Porsche 993 twin turbo

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Re: Porsche 993 twin turbo

Post by BLSTIC »

I'd call lag the delay between throttle application and the expected percentage of available torque arriving.

My 1l 3 cylinder with a mismatched turbo had lag at most rpm.

My 4th generation legacy had notable lag at low rpm but minimal at high rpm. The turbo being a metre away from the exhaust ports will do that. It was auto though, so it took care of the downshifting and responded pretty well, except off the line.

My stock barra turbo had moderate lag at most rpm, but it wasn't really a big deal. As a 4l engine built for low-mid range torque that runs all of 6 psi, you didn't really have to wait for "enough" torque and the delay was merely the difference between "enough" and "lots". It ran a gt3582r with a 1.06 housing, so it was very much overkill for the stock power output
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Re: Porsche 993 twin turbo

Post by ptuomov »

rp930 wrote: Sun Aug 14, 2022 3:19 pm It’s not how much power, it’s power delivery. High HP turbo motors can get very peaky with narrow big power bands. Just like old muscle cars older Porsches have nostalgic value but the newer the better for power and handling.The Turbo S’s were 450 and quick so that’s a good guideline. At some point you have to enjoy it for what it is.
So something like moving the base 408hp curves to the Euro S / WLS 2 levels as below?
Porsche450TorqueCurve.jpg
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Re: Porsche 993 twin turbo

Post by rp930 »

ptuomov wrote: Wed Aug 17, 2022 7:03 am
rp930 wrote: Sun Aug 14, 2022 3:19 pm It’s not how much power, it’s power delivery. High HP turbo motors can get very peaky with narrow big power bands. Just like old muscle cars older Porsches have nostalgic value but the newer the better for power and handling.The Turbo S’s were 450 and quick so that’s a good guideline. At some point you have to enjoy it for what it is.
So something like moving the base 408hp curves to the Euro S / WLS 2 levels as below?

Porsche450TorqueCurve.jpg
Yes more like that.

You are missing the point about lag. It can get so bad the car will will be miserable to drive. Yes you can increase rpm for the highway but what happens in first gear - you plant your foot and wait and wait and wait. For a race car great, for anything else annoying. The beauty of a twin turbo was the almost complete elimination of lag, great fun to drive because the torque was available at low rpm. The more you try to tune it as a race car the worse it gets. You add big turbos, then add the big cams, then port the heads and it’s fast but everything on the street blows by you while you wait to get the right rpm. Been there done that, no thanks.
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Re: Porsche 993 twin turbo

Post by ptuomov »

For a reference, here is the relevant transmission graph from the stock transmission 993 Turbo:
DE7BF418-79A3-4283-80C1-339BB0C5304F.jpeg
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Re: Porsche 993 twin turbo

Post by Fusion Works »

That doesn't tell me gear ratios and final drive specs.
Also what are your tire sizes?

What top speed do you need to hit?
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Re: Porsche 993 twin turbo

Post by ptuomov »

Fusion Works wrote: Wed Aug 17, 2022 11:22 am That doesn't tell me gear ratios and final drive specs.
Also what are your tire sizes? What top speed do you need to hit?
http://www.gboxweb.com/993specs.html

I think the overall tire diameter is 25 but let me check.

Over 300 km/h would be fun on a trip next summer.
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Re: Porsche 993 twin turbo

Post by rp930 »

ptuomov wrote: Wed Aug 17, 2022 12:10 pm
Fusion Works wrote: Wed Aug 17, 2022 11:22 am That doesn't tell me gear ratios and final drive specs.
Also what are your tire sizes? What top speed do you need to hit?
http://www.gboxweb.com/993specs.html

I think the overall tire diameter is 25 but let me check.

Over 300 km/h would be fun on a trip next summer.
It will go at least 290 kph stock.
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Re: Porsche 993 twin turbo

Post by englertracing »

ptuomov wrote: Wed Aug 17, 2022 4:51 am Here's a question about turbo lag.

When I read posts about turbo lag, I never know exactly is the accepted forum definition of turbo lag.

None of the _street_ turbo cars that I've worked have any appreciable turbo lag to speak of in the following sense: When you're driving hard and shifting near the redline, the engine is simply on after the shift and there's no lag. All of them have a turbo lag in the sense of starting with a high gear cruise and flooring the car at low rpms without downshifting.

But well tuned naturally aspirated engines also bog down if one floors the gas pedal at high-gear low-rpm highway cruise. In my experience, carbureted and mechanical throttle cammed engines can really lose the plot, electronic throttle ones often save the driver by effectively ignoring the driver's right foot until rpms are up. FBW throttles simply don't open fully and dual throttle ITBs (like Suzuki) have the computer not fully open the second set of throttle valves.

