Gear Ratio Changes?

Wanting to get some advice or examples of anyone changing to Lower gears in 1/8 mile.

Info:
1/8 mile only footbrake bracket car
Current 4.11 gears
1.80 Powerglide
32x14s
Shift at 6700 and cross right around 6700-6800
6.0s at 112mph in 1/8

Any issues going to a 5.00 gear ratio? Thoughts?

I don't have tons of race experience but that ratio change will increase your RPM at 112mph to over 8000

moonshine,

Interested to know how you came how with that number?

5 ÷ 4.11 = 1.219

1.219 x 6800 = 8,292

Do you know where the engine makes peak power? What is the redline for the engine? Gear changes are a little hard to predict in a drag car with a torque converter since the converter slip changes at the same time. 4:11 to 5.0 is a big change. Racers usually only go one step or two steps at a time but if you have a high RPM engine and you need to move the RPM up a bunch then that change will do it.

The reality is that there is a bunch of converter slip at the 1/8 with the current combo(about 2000rpm worth……..+/- 30%).

That situation should improve noticeably with the lower gears.

Mathematically, the 5.0 gear would still have the drive shaft rpm under 6k @112mph.

Apologies, I should probably keep my mouth shut lol.

Do you have any converter slip info? The best info would be driveshaft RPM from a data logger but if you are sure of your RPM and MPH at the light you can also figure it out that way. When I figure it out I get 35% slip based on 6700 rpm, 112 mph, 4.11 gear and 32 inch tire.

A 5.0 gear with the same 32 inch tire and 6700 RPM would end up being 115 mph in the lights with 10% slip. So you either need to talk to your converter mfg to see what they say, or just try it. If the converter tightens up that much then you'll be golden, if not, you'll need to buy a different converter.

A starting line ratio of 9/1 is very close to optimum, maybe a tad high. 4.88 x 1.8 would be 8.78, not too much unless you have very large stroker motor. I get away with 11.17 due to using a soft launch (low rpm) and 33 inch tall tires.

What is your improvement goal? That is a big change in gears that could also create issues down low with suspension setup and traction, with a glide I wouldn’t go any steeper than 4.56. Wallace Race Calculator has a bunch of categories that should get you a close idea. Doug

This would be true with a manual trans as long as the clutch wasn't slipping. The problem with the OP setup is that it appears that his torque converter is slipping a ton at the 1/8 mile mark. As far as we can tell from the numbers he posted, his driveshaft is turning about 4400 rpm while his engine is turning 6700 rpm. Could be a bad converter, could be the wrong gears, could be something else.

My goal was to accelerate out of the launch harder or faster. If that was to give up a little MPH at the 1/8 I was ok with it. I’m confused some on the converter slipping a “ton”. I told BTE/abruzzi what they flash at, what I shift at and what RPM is at the 1/8 and they both run about the same/setup similar.

Heck no, the rest of us too timid too speak up wouldn't be able to learn anything from others stupid comments.

I'm just kidding, don't take it personally.

The car will accelerate out of the hole harder as long as you have traction. You have huge rear tires so I assume you have plenty of traction but I guess you'll find out. A simple data logger would answer a lot of your questions but if you don't have access to one you can get a lot sorted out by just using simple math. The math right now shows your converter is slipping about 35% at the 1/8 mark. That isn't a huge surprise if the car is still accelerating hard at that point. Cars set up for the 1/4 mile are often slipping the converter a bunch at the 1/8 mile, especially if they have just shifted into high gear.

How fast should your car go in the 1/8th based on weight and power? That number should be your "guide". Once you know that number you can figure out what your gear ratio should be. It really helps if you know what the power curves for your engine are. If you don't have any dyno data then you have to work off how the car feels. Is the car laying over at 6700 rpm or is it still pulling?

When do you pull into high gear? Pretty close to the finish line or halfway down the track? With the 5.0 gears you might be pulling into high gear pretty soon after you leave the line.

I apologize as I don’t have all the numbers.
The car weighs 2900 with me in it and about 650HP.
It shifts pretty close to half track.
It is still pulling really good at 6700.

