Rear End Ratio Discussion
 

A lot to ask here so I'll try to be direct.


1968 GTO
Current Engine 462 Stroker with Iron Heads, full Roller
10.25 Comp, Run mostly on 93 Octane, sometimes race gas for the smell.
4000 pretty hard miles on the engine.
236 / 242 112 LSA Roller Cam
Doug Headers, Performer RPM Intake, Cliff built Quadrajet
Engine Dyno at 495 HP at 5,200 and 600 foot pounds at 4,200 peaks
TH400

Rest of drivetrain last year was stock, with 3.36 rear end.
Last year I had an Auburn Clutch Posi installed and used MT 255/60/15 SS on 15"
My best time was around 12.7 at 108mph

Decided for fun and to improve before I broke something to do a rear end upgrade over the winter. Added:

All Spohn Control arms, Delsphere, QA1 single adjustable shocks, new driveshaft and Strange S60 Rear end. Moved ratio to 3.70 Lots of $

I thought that by moving RPM a bit up the curve I would increase horsepower to the wheels and improve ET. I have since purchased MT Street Pro Drag Radials as well, 275/60/R15

To my Suprise, and I think I'm hooking ok, I've not been able to get under 13 seconds this year, and my 60' time has gotten a lot worse, even with what feels like decent launches.

I'm highly suspicious I shot myself in the foot by going to 3.7 - she seemed, in hindsight, to like the 3.36 ratio.

What do you guys think?

Ed

my guess, with 336s you were still pulling in the power band or not too much avove it at the line. Now with steeper gears your running out of breath sooner and hanging out above the 5200rpms before the line and really not pulling
anymore. thats my guess. i would bet going back to the 336s or in the middle somewhere would fix it....or more top end parts to make it pull higher in the rpms.

Automatic or stick ?

If automatic here is a interesting article with a formula......

https://www.circledspecialties.com/b...g-applications


Also noted there are numerous online calculators available that allow you to simply fill in the data fields for these parameters & then will give you the mathematically ideal final drive ratio as a result.

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its all in your 60ft. What was your 60 before and after? Are you flashing the converter or leaving at a specific rpm?

TH400

Great question.

I had my 60' down to about 1.7 - now its consistent about 1.95 or 2.

Previously I was bringing it up to around 2300 at launch, but I've been experimenting with just off idle (1,200 or so) - no improvement in 60' time, but trying to re-adjust with that approach to get my reaction time back down, but that's not really important for this.

I should also say, I've lost about 5 mph in my final speed across the line. Feels like, as someone said, she is grasping for air. I'd say that is just the way it goes with iron heads and a quadrajet, BUT it was doing better before the rear end conversion.

28 inch tire diameter
88 inch wheel rollout
5200 RPM (engine peak power)
1.00 final drive ratio
Rear gear ratio 4.01
0% torque converter slippage
Speed 108 MPH (trap speed )

https://www.markwilliams.com/gearratiocalculator.html

Discussion... going thru the traps at engine peak power rpm is up to debate.


( Information provided in this post does not represent any endorsement. And unless specified it is not based on personal experience and is offered for general interest only )


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Thanks for the calculators. I think what missing in them though is they don't take into account the HP / Torque curve - where you peak out on either - seems to me like that's the "magic" in finding what's best for your situation. This is kind of new to me is why I ask. For example, these calculators could be applied to a 10,000 RPM chevy small block as well as a 462 Pontiac stroker, but they are different situations by far, at least I think (?)

theres a few that will give you the option to enter hp and trap rpms, along with the gear you have to estimate your et times. theres some that will let you enter the et, mph, tire size and rpms and estimate the hp needed. if you double the numbers against themselves i think you can get a pretty good idea of the gears you need.

Clearly 60ft here is your loss in ET, actually if you get your 1.7 back you'll be faster.

Are those 275 dead hooking?
You might need to leave higher, 3200rpm etc and take advantage of this tire hooking and higher rear torque gear multiplication.

https://tiresize.com/gear-ratio-calculator/

Your 3.36 on a 27" tire was the same as 3.48 vs your new 28" tire on 3.70.

What's your rpm through the traps now vs before?

Your converter is at best coupling 3".

