Picking gears

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273

273

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Just got a 68 Valiant not too long ago
360/380hp create engine, 2800 stall 727 with 2.96 gears. I want to take it down the strip a few time with the 2.96 gears but looking to upgrade somewhere between 3.55 - 4.10 haven't narrowed it down yet.

From what I understand you should be 500 - 600 rpm over peak power at the finish line which would be about 6000 rpm with 4.56 gears.

My question I assume that performance gain with gears is gonna be non linear.
Like going from 2.96 to 3.73 is gonna be a bigger jump than 3.73 to 4.56 in performance??

Assuming 4.56 is peak performer in this e.g.
Plus what et and mph should I be able to run with the right gear ?
 
How tall is your tire?
A 4.56 gear is 111 MPH with a 28 inch tall tire.
If your car is 3200 pounds and you make 380 HP, you have enough power to run 115-117 MPH. That tells me you need about a 4.30 gear.

Just throwing numbers out there.
 
Well,
you could do what I do;
run what I brung.
I get an MPH number, and translate that to a theoretical SuperStock ET.And then I be happy with knowing that if I spend say an awful lot of money on my suspension, I might get close to that ET. Since I don't care about ET, I'm just happy with 93 mph in the 1/8th.

But once you have an MPH number that won't change all that much . So then you can gear your car correctly, for the MPH.
If you do not run the optimum gear, then it won't matter much how far you are off, cuz you are always trading bottom end for top end. Baring tire-spin,the closer to optimum you get, the more average power you can put down during the run.
IMO it is more beneficial to get the 60ft sorted. There is about a half a second to be gained here, with perhaps another .2/.3 with the correct trap gear. But they kindof go hand in hand.And hand in hand with the TC stall.
It's easy to go crazy and suddenly have 4500 or 5500 or more ftlbs on the starting line, and the tires won't have anything to do with that,and so your 60 ft goes to 2.2 or more. I found this out with 4.88s and a 3.09 low and launching at peak torque.BFG DRs, in 325/50-15s, weren't having any of it. And once they let go, there was no saving it; they spun the whole 660'. The car was just not transferring weight. I had a lot of fun tho.......I hope you do too.



Just throwing chit out there.
 
AJ/Forms I like how you always boil it down to the practical.
 
my opinion for a bracket car, you would want to cross the finish 200 rpms before shift point. In other words, per say, if you shift a car at 6000 rpm's and it's a 727, you'd want to cross the line at 5800 rpm's in 3rd gear. You want the car to be pulling at the finish incase your the chaser, and it's easier on the engine for longer life.
 
Like anything else, it's a game of diminishing returns as you get closer to "right", but as AJ said - once the gearing is "right", other weaknesses will rpesent themselves. If you own a race car the assumption is a peakier power curve and move race oriented suspension and tires, then yes, gear to be a few hunrred rpm above your peak HP. Once you say "it's a street car, and I race it" that kind of goes to pot because the "right" gear will not be very streetable unless you live in Manhattan and never drive on the highways.

I put "right" in quotations because it's very subjective.

My advice would be chose a gear that keeps your highway crusiing rpm just above the convertor's stall speed. So a convertor of 2800 rpm, I'd be looking at 3.91s or 4.10s. Your best ET will be totally dependant on your ability to tune the engine and suspension, and how much money you put into the suspension and tires. With a set of slicks, 4.10s, and a decent 360 I would expect you to be in the 12s, with potential for high 11s at sea level with a good track, assuming you suspension is optimized.
 
273 said:
Just got a 68 Valiant not too long ago
360/380hp create engine, 2800 stall 727 with 2.96 gears. I want to take it down the strip a few time with the 2.96 gears but looking to upgrade somewhere between 3.55 - 4.10 haven't narrowed it down yet.

From what I understand you should be 500 - 600 rpm over peak power at the finish line which would be about 6000 rpm with 4.56 gears.

My question I assume that performance gain with gears is gonna be non linear.
Like going from 2.96 to 3.73 is gonna be a bigger jump than 3.73 to 4.56 in performance??

Assuming 4.56 is peak performer in this e.g.
Plus what et and mph should I be able to run with the right gear ?
Click to expand...


