Choosing the correct rear gear ratio

Posted By: Ethan

Choosing the correct rear gear ratio - 06/29/20 09:12 PM

I'm getting my rear end done over soon, (better axles, carrier, c-clip eliminators) so I can start spraying my car.

The kit gives me a gear. Choice and I'm thinking the 4.10 (which I currently run) might be too much once I spray it.

Car still has a non stock 200r4

My cams power range is 2500-6000. Shifting at 6000 On motor my shift light came on before the end of the 1/4. My shift light is set to 6000. (The parts can handle higher rpm, but I figure if the cam is out of steam why ring it out?)

I just feel on nitrous it'll be too much gear. (Maybe im wrong? Wouldnt be the first time) So if anyone has a suggestion on what gear to go with, or how I can figure it out I'd appreciate it.

I've also been told on nitrous the motor will make power past the cams range, so maybe the extra rpm will be enough?

Thanks in advance guys
Posted By: SpookShow'84

Re: Choosing the correct rear gear ratio - 06/29/20 10:32 PM

How tall are the tires?
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 06/29/20 11:24 PM

The tires are 28s
Posted By: SpookShow'84

Re: Choosing the correct rear gear ratio - 06/30/20 01:32 AM

I'd go 3.90, maybe even 3.73. You'll be moving through the gears faster with nitrous, and you're already out of gear now with an engine that's not spinning to the moon. Spend more time in the powerband, and you're not heavy enough to really need 4.10's.
Posted By: BadSS

Re: Choosing the correct rear gear ratio - 06/30/20 11:15 AM

Here's a pretty good calculator - comes pretty close to a good dragstrip simulation. You'll need to add in your nitrous HP to whatever your engine is making. For instance if your engine makes 600HP and you're spraying 300HP and making 900, it looks at rearwheel, so with an automatic multiply by .80 for an estimate - in this case it would be 720HP in the calculator. If you already know rear wheel, put that in.

DO NOT over-rev with nitrous. Shift at the same point OR less than you shift on engine alone. Your ETs will typically be better shifting at a lower RPM as the nitrous makes more torque down low than it makes HP up high. So no need to put more wear and tear on the engine as necessary.
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 06/30/20 12:14 PM

Im all for not ringing out the engine unnecessarily, but if i had to turn it to 6500 in third tim good with that. I need to dyno it and see where it peaks to be honest.

I haven't had the engine dynod, but from the builders estimate and my 1/4 mile times I have a pretty good idea so that calculator should be just what I need.

Thanks for the help guys.

BadSs, you're always helpful and full of great info so thank you
Posted By: Travis Jones

Re: Choosing the correct rear gear ratio - 06/30/20 12:18 PM

I made a tool to help with this, and you can compare RPM at speed, Shift drop etc...
Posted By: BadSS

Re: Choosing the correct rear gear ratio - 06/30/20 07:29 PM

Originally Posted by Ethan
BadSs, you're always helpful and full of great info so thank you

Thanks for the kudos, I appreciate it. I try to help when I can.
Posted By: mmc427ss

Re: Choosing the correct rear gear ratio - 07/01/20 03:56 AM

There are a few pretty good calculators here,. easy to use.

My car is slow, only 114-115 trap speed. Running a 27" QTP I went with 4:11 to get the trap rpm a little higher in 4th gear., 5900, about a 500 rpm more than the 3.73s . The engine noses over at 6K, no sense beating it up.

This is the simple plug and play from Wallace, accurate.
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 07/01/20 09:47 AM

You shift into overdrive during the pass? (Not judging, just asking. Ive been scared to shift OD under full throttle, for frear of breaking the trans.) Or you've got a stick shift?

I think my car will mph in that range. For some reason with this tranny,gear combo the car has always had a mph lower than you'd think for the times it would run. Even with the old engine, and when my uncle still owned the car.

I was at 108 at the 1000 foot, then let off just after that because the tire threw something into the bottom of the car and I thought I broke something. So I ended up going 108 to the 1/4
Posted By: Hunter79764

Re: Choosing the correct rear gear ratio - 07/01/20 03:53 PM

Quick times and low speed is usually too much gear. I'm on the other end with about 2 seconds I could gain by getting a reasonable gear (I've got a 2.14 right now, and run about 15.5 @100 mph, barely getting into the power band in 2nd gear). My guess is that a 3.73 is going to be more appropriate for you with and without a reasonable amount of nitrous, but the math and calculators and advice above will get you in good shape.
Posted By: mmc427ss

Re: Choosing the correct rear gear ratio - 07/02/20 03:06 AM

The car has T56, 4th gear is 1 to 1, runs 12 teens with the QTP. But I never bang shift the trans, wind it out, let off, shift, stands on it. Kinda like when you make a run over to the mall for ice cream. It's not advisable to bang shift a 20 year old T56.

