275 tires - 3.70 vs 3.89 gears

[Gaugster]

Looking for folks opinions and experiences about tire size vs gear ratio. I'm setting up my Cougar with 275/50R-17s out back. The tires currently are Nitto NT555 Ultra High Perf. but I may switch to M&H drag radials later on.

Tire diameter is in the 27" to 28" range. Is this "a lot" of tire and capable of hooking or will I be smokin' them in every gear? I realize there are many variables but I'm wanting to nail down which ring and pinion I should end up with. Note that the 6R80 1st gear ratio is 4.17 : 1. 99% street driven but I want it to be capable at the drag strip without being a "white knuckle" ride.

650HP stroker FE
3500lbs
Caltracs, Composite leaf springs, single adjustable shocks
~2500 stall trans brake, 2000 stall foot brake

At 2000 RPM and a 0.69 overdrive the 3.70 would put 63MPH whereas the 3.89 would drop this to 60MPH.
2350 RPM puts me at 74MPH or 70MPH.

[jayb]

John, as I'm sure you know the 6R80 sixth gear is a 0.69 overdrive.  If you put 4.11 gears in your car, that's the equivalent of a 2.83 ratio in sixth.  I have a 0.63 ratio sixth in my Tremec six speed, and with 4.11 gears and 26" tires I'm only turning 2200 RPM at 65 MPH in sixth gear.  With your tires I'd be going 70!  I'd say 3.89 is a minimum gear, and I think you'd be just fine with 4.11s.

[Gaugster]

John, as I'm sure you know the 6R80 sixth gear is a 0.69 overdrive.  If you put 4.11 gears in your car, that's the equivalent of a 2.83 ratio in sixth.  I have a 0.63 ratio sixth in my Tremec six speed, and with 4.11 gears and 26" tires I'm only turning 2200 RPM at 65 MPH in sixth gear.  With your tires I'd be going 70!  I'd say 3.89 is a minimum gear, and I think you'd be just fine with 4.11s.

Thanks Jay! I was leaning towards the 3.xx ratios since the 6R80 has a steep 4.17 first gear. Does your Tremec (Magnum?) have a 2.66 first gear? I was using 2000 RPM as my cruise speed. I'll crunch some numbers at 2200RPM.

[My427stang]

This is a half-empty/half-full dilemma, I do agree with Jay that you don't want too tall in top gear .  I had a 2.36 final in my car (3.70/.64) and switched to a 2.63 (4.10/.64) and it does everything better.

However, your 1st gear is silly deep at 17:1, BUT, not being a manual tranny, it'll shift hard and fast when it's ready, but honestly, you are asking which gear you won't be using a lot, 1st or OD depending on gear ratio

My Tremec is a 2.87/4.10 = 11.76, 275/60-15 tire, and I often start in second in traffic.  First gear at full throttle is very short, but good..., yours is 45% lower, and will act slightly lower than that due to the converter slip

To make the decision, I would look close at how you plan on driving it the most, AND see if you can get it with a locking converter.  One end is going to be sub-optimal whatever you do, but I think I would lean toward deeper gears. 

Adding to things to think about, also think how fast you plan to go, deep gear means increased driveshaft speed. With the capability to go REAL fast with that combo, that's a concern too and make sure you have a driveshaft that can handle it if you go deep gear.  Might be wise in that case to lean toward Starting Line Ration and accept you won't use top gear all the time

If you gave all the details of the build it would help too, remember though SLR is important.  A Drag Pack CJ had an SLR of 4.30*2.32=9.97......your car would launch that hard with a 2.39 rear axle (actually harder because the converter would help you)  Not advising that, but you have a wide range of "will work good"

[Falcon67]

I'd go with the bigger gear 3.89 also.  Your converter is going to be more of a factor at the track than the gear ratio.  So I'd plan more for whatever RPM @ cruise speed you'd prefer.  But with the OD, bigger will be better. 

Tires - the taller the tire, the larger the contact patch.  Should be close to the same diameter as most 275-60x15 drag radials, which is a 28" tire.  I don't see much white knuckle in any of those tires as you'll need air pressure in the 16 lb up range.  If your caster in the front is decent and whatever pressure in the back gives the best face of the tire pattern, it should go right from one end to the other. 

[gregb]

What power range for the engine?  Will the engine be happy at 2000 or even 2200 when cruising?  That's what I'd use as a bottom number for figuring out final gear, I had 3.91's and a four speed, and when I changed to a 5 with .68 overdrive I ended up changing the rear to 4.11's......although with a 4.17 first in the trans it will shift out of first pretty quickly when just driving for fun......

[Gaugster]

This is a half-empty/half-full dilemma, I do agree with Jay that you don't want too tall in top gear .  I had a 2.36 final in my car (3.70/.64) and switched to a 2.63 (4.10/.64) and it does everything better.