To add to the other comments, if we "assume" you have 350fwhp and a race weight of say 3100lbs, with a chassis that can hook reasonably well, with 28" tall tyres / 4.10 gears, a good run would look something like this>
60 Foot E.T. : 1.68
1/8 Mile E.T. : 7.60
1/8 Mile Trap Speed : 89.39
1/4 Mile E.T. : 12.05
1/4 Mile Trap Speed : 111
1/4 Mile Trap RPM : 5,475.....w/4.56's 6049
But with that 2800 verter you may not 60ft in the 1.6's, your mph should still be there but your ET may suffer some, according to some dyno testing max hp was at around 5300rpm with a 750 Holley and hdrs.
 
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If it's a Street Car - go with 3.55s, and get what You get, at the strip. - No one cares, but You. -- My 2 cents.
 
I kindof agree with Magoo there.
The mpg will tell the story if you're making power or not. From the mph you can estimate a minimum ET. Then you can start budgeting for suspension mods to get you close to that. And the rest is tuning, tuning, and more tuning.
People always ask me how fast is it? They never ask how quick? So I give them the mph.
 
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If it's a Street Car - go with 3.55s, and get what You get, at the strip


Yes we all know that putting 4.10 gears in a street car can ruin it, been there and done that, but the OP was showing an interested in lower gears and what they could do to enhance his performance in his question so the responses are to his question.
 
For me highway is no big deal
I live just outside of Toronto, even if I do highway it into T.O. its for 10-15 mins max.
Now to go to the drag park from here is a different story it's a couple of hours away.

I'm torn with going straight to 4.30 and maybe go overdrive over winter or compromise with 3.55/3.73/3.91.

I would like to do some traveling with the car but could always swap in 2.96 before hand if I don't go O.D.
 
Getting back to trapping
With a 110LSA and good heads,the power curve over the top is often nearly dead flat. There may be 500 or 600 rpm on either side of the power peak. It behooves you, for low ET to hang in there as long as you can. In the lower gears especially, as on the shift, you can come in,in the next gear, at a higher rpm where there is more power. Short shifting is not gonna get you max mph.
Once you have blasted your way to top gear, in the shortest possible time, now begins the long-march to the traps.When you get there, you want to be at a part of the power curve that is maxed out somewhere toward the middle of the trap. If you enter at 300hp, peak at 305 and exit at 300, you get 302.5 hp average, and you can't get much better than that.
If you enter at 290 and exit at 305, your average will be 297.5.
If you enter at 305 and exit at 285, your average will be 295.
All numbers are of course theoretical.
302.5hp, of course will probably post the highest mph.
Those first three gears are just to get you down the track as far as possible, and set you up to max your average hp thru the traps.This is why leaving a half second in the 60 ft is so critical. In the first three gears most of the power gets onto the track. But in top gear,the power has to be shared with wind-resistance.
 
273 said:
For me highway is no big deal
I live just outside of Toronto, even if I do highway it into T.O. its for 10-15 mins max.
Now to go to the drag park from here is a different story it's a couple of hours away.

I'm torn with going straight to 4.30 and maybe go overdrive over winter or compromise with 3.55/3.73/3.91.

I would like to do some traveling with the car but could always swap in 2.96 before hand if I don't go O.D.
Click to expand...
Been there done that;not going back,lol

I got a compromise for you, that worked pretty good for me; namely trap in third gear.
But first run the 2.96s to get an MPH number. Then gear it to trap in third gear. Then get yourself a 3.09 low to blast off with. Then move your power curve to the middle of the trap, and you are done.
So if 4.30s were the right gear,the math says 4.30 x 1.00top gear= 4.30final drive, and 4.30/1.39 third gear =3.09 gears to trap in third. The final drive ratio is the same 3.09 x 1.39=4.295
Your required gear is between availables, (2.96 and 3.23s), so I would try the larger one to help with the 60ft. Now 3.23 x 3.09=9.98 starter gear. This would be about equivalent to 2.66low x 3.73 rear=9.92. Now you have both ends covered, allbeit with just 3 gears.
And you still get to cruise to the distant track at 65=2700ish, with those 3.23s. Problem solved.
Now the above was based on the 4.30s being the optimum gear. Your first run with the 2.96s will sort that right out. This is a tuning opportunity you shouldn't miss,IMO.
Furthermore the 3.23s will not get you low-ET cuz you are missing a gear. Which has decreased your average power put down during the run, as the engine is forced to spend more time in each gear. I call this no big deal, cuz the option is to spend a wheelbarrow full of money, chasing low-ET. I'm happy just knowing my car could be faster if I was rich. And on the street, it matters not a whoot!
Oh chit, it's a 727/2800 :( Sorry about that