Also on Wallace are some HP calculators to see effects on ET and mph. I have well over a hundreds time slips in the drawer. Lots of useful info on them when I was running the car 3 or 4 times a season. Where the car was strong and where weak. The good info i get also was also from the other car on the slip. Cars a second faster than me I could see where they were strong and compare the splits time wise. I found my car was equal to many mid 11 cars in the second half of the track. Getting killed out to 330'.
So time slips you can learn a lot about your car.

One of the problems with drag racing is you are never happy, always looking for more. Keep that in mind when choosing gears. Without a NO hit on the car it will run close to the same mph all the time, especially with an auto. Oh, and gear it for no OD. Have never given an engine a dose of NO, but have seen the hunger for faster apply to much, more is better, ok. This will effect top end mph and of course gearing.
Posted By: BadSS

Re: Choosing the correct rear gear ratio - 07/03/20 03:33 PM

Originally Posted by Travis Jones
I made a tool to help with this, and you can compare RPM at speed, Shift drop etc...

Travis, this is one of the best (if not the best) drive-train tools I've seen. Absolutely awesome - one stop shop for an extensive amount of drive-train information. THANKS!!!!!!
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 07/03/20 10:30 PM

Mmc427ss, ah okay that makes more sense haha.

My car actually 60' better than I ever thought considering I come off the foot brake, stock suspenaion other than super budget jegs adjustable rear shocks,and it's a well built engine, with good parts, but still a pump gas motor, not a full on race motor. It went 11.47 with me letting out at probably 1100-1200 foot so I was very happy with that, out of a car I can drive everyday if I wanted to. But you're right. I'm always looking to go faster. Already have my next setup blueprinted in my notes section on my phone haha. Just saving up for it

I think I need an actuall computer to use that tool travis posted. I'm on mobile and it didn't open anything when I clicked the link, so I'll try it out next time I'm at a friend's house.

I'm still trying to decide between 3.90 and 3.73. I'd like to be able to hit the car with a 250 shot if I want to get froggy, but the cars going to have a 100-150 in it normally the 3.90 might still be a bit much for the 250 according to the calculators I've used.
Posted By: mmc427ss

Re: Choosing the correct rear gear ratio - 07/04/20 04:50 AM

As you know 11.47 will get you kicked off most sanctioned 1/4 mile tracks, after a warning of course, cage time.

Trap mph is a good indicator of horsepower, ET is a good indicator of how the car does in the first 100', one tenth in the 60' can net 2 tenths at the big end.
Mid 11s hauling 3500 lbs is 500 flywheel give or take 50 hp, a zillion variables, but a good guess. Adding another 250 with a shot pumps that to 750.The mph is going to go up a little, yea, right, could get into the 130 if everything goes right. So R&P choice becomes important, tire diameter, converter choice also.

Not being really a drag racer, just someone who use the track to gauge engine performance. Usually made the trip to the track a couple times a year just to legally do 120, in 4th gear. If the trap mph stays in the 112 -114 range the 15 year old build is still healthy. Best 60's were 1.7s on the QTP, granny shifting 12 teens at 114. Tried the M/T drag radials, gave them away to a friend, couldn't hook them with the clutch I had in the car. As with most motorsports tires are important.

Next time at the track have the car scaled "as raced", you, sitting at the line. Important to know weight to make calculated guesses at mph, ET, hp, trap rpm,
At the track watch other car that are running mid 10s to 12 second range and look at their mph, the way they 60', are they doing those ETs on hp or are the 330's that good.

By the way, what rear are you getting built?
Posted By: BadSS

Re: Choosing the correct rear gear ratio - 07/04/20 03:16 PM

Ethan, I'm plugging some numbers for you into a drag strip simulation that I've found to be really accurate when you know all the factors. It can also let you know with near certainty what the HP is if you know all the rest of the inputs and have a couple consistent times slips.

What I don't know:
Weight - is it mainly stock - meaning no weight reduction and nothing like a sub box or anything added?
Temp - Need to know approximate temp when the 11.47 run was made
Tires - verify you were running 28s (6,000rpm with 28s should let you pull to 121mph / 26s around 113)

Things that can help REALLY dial it in is the time-slip data: 60ft times, 1/8 ET and MPH, 1,000 ft info.

You are making extremely good power from your combination (even if temps were in the 30s). PM me if you would rather take it offline - I know when I was grudge racing, I wouldn't want a lot of info out on the interweb - lol

General comments - I don't like running over 150 shot on a dual plane intake. I would hate to see you change much as good as it is running with the dual plane (did you by chance try that Street Sweeper spacer for it?). However, it is best to run a single plane for higher nitrous levels and an intake swap would add add about 300 rpm to the shift point. I'd hate to see you change anything though as it seems the converter matches up really well now.
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 07/04/20 08:01 PM

Bob, I'm getting the 8.5 grand national rear I have in the car now built. Better axles, c-clip eliminators, better carrier, getting braced, and welding the axle tube.

