1/4 miles times and horsepower - seeking expert opinions

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Joined
Aug 14, 2014
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Location
Madison, WI
My Car
1971 Mach 1 w/408C stroker
I am not an expert on 1/4 mile passes so seeking explanations on my times.
The picture shows 3 different time slips over 3 years. In 2021 I dyno my car just after the 1/4 mile run and it was making 366 rwhp. My ET was 13.57. In 2023 I dyno the car a day later on the same dyno for 404 rwhp. The ET was 13.64. Last week I dyno the car the day before yet in the same dyno for a whopping 471 rwhp. My ET was 13.39.
All these have been at the Hot Rod Power Tour so I keep dynoyng the car with the same vendor to track how the different mods affect performance. So the question is why is the ET not improving as rwhp is improving?. All these have been with same transmission, rear ratio and tires. Temperatures have been in between 80s and 90s for all the runs.
PS: i know i am behind on posting about improvements. I was very pleasantly surprised with the latest numbers. I will write about it in a separate post. In this post I am trying to make sense of the lack of relationship between ET and power.
 

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Many things going here Tony. First off your reaction times are all over the place (not being negative) but with that said the first thing that comes to mind are you putting weight on your car over the years? Also are you running different tires or pressures during your timing sessions (different traction)? I will also add I would analyze your dyno power curves and do appropriate adjustments for the drag strip (I know you like road race-totally different animal to drag race) Many racers find out there is not always a direct correlation between horsepower and ET's depending on many, many variables. With your substantial power increases my opinion is your tune is not optimized for for 1/4 mile at the top end. Just my opinion.
 
It would help if you could scale the car before each race or session. Preferably with each wheel 🛞 weighing separately. That way you can compare the time slips to the weight and cross weight.
 
Could be any number of things. My first guess would be that you're not that consistent of a driver. You're gonna need to run more than 3 quarter mile passes over 4 years to develop the skills to drive the car consistently.

Then after the driver, I would blame different track conditions. Your car may be hooking up better on one or something like that.
 
Thank you. Very good points. The weight has been about the same, maybe a bit lighter after the fiberglass hood and aluminum heads. My driving skills for drag racing are very basic since it is not my forte and definitely inconsistent. However, i did 7 runs in total last week and i posted the best ET. All the others are within 1/2 a second. They dont prep the tracks for the HRPT. I need to check the one run i recorded with my EFI system. Maybe the AFR is off compared to the dyno. The dyno runs are short compared to the drag strip so I need to check that fuel delivery is not an issue.
 
also consider the dyno and its variability. I’ve seen significant differences in results that boiled down to calibration errors by the dyno operator regarding critical inputs of temperature, atmospheric pressure and altitude.
Based on your times and speeds I’d say the car in its current condition is running pretty consistently.
 
I have a little experience. Two things, already mentioned, don’t trust the Dyno’s, they vary due to age, handling, use, and who knows if you are using the exact same one every year..l

Also, 60 ft times make or break the rest of your run. Below 2 seconds is preferred…. if your engine is bogging, raise the RPMs at launch, if your wheels are spinning lower the RPM at launch. You just have to do it to see if you can improve it. The lower you can get 60 ft times, the better your end result will be…

Beyond that, it gets complicated with slight gains, tire compound, tire pressure, weight and balance, fuel, engine tune and engine temp. Gearing, shift points, track prep, throttle application…and on and on…
 
There’s lots of variables, gear ratio, tire size, launch rpm, traction, shift points, weight.
I think if you have 471 rear wheel hp, and if the car is optimized for drag racing, that’s enough hp to be in the low 11 second range. Assuming the car isn’t set for drag racing, it should run in the 12’s.
 
As others said, there's a lot to unpack here...

First off, reaction times don't have anything to do with E.T.. The clock doesn't begin timing you until you roll out of the beams. You could sit there after the lights turned green and eat a sandwich, and THEN go, and run a quick time. You'd just lose, because the guy next to you didn't take the same sandwich break LOL..

