back from track (density altitude lesson)

[bhuff30]

Ever come back from the track after making some changes to the car, and wonder what the heck happen? Why are you SLOWER?!

I'm back from the track. I only got to make 2 passes, and they were both slower than my previous best. I ran [email protected] which is slower than the time in my signature by 2 mph and several tenths. But how's that possible? I added underdrive pullies, and drop in K&N, nitto drag radials and lost that much? That's why you have to calculate the density altitude!

The DA last night was 2100. Not that great. The DA when I ran the 14.26 was -600! My track is at 800' elevation, but with cold air and rediculously high pressure, the DA dropped to -600. So adding correction factors (can be found from NHRA) I actually gained .260 seconds and 1 mph.

So there you have it. Under drive pullies and a drop in K&N are worth about 1mph.

The car has more to give, but I had some other issues. First, the clutch is getting a bit soft. It doesn't grab hard enough these days, and on a really hard 3500+rpm launch, the rpms won't drop to 3500. They'll hang around 4000-4500 until the speed cetches up, even though the clutch is all they way out. It also took longer than normal for the rpms to drop after a hard power shift. So note to self: don't abuse clutch until replaced!

 

[98COBRA281]

DA is my worst enemy!!! my best run from last saterday was a 12.81 with a 3100+ DA!!!!

alot of people dont under stand that a bad DA can slow you down by 2 or 3 tenths EASY!!!

 

[Barnaby]

I envy your DAs. Last time I ran the DA was over 8000ft.

 

[98COBRA281]

I envy your DAs. Last time I ran the DA was over 8000ft.

thats sucks... talk about 12 second cars running 15's

 

[bhuff30]

Exactly why I posted this. I can't count the number of times someone has posted up "my car ran slower, even though I added XXX parts". Or the classic case of what the magazines publish vs what you can actually do in the real world. Weather is not perfect in the real world.

I had a guy with a new SS camaro at the track last night telling me expects to run low 12s with a tune, and deep 12s as it sits bone stock. I then watched him run a series of 13.4 and worse times... and with the worst track manners I've ever seen. He stayed in the gas after the finish line to try to pass my friend after losing.

DA is my worst enemy!!! my best run from last saterday was a 12.81 with a 3100+ DA!!!!

alot of people dont under stand that a bad DA can slow you down by 2 or 3 tenths EASY!!!

 

[BurningRubber]

Good post.

I have never ran my car in ideal conditions.

It's either been stupid hot AND humid, or cool weather but a ton of moisture in the air.

I would love to hit the track with a nice day, and since the tracks around me are right around sea level, weather is really the only thing for me to worry about.

 

[Pearl02]

Exactly why I posted this. I can't count the number of times someone has posted up "my car ran slower, even though I added XXX parts". Or the classic case of what the magazines publish vs what you can actually do in the real world. Weather is not perfect in the real world.

I had a guy with a new SS camaro at the track last night telling me expects to run low 12s with a tune, and deep 12s as it sits bone stock. I then watched him run a series of 13.4 and worse times... and with the worst track manners I've ever seen. He stayed in the gas after the finish line to try to pass my friend after losing.

Did you run at Topeka? Pearl02.

 

[bhuff30]

KCIR

Did you run at Topeka? Pearl02.

 

[GettinThere]

He stayed in the gas after the finish line to try to pass my friend after losing.

What a douche, he needs to be banned from the track.

 

[Winters98GT]

Brian, how is the car getting out of the hole with the nittos? Can you post up the full runs of each with 60fts if you have them?

A while back when I was doing my write up on my 97, the pulleys were worth close to .2 and 1mph if I can recall when I ran at the same track, similar 60ft's and DA's. I doubt the KN helped any, though. Was there any signs of the clutch slipping at the top end of the track, or did it stick or feel like jello after any runs?

What's weird is, tonight I trapped an identical MPH that I did last year, but with 1000ft more elevation, and no mods. I'm still looking at the runs and weather to try and figure out why.

DA does play a huge factor though, great info for those getting into drag racing.

Mark

 

[VEE EIGHT]

You guys ever want to see C6 Z06's run 13's and even 14's? Come to west Texas. DA is a major killer.

 

[bhuff30]

Mark,

The first pass, I bogged down a bit. Probably surprised by the traction of the nittos. I held 2500 rpm on the launch and with the shallow 3.27 gave me a poor 60':
60 2.221
330 6.164
1/8 9.428 @ 75.34
1000 12.241
1/4 14.623 @ 94.77

Here's the best time I made with the nittos on. On launch, was slipping the clutch from high rpms, and the clutch was slipping steadily at 4500 until I gained enough speed. I would rather slip it from 3500, but the clutch just couldn't pull the rpms down because of the engines momentum:
60 2.120
330 6.000
1/8 9.261 @ 75.37
1000 12.073
1/4 14.456 @ 94.43

The clutch holds fine in high gears and even at 100 mph. The problem is when at full throttle, the engine has enough momentum, it takes a little time to bring the engine rpms down. It would usually bark 2nd, but powershifting 3rd and 4th would take about 2/10ths of a second for the rpms to drop to where they should be. It should be an instant drop in rpms if the clutch is strong. The pedal never really felt mushy, like I was overheating it, but it has a limited life now. It has 75k miles, 30 full passes, and countless daily street abuse.