Downshifting the transmission on this 993tt to the correct gear for maximum acceleration with the car already moving will result in an engine speed higher than 4300 rpm. Based on my driving this car in its stock form, the power is on instantly at that 4300 rpm and I can't perceive any lag. Maybe my senses aren't that finely tuned.

Furthermore, if one is willing to downshift, then the way to combat turbo lag is to install a larger compressor wheel and increase the engine redline. This allows one to productively downshift to a higher initial rpm. This on the face of it is the opposite of the usual turbo lag advice.

So what is the best definition of a lag? Do people not downshift their cars when they get into it on a highway? Am I missing something?
If your driving a car on a winding road or on a track and entering corners close to the point it will understeer, you rely on throttle response to balance the car, not all corners are conductive to downshifting.... so for someone who likes instant throttle response, or relys on it any delay is massively noticeable or problematic.
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Re: Porsche 993 twin turbo

Post by ptuomov »

englertracing wrote: Wed Aug 17, 2022 1:42 pm If your driving a car on a winding road or on a track and entering corners close to the point it will understeer, you rely on throttle response to balance the car, not all corners are conductive to downshifting.... so for someone who likes instant throttle response, or relys on it any delay is massively noticeable or problematic.
That makes sense. You want an instantaneous response to pedal application that results in predictable torque.

Someone told me that this car likes to go into the corner loaded with brakes and negative accelerations and exit it loaded with engine and forwards acceleration but it doesn't like to coast in between. I don't know much about driving cars fast, but I can believe that having driven the car a little bit now. Given that the forward/backward plus lateral acceleration budget is limited, I'm thinking one wants to increase torque gently if that's the driving technique one employs. But as I said, I don't understand driving.

I am looking at the acceleration and traction limits now using an Excel spreadsheet. I'm assuming that with four wheel drive and sticky tires, the car can accelerate at 0.9g before loosing traction.

Even without any boost, the car is close to the traction limit on first gear. Hypothetically, with the turbos fully spooled, the engine would deliver 38% more torque to the rear wheels than what the traction could handle on first gear and 40km/h speed. Therefore, turbo lag is making the car faster on first gear, the lag is a blessing that prevents the acceleration from going up in smoke (pun intended).

On the second gear, if fully spooled and loaded, the car would be about 23% (factor of .77) from the traction limit at 3800-5100 rpm or, equivalently, 60-80 km/h. Here, there's some chance that the turbos are already working, so increasing the torque from 540 Nm to about 700 Nm would be clearly productive in ideal traction conditions but after that it might cause traction problems even on dry asphalt. That's why I believe Porsche cut down the boost in the S model such that the 3500-5000 rpm torque is flat at about 590 Nm (give or take) -- traction might be less than ideal. Above 5500 rpm and all the way to the redline, one should in my opinion try to add as much torque, the traction limit is not very binding.

How to add torque about 5500 rpm? I think the answers are bigger compressors, longer duration cams, and the Bosch Motronic boost controller algorithm tuned to ramp up the boost with rpm above 5500 rpm to offset the declining N/A volumetric efficiency of the engine.

Does this make sense?
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Re: Porsche 993 twin turbo

Post by rp930 »

ptuomov wrote: Wed Aug 17, 2022 2:31 pm
englertracing wrote: Wed Aug 17, 2022 1:42 pm If your driving a car on a winding road or on a track and entering corners close to the point it will understeer, you rely on throttle response to balance the car, not all corners are conductive to downshifting.... so for someone who likes instant throttle response, or relys on it any delay is massively noticeable or problematic.
That makes sense. You want an instantaneous response to pedal application that results in predictable torque.

Someone told me that this car likes to go into the corner loaded with brakes and negative accelerations and exit it loaded with engine and forwards acceleration but it doesn't like to coast in between. I don't know much about driving cars fast, but I can believe that having driven the car a little bit now. Given that the forward/backward plus lateral acceleration budget is limited, I'm thinking one wants to increase torque gently if that's the driving technique one employs. But as I said, I don't understand driving.

I am looking at the acceleration and traction limits now using an Excel spreadsheet. I'm assuming that with four wheel drive and sticky tires, the car can accelerate at 0.9g before loosing traction.

Even without any boost, the car is close to the traction limit on first gear. Hypothetically, with the turbos fully spooled, the engine would deliver 38% more torque to the rear wheels than what the traction could handle on first gear and 40km/h speed. Therefore, turbo lag is making the car faster on first gear, the lag is a blessing that prevents the acceleration from going up in smoke (pun intended).