Secondly, you're kind of ignoring your trap speeds. Trap speeds are the better measure/indicator of making power. E.T. is power combined with traction and gear. And it's the trap speed on your quickest and ostensibly highest-dyno rating run that puzzles me. You had a lousy launch (60-foot), but then the car REALLY started to accelerate down the track much stronger than the other prior two runs.. And then, at some point prior to going through the traps, it noses over and posts a pretty mundane trap speed.

Run # 1 makes sense to me... Lackluster 60-foot (launch) means you're not going to run the low-13's that you'd hope to. The whole run makes sense.

Run #2 makes sense to me... Ostensibly more power, but just gawdawful at getting that power to the ground in the 60-foot. That rotten 60-foot is why you ran slower, 13.64. But look at your trap speed, 107.46, 2.5 mph faster than the first run. That proves you were making more power than before, because the car OVERCAME a pokey beginning and gradually built better speeds at the 1/8 mile and 1/4 mile. That run makes sense.

Run #3.... First off, as others said, just because it's the same dyno, doesn't mean it's not at some point been recalibrated/corrected since the prior two years. Dynos are good for measuring differences in tune/performance in the right-here and right-now. Not over years. But looking at this 3rd pass, it appears that you INDEED were making yet-more-power than Runs 1 and 2. Your 60-foot was pretty much just as poor as run #2. But then, look at how you methodically improve on run #2, not just in E.T. at the 330 and 1/8 marks, but in MPH.. The car was almost 2mph faster at the halfway point, which SHOULD have compounded itself into a 4 mph advantage (at least) at the finish line. But that didn't happen... Everything was again, "making sense", until somewhere at the end of the run where your car posted a "quicker" E.T. than the prior two, but it LOST GROUND in MPH at the end.

"Something" happened as you were approaching the final traps. Maybe you let out of the throttle prior to having broken through all of the beams at the end (common rookie mistake, lifting early, not realizing how many timing beams there are to run through). Or maybe the car for some reason started starving-out on fuel.

Something happened. If you had CONTINUED the momentum you showed on that time slip, and had you continued to BUILD on the 1/8 mile trap speed, you would/should have run a 10th or 2 quicker with a 110 +/- mph trap speed.

Something got done wrong, or went wrong, right at the finish of run #3.

You're not going to run 12's without a better tire for the strip, the power you're making should easily post 1.9-ish in the 60-foot, which would buy you easily another half-second at the end, probably more. Early gains compound/amplify at the end.
 
As others said, there's a lot to unpack here...

First off, reaction times don't have anything to do with E.T.. The clock doesn't begin timing you until you roll out of the beams. You could sit there after the lights turned green and eat a sandwich, and THEN go, and run a quick time. You'd just lose, because the guy next to you didn't take the same sandwich break LOL..

Secondly, you're kind of ignoring your trap speeds. Trap speeds are the better measure/indicator of making power. E.T. is power combined with traction and gear. And it's the trap speed on your quickest and ostensibly highest-dyno rating run that puzzles me. You had a lousy launch (60-foot), but then the car REALLY started to accelerate down the track much stronger than the other prior two runs.. And then, at some point prior to going through the traps, it noses over and posts a pretty mundane trap speed.

Run # 1 makes sense to me... Lackluster 60-foot (launch) means you're not going to run the low-13's that you'd hope to. The whole run makes sense.

Run #2 makes sense to me... Ostensibly more power, but just gawdawful at getting that power to the ground in the 60-foot. That rotten 60-foot is why you ran slower, 13.64. But look at your trap speed, 107.46, 2.5 mph faster than the first run. That proves you were making more power than before, because the car OVERCAME a pokey beginning and gradually built better speeds at the 1/8 mile and 1/4 mile. That run makes sense.

