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#1
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back half mph
can someone explain why most 700 HP and up cars pick up 25-30 mph in the last 1/8 and some are a little more or a little less 27-28 seems the most common
i believe it is mostly a mathematical problem, effected by: vehicle weight? gearing rpm thru trap and thus shift points earlier or later on the previous 1/8 mile track rich/lean what else would effect that MPH? thanks
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1969 carousel red firebird 455, richmond 5 speed 1964 540 gto 1971 lemans sport convertible 1972 Maverick under slow construction |
#2
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No idea..... My brother picks up 27 mph and I pick up 29. My car is heavier and makes more power but they ET about the same in the 1/8 and I'm quicker in the 1/4..... Things that make you go hmmmm.
PS when I ran 12's they didn't give you 1/8 track data |
#3
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It’s very hard to nail that down, but one thing that I think you should add to what you have questioned about would be the amount of internal fiction a given motor has.
How many folks actually build with keeping that factor as low as they can? If your motor for example is eating up 80 Hp at 7000 rpm ( which I think is kind of a low number ) then what can be said? In terms of gearing which you mentioned, that I think is a big point of interest also. I think most of the larger mph cars are set up for a narrow max power band of like 2000 rpm and the gearing chosen keeps that motor in that narrow range. So you then have a car that’s applying maximum power for the the entirety of the time that the run takes place. How many builds that run only low 11s use Aluminum rod’s for for far faster reciprocating assembly acceleration? And all of this of course assumes that the timing and fuel mixture are dead on for what the motor wants.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! Last edited by steve25; 07-07-2023 at 03:03 PM. |
#4
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Vehicle coefficient of Drag
Vehicle Frontal Area Auto Trans % converter slippage Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#5
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if converter was tight it would bring it up down and loose less mph? if its uncoupled loose, its probably not accelerating as fast as if it was tight
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1969 carousel red firebird 455, richmond 5 speed 1964 540 gto 1971 lemans sport convertible 1972 Maverick under slow construction |
#6
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This isn't an answer to the specific question, but for general knowledge, a couple of competing factors are at play as the speeds go higher.
The obvious one is that aero drag is going up. The less obvious one has to do with power delivered to the ground. The power that an engine makes is constantly getting divided between accelerating the car and accelerating the internal engine components. At higher speeds and in higher gears, the engine is accelerating more slowly so more power is available at the rear wheels. The same effect shows up on the engine dyno. Set it for a faster accel rate (higher rpm/sec) and it will register less indicated power at the waterbrake. The same effect also shows up if you gear a car too (numerically) high and it slows down. You expend more of the available power zinging the engine internals up to speed and less is going to the track. FWIW, Eric
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"Everybody has a plan until they get punched in the mouth" noted philosopher Mike Tyson Life begins at the end of your comfort zone. “The mind, once stretched by a new idea, never returns to its original dimensions.” |
#7
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I say convertor.
1. If convertor is loose or inefficient up top it will not mph but will ET. 2. If convertor is efficient it will pick up more mph but not load the engine enough to drag it down. I'd say this is a perfectly executed convertor design to the combo. 3. If the convertor is too tight it will not pick up. It will not ET, specifically it will not 60 ft. I am certain each 1-3 above has a window of error to accomplish nearly the same results. You can even discuss transmission, there is a reason why Stock Eliminator guys use a metric 200. I remember when Pontiac Dude went changed heads. He said the shift recovery was not great. This was the first time I was exposed to that piece of data. Makes sense, if it slow to recover from the shift change the MPH and ET will suffer. |
#8
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I always found that when the back of my car picked up speed over the front, it was usually a sign you have just spun
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#9
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My car runs 10.01 and the mph difference from 1/8 to 1/4 is 25.3 mph with 6.8% converter slip.
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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule. |
#10
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My car is slow and I don't think about these things. Some day maybe I will??
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DragStarLeMans |
#11
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Quote:
Other ET's would have a different back half range. A better way to look at it would be multiply your 1/4 ET by the 1/4 MPH. It should be around 1325. Above or below that shows different circumstances. For recreational use only
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John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
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#12
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Also, multiply the 1/4 MPH by .813 to get a rough 1/8th MPH.
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John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
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#13
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Do manual transmission cars pick up more than automatic ones?
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Be carefull of the feet you step on today.They may be attached to the a$$ you kiss tomorrow. |
#14
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Does anyone here run or know someone that runs a lockup convertor?
Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#15
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Low 11’s pick up 23 mph
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#16
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Yes , my wife does in her Grand National
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The Following User Says Thank You to PAUL K For This Useful Post: | ||
#17
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Great thread!
I’ve also wondered the same thing. Dads car regardless of hp level will only pick up 25 mph in the back half. 650hp 462 will pick up 25mph 900+hp 535 it will only pick up 25 mph in the back half. We’ve swapped converters and it’s now picking up 27 in the back half. But it has me scratching my head. We took a motor out of my car and dropped it in his and and it slowed down almost 2.5 tenths. Dads car with the a 535 was 350lbs lighter than my car and has a T-400 Trans, where mine only has a glide. My car has a 7AL-3 and dads has a 7AL-2. Same ign coils, same fuel systems, same converters,. Same rear gear and rear tire size, my car with the same motor picked up 30-33mph in the back half.,dad would only pick up 25mph. If I would have had a racepack in my car when I had that motor in it I would Be able to see a lot more and maybe have some answers to my back half questions. Dads car doesn’t have racepack. It is very handy when looking for answers.
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"You have to evaluate the past,Focus on the future,and that tells you what you have to do in the present"--Lou Holtz “It’s the process it takes to get to goals that sets us apart, the execution on every single play, one play,one life” Notre Dame Head Coach Marcus Freeman 69 GTO NHRA Super Street Car 2860lbs 10.854@160.12 in Brainerd MN 7/2024 |
#18
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What's the ET for the MPH?
Not throttle stopped? I'd say off-hand the TH400 is wasting a lot of power with the inner rotating mass compared to the glide? (or keeping the engine power range too low on shifts?)
__________________
John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
The Following User Says Thank You to johnta1 For This Useful Post: | ||
#19
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I run a 4l80E with a billet lockup converter. Last year at Norwalk I did some playing around.. got lockup to engage after completion of 3rd gear shift and car picked up a solid 3 mph in the 1/4. Im going to play with my setup again this year and try to get lockup to come in and stay in after 1-2 shift. Last year the car was running 10.74-10.80 @125
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#20
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Quote:
Right I know it confuses a lot of people how a car that’s 350lbs lighter goes slower all the way around. I know I don’t spend as much time on the old mans car as I do mine. But since Jeff brought up the thought I thought I’d throw that out there since it was relevant. I’ll figure something out on dads car before too long.
__________________
"You have to evaluate the past,Focus on the future,and that tells you what you have to do in the present"--Lou Holtz “It’s the process it takes to get to goals that sets us apart, the execution on every single play, one play,one life” Notre Dame Head Coach Marcus Freeman 69 GTO NHRA Super Street Car 2860lbs 10.854@160.12 in Brainerd MN 7/2024 |
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