He'd a chasis guy, so going to have him throw in an anti roll bar whiles he's at it.
Posted By: mmc427ss

Re: Choosing the correct rear gear ratio - 07/04/20 09:15 PM

Did my 8 1/2 for the second time a few years ago. Tubes welded, sandblasted, Eaton posi, stock 3.73 (Olds 442 donor), bearings, 1350 yoke, LPW girdle were done the first time around, Moser 28 spline axles later when a journal went south on an axle. Second time around was 30 spline Truetrac, 30 spline Moser, 4.11 Yukon gears, new bearing again. Eaton posi did well on the street and no problem at the drags. Raced the QTP tires and Truetrac a couple times, no problem. Second build got a great deal on all but the 4.11 gears. Did all the installs myself and a few other people's rears along the way.
One of the reasons I asked was changing R&P on some rears is more of a PITA, 9" being a little less of a PITA. Picking the gears and posi and not doing it twice should be a primary goal when someone else is doing the labor.

Rear sway bar. Another choice to be made. Several link types available, some for street and cornering carving, some with adj rates, some only considered drag race bars. Fat, 1" and bigger rear bars usually don't work well on the street, but some of the really fat rear bars can do wonders on a soft frame G body or the A bodies to get the front end lifting evenly on each side.
Never had to worry much about how evenly my car would launch, not enough hook, but ran an airbag in the RR @7psi and set my 7/8" rear bar to full firm, it left pretty level.

Lots of choices today on parts.
Posted By: Ethan

Re: Choosing the correct rear gear ratio - 07/04/20 09:35 PM

Yea I'm doing the 30 spline moser axles with the truetrac.

I'd love to do a 9" but they're pricey, and since I already have the 8.5 parts and labor to build this one still comes out cheaper.

I don't corner the car hard anyways with the skinny tires upfront. Im a very calm driver i chill at 55-60 on the highway. car is definitely more toward strip, my tolerance for dealing with that cruisin around is high haha. I live 10 minutes from work, so even when I take the car it's not bad. Never really drive more than 20 minutes away, I can if I need tho.

My friends monte twist pretty hard on spray, so we're both doing the anti roll bars haha.

I've been thinking about the airbag thing actually. Going to see what the chasis guy says when I bring him the car to do the rear, and anti roll bar.
Posted By: mmc427ss

Re: Choosing the correct rear gear ratio - 07/06/20 04:44 AM

Currently and for the future my car is setup to go quicker left or right, not for launching. Air bags are out of the car now. For several years I just left them install but squeezed them until collapsed, negative pressure, enough that they didn't contribute anything to spring rates. Separate air lines to each makes that easy enough.

Couple other things for the 8 1/2. Are you doing LCA relocation mounts on the rear? It's another plus, tuning tool. The angle of the LCA can effect several things, IC being the big one. Anytime you change the ride height that angle will change. There are bolt on and weld on relocation kits but didn't like the choices and made my own, while the rear was still in the car. More about that later if you need to go that route.

The height of the spring seat on the rear for the pigtail is short, adding 3/4" to the height is a good thing. All the 8 1/2 G rears had short seat height, GN and 442. Softer spring rates and longer free length rear spring got away with short spring mounts. On the SS 7 1/2" they double up on the seat height, higher rate and shorter free length. The cup used as a upper pigtail locator was welded to the SS rear's spring seat to make it taller. A cheap fix for GM. A quality piece of 2 1/4" exhaust pipe will weld nicely to the mount to make it any height you want. I had extra cups laying around and added them later, with the rear in the car.
The name of the game is you don't need coil springs popping out of their seat.

Truetrac, was that you only choice? I run one mostly for it's better street habits and hopefully better at the autox, not for drags. Have no issues with it at all but if the car was drags primarily or straight ahead blasts I might think about a locker with the correct springs in it. A spool is out. The Eaton posi I had initially in the 8 1/2 newer gave me any trouble, and it was exercised many times. The Truetrac replaced the Posi because i felt the Truetrac's "torque sensing" drive would make my car a little easier to drive. And I got a SUPER deal on all the new build parts.

I installed a Quaife diff in my son's hillclimb car several years ago, amazing how much quicker it was in the corners. Quaife, Truetrac, Torsen all helical diffs.

Posted By: Ethan

Re: Choosing the correct rear gear ratio - 07/06/20 07:50 PM

I'll bring up the relocation mounts when I see my chasis guy, if it doesn't break the bank I'll definitely consider that.

Yea the truetrac was just a good, affordable option for now. With the axles and carrier, and all the parts that came in the kit I'm into the rear end about a grand. It should hold me over for a while until I do the 9"

I'm going to go over some things with the chasis guy once I bring him the car. See what he else if anything he reccomends I do,

I'll get back to you after I see him and ill have more info for you bob. I'm not very knowledgeable on suspension tuning as of now so hopefully I'll have a better idea of the final product after I see him
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