Run #3.... First off, as others said, just because it's the same dyno, doesn't mean it's not at some point been recalibrated/corrected since the prior two years. Dynos are good for measuring differences in tune/performance in the right-here and right-now. Not over years. But looking at this 3rd pass, it appears that you INDEED were making yet-more-power than Runs 1 and 2. Your 60-foot was pretty much just as poor as run #2. But then, look at how you methodically improve on run #2, not just in E.T. at the 330 and 1/8 marks, but in MPH.. The car was almost 2mph faster at the halfway point, which SHOULD have compounded itself into a 4 mph advantage (at least) at the finish line. But that didn't happen... Everything was again, "making sense", until somewhere at the end of the run where your car posted a "quicker" E.T. than the prior two, but it LOST GROUND in MPH at the end.

"Something" happened as you were approaching the final traps. Maybe you let out of the throttle prior to having broken through all of the beams at the end (common rookie mistake, lifting early, not realizing how many timing beams there are to run through). Or maybe the car for some reason started starving-out on fuel.

Something happened. If you had CONTINUED the momentum you showed on that time slip, and had you continued to BUILD on the 1/8 mile trap speed, you would/should have run a 10th or 2 quicker with a 110 +/- mph trap speed.

Something got done wrong, or went wrong, right at the finish of run #3.

You're not going to run 12's without a better tire for the strip, the power you're making should easily post 1.9-ish in the 60-foot, which would buy you easily another half-second at the end, probably more. Early gains compound/amplify at the end.
Excellent analysis. Thank you.
You are very right. There was a run I remembered the car loosing power towards the end, hence I was thinking of fuel starvation. So most likely that was run #3. I dont have the slip right now to take a picture, but my fastest speed last week was about 108.
Throughout the week I learnt to start once the third light lights up. That made a huge improvement in my reaction times. Definitely dont have that good of traction with these tires. I was trying to balance between spin and no spin.
 
What are your shift points? Rear gears? What RPM does the car make max power?

An old trick is to take Trap Speed x ET. It should be around 1320, higher means you are not making efficient use of the car's power. Traction, shift points, rear gears. Yours are all well into the 1420+ range
 
What are your shift points? Rear gears? What RPM does the car make max power?

An old trick is to take Trap Speed x ET. It should be around 1320, higher means you are not making efficient use of the car's power. Traction, shift points, rear gears. Yours are all well into the 1420+ range
I tried a few runs shifting at about 6,200 and others at about 5,750. Not much change in ET.
Here is my dyno pull. This is about 250 rpm off since I let off near 6,500 instead of 6,250 as it shows. That said top HP is very flat between 5,250 and 6,250.

Dyno HRPT 2024.jpg
 
Best way to lower ET is power versus traction. As others have said your run is influenced by your 60 foot. If you have unlimited power then you could roll into the power as traction permits. Most of us don’t have unlimited power so practice practice and more practice drag racing can pay off with smaller gains
 
It's the 60' times. They are not great. You need to work on the launch.
I agree. You have plenty of HP but you need to improve traction and launch. MT drag radials should be on the list. Launch with no tire spin first, then work on more RPM sooner with no spin. Get 60 foot times into 1.8 territory. That's a start. Ignore reaction times until 60 foot is consistent and into the 1.xx's.
 
Like others have said.
I would get a set of wheels and tires to eliminate the inconsistency of traction.
Switch them when you get there.
But be prepared for possible breakage 😂
I remember the first time trying slicks I was shocked. When I did my first little hop out of the water box the tires bit so hard my hands came off the steering wheel and the shifter.
I was instantly hooked! 😍 no pun intended.
 
Thank you. I was already launching close to the tire spin limit. My focus is road course tracks and autocross so no drag radials in my future. I was doing the drag strip just for fun during the Tour so i was intrigued by the results. I have learned a lot from you all. Thank you😃
 
Like others have said.
I would get a set of wheels and tires to eliminate the inconsistency of traction.
Switch them when you get there.
But be prepared for possible breakage 😂
I remember the first time trying slicks I was shocked. When I did my first little hop out of the water box the tires bit so hard my hands came off the steering wheel and the shifter.
I was instantly hooked! 😍 no pun intended.
First time you put slicks on it and launch it, you learn which parts were the weak links. Probably don't wanna be learning that on power tour unless there are a lot of vendors there with all sorts of spare parts and tools for sale. :D
 

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