Tyre pressures and 1/4 mile times....
Discussion
Hi all,
I think I have an issue with my car after some poor 1/4 mile times.
My Skyline (R33 GTR) usually runs late 11s, personal best is 11.76 @ 116 mph (engine, injectors and turbos all standard), the normal times are 11.9s mostly.
But, I recently went to Shakespeare raceway, then Santa pod a few weeks later, and now my car is running 12.3s pretty much every run!!!
Everything appears ok though, it's boosting ok, there is little sign of any detonation on my power FC hand commander, no sign of misfire or clutch slip, I'm going through the gears nicely, same as ever, but I've already lost 6/10ths of a second to the 1000ft marker over my personal best.
The only difference in the car is that I usually run 16psi in the rear tyres, but on the last 2 outings I've dropped the front tyre pressures to 19psi as well.
My 0-60fts are better, my last run out at Santa pod produced two new 0-60ft personal bests.
My personal best 1/4 mile run was with a 1.64, and now I can run 1.60s with the pressures dropped on the fronts, but then it all falls apart. My terminal speeds are down from 115-116mph, to 107-109mph too.
Could this be down to the front tyre pressures? Can it really make that much difference or do you suspect I have an underlying fault with the car not related?
Any advice or input appreciated.
Cliff
I think I have an issue with my car after some poor 1/4 mile times.
My Skyline (R33 GTR) usually runs late 11s, personal best is 11.76 @ 116 mph (engine, injectors and turbos all standard), the normal times are 11.9s mostly.
But, I recently went to Shakespeare raceway, then Santa pod a few weeks later, and now my car is running 12.3s pretty much every run!!!
Everything appears ok though, it's boosting ok, there is little sign of any detonation on my power FC hand commander, no sign of misfire or clutch slip, I'm going through the gears nicely, same as ever, but I've already lost 6/10ths of a second to the 1000ft marker over my personal best.
The only difference in the car is that I usually run 16psi in the rear tyres, but on the last 2 outings I've dropped the front tyre pressures to 19psi as well.
My 0-60fts are better, my last run out at Santa pod produced two new 0-60ft personal bests.
My personal best 1/4 mile run was with a 1.64, and now I can run 1.60s with the pressures dropped on the fronts, but then it all falls apart. My terminal speeds are down from 115-116mph, to 107-109mph too.
Could this be down to the front tyre pressures? Can it really make that much difference or do you suspect I have an underlying fault with the car not related?
Any advice or input appreciated.
Cliff
Edited by Cliff J on Thursday 9th September 11:59
catch up with trackday addict when he gets back from the Euro finals, he may have some ideas for you...
I'm slightly reluctant to run the car up the strip again if there is a problem, but then I'm stuck with the dilema of paying for track time or rolling road dyno time, possibly by going to the track I could end up paying for both. Not a great situation when money is tight in life.
I saw John Bradshaw at Santa pod when I was having my issues, he was competing in the HKS drag series and I was in the HKS drag series academy. I mentioned my problems to John and also to his Abbey motorsport support team, who also look after my car, but there wasn't much feedback, I guess because there isn't anything obviously wrong and it's impossible to diagnose without the full facts and figures, maybe also without the dyno too to see everything that maybe I can't in the heat of the 12.3 seconds I was running for.
Tyre pressures were mentioned for the lower terminal speed, but I just can't see them being responsible for the 1/4 mile time, but have to keep thinking about the fact that it's the only change I've made to the set-up.
I saw John Bradshaw at Santa pod when I was having my issues, he was competing in the HKS drag series and I was in the HKS drag series academy. I mentioned my problems to John and also to his Abbey motorsport support team, who also look after my car, but there wasn't much feedback, I guess because there isn't anything obviously wrong and it's impossible to diagnose without the full facts and figures, maybe also without the dyno too to see everything that maybe I can't in the heat of the 12.3 seconds I was running for.
Tyre pressures were mentioned for the lower terminal speed, but I just can't see them being responsible for the 1/4 mile time, but have to keep thinking about the fact that it's the only change I've made to the set-up.
Edited by Cliff J on Thursday 9th September 12:30
Tyre pressure can make all the difference. It's basic Physics. The lower the air pressure in the tyre the more "Rolling Resistance" there is and there would also be a bigger "tread print" on the road. Put the tyres back up to around 28 or 30+ and your ET's will go down because there would be less "Rolling Resistance". Ask any of the Street Racers.
Thanks for the repy Geoff, that's really very interesting information.
I'm going to book a rolling road dyno check up, which I guess I should do anyhow as it's been over a year since it was set-up and mapped, and if it's all ok I will head back to the strip with my front tyre pressures back to their normal road pressures.
I think I'll still keep the rears down low as it took me an age to perfect them and get my starts perfected, backed up with an excellent ET for my state of tune.
