GM's new 4.2L DOHC twin turbo V-8 !!

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Truckedup » Mon Dec 10, 2018 10:05 am

So all the theory of mid engine cars having a tendency to spin out faster due to "polar inertia" is not rue? Obviously modern suspension can nullify this...
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by mk e » Mon Dec 10, 2018 10:20 am

Truckedup wrote:
Mon Dec 10, 2018 10:05 am
So all the theory of mid engine cars having a tendency to spin out faster due to "polar inertia" is not rue? Obviously modern suspension can nullify this...
Oh that's mostly true, they change directions very quickly.
But throttle on it a turn tends to make them understeer, at least the ones I've drive hard enough to find out so they feel stable.

The problem it that due to the inherent balance getting all 4 tires to full load at about the same time and the fact that the low polar interia means you can turn in faster, once it let's go there is basically no saving it......at least not in mortal hands. I saw in-car video of Kimi Raikkonin's f1 Ferrari getting out of shape and he was moving the steering wheel so fast his hand were just a blur. A few laps before a lower teir car got out of shape in the same spot....and ended up in the wall.
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by MadBill » Mon Dec 10, 2018 3:48 pm

Fast hands; one of the sport's critical attributes... :)
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by peejay » Mon Dec 10, 2018 3:51 pm

mk e wrote:
Mon Dec 10, 2018 10:20 am
Truckedup wrote:
Mon Dec 10, 2018 10:05 am
So all the theory of mid engine cars having a tendency to spin out faster due to "polar inertia" is not rue? Obviously modern suspension can nullify this...
Oh that's mostly true, they change directions very quickly.
But throttle on it a turn tends to make them understeer, at least the ones I've drive hard enough to find out so they feel stable.

The problem it that due to the inherent balance getting all 4 tires to full load at about the same time and the fact that the low polar interia means you can turn in faster, once it let's go there is basically no saving it......at least not in mortal hands. I saw in-car video of Kimi Raikkonin's f1 Ferrari getting out of shape and he was moving the steering wheel so fast his hand were just a blur. A few laps before a lower teir car got out of shape in the same spot....and ended up in the wall.
Throttle in ANY vehicle tends to make them understeer. The difference with mid-engine versus front-engine rear drive is that you can generally get more acceleration before "load transfer understeer" turns into "wheelspin oversteer".


Traditional mid-engine cars, especially the ones with a reputation for swapping ends on you, have had shortish wheelbases. In the 85-95 inch range. Modern design tends towards much longer wheelbases, 100-110" and even higher, so they are much less likely to snap on you.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Bazman » Mon Dec 10, 2018 3:58 pm

My buddy has a Lambo 560-4. It's a great car and has 000's of closed road and track kms on it (about 25k of events on it). It is catchable on road tires - but you need very quick hands.... I'd liken it to driving a rear wheel drive car on slicks that are a bit old and hard.... it grips, then it lets go, but will normally offer an early warning wiggle before it get's to that if you are pushing. The problem is when you get on it too hard out of a turn, or enter too fast and brake too late - either of those and you'll be busy.

The best invention to make high power cars safe has been the smart diffs like Wavetrac, Trutrac, Quaife etc that allow both wheels to be driven in a straight line, but put power to the OUTSIDE wheel out of turns. This one thing alone is the difference between being able to drive 6-700rwhp hard in the wet or dry vs going sideways in the dry passing a car on the old spool or older style LSD's.... The smarter diffs allow you to steer through it, the older diffs you had to steer on the throttle - very dangerous on public roads.

I feel safer driving 1000hp with a Wavetrac or similar than 500hp in an old Mopar A body and a spool/lock up.

As for traction control - takes it another level. AMG has the best I've experienced - push the button to turn it half off - keep alert and enjoy - wet or dry is all good.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by peejay » Mon Dec 10, 2018 4:00 pm

Truckedup wrote:
Mon Dec 10, 2018 9:04 am
Uh, public roads closed off to traffic are race tracks... :D

I call then rally stages, as do most of the rest of the world :) Sadly, stage rally is not quite as popular in the US as in other countries. If half the people at any given test and tune night at the local dragstrip, or half the people at any given circle track, were interested in stage rally instead, national participation would practically double.

It doesn't hurt that the majority of enthusiast vehicles in the US are quite unsuitable for it. People have rallied Mustangs, mostly to prove they could do it.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by peejay » Mon Dec 10, 2018 4:04 pm

Bazman wrote:
Mon Dec 10, 2018 3:58 pm
The best invention to make high power cars safe has been the smart diffs like Wavetrac, Trutrac, Quaife etc that allow both wheels to be driven in a straight line, but put power to the OUTSIDE wheel out of turns. (...)
I feel safer driving 1000hp with a Wavetrac or similar than 500hp in an old Mopar A body and a spool/lock up.

