Half Track (660ft) E.T. & MPH
Started by
LXCHEV
, Jan 29 2007 03:14 PM
59 replies to this topic
#51
Tiny
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Posted 15 April 2008 - 09:17 PM
#52
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Posted 17 April 2008 - 12:21 PM
Is this a general rule of thumb for all cars - that you want to cross the finish line around 400 RPM above your peak power RPM level???Ideally you want to cross the line at about 7000rpm if your engine makes peak power at 6600.
Also - I'm curious about all this talk of "convertor slip". It's easy to say 'assuming 4-5% slip' but where do these figures come from? How can the general hack like me find out how much slip my convertor has? Does this come back to mathematical equations too?
#53
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Posted 18 April 2008 - 07:48 AM
Hi LXCHEV, 400rpm past peak HP works for me. To get the very best ET you can rev the engine well past peak HP. Some use an average that centres on the peak. ie convertor stalls at 5500, peak HP is at 6500, cross the line at the rpm that has the same HP as the stall speed (in this case it would be about 7200 as HP drops quicker after the peak than it does before). I hope this makes sense, I'm not sure I have explained it clearly enough.Is this a general rule of thumb for all cars - that you want to cross the finish line around 400 RPM above your peak power RPM level???Ideally you want to cross the line at about 7000rpm if your engine makes peak power at 6600.
Also - I'm curious about all this talk of "convertor slip". It's easy to say 'assuming 4-5% slip' but where do these figures come from? How can the general hack like me find out how much slip my convertor has? Does this come back to mathematical equations too?
To calculate the convertor slip use the calculator to work out RPM from your tyre height and gear ratio. This will give you a zero slip figure. Then see what your actual RPM is and calculate the difference as a percentage. These days most quality convertors are around 5-6% however some old school ones are at about 15%. Ten to twelve years ago 10% was acceptable.
#54
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Posted 18 April 2008 - 01:29 PM
#55
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Posted 18 April 2008 - 02:13 PM
Awesome, thanks Struggler... very interesting info and good food for thought....
#56
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Posted 26 May 2008 - 08:32 PM
Alright, so I was out at Heathcote yesterday and managed to fight my way to 4 whole runs in the whole day.
Disappointingly I was not able to improve on my PB of 12.16@155mph and only managed a best of 12.34@109mph. Comparing the two timeslips the 60fts are identical, with losing most of my time after the 660ft. The big loss in mph is obviously having its effect the further I get down the track.
So the question is, what can cause the 6mph loss? I changed 2 things, full exhaust instead of dumped at the diff and dropped my ET Streets to 15psi cold from 18psi last time. While the temp was 4 degree's colder (14.7) the humidity was up 22% to 55%. Am I right in thinking the track temp is what has robbed my mph?
I did manage to confirm the need for the 4.56 rear ratio - leaving at 1500-2000rpm and shifting right on the knocker at 6500rpm I was still crossing at 6100.
Cheers,
Disappointingly I was not able to improve on my PB of 12.16@155mph and only managed a best of 12.34@109mph. Comparing the two timeslips the 60fts are identical, with losing most of my time after the 660ft. The big loss in mph is obviously having its effect the further I get down the track.
So the question is, what can cause the 6mph loss? I changed 2 things, full exhaust instead of dumped at the diff and dropped my ET Streets to 15psi cold from 18psi last time. While the temp was 4 degree's colder (14.7) the humidity was up 22% to 55%. Am I right in thinking the track temp is what has robbed my mph?
I did manage to confirm the need for the 4.56 rear ratio - leaving at 1500-2000rpm and shifting right on the knocker at 6500rpm I was still crossing at 6100.
Cheers,
#57
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Posted 26 May 2008 - 09:08 PM
Firstly, your car will not run 155 mph.
Post your old and new incrementals so we can make an assessment.
It would be easy to believe the extra exhaust would cost you MPH, both in lost power and additional weight.
Post your old and new incrementals so we can make an assessment.
It would be easy to believe the extra exhaust would cost you MPH, both in lost power and additional weight.
#58
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Posted 26 May 2008 - 10:58 PM
Hehe, you're right 155mph is a tad out of reach!
P.B / Latest
1.78 1.78
5.05 5.07
[email protected] / [email protected]
10.15 10.27
12.16@115mph / 12.34@109mph
Interesting that 3ft of tailpipe would make that much of a difference. BTW the extra weight of the exhaust would have been cancelled out by the 15lt less fuel I had in the tank yesterday.
Thanks for your help.
P.B / Latest
1.78 1.78
5.05 5.07
[email protected] / [email protected]
10.15 10.27
12.16@115mph / 12.34@109mph
Interesting that 3ft of tailpipe would make that much of a difference. BTW the extra weight of the exhaust would have been cancelled out by the 15lt less fuel I had in the tank yesterday.
Thanks for your help.
Edited by MAWLER, 26 May 2008 - 11:00 PM.
#59
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Posted 27 May 2008 - 08:04 AM
Your early loss is quite minimal suggesting the torque is still there, but you are losing quite a bit of HP up the top.
It is not just exhaust backpressure that has an effect, the total length of the system has a bearing on output as well. You must have gone from optimum with the dump at the diff, to worst case with the tailpipes on.
How was the wind both days ? A head or cross wind usually affects the second half of the run a lot more than the first half.
It is not just exhaust backpressure that has an effect, the total length of the system has a bearing on output as well. You must have gone from optimum with the dump at the diff, to worst case with the tailpipes on.
How was the wind both days ? A head or cross wind usually affects the second half of the run a lot more than the first half.
#60
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Posted 27 May 2008 - 07:32 PM
Interesting, i'd be interested to do a back to back dyno run to get a measureable on just how much difference the exhaust makes. Obviously the exhaust thing makes a difference, but I'd always thought to make gains from dropping the system it would require tuning in that format.
Not sure which direction the track runs at Heathcote but yeah wind is almost opposite - 301 degrees for PB and 154 degrees on sunday.
Not sure which direction the track runs at Heathcote but yeah wind is almost opposite - 301 degrees for PB and 154 degrees on sunday.
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