Wide Vs. Close Ratio?

[James B.]

Confessions: 1. I prefer close ratio transmissions, 2. I don't understand gearing, 3. I'm not the sharpest crayon in the box technically.
I have had many Corvettes with several drivetrain combinations. I currently have a '67 L79 with 3.55 and a wide ratio trans and a '64 L84 with 3.70 and a close ratio trans. I prefer driving the '64 from the standpoint of the gearing combination. If I compare the feel in the two cars on making the 1-2 and 2-3 shifts it feels like the '67 loses more RPM in the shifts-like I've dropped into a hole. By comparison the '64 with 3.70 and close ratio trans feels like I lose very little RPM and the motor is still in it's torque 'sweet spot' (and yes, I know this is an 'apples and oranges' comparison as I have different rear end ratios in the two cars).
However, my sensations don't seem to be borne out by a gearing chart I put together (if I did it right?)

Wide ratio vs. close ratio- RPM drops on upshift
Shift point	4000	RPM
Tire Revs	776	Revs/mile
Wide ratio		Split			Close ratio
1st	2.52				2.20
2nd	1.88	25.40%			1.64	25.50%
3rd	1.47	21.80%			1.27	22.60%
4th	1.00	32.00%			1.00	21.30%
Wide Ratio w/ 3.55:1 Rear					Close Ratio w/ 3.70:1 Rear
	3.70					3.70
2.52	9.32				2.20	8.14
1.88	6.96				1.64	6.07
1.47	5.44				1.27	4.70
1.00	3.70				1.00	3.70
RPM at shift- first to second
(RPM/overall ratio)*60= Revs per hour
Revs per hour/ Revs per mile= speed in MPH
	33.2	MPH in 1st gear at shift point				38.0
RPM in Second gear after clutch engagement
(MPH/60)=miles/minute*revs per mile= Revs/minute*overall ratio=RPM
	2984	RPM				2982
Rev Difference	1016					1018
2nd gear to 3rd
	44.5	MPH in 2nd gear at shift point				51.0
	3128	RPM				3098
Rev Difference	872					902
Note: Torque Peaks
L-79 360@3600
L-84 350@4400/4800
Note: 1st Gear ratios
	2.20X3.70=	8.14			2.52X3.36=	8.47
	2.20X4.11=	9.04			2.52X3.55=	8.95
					2.52X3.70=	9.32

As this indicates, the splits are essentially the same percentage in the 1-3 gears and if I shift at 4,000 RPM the rev loss is the same (road speeds are different) going from 1st to 2nd or 2nd to 3rd. So, I guess my perception isn’t real. Maybe it is the road speed difference and not rev loss I am sensing??
Sorry to all, just realized that on the