Thanks - I'm not completely sure if it is dead hooking, sorry if that sounds novice. I recall one launch last year that was an actual head snapper, and I've never felt that since, maybe it's a "if you have to ask, it's probably not". I have my tires at 20 lbs. and my single adjustables at 2 (of 16) but have moved both all over the place with little difference. Last year I launched at pretty good RPM, I'm trying now to come just off idle to be more gentle. I've had a video of a run this year, and I'm sure I saw white smoke like 30' down the track at the wheels.

https://www.mickeythompsontires.com/...et-drag-radial

The MT Pro Drag Radial is a beast of a tire, leaving off idle if you're doing any sort of a burn out to heat these up is hooking hard.

You most likely are too soft out the hole, you need to do a test of 2500rpm, 2800rpm etc type launches and see what the car does as you go higher rpm not lower.

255 anything vs 275 Pro isn't in the same league unless it was a Pro.

I've got 295/55/15 MT SS and these can't hook at any launch unless I'm dedicate but on a Pro, damn.

Sorry if I said the wrong thing, this is the tire:

https://www.mickeythompsontires.com/...eet-radial-pro

275/60/15 on a 15-8 Wheel

Ya post 1 says Pro Drag Radial like my link. But...

Street Radial Rro is a Street R with less grooves for water run off etc, it's barely street legal and while it states not Street Legal there's definitely people using them on dry days. It's still a killer tire and probably hooking hard.

I'd try pushing the launch rpms until you have proof the cars spinning and losing 60ft times.

Can you post the dyno sheet? A WAG would be, you should be shifting at a min of 5600 RPM and crossing the finish at 5600 RPM or higher.

Stan

PS Can you also post a before and after time slips? Also how much different was the weather between these runs?

small block mentality with a big Pontiac , my old 68 GTO automatic TH400 on column and the original 2.93 Safe Track on 26x9.5x14 Hoosier QT Pro DOT on 14x7 Rally 2's with my also 10.5:1 iron head 462 cid with just mild flat tappet hyd cam .465 v.488 lift 224/234 @050 LS 112 , iron 68 intake , 800 Qjet , iron HO exhaust stock 1972-1978 style Carter mechanical fuel pump..... sticking the car on the launch with a stock 13" converter and shifting at 4900 RPM went 12.6@109 even got into the 1.8 60 ft .. Ya got too much gear for a mild Combo and flying past your torque curve therefore your not "riding the torque train" as I like to refer it. Think like ya got a diesel and dont over speed the rpm of the engine that can't really breath and let it stay in the torque curve thru the run in the midrange , no reason it cant run a little quicker than the 3.36 did but since its already changed to 3.70 the other way is a tall as tire change from what ya got or stick some big ass KRE heads on it and a bigger cam

My previous 4.210 stroke 462 combination made peak power on the dyno at 5800 rpm (580 hp) and we used a 6200 rpm chip in the shift light.
I can assume it was closer to 6000 rpm (?) There is reacting to the shift light and the physical time to make the shift.
I have a time slip for a 1.6 60-ft and 10.88 at 124.28 mph.

Again for conversation only, but entering the numbers into the calculator I posted works out close:

28 inch tire (88" circ.)
6000 rpm
3.73 gears in use (TH400 trans)
6.7 Torque Converter Slip (%)
124.28 mph


That amount of converter slippage the calculator indicates is not out of the question for the converter at the time.
I sent my dyno sheet and car specs to Marv Ripes at A-1 and he built a custom 8-inch unit for us. It had about a 4200 rpm stall.


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Steve C
What tire and pressure?
What launch rpm, 4200?

My memory doesn't go back that far :)
This was in 2003

Dyno results: 589.1 CB Torque @ 4400 rpms and 580 CB Power @ 5800 rpm

Quarter mile: Running 8" converter and 90/10 shocks on the front and no sway bar in place: 1.60 60" times, 10.88 ET at 124.28 mph

The rear tires were M/T ET Drag 28/10.5-15W at the track

At the time I had fiberglass race seats installed with the fiberglass nosepiece to save weight. On one time slip I wrote down about 3710 lb race weight.
I have no record of launch rpm or tire pressure handy, sorry.

I used a 10-inch Continental torque converter for the street

Correction: That wasn't the converter done by Marv Ripes at A-1
It was a custom Huges 8-inch converter.

And I found a 10.94 at 123.59 and 1.59 60-ft with the 10-inch Continental.



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