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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I agree that like you posted , you shot yourself in the foot either way those 3.7 gears and only that size cam.

The motor is reving thru the motors torque cradle too fast.

Attachment 654779

Hopefully that worked. This was the first (engine dyno) run. Did a little better in the next few runs, HP topped at 5,200

This is great, thanks! Turns out the smallest I can get on there (per Strange) is 3.54 - But seems an improvement. Great post, thanks!

See above
MT Street Radial Pro 275/60/15
Trying to Launch just off idle, say 1,200

Gotcha E but......

My question was for details on Steve's 1.5s 60ft.
If he's running a 4200 stall converter he's most likely not doing an idle stomp and flash and he was on slicks so that gives him an edge over Radial Pros but some comparison.

Since you're 485hp and 590tq you got a nice 12 sitting in there for sure even at say 4000lbs.

Could it be possible he should re'visit jetting ? I am not implying gearing effects jetting but more RPM'S means more air pumping and if fuel delivery or A/F ratio is marginal to start with then it would be aggravated with more gear?

Secondary air flaps fully opening ? = top end extension

Qjet dashpot functioning properly not leaking?

The 60" loss seems to me shouldn't be happening with full traction and .4 gear change

The DYNO power says 10's. Go back to 3.31:1 posi. See signature. There must be a fuel problem (maybe the timing backed-off?).

Get a fuel psi gauge. I missed top end mph?

My top MPH went from about 108 last year to about 104 this year. This carburetor was rebuilt by cliff, and I’ve had zero problems with it, I’m pretty sure everything is opening properly, and I have verified the full plates open when foot on the floor. again, I know there are thousand things that could be wrong with an engine, but I think my case is simple because I’m only comparing where I was last year, to where I am now with the gearing change. At least that’s what I think. I have the Rob Mick 550 hp fuel pump.

I like the signature, actually that’s pretty helpful. It gives me hope! 😁

This is great, thanks for the info.

You need to video rear tires at the track to see if you're spinning. When I was setting my car up my 60 went from 1.70s to 1.90s. New converter and trans and it went down in the 1.60s.

I talked to ebartone earlier today and does anyone think this air filter could be an issue?

https://www.summitracing.com/parts/edl-43662

For interest only....

Your 1/4 ET is 11.59 seconds and 117.61 MPH
Using vehicle weight of 3900 pounds and HP of 495

http://www.wallaceracing.com/et-hp-mph.php


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I didn't see in any of your posts where you mentioned your fuel system or your fuel pressure. You HAVE TO have an adequate volume of fuel to a Q-jet.

Thanks, yes Rob Mc pump at 7 psi.

Thanks Joe, I’m picking up a 14x3” Wix element tomorrow to try next time out, that’s a great idea. Again, though, pre gear change I was seeing hig 12s, so I know it’s capable. But I do notice that the top end of the track I seem to be gasping for air, so I think this is definitely worth trying. Thank you!

I guess I should have worded it differently….

What is your fuel pressure during a pass?

I don’t have a way to know that, however, I can tell you that there is no feeling of starving for fuel, no stumbling, etc. but I do not have a fuel pressure gauge that I can monitor while I’m making a pass.

To lose 2/10s in 60ft. and lose 4 mph in the 1/4 mile is a lot . You’ve lost it on both ends. Hard to believe only going from a 3.36 gear to a 3.70 would affect 60 ft so much and to blame it all on the gear change. And you went with a taller tire which would have cancelled out some of that gear change. Something else has to be going on here. Is that new rear end robbing horsepower compared to the other rear?

Also you might not feel a complete loss of power at the other end but still be running out of fuel. For the heck of it you can hook up a temporary fuel pressure gauge to see if you’re dipping below 4 psi or so at the other end. Not saying that’s your problem, but it’s always nice to know that information.

They say going from a c4 trans to a c6 robs about 20-30 hp just because of the internals.

Thank you, Scott, and everyone else for the ideas. I do agree with your post, though, it does seem to be a lot just to blame on a gear change. I’m gonna work through a few things for the rest of the summer such as finding a way to measure fuel pressure, and changing out this Air filter as Joe suggested. A few other things as well, thank you everyone for the tips! I really appreciate it.

I'd check all the things you changed, instead of what was there already for problems?

The Strange S60 Rear end like the Dana60 rear end?
If so that is a real heavy rear and possibly uses more HP to use it?
You don't have that much HP to give up I think.

The higher gear probably makes the converter act like a lot lower stall converter? Try a higher stall converter?

Is the new rear and other chassis stuff set up properly?

Try spinning the rear with it jacked up and see how hard it is to rotate?


:confused:

...throttle not opening all the way is a classic fix. Check it?

Thank you for saying this! That’s actually something I checked about a week ago, it had crossed my mind. Like I said, towards the end of the track I feel like there should be more and it’s holding back. Throttle plates not opening up all the way, definitely crossed my mind and I recruited the wife to hold her foot down while I double checked, thank you! I’m also swapping out the air cleaner just for grins to see what happens.

Whoever said it made a good point, what is it that changed since last season, rather than issues that would have been there before as well. It does occur to me that the air filter that I was using, late fall after I put her away, I cleaned it out, then oiled her up pretty good. Maybe I over oiled and it’s already clogged up, I don’t know, but Ill find out.

A fwiw.....

I'm not sure if it's brand specific, but related to testing a K&N X-Stream filter lid with a q-jet carburetor. Years ago when I spoke with a K&N Tech Rep he supported comments made within in an article that the entrance on a Holley 'HP' series carb with its smooth contour does in fact act like their Stub Stack and will help it work with the X-Stream lid. Whereas apparently a Holley carb design without a smooth contour (w/ choke tower) and like a Q-jet doesn't work as well with a X-Stream lid.


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Just take off the air cleaner, or at least remove the filter and the top leaving the base in place (secure it) and see how it runs. No money spent.

3.70 should be a good gear, you have other issues. It’s not like you went nuts and put 4.88s in it.

Fuel delivery even with the big 550 mechanical pump may be an issue, especially if you have not yet upgraded the fuel line from the tank to 1/2”. Keep your tank at least half full. You really need to hook up a fuel pressure gauge even just temporarily on the cowl where you can see it.

Sort of related....

Out of interest I talked with a tech rep at K&N about the air flow requirements for my 505cid engine and if the small air cleaner assy I was using is adequate. According to their formula a 505 cubic inch engine at 6000 rpm requires about 745 cfm of air flow. Because of hood clearance issues I'm using a 'small' 9-inch diameter K&N filter assy with the K&N X-Stream filter lid. According to him both used together will support about 650 cfm. So he said on the street I'm fine. If I went to the track I'd just remove it. The was with the cut down Victor intake and my custom HP950 carb.

When I changed to the direct port fuel injection I put the car on a Mustang chassis dyno for interest. To the wheels it made 591 hp at 6000 rpm with the filter assy in place.
Removing the filter assy it made 608/609 hp at the same 6000 rpm. The 18 hp difference on the street is a moot point !


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I was having problems towards the end of the 1/4 mile running a RobMc and Qjet. It felt like it was starving for fuel. Switched over to an 850 dp. Had the same issues. Took the car over to the dyno shop. On the dyno the tech found an ignition problem around 5400rpm where the module was starting to cut out. We replaced the old distributor with a new MSD. The problem went away and the engine now makes power up to 6300rpm. The ignition might be something else to check.

IMO, put an AFR gauge in it and know for sure since you track it.