and a bit more boost/rpm being on tap, but when you _really_ study how they accelerate, they've just got instant torque from a very modest rpm point, and pull like a train up through the gears. With a laggy turbo, coming out of a turn in the wrong gear, you'll completely stuff up any hope of a good lap time, and esp struggle not to get overtaken in the straight, but (and I must stress that this is specifically 'relatively speaking') in an ipra (or even former WRC car) style setup, they'll still pull out of that corner well. Not ideal, but well.
I realise you aren't on a circuit (you specified street and occasional 1/4mile) - but this whole 'deal' of coming out of hte corner optimally on a circuit - well even when 'not' cornering, but whenever you need to get the car accelerating immediately and at a decent rate, well these general features (the favouring of spoolup and mid range over peak power)
Just to play devils advocate - if you happen to be someone who has a lot of experience with high performance 2 strokes (be it older race bikes, or trail bikes or whatever) and really have an affinity for getting it right with gear selection and shift points for a peaky motor with what is almost like an all or nothing powerband, then relatively speaking, you'll have less trouble getting the most out of a laggier turbo combo. But eventually there's a crossover point - lower in the rpm range than most would initially suspect - esp for a streeter where flexibility/predictability and so forth 'out gun' a setup that makes more peak power.
Posted on: 2012/4/29 4:38