Wow. Great read. Here's my Reader's Digest version. You need to examine the compressor maps for the turbos. The reason some poop out at the high is because they are "too small, too hot". They can't efficiently handle the higher air flow. Once you figure out at what rpm range you are going to be shifting, then you can calculate the air flow and match it with the proper turbo.

 

In my work, I usually am approaching it from the other side. I always ask my customer if s/he's had the car on the dyno and if they can produce a dyno chart for me in addition to the other standard things like tire size, redline, what tracks to do they run on etc. Then I can run a gearchart and suggest ratios that meet their specific needs, including what their engine does.

So often on the web I see people saying things like,"You want to run gear ratios x,y,z. That's what I've got in my car". But the poster saying that drives a GT3 with 8400rpm redline and is advising someone with a mostly stock 996TT with a totally different power curve than their car.

I recently consulted a guy with a pair of really big turbos on his car. He was bothered because on corner exit the GT3 drivers were walking away from him even though he had close to 200bhp more than they did. What he was fighting with in particular was trying to stay on the 3-4 shift plane and not dropping it into 2nd on the couple of slow corners on the his "home" track. When he stayed in 3rd, his turbo would come off boost and the GT3's were getting the jump on him in that half a second he was waiting for the power to come on. He'd catch them by the end of the straight, but it was the difference between being able to catch them and being able to pass them.

So what we did was look at 3rd gears that were shorter that would keep him on boost and more in the sweet spot of his motor. The problem there is the shorter you get with 3rd gears, the smaller the diameter of the idler gear gets and it gets weaker. He was looking at a 3rd gear that GT3 guys use all the time, but with his power levels it was a risk. And it was short enough that he would have to change 4th as well or else he was going to have too big of an rpm drop on the 3-4 shift. He would just move his 3rd gear problem up to 4th.

We went back and forth about it for a few days and traded gearcharts and what not. The final conclusion we came to was that his car was going to be faster and more likely to be able to beat the GT3s if he took off his big turbos and went back to a set of smaller (though larger than stock) turbos that he had previously been running that would spool up 500rpms lower and still had good boost in his 3rd gear corners. He figured out that his build, which would have been fast in the Texas Mile, wasn't right for the road course that he ran. It's always a balancing act. We can't have it all.

 

Yes, that does clarify things along with some of the other posts. Thanks.

Makes me wonder about F1 engines with no turbos and shifting at 19,000 RPM's. How can they breathe?

 

The F1 engine goes back to the formula. It's not making a ton of torque at 19,000rpms but because of how fast the engine is spinning it makes a lot of horsepower. The make upwards of 700bph because they are making close too 200ft/lb of torque at those engine speeds.

 

See, now that doesn't compute in my world. I know they are extremely light weight, but 200 ft/lbs of torque seems weak compared to the torque that our turbos are able to produce, and yet they would probably just run away from us. I should have been an engineer so I could understand this stuff.

 

Power = torque * rpm.

Because of the much higher rpm, the F1 can run a much shorter gearing.

At 4.000 rpm, your car is much quicker in 1th, then in 6th gear. Why? Not because of there being more torque, but because of gearing.

Since a F1 car can do about triple rpm's of our cars, our 1th gear in a F1 would get an F1 to 180 km/u... So that would be in between 3th and 4th gear in an F1!

And that is why power is all that matters... because of gearing.

 

Excellent info. Thanks ...I had the same exact problem with my 18g's coming out of corners! ...on the short straights the stock 996gt3 pulls ahead by 1 car after exit ...when my boost comes on 3rd gear i am at least 7 cars ahead! Then when i brake he catches up because my 4 piston stock brakes arent as good as his 6 piston PCCB's.

So that 500 rpms spool does wonders? Tial Gt28 maybe the answer here..My 16g's spooled faster but ran out of breath faster...GT28's will be perfect for this scenario

 

What you are describing here is not all about the hardware... The distances between you and the car ahead always change quickly at the entrance and exit of corners. The car in front gets on the throttle first, so he is going faster than you are at the exit. That makes the space open up. Similar thing happens in reverse under braking.

Here's a simple example: Let's say that you are 1 second ahead on the straight at 100 mph. That is roughly 150 ft, or almost 10 carlengths. Now, you both slow to 50 mph at the corner. If you are still separated by 1 second, that corresponds to 73 ft, or about 4-5 carlengths. If the distance gets smaller than that, it is because you slowed down more than he did.

Also, the limits of your braking are set by your tires, not your brakes. You have enough brake there in the stock caliper to take you to the threshold of locking the wheels. That is the best you can do.

Jon

 

Jon , That makes perfect sense. So in summary , i am braking much harder (to stop my 3500 lbs car vs his 3000 lbs car) before entry than the Gt3 but the Gt3 catches up because he can carry much higher speeds into the corner than my stock suspension car. At the exit , even though i have more power , his reponse is faster ...That's why i was referring to the spool time of the turbo.

Both our cars had the same sumitomos tires....

 

I believe it has more to do with your rpm's when you come out of the apex. Your gearing on that particular turn needs to have your rpm's in a position of having good torque at the moment you want to apply gas.

Your turbo's won't have the response if your motor is on in a rpm range that is up in your torque curve.

 

It could be , I always struggle when shifting from 4th to 3rd braking and exiting the corner. I cant heel and toe yet...sometimes i get exit rpms right sometimes i dont. Maybe my trial braking is hurting the time to get the right rpms...lot to learn!

what is the right technique and sequence for this scenario?

 

Next time you come out of that corner, note your rpm's. If you have a dyno graph look up your torque numbers at that rpm. You need to come out of that turn in a rpm range that is up in the curve. Commonly referred to as the "sweet spot" This is the only way you will see the "response" you are looking for.

If you can't get to that spot you need to look a modifying your gearing.

 

There is also the issue of driver experience. It takes quite a while to learn to carry a lot of speed into corners - much longer than it takes to learn to build speed coming out of them.

I'm hoping to meet Vinod on track sometime so I can see what he and his car can do. C'mon buddy, Putnam Park, July 25-26 - drag Frank along with you.

Jon

 

Excellent point. I had assumed that wasn't the issue and he was carrying the proper speed into the turn.

 

Driver issue ...that's a sure one My car is way too fast for my skill level.

Will defintely try to work putnam park ! But i am planning on doing PCA Road america DE July 14 too...thats 2000$ in 2 weeks ..ouch! we will see.