Well in that case do what I did; put a 4 gear in it,lol.
But seriously;
Trapping in second gear with an automatic and a small engine is like watching grass grow; who does that?Well you're a bit ahead with the 360/380
The only cheating you can do is to race with short tires and cruise with tall tires. But there is barely one final drive ratio equivalency to be gained. Been there done that, ain't going back.
 
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yellow rose said:
How tall is your tire?
A 4.56 gear is 111 MPH with a 28 inch tall tire.
If your car is 3200 pounds and you make 380 HP, you have enough power to run 115-117 MPH. That tells me you need about a 4.30 gear.

Just throwing numbers out there.
Click to expand...
This is about as dead on from what I have seen and asked about at the track. (E-town) I have spoken with a number of racers with that exact 360/380 create engine and the best times returned have been with the 4.10 - 4.30 gear sets. Tire size, car weight, converter, etc.... all play into it.

RPMagoo said:
If it's a Street Car - go with 3.55s, and get what You get, at the strip. - No one cares, but You. -- My 2 cents.
Click to expand...

Excellent street choice gear.

273 said:
AJ/Forms I like how you always boil it down to the practical.
Click to expand...
Now, IF he could ONLY do it in a short paragraph. Or less.


For a best of both... 4.30's and a OD trans.
 
273 said:
Just got a 68 Valiant not too long ago
360/380hp create engine, 2800 stall 727 with 2.96 gears. I want to take it down the strip a few time with the 2.96 gears but looking to upgrade somewhere between 3.55 - 4.10 haven't narrowed it down yet.

From what I understand you should be 500 - 600 rpm over peak power at the finish line which would be about 6000 rpm with 4.56 gears.

My question I assume that performance gain with gears is gonna be non linear.
Like going from 2.96 to 3.73 is gonna be a bigger jump than 3.73 to 4.56 in performance??

Assuming 4.56 is peak performer in this e.g.
Plus what et and mph should I be able to run with the right gear ?
Click to expand...

I can't tell you what gear to use but I can tell you what RPM I figured out to shift at. I started out shifting at 6000 and starting going down on my shift points and I ran the quickest ET shifting at 5400 rpm. When I was shifting at a higher RPM it would nose down before it shifted. I am using a air shifter so all I had to do was change RPM chips. I only had 1.5 inch block hugger headers with 2 1/2 inch collectors on it so that had to slow me down.
 
D-50394 said:
I can't tell you what gear to use but I can tell you what RPM I figured out to shift at. I started out shifting at 6000 and starting going down on my shift points and I ran the quickest ET shifting at 5400 rpm. When I was shifting at a higher RPM it would nose down before it shifted. I am using a air shifter so all I had to do was change RPM chips. I only had 1.5 inch block hugger headers with 2 1/2 inch collectors on it so that had to slow me down.
Click to expand...
Is that Normally Aspirated?
 
AJ/FormS said:
Been there done that;not going back,lol


The only cheating you can do is to race with short tires and cruise with tall tires. But there is barely one final drive ratio equivalency to be gained. Been there done that, ain't going back.
Click to expand...

Thinking about cruising here!
If he chooses this way, should he stay with the 2800 stall converter? 3.55 Gear ratio ? Tire diameters ?
This always loses me, I know nothing about choosing a tq converter.
What about a GV ?

Mopar Performance 360 cid 380 hp rated

RPM TQ HP
2500 380.6 181
3000 402.9 230
3500 415.1 277
4000 437.8 333
4100 439.5 343
4500 437.1 375
5000 416.3 396
5400 398.6 409
5500 388.6 407
6000 343.9 395
360 Crate Engine Buildup & Dyno - Mopar Muscle Magazine
 
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My.combination 408 ci 727, 4:10 rear with 26.5 in dia tire. Cross the stripe at 6250. At 113mph, ptc 9.5" coverter flashes to about 3000rpm. Has about 6% converter slippage
 
70 DUSTER CRATE said:
Thinking about cruising here!
If he chooses this way, should he stay with the 2800 stall converter? 3.55 Gear ratio ? Tire diameters ?
This always loses me, I know nothing about choosing a tq converter.
What about a GV ?