I'm going to book a rolling road dyno check up, which I guess I should do anyhow as it's been over a year since it was set-up and mapped, and if it's all ok I will head back to the strip with my front tyre pressures back to their normal road pressures.
I think I'll still keep the rears down low as it took me an age to perfect them and get my starts perfected, backed up with an excellent ET for my state of tune.
Sorry i should have said put the front up only. Keep the rears as they are. I don't know if it's still available but there used to be a book about the basics of drag racing and how to set your car up. Try searching the net for "The Basics of Drag Racing" you may come up trumps.
Pumping up your front tyres will reduce the rolling resistance. The rear tyres i assume are low profile but as you accelerate they do "grow" albeit very little thus it changes the gearing slightly giving you more mph. If you look at any race car with slicks on you will see a "Wrinkle Wall" on the tyre. As they accelerate the tyre grows and it helps give the car more mph. ie the rear axle ratio becomes taller. There is actually a mathmatical formula you can use which you can work out what your technical top speed would be given certain factors such as height of tyre, gear ratio etc etc. Also it might help you if you got a copy of the construction rules from the Santa Pod Supporters Club.
Pumping up your front tyres will reduce the rolling resistance. The rear tyres i assume are low profile but as you accelerate they do "grow" albeit very little thus it changes the gearing slightly giving you more mph. If you look at any race car with slicks on you will see a "Wrinkle Wall" on the tyre. As they accelerate the tyre grows and it helps give the car more mph. ie the rear axle ratio becomes taller. There is actually a mathmatical formula you can use which you can work out what your technical top speed would be given certain factors such as height of tyre, gear ratio etc etc. Also it might help you if you got a copy of the construction rules from the Santa Pod Supporters Club.
Also another thought...go through the car to make sure nothing is binding. ie your brakes are adjusted properly, Believe me it can be something really silly that has caused the car to slow. It might even be the driver! I don't know about other racers but Shakespeare Raceway and Santa Pod do differ at least they did when i raced. I always got 5 or 6 mph more at Shakespeare than at the Pod. As i said go through the car, Set it up to Drag Race not to go round corners!! You will soon be back done running 11.7's
Superb Geoff, excellent posts again thanks.
I hadn't considered brakes at all in my problem thinking. My PB was set at Santa pod with the 116mph speed, but now at the pod I'm 6/10ths down and 7-9mph down, but 60fts are at an all time high, which is very annoying to say the least.
I'll have a search around now for "the basics of drag racing", see what I find.
My car is on coilover suspension, not the best for dragging I know, but I've set it to the softest rebound setting for the rear and quite hard on the front.
Tyres are Toyo Proxies R888, not a great profile or size either at 285/30/18.
Something works though, I have one of the best times in the world for a Skyline on standard ceramic turbos, most who have bettered my time are running insane kamikaze boost levels (1.5bar), where mine is a safe 1 bar, but it's quite meaningless right now running 12.3s, which used to be my worst times with the current set-up, that used to be a time I'd achieve if I missed 3rd gear!!!
I hadn't considered brakes at all in my problem thinking. My PB was set at Santa pod with the 116mph speed, but now at the pod I'm 6/10ths down and 7-9mph down, but 60fts are at an all time high, which is very annoying to say the least.
I'll have a search around now for "the basics of drag racing", see what I find.
My car is on coilover suspension, not the best for dragging I know, but I've set it to the softest rebound setting for the rear and quite hard on the front.
Tyres are Toyo Proxies R888, not a great profile or size either at 285/30/18.
Something works though, I have one of the best times in the world for a Skyline on standard ceramic turbos, most who have bettered my time are running insane kamikaze boost levels (1.5bar), where mine is a safe 1 bar, but it's quite meaningless right now running 12.3s, which used to be my worst times with the current set-up, that used to be a time I'd achieve if I missed 3rd gear!!!
Forget your 60 foots mate, there is still 1260 feet after that...
Try to look at getting your 330 foot and 1/8th times down, rest will follow
Like has been said, get the front PSI right up, or if you can fit skinnier wheels/tyres on front (remember weight of car, to do it safely)
Sounds a quick motor though, good luck with it
Try to look at getting your 330 foot and 1/8th times down, rest will follow
Like has been said, get the front PSI right up, or if you can fit skinnier wheels/tyres on front (remember weight of car, to do it safely)
Sounds a quick motor though, good luck with it
The probable reason your 60ft times "look" better is because you lowered the front tyre pressures. This will make the tyre bulge a bit more at the bottom and will lengthen your roll out. Providing you stage shallow every time, lowering your front tyres will improve your 60ft times (but not really, it just reduces the 60 ft to 59ft 10 inches) but, as others have said, rolling resistence will slow you down for the rest of the trip.