Funny, I feel the opposite. A spool or a nice tight clutch pack diff feels much more predictable than a helical-gear diff, because you have no idea where the gear type diff is going to be shuttling torque at any given moment.

Audi does use them, but only on AWD cars, which are not traction limited. In a traction limited situation, they are nearly undrivable if you like to control the chassis with throttle inputs.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by MadBill » Mon Dec 10, 2018 4:16 pm

Several guys I know that race in Trans Am, GT1, etc. have tried Torsen, Wavetrac, Salsibury, etc. diffs, which seem to have much to offer, but most have returned to the simple old Detroit Locker. Ironic, since I believe it was designed mostly just to avoid broken axles from U-turning off the strip with hot slicks and a spool. (One benefit is they either work or don't; there's no slowly fading away and sucking up lap times.)
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Truckedup » Mon Dec 10, 2018 4:48 pm

peejay wrote:
Mon Dec 10, 2018 4:00 pm
Truckedup wrote:
Mon Dec 10, 2018 9:04 am
Uh, public roads closed off to traffic are race tracks... :D

I call then rally stages, as do most of the rest of the world :) Sadly, stage rally is not quite as popular in the US as in other countries. If half the people at any given test and tune night at the local dragstrip, or half the people at any given circle track, were interested in stage rally instead, national participation would practically double.

It doesn't hurt that the majority of enthusiast vehicles in the US are quite unsuitable for it. People have rallied Mustangs, mostly to prove they could do it.
Rally makes me think of the World Rally cars. small 400 HP boxes with the drive wheels always spinning and at the edge of reality at all times... that's racing...
The US also has a huge off road following from motorcycles, quads, side by sides, Jeeps, and PU trucks.. Off road action draws many people away from street performance cars..And all the young guys in modified Diesel pu trucks..
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by peejay » Mon Dec 10, 2018 6:42 pm

Truckedup wrote:
Mon Dec 10, 2018 4:48 pm
peejay wrote:
Mon Dec 10, 2018 4:00 pm
Truckedup wrote:
Mon Dec 10, 2018 9:04 am
Uh, public roads closed off to traffic are race tracks... :D

I call then rally stages, as do most of the rest of the world :) Sadly, stage rally is not quite as popular in the US as in other countries. If half the people at any given test and tune night at the local dragstrip, or half the people at any given circle track, were interested in stage rally instead, national participation would practically double.

It doesn't hurt that the majority of enthusiast vehicles in the US are quite unsuitable for it. People have rallied Mustangs, mostly to prove they could do it.
Rally makes me think of the World Rally cars. small 400 HP boxes with the drive wheels always spinning and at the edge of reality at all times... that's racing...
The US also has a huge off road following from motorcycles, quads, side by sides, Jeeps, and PU trucks.. Off road action draws many people away from street performance cars..And all the young guys in modified Diesel pu trucks..
I think of Ford Escorts myself, not WRC. Other places they think of Peugeot 205s, other places Volvo 240s...

I wouldn't say the drive wheels are always spinning. They have something like 13" of suspension travel and some very interesting/bizarre shock tuning and suspension geometry. They're clinical and boring to watch.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Bazman » Mon Dec 10, 2018 7:05 pm

peejay wrote:
Mon Dec 10, 2018 4:04 pm
Bazman wrote:
Mon Dec 10, 2018 3:58 pm
The best invention to make high power cars safe has been the smart diffs like Wavetrac, Trutrac, Quaife etc that allow both wheels to be driven in a straight line, but put power to the OUTSIDE wheel out of turns. (...)
I feel safer driving 1000hp with a Wavetrac or similar than 500hp in an old Mopar A body and a spool/lock up.

Funny, I feel the opposite. A spool or a nice tight clutch pack diff feels much more predictable than a helical-gear diff, because you have no idea where the gear type diff is going to be shuttling torque at any given moment.