However, your 1st gear is silly deep at 17:1, BUT, not being a manual tranny, it'll shift hard and fast when it's ready, but honestly, you are asking which gear you won't be using a lot, 1st or OD depending on gear ratio

My Tremec is a 2.87/4.10 = 11.76, 275/60-15 tire, and I often start in second in traffic.  First gear at full throttle is very short, but good..., yours is 45% lower, and will act slightly lower than that due to the converter slip

To make the decision, I would look close at how you plan on driving it the most, AND see if you can get it with a locking converter.  One end is going to be sub-optimal whatever you do, but I think I would lean toward deeper gears. 

Adding to things to think about, also think how fast you plan to go, deep gear means increased driveshaft speed. With the capability to go REAL fast with that combo, that's a concern too and make sure you have a driveshaft that can handle it if you go deep gear.  Might be wise in that case to lean toward Starting Line Ration and accept you won't use top gear all the time

If you gave all the details of the build it would help too, remember though SLR is important.  A Drag Pack CJ had an SLR of 4.30*2.32=9.97......your car would launch that hard with a 2.39 rear axle (actually harder because the converter would help you)  Not advising that, but you have a wide range of "will work good"

Yea... I always forget to consider critical & 1/2 critical driveshaft speed so thanks for the reminder. I'm using a 3.5" chromoly shaft that's on the long side. 57" total length from u-joint center to center. The Spicer calculator I have been using shows 4000 and 3500 RPMs respectfully. The driveshaft has a slip section so the 3.5" diameter section is 46" actually which increases the critical RPM to 5000+. Might not be the correct way to think about it from a harmonic standpoint but reality is probably some RPM in the middle. For certain I will be spinning the driveshaft through those RPMs. My understanding is that I should just avoid lulling at those RPMs during a cruise speed condition.

The computer I am using let's me lock the converter in a variety of situations. Currently it will lock up in 4th, 5th and 6th with light loads. I'm guessing letting it slip and getting the torque multiplication will be best in a drag race.

[Gaugster]

I'd go with the bigger gear 3.89 also.  Your converter is going to be more of a factor at the track than the gear ratio.  So I'd plan more for whatever RPM @ cruise speed you'd prefer.  But with the OD, bigger will be better. 

Tires - the taller the tire, the larger the contact patch.  Should be close to the same diameter as most 275-60x15 drag radials, which is a 28" tire.  I don't see much white knuckle in any of those tires as you'll need air pressure in the 16 lb up range.  If your caster in the front is decent and whatever pressure in the back gives the best face of the tire pattern, it should go right from one end to the other.

Yup. It seems the 3.70 is off the table considering it would be a very tall 2.55 final drive ratio. I do want to use all the gears as best I can. Good comments about these tires as they don't get aired down like a bias ply slick.

[Gaugster]

What power range for the engine?  Will the engine be happy at 2000 or even 2200 when cruising?  That's what I'd use as a bottom number for figuring out final gear, I had 3.91's and a four speed, and when I changed to a 5 with .68 overdrive I ended up changing the rear to 4.11's......although with a 4.17 first in the trans it will shift out of first pretty quickly when just driving for fun......

Well that is a great question. Jay is aiming for a non-aggressive 650HP 390 based stroker using his components. I intend to copy/repeat that effort but he's using a 4.25" stroke I believe. Motor shouldn't be cammin' hard at 2200 RPM

Redline no higher that 6250RPM
390 Block 0.040" over, studded with girdle
460 CID via a 4.375" scat crank
10.25 : 1 CR
FE Power heads - SE as cast. 1.76 rocker ratio
FE Power cross ram, EFI, Electronic Ignition
Camshaft - TBD
I'm keeping my shock towers so Tri-Y headers most likely.
Power steering, Power Brakes and Air Conditional capable. 

Basically I am doing my best to setup my chassis and powertrain with the engine design being the last piece of the puzzle.

[Falcon67]

>Will the engine be happy at 2000 or even 2200 when cruising?

This is more of a converter question as you don't want to be loafing along the road below normal stall speed as that will just heat fluid and raise trans temperatures.  But yes, a big overlap cam type deal won't be too happy at 2K LOL. 

[Gaugster]

>Will the engine be happy at 2000 or even 2200 when cruising?

This is more of a converter question as you don't want to be loafing along the road below normal stall speed as that will just heat fluid and raise trans temperatures.  But yes, a big overlap cam type deal won't be too happy at 2K LOL.

Converters are an interesting topic. I had a thread in the Non-FE section and we hashed that out pretty good. I have the ability to 'tune' the slip/lock of the converter via software. So another level of flexibility which is cool. It's been very rewarding to mix a FE with a modern trans.