That is enough info to put into Wallace Racing calculator. The 1/8th mile calculator at Wallace says 650 hp and 2900 lbs will run 6.09 in the 1/8th at 109 MPH and the ideal gear ratio for 6700 rpm and a 32 inch tire is 5.83

I was trying to use some of those calculators before my original post and would come up with numbers like that. So there seems to be some slight concerns going to 5.00 but this says ideal 5.83 😳

I’m interested in what you are trying to say here. It’s a 500ci Indy maxx aluminum block, just 4.15 stroke.

just from my experience you will not change much by dropping to the lower gears. Your RPM's are close to where you need to be. My experience is from two drag cars. I had a 48 Anglia with a 340 4-speed, used to race 1/4 and 1/8 had 5:12 gears for the 1/8 and 4:56 for the 1/4 one time did not have time to put the 5:12's back in for the 1/8 and it ran exactly the same, I just did not shift into 4th. I recently had a 64 NSS car ran 9's It liked 4:30 gears 1/8 or 1/4 tracks tried several ratios. Just my experience others may vary.

In my the case, i have 549 cubes, 52% on the front, 2.45 low gear torqueflite, 4.56 rear gears, 30 inch ladderbars, 106 inch wheelbase.
Converter flashes to 6200, shift 7600 rpm or more. The best 60 ft so far is 1.224, and that was with a throttle controller.
The throttle controller keeps throttle at about 10 percent open ,or 3800 rpm till trans brake is released, then takes about .10 to fully open, resulting in a very soft hit and no violent hit on the wheeliebars. This was my first shot at a low rpm launch, but gave me the best 60ft ever.
In your case, with a 1.800 first gear you could leave at 4000, to 4500rpm, run a 4.56 to 4.88 rear gear, and have a KILLER consistant car. The 4.56 might be my choice, because it will be deep enough for near best performance, yet allow your shift to happen further down track for great consistancy, and you stll will be able to run 1/4 mile if desired.
Good double adjustable shocks are important . Consider that an 8.0 starting line ratio multiplies engine torque EIGHT times after the converter doubles it! 700 ft lbs becomes 11, 200 ft lbs at the axles, and. Your springs and shocks have to tame that, and apply it smoothly .

It’s a 4link with double adjustable afcos front and rear.
I will never be running 1/4 mile with this car.
I’m aligning with what you are saying. I was thinking 4.56 or 4.88s myself.

Are you planning on staying at 650 hp or are you thinking of adding power to the car? A 500 inch aluminum block engine can easily handle a lot more than 650 hp so you might only be a cam swap or head swap away from a lot more power.

Are you running a 9 inch rear? That makes it easier to swap gears and there are a lot more choices. It also gives you the ability to have a spare chuck all set up and ready to drop in.

If I could work on it launching harder, I might be ok with the current HP. I am wanting to buy some bigger heads, but that might be 1-2 years away.
It’s a ford 9”

Which heads do you have on the engine now? My old 514 made 900 hp with Indy EZ heads. https://www.motortrend.com/how-to/modify-carburetor-run-oxygenated-race-gas/

Along with Andys question, can you provide engine specs, as IMHO It would be a good idea to formulate a long term plan hear. If that works for you, selecting parts to compliment your end goal will help you get there and save $$ in the process.
Right now i am wondering how much torque your trans can take, bore /stroke combo, etc. For a glide/bracket car, too many cubes is about right. That may be a better place to start? Especially if there is a crank/rod combo that will use current pistons.
An example would be a 572 shortblock with small heads, like ported SRs. Low maintenance, tons of torque, all done by 6500 rpm?

I have tested several different rear gear ratios in my old pump gas Duster, 3:73, 3:91,4:10 and 4:30 in the 8 3/4 housing using 742 carriers for all of those ratios, I saw no noticeable change in the time slips in the 1/8 mile and 1/4 mile. That was with a six pack intake and magic carbs That car weighed 3450 lbs. with me in it with a full tank of gas in the stock gas tank
That car ran a best of 9.99 at 134.+ MPH corked up using Oregon 92 octane pump swill with the air cleaner on through the full 3.0 exhaust system exiting at the rear bumper
My last bracket car weighed 2850 lbs. with me in it and it would run 5.50 in the 1/8 mile at 125.MPH, 8.86 ET at 150. MPH in the 1/4 mile on E85 with a very similar motor as the Duster with higher compression running on E85. That car was a 1970 Cuda all out full chrome moly chassis with a glide and several different converters in it, stalled at 5600 RPM against the trans brake at WOT and shifted between 7000 RPM at first and later shifted it at 7300 RPM which picked the MPH only up in the 1/4 mile by 2.0 MPH
That car had 4.56 gears in the Dana 60 with a four link coil over rear suspension that worked very well,1.23 60 fts. 15x33x15 inch Goodyear slicks on 15 inch wide rear rims