[mysticmissle]

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

[PAUL K]

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

[steve25]

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.

[Stan Weiss]

Vehicle coefficient of Drag
Vehicle Frontal Area

Auto Trans % converter slippage

Stan

[mysticmissle]

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

[Elarson]

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

[ponjohn]

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.

[Trevor78]

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

[AG]

My car runs 10.01 and the mph difference from 1/8 to 1/4 is 25.3 mph with 6.8% converter slip.

[Drag Star Le Mans]

My car is slow and I don't think about these things. Some day maybe I will??

[johnta1]

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

That range of mph is only good for a roughly 10 sec 1/4 mile ET?
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

[johnta1]

Also, multiply the 1/4 MPH by .813 to get a rough 1/8th MPH.

[chiefbigb]

Do manual transmission cars pick up more than automatic ones?

[Stan Weiss]

Does anyone here run or know someone that runs a lockup convertor?

Stan

[grandam1979]

Low 11’s pick up 23 mph

[PAUL K]

Originally Posted by Stan Weiss

Does anyone here run or know someone that runs a lockup convertor?

Stan

Yes , my wife does in her Grand National

[S/st 54]

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.

[johnta1]

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?)

[Slick Poncho]

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

[S/st 54]

Originally Posted by johnta1

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?)

Throttle Stop was not used. These were wide open passes.

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.