Brian, how is the car getting out of the hole with the nittos? Can you post up the full runs of each with 60fts if you have them?

A while back when I was doing my write up on my 97, the pulleys were worth close to .2 and 1mph if I can recall when I ran at the same track, similar 60ft's and DA's. I doubt the KN helped any, though. Was there any signs of the clutch slipping at the top end of the track, or did it stick or feel like jello after any runs?

What's weird is, tonight I trapped an identical MPH that I did last year, but with 1000ft more elevation, and no mods. I'm still looking at the runs and weather to try and figure out why.

DA does play a huge factor though, great info for those getting into drag racing.

Mark

 

[98COBRA281]

try launching a little higher, with my M/T, i leave at 4000-4500prms, and just dump it, even with my power level and 4.30's it still almost wants to bog, but i have cut plenty of 1.7 60's.....

 

[bhuff30]

The last run, I didn't dump the clutch, but I did let it out very aggressively and fast. That run, the clutch was slipping right at 4500 until the speed of the car caught up. Maybe if I actually dumped the clutch, it would have grabbed and allowed a little tire spin, and put the power to the ground instead of making heat in the clutch. Playing around on the street, I did get tire spin on one launch with the nittos. It was damn fun too! It's amazing how fast the shift to 2nd gear comes, even with 3.27 gears.

Anyway, I don't much feel like replacing the clutch *quite* yet, so it is easy driving for a while. Long as I don't launch above 3500 or powershift, the clutch shows no signs of weakness.

 

[Winters98GT]

Brian,

Thanks man. I know you have quite a bit of track experience, so I am sure you will get that sixty foot down and nail your goals. Have you tried dumping the clutch and getting some wheel spin? Some times, it works better for me working backwards. See where the car spins and use a little spin to your advantage, and see how much you need to slip it. What PSI are you running?

BTW, how is the 4banger running?

Mark,

The first pass, I bogged down a bit. Probably surprised by the traction of the nittos. I held 2500 rpm on the launch and with the shallow 3.27 gave me a poor 60':
60 2.221
330 6.164
1/8 9.428 @ 75.34
1000 12.241
1/4 14.623 @ 94.77

Here's the best time I made with the nittos on. On launch, was slipping the clutch from high rpms, and the clutch was slipping steadily at 4500 until I gained enough speed. I would rather slip it from 3500, but the clutch just couldn't pull the rpms down because of the engines momentum:
60 2.120
330 6.000
1/8 9.261 @ 75.37
1000 12.073
1/4 14.456 @ 94.43

The clutch holds fine in high gears and even at 100 mph. The problem is when at full throttle, the engine has enough momentum, it takes a little time to bring the engine rpms down. It would usually bark 2nd, but powershifting 3rd and 4th would take about 2/10ths of a second for the rpms to drop to where they should be. It should be an instant drop in rpms if the clutch is strong. The pedal never really felt mushy, like I was overheating it, but it has a limited life now. It has 75k miles, 30 full passes, and countless daily street abuse.

 

[bhuff30]

Next time I'm out, I'll try dumping it. It is my daily driver, so I probably won't take it out again for a year or more; I need that car. I definately know what you are saying about a little wheel spin helping. I was running 16psi in the tires on those runs since it worked well on the 4cyl. I was planning to make a 3rd run, at 29psi in the tires, trying to encourage a bit of wheelspin and help my mph from less rolling resistance, but the track got oiled down at the top end before I could run. I didn't feel like waiting an hour and a half, so I left.

The 4cyl is still broken. Last time out, I ran a 12.36 and cracked a cylinder wall in the process. I was expecting a blown head gasket... it was running on 3cyls and mixing coolant/oil, but no such luck. I found a 6" long crack in the #2 cylinder. The problem is, I'm running the NA block and boring .030 over. The next engine will probably get partially filled and/or standard sized pistons. But I'm not sure if and when I can get it back together. I may just get a stock turbo shortblock for 150$ and throw it together for some more cheap fun.

Brian,

Thanks man. I know you have quite a bit of track experience, so I am sure you will get that sixty foot down and nail your goals. Have you tried dumping the clutch and getting some wheel spin? Some times, it works better for me working backwards. See where the car spins and use a little spin to your advantage, and see how much you need to slip it. What PSI are you running?

BTW, how is the 4banger running?

 

[Winters98GT]

Sounds great man! I'm sorry that the turbo is still down. You came such a long way in that thing, but $150 bucks for a shortblock and low 12 second fun seems like a good idea to me. Good luck, let me know how it turns out.