If you can't explain roll-out to yourself and undertand why it will affect your 60 foot times, then you'll need to put that hole in your knowledge right. It is important and will stop you misleading yourself from your own timing slip information.
If you can't explain roll-out to yourself and undertand why it will affect your 60 foot times, then you'll need to put that hole in your knowledge right. It is important and will stop you misleading yourself from your own timing slip information.
Edited by ribaric on Friday 10th September 12:28
Just to add a little on this for those who dont know the GTR set up. This car is 4 wheel drive when rear slip is detected (ecu controlled centre diff) The rolling resistance theory is a good one and worth putting pressures back up to try.
Have Abbey done a leak down test since your slower et/speeds?
My gut feeling is you may be down on power a tad giving the lower mph so be careful at full boost in case no5 or 6 piston is damaged (im assuming stock pistons but even forged can fail when temps are high. Just my 2p
Tony
Have Abbey done a leak down test since your slower et/speeds?
My gut feeling is you may be down on power a tad giving the lower mph so be careful at full boost in case no5 or 6 piston is damaged (im assuming stock pistons but even forged can fail when temps are high. Just my 2p
Tony
I was always taught that 1/10th off your 0-60ft time at the start, roughly equals 2/10th's at the finish, hence my 0-60ft fascination. I will return the front tyres to normal pressures once it's been checked over by Abbey Motorsport though.
As 'Tony g' explained, the front wheels do come into play when I lose traction on the rear and have very often been in a 4 wheel spin off the line with the fronts at full pressure, hence playing with them after some advice.
I haven't had a leak down test, I suppose I should get it done, but going by the in-car gauges I've never thought it neccessary. My oil pressure is superb, as always, and no sign of detonation on the Power FC commander. My pistons are standard.
As 'Tony g' explained, the front wheels do come into play when I lose traction on the rear and have very often been in a 4 wheel spin off the line with the fronts at full pressure, hence playing with them after some advice.
I haven't had a leak down test, I suppose I should get it done, but going by the in-car gauges I've never thought it neccessary. My oil pressure is superb, as always, and no sign of detonation on the Power FC commander. My pistons are standard.
Somebody made a reference to 'basic Physics' regards front tyres and effect on MPH and it made me think. So I'm going to stick my neck out -
I've just done a quick calc for a 2500lb cars horsepower requirements going from 108mph to 115mph and i get 243hp and 295hp - a difference of 52hp. If the 7 mph lost is solely due to front tyre drag, as has been speculated, each front tyre would be absorbing about 26hp. 1 horspower is roughly equivalent to 750 watts, so each tyre would be heated by the equivalent of nearly 20 kilowatt, or a 20 bar electric fire!
Unless the tyres are red hot after the run, the front tyres ain't responsible for all the power loss!
I don't know what weight your car is so the above calcs may be slightly different one way or another, depending whether your car is heavier or lighter, but I don't think they are wrong by much...
I've just done a quick calc for a 2500lb cars horsepower requirements going from 108mph to 115mph and i get 243hp and 295hp - a difference of 52hp. If the 7 mph lost is solely due to front tyre drag, as has been speculated, each front tyre would be absorbing about 26hp. 1 horspower is roughly equivalent to 750 watts, so each tyre would be heated by the equivalent of nearly 20 kilowatt, or a 20 bar electric fire!
Unless the tyres are red hot after the run, the front tyres ain't responsible for all the power loss!
I don't know what weight your car is so the above calcs may be slightly different one way or another, depending whether your car is heavier or lighter, but I don't think they are wrong by much...
Well, I returned to the drag strip yesterday and left my front tyre pressures alone, just dropped the rears to 16psi as per usual, ....... guess what? All my times and terminal speeds have returned to normal!!!
I ran 4 times and got quicker each run, 12.1, 12.0, 11.9, 11.8, all at 115mph.
So, in answer, tyre pressures make a hell of a difference, especially if you don't have monster power like my own car.
Lesson learnt
I ran 4 times and got quicker each run, 12.1, 12.0, 11.9, 11.8, all at 115mph.
So, in answer, tyre pressures make a hell of a difference, especially if you don't have monster power like my own car.
Lesson learnt
Cliff J said:
Well, I returned to the drag strip yesterday and left my front tyre pressures alone, just dropped the rears to 16psi as per usual, ....... guess what? All my times and terminal speeds have returned to normal!!!
I ran 4 times and got quicker each run, 12.1, 12.0, 11.9, 11.8, all at 115mph.
So, in answer, tyre pressures make a hell of a difference, especially if you don't have monster power like my own car.
Lesson learnt
Nice one mate, glad to hear itI ran 4 times and got quicker each run, 12.1, 12.0, 11.9, 11.8, all at 115mph.
So, in answer, tyre pressures make a hell of a difference, especially if you don't have monster power like my own car.
Lesson learnt
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