On the second gear, if fully spooled and loaded, the car would be about 23% (factor of .77) from the traction limit at 3800-5100 rpm or, equivalently, 60-80 km/h. Here, there's some chance that the turbos are already working, so increasing the torque from 540 Nm to about 700 Nm would be clearly productive in ideal traction conditions but after that it might cause traction problems even on dry asphalt. That's why I believe Porsche cut down the boost in the S model such that the 3500-5000 rpm torque is flat at about 590 Nm (give or take) -- traction might be less than ideal. Above 5500 rpm and all the way to the redline, one should in my opinion try to add as much torque, the traction limit is not very binding.

How to add torque about 5500 rpm? I think the answers are bigger compressors, longer duration cams, and the Bosch Motronic boost controller algorithm tuned to ramp up the boost with rpm above 5500 rpm to offset the declining N/A volumetric efficiency of the engine.

Does this make sense?
Despite what I and others have stated you seem intent on turning this car into a peaky Ill driving track car. Why not sell it before you ruin it and buy a track car that’s already been beat to death? Your “spreadsheet” doesn’t consider all wheel drive or traction control. Those same “bigger compresssors and longer duration cams” you want will move torque to a higher rpm, likely kill the low end and make it very difficult to modulate when cornering. But hey it’s your car let me know after you’ve screwed ot up how much you’re “enjoying” the car. I’m out.
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Re: Porsche 993 twin turbo

Post by ptuomov »

rp930 wrote: Wed Aug 17, 2022 3:09 pm Despite what I and others have stated you seem intent on turning this car into a peaky Ill driving track car. Why not sell it before you ruin it and buy a track car that’s already been beat to death? Your “spreadsheet” doesn’t consider all wheel drive or traction control. Those same “bigger compresssors and longer duration cams” you want will move torque to a higher rpm, likely kill the low end and make it very difficult to modulate when cornering. But hey it’s your car let me know after you’ve screwed ot up how much you’re “enjoying” the car. I’m out.
Good bye!
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Re: Porsche 993 twin turbo

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My little spreadsheet uses an AWD assumption with 0.90g maximum acceleration. The 993 Turbo "traction control" does not prevent all the wheels form spinning up a smoke cloud, the ABD system only selectively brakes the fastest spinning wheel when the estimated speed is under 44 mph. It's a useful system but it's not a traction control system in the sense of reducing engine torque to prevent wheel spin. This system will simply be overwhelmed by the engine if the traction is poor, speed low, and engine power high. This is not the PSM system that later model Porsches use.
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Re: Porsche 993 twin turbo

Post by Fusion Works »

If you could find a 3.6,3.7, 3.8 final drive you could offset the drawbacks of larger turbos, bigger cams, etc. You can create more torque at the contact patch. Yes you will lose some top end, but the reality is, normal driving doesn't allow for speeds in excess of 150mph on a regular basis.
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Re: Porsche 993 twin turbo

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Fusion Works wrote: Wed Aug 17, 2022 10:35 pm If you could find a 3.6,3.7, 3.8 final drive you could offset the drawbacks of larger turbos, bigger cams, etc. You can create more torque at the contact patch. Yes you will lose some top end, but the reality is, normal driving doesn't allow for speeds in excess of 150mph on a regular basis.
The two turbine hot sides under consideration are K16 and K24. Both came factory stock with this car, the European S model getting the larger K24 hot side. The compressor wheel of K16 is too small, so staying with the K16 turbos would mean installing a K24 (or similar) compressor wheel. The compressor wheel itself isn’t a big factor in lag or transient response, turbine geometry is. Compressor wheel does to a large extent determine the peak hp, however.

On the cams, I think that by increasing the duration by increasing overlap one can have one’s cake and eat it, too. This is because the increased overlap will in my opinion help across the whole rpm range above idle. The constraint here is passing the emissions at idle. Again, not planning to go overboard here, just the types of cams that the normally aspirated 993’s have. Like hydraulic 993 Euro RS emissions passing street car cams.

The third physical change would be a free-flow exhaust. Many wools say that reducing the flow restriction after turbo helps the turbo spool up. I don’t think this is true, because the mass flow of exhaust at say 2500 rpm is so low compared to 6500 rpm that I don’t see how the exhaust can be chocking off anything at 2500 rpm. Free flow turbo back exhaust will help the top end, though.

With these changes and the software calibration to complement them, I think one could add more power everywhere while keeping the lag at most at the factory S level and possibly better. The gearing changes might not be required.
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Re: Porsche 993 twin turbo

Post by Fusion Works »

The gearing changes can make the car more fun to drive everywhere. It will accelerate faster and have less "dwell time in the lag zone". Porsche loves themselves some long gears. I would make that change anyways regardless of everything else. Plus you can rev higher with the longer duration cams so you may not lose that much on the top end.
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