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Discussion in 'Trucks and Trailers' started by Travis Followell, Nov 29, 2005.
What is Turbo Lag?
The time it takes for the turbo to spool and begin to provide boost pressure to the engines intake manifold. Intercooled engines tend to have more turbo lag that non intercooled because you are trying to pressurize more air...the intercooler holds more air.
My understanding, is that its the time between when the motor accelerates, and the turbo spools up to provide the appropriate boost to the motor.
Well the turbo runs off spent exhaust gases so the engine has to be revving up to spin the turbo faster to make more power causing a little pause between the turbo to spin and getting it back into the intake manifold.
I always thought it was the observed difference in time between when you put the hammer down, and your engine responded. Used to be very noticable in early turbo systems, especially in passing or crossroads driving situations. This is why my Maserati has two small turbos, one for each bank of cylinders. The smaller the mass of the turbine, the less hangtime at redlights.
StealthDumpTrucks, usually, instead of using two small turbos, you make a compound set or twins. This consists of one large charge on the bottom and one small on the top. The small will spool quickly while feeding the large, then the large comes in for your real boost.
Really? I've never seen a setup like that, got a link or pics? If they are in series, I guess they would need an intercooler rather than the Maserati system dumping into a pressurized box.
Stealth, There are two ways to set up a twin turbocharged system.
The way your maserati is set up is called Parallel turbo's. It works by having a turbocharger off each engine bank, while this method does not reduce turbo lag, or overall boost generated, it is still able to pump more air in at the same psi (More CFM, same PSI) resulting in more power.
A sequential turbocharger setup is what LawnManiac is describing. This is usually ran off both banks and consists of a small turbo and a larger turbo.
The small turbo spools almost instantly, but goes out of its efficient range quickly, by the time you it does the large turbo is spooled.
The result is a almost the low lag of a small turbo with the top end power of a large one.
The Mazda RX-7 and Toyota Supra both used this setup.
The main problem with it is the incredibly complex vacuum lines/waste gate/plumbing setup for them, they can be picky and hard to get to work right.
I think i understand how it works, but I'd like to see how they handle an obvious pressure/vacuum differential on both sides of the spools. So they don't use the same plenum or waste gate? Ducting must be a nightmare.
Usually on turbo'd engines a blow off valve will release the extra boost into the atmosphere so as not to cause harm to the engine by overboosting, which can send rods through the side of the block and all sorts of other nasty stuff. In a sequential setup the turbo's use the same wastegate/dump valve/blowoff valve to release extra boost. The best idea to minimalize turbo lag aside from a sequential setup is Banks splined turbo system. It's very difficult to explain, but basically what happens is that the vanes of the turbo open and close, allowing more or less exhaust in and out automatically. It's a centrifugal system in which the faster the turbo spools, the wider the vanes open and when the turbo is spooling slighty, it lets in enough exhaust to boost quickly without lag. I'm not sure if they've released this product yet, saw it on Trucks or something on TV.
On turbo'd Audi's, a blow off valve cannot be used as the engine needs a constant supply of exhaust to run the engine correctly, so a diverter valve in used. Instead of blowing off the excess boost into the atmosphere, it recirculates it through the intake to maintain a minimum amount of boost. If there's problems with overboosting, there's problems with underboosting and that's what the diverter valve basically eliminates. When an upshift is made the engine negatively boosts, blowing all of the exhaust out through the wastegate and technically sucking some air back in through the turbine. With a constant positive boost pressure the turbo is always spooling in the right direction. Haha. For some reason on Audi's, internally, they need that consistent boost to keep the engine running through an upshift.
Anyone know of any other engines that use a diverter valve instead of a blow off/wastegate? Audi and maybe a couple other car MFG's are the only ones I know of.