Mopar Performance 360 cid 380 hp rated

RPM TQ HP
2500 380.6 181
3000 402.9 230
3500 415.1 277
4000 437.8 333
4100 439.5 343
4500 437.1 375
5000 416.3 396
5400 398.6 409
5500 388.6 407
6000 343.9 395
360 Crate Engine Buildup & Dyno - Mopar Muscle Magazine
Click to expand...
What's the question?
Dragracing with short tires is a bad idea. You need the tallest tires you can fit and work in those wheelhouses, even if it means a little narrower.
If the ideal gear is a 4.30 or whatever, and if you run it, then cruising will be a heartache, without an overdrive.
This is why I have a manual trans
 
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One way it can be done without the expense of an OD to save $$$ is if we base this engine on a true 380fwhp at the track and 3200lbs with a 26" tall slick and 3.73 gears, the result if the car hooks should be a high 11 sec pass at around 113mph, trapping at 5455+ say 5% verter slip which = 5800 which I would've thought was ok for that motor. For street use some 28" tyres should handle the cruise rpm?.:)

A taller slick like a 28" with 4.10's would be better and work out around 6000 trap but then your borderline for street cruising with the same height street tyre...its all a compromise when you bring street into the equation as we all know.
 
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I really like the 4.30 gear works great at the track, and is good around town, i drive to the track but no freeway.
 
I had a 4.30 gear with 28" Good Years on my '71 340 Cuda (365 on track hp), weighing 3550lbs with a 4200 verter and trapped at 6100 at 12.3's@108, with 26" street tyres which is all we had (1984) it was murder on the long runs, 53mph@4000rpm:(, but boy was it fun 0-60:D
 
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Mopar Performance 360 cid 380 hp rated

RPM...TQ.... HP
2500 380.6 181
3000 402.9 230
3500 415.1 277
4000 437.8 333
4100 439.5 343
4500 437.1 375
5000 416.3 396
5400 398.6 409
5500 388.6 407
6000 343.9 395

The thing about race-gears on the street is that with 400 ish HP, any more than 10 or 11 to 1 Starter gear is kindof a waste. What I mean is a if your 400hp engine has a stout bottom end, say on account of it's running nearly 11/1 SCR, then an 11/1 starter-gear will go up in smoke real easy. With an automatic there is an initial TM hit in the TC,on the line as well; meaning a 10/1 starter is more than you need.
With that in mind; 10/2.45=4.17s is the gear of choice. So you can round that either way; 4.30s or 4.10s.
Now this stout 400 hp-SBM is gonna start making power around 4000 rpm, which is about 30 mph, unless of course it is spinning. And it's gonna want to rev at least 6500 at the shift to come in at 3850.Well 6500 at the top of first,not spinning is about 50 mph.So now you drop into second at 50 mph doing 3850. 60mph will be 4620. So during the hardest part of the run,from 50 to 60 mph, the engine will be stuck at about 300 to 380 hp. Quick math says average hp from 50 to 60mph is 340 :(
So you smoked off the line to 50, slammed it into second, and now are struggling to get out of that hole.
Now work it out with 3.55s
The starter gear is 3.55 x 2.45=8.7 plus say 10% in the TC for a few feet =9.57,Perfect. Then the TC gets down to business and you hit 5670@50mph. This being about 400hp. By 60 the engine is well past peak-power, spinning 6800, and probably still making 375hp. So now, during the hardest part of the run, the engine will be putting out 400 to 375hp. Quick-math says average hp from 50 to 60 mph is 387.5hp :)
If you can't make race gears stick on the street,then, there is no point having them. I've run: 5.13s,4.86s,4.30s,4.10s,3.91s,3.55s, 3.23s.2.94s,and 2.76s. Guess which are my favorites.

Choose wisely grasshopper
 
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Good GOD that could have been shorter without all the negitivity shoved in with assumption as a cherry on top!
 
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