Audi does use them, but only on AWD cars, which are not traction limited. In a traction limited situation, they are nearly undrivable if you like to control the chassis with throttle inputs.
If you have a smooth surface (like a circuit) that's one thing. On a street with big camber for rain run-off, bumps dips, and uneven surfaces - no way. It'd take a way better driver than me or any of the guys I hang with to keep up on our closed roads with a spool or locker and not crash. Not saying you couldn't... just have yet to see anyone up front with that ol tech on our roads, they're all way behind trying not to die :lol:

Funny you mention Audi - the most intrusive awful traction control out there, they're uncontrollable not because they have a Wavetrac style diff but because they're constantly braking to "assist" traction.... they suck for serious closed B road work unless you turn everything right off. Try a good AMG with traction in sport mode then you can wring its neck and drive on the throttle

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Brian P » Mon Dec 10, 2018 7:06 pm

Truckedup wrote:
Mon Dec 10, 2018 10:05 am
So all the theory of mid engine cars having a tendency to spin out faster due to "polar inertia" is not rue? Obviously modern suspension can nullify this...
Lower polar moment of inertia means the car can get turning quicker. It also means that if it gets out of shape, it does that quicker, too. It's harder to build in an understeer margin if there is rear-biased weight distribution. A lot of them have wider rear tires than front in order to help with that. Good suspension design helps. For a street car, the enabler is the traction and stability control system.

That same rear-biased weight distribution will also mean it will take more acceleration before it gets into wheelspin-induced oversteer. It should cut down on the number of idiots who spin the tires (at relatively low road speed) and end up tossing the car into the bushes ... or pedestrians. (Mustangs are even more prevalent for doing that)

The stability issues with mid-engine happen at speeds and cornering loads that are well past what those inexperienced drivers are likely to ever see when exiting Cars and Coffee.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by mk e » Mon Dec 10, 2018 7:12 pm

peejay wrote:
Mon Dec 10, 2018 3:51 pm
Throttle in ANY vehicle tends to make them understeer. The difference with mid-engine versus front-engine rear drive is that you can generally get more acceleration before "load transfer understeer" turns into "wheelspin oversteer".


Traditional mid-engine cars, especially the ones with a reputation for swapping ends on you, have had shortish wheelbases. In the 85-95 inch range. Modern design tends towards much longer wheelbases, 100-110" and even higher, so they are much less likely to snap on you.
I think its more subtle than that. If its stupid front heavy then it plows until you apply a little throttle to get a little weight off. If it's stupid light in front you probably need to brake deep to keep weight on the front.

I think wheel base is mote about stability at speed. My car is a 92" and doesn't spin. I've driven a couple FSAE cars in the 60-65" WB range, they didn't spin....but that is all at autocross speed (street speed), slide yes, spin no. But at speed they both get increasingly unstable. The FSAE car was stupid fidgety at 80+, the 308 is stupid light up front at 120+....I changed the front valance and it helped some but its not stable at high speed and the cockpit is cramped and awkward vs the newer longer models that have space to sit properly and are aero stable at speed.

I think there are a lot of reasons cars end up the way they end up
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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Bazman » Mon Dec 10, 2018 7:15 pm

MadBill wrote:
Mon Dec 10, 2018 4:16 pm
Several guys I know that race in Trans Am, GT1, etc. have tried Torsen, Wavetrac, Salsibury, etc. diffs, which seem to have much to offer, but most have returned to the simple old Detroit Locker. Ironic, since I believe it was designed mostly just to avoid broken axles from U-turning off the strip with hot slicks and a spool. (One benefit is they either work or don't; there's no slowly fading away and sucking up lap times.)
Yes the track circuit guys typically do, except for the high dollar enduro guys running GT3 McLarens, Lambos, Ferraris, R10 Audis, GT3 RS's etc - they run the higher tech diffs. I'm talking about public roads and tarmac rally - you're welcome to try your spools and lockers in that environment - bring a diaper. The big difference is not only road surface but the fact you have NO IDEA what is around any corner, you don't get to practice, you just run as you see it and read the road as you go. The high tech diffs keep the car planted with high throttle loads out of turns while the spool/lockers will go sideways the moment you over power them. Drifting can be fun but not unintentionally, with trees and buildings and banks/rocks etc to hit, it's also slower to slide. On a track you can master it to the point its predictable - not public roads you might see once a year or are new.

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Re: GM's new 4.2L DOHC twin turbo V-8 !!

Post by Truckedup » Mon Dec 10, 2018 8:12 pm

peejay wrote:
Mon Dec 10, 2018 6:42 pm
Truckedup wrote:
Mon Dec 10, 2018 4:48 pm

I think of Ford Escorts myself, not WRC. Other places they think of Peugeot 205s, other places Volvo 240s...

I wouldn't say the drive wheels are always spinning. They have something like 13" of suspension travel and some very interesting/bizarre shock tuning and suspension geometry. They're clinical and boring to watch.
I don't know much about rally but to me 380 HP 2400 pound cars running flat out on narrow roads throwing dirt and stones and flying off the road surface and bouncing off of berms and trees sound pretty exciting for 10 minutes..Real excitement is on two wheels... :D
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