Impreza Off Boost?

Steve Walker ( snipped-for-privacy@otolith.demon.co.uk) gurgled happily, sounding much like they were saying :

Umm, why on earth would you want to plant it at 30 in top?

Turbos need engine revs to give boost. I wouldn't expect any turbo to give much boost, given the low revs of an engine in 5th at 30mph. I think maybe you're expecting too much. Driven normally, my Celica turbo has no noticeable lag. Of course more power comes in as the revs rise, but it's smooth and progressive. I would expect an Impreza turbos delivery to be very similar. Mike.

Becasue in a sensible car with torque instead of nutty rev levels you can happily put it in 5th at 25 and leave it there all the way to 150+...

In a turbo'd engine, the compression ratio is often lowered, this means that a turbo'd engine produces even less torque at low revs than a similar capacity normally aspirated one.

At low revs there is not enough exhaust gas to push a usefully sized turbo hard enough to make it push more air into the engine. In fact, turbos are actually a restriction in the exhaust and the inlet at low (and high) revs.

I'm not sure I would class what you describe as lag. Try driving something turbo'd from the 1980s for more details. (c: I'd describe lag as the delay between being off boost but at revs within the range of the turbo, flooring it, and waiting for full boost and the turbo 'kick'. On older cars, the delay is quite marked and could easily take an inexperienced driver by surprise.

Many modern turbo'd engines seem to have this delay engineered out to some extent, by using clever turbos and electrickery, but I don't think there's a lot they can do to improve driveability at really low revs.

Douglas

"Mike G" wrote in news:42ff40d7$0$1316$ snipped-for-privacy@ptn-nntp-reader02.plus.net:

Like Mike says, you need to gear the car correctly. At 30 in fifth, you don't have enough revs to spin the turbo and it's just labouring the engine. When you have an engine speed of approx 2500rpm, the point at which a turbo begins to have real effect, try putting your foot down then. Then you can measure the true turbo lag. The real advantage of the turbo is the high torque output at relatively low revs. My S70 T5 hits maximum torque at about 2500 and sustains the same torque all the way to

Im my 307 D-Turbo put it in 5th at 25 and you'll stall the engine! Gotta be doing at least 35-40 before engaging 5th just to make sure the engine is above 1100RPM then it'll pull all the way. Could really do with a 6 speed box to help things mid range but not enough pennies to buy a new model.

Chris

Without sacrificing good acceleration? Most cars would still be comparitively gutless, regardless of how much torque the engine had. Mike.

Well obviously if you want *good* acceleration, change down.

But for pottering on A and B roads being able to put it in 5th and leave it makes for a relaxing drive.

My Carlton had better acceleration in 5th at 40 than my Astra (1.4i) had in

Yup. My Celica has a 8.5 to 1 comp ratio. If there is less torque at low revs, it's not something that is apparent in normal driving.

I understand the Sierra Cosworths are like that. Boost comes in so quickly that on a wet road, you can easily lose the back end. Mate had a '90 Saphire Cosworth in excellent nick. I could have baught it for £1500 a couple of years ago, but the turbo delivery characteristics were enough to put me off, so I got a Celica turbo instead. Not as quick, but better equipped and more rewarding to drive OMO.

My Celica GT4 turbo is a 1990. Turbo delivery comes in so smoothly and progressively, that I doubt anyone who didn't know would realise that it had a turbo. Poodling around town, it feels like, and is just as driveable as any non-turbo car. Mike.

In message , Adrian writes

Curiosity, principally.

In message , Mike G writes

After the slagging I've heard people give VTEC units in favour of sporting blown engines, I was expecting to see some of the flexibility I keep hearing about.

Oh, it's fine when it comes in, a bit less frenetic than what I'm used to. I got the impression one could very easily lose one's license without having any fun in the process. Nice car, though.

Ever considered an automatic...?

That's not lag, that's just off-boost. At low RPM there's not enough exhaust gas flowing to spin up the compressor. Turbo lag is the delay from hitting the accellerator at a speed where it is capable of spinning the compressor - say 3000rpm, and the turbo spooling up to pressure.

Steve Walker ( snipped-for-privacy@otolith.demon.co.uk) gurgled happily, sounding much like they were saying :

I don't think anybody - or at least anybody sane - would ever suggest a "conventional" turbo lump would be as flexible as an equivalent power big- capacity normally aspirated lump.

I've driven VTEC civics in the past - the original shape one, 1.6/160bhp, about L-reg - and they went like s*it off a shovel. But only really once they hit loonyrevs. Below that, they were just... unexciting.

Likewise, no turbo off-boost will be all that, as the CR has to be lowered to stop the thing eating itself.

If you want to make a turbo "flexible" on- and off-boost, then you've got to start either managing the boost electronically or using lower boost pressures. For example, the petrol turbo in my Cit XM manages the boost to allow the use of a relatively high CR - as a result, the absolute power's fairly unexceptional for a turbo'd 2.0 petrol (150bhp), but the off-boost performance is good, with a ludicrously flat torque curve - 180lb/ft peak is reached across a range of about 3,000rpm

In message , Adrian writes

They aren't like that now. You have to bear in mind that the 1.6 litre VTi Civics were in the market against 2.0 litre naturally aspirated lumps giving about 150 bhp. Of course they lacked torque relative to something like a 16V Astra GTE. The 2.0 i-VTEC engine gives similar low-rev torque to any other normally aspirated two litre.

Yep. But when I'm not in a relaxed all-day-in-5th mood I tend to be in a 'steering with the throttle' sort of mood and a manual works far better at that point... ;-)

Try 25-70 in 3rd. Celica zoom time.

Bit boring though. I miss the "lag, lag, boom, oh jesus=A3 boost of my old Saab. Mind you it was fun arround the derbyshire lanes yesterday, at the=20 Bubble Inn charity car meet.

--=20 Carl Robson Car PC Build starts again.

Modern turbo installations use a bypass valve downstream of the turbo in addition to the wastegate. This allows the turbo to maintain spin while the throttle is closed. This dramatically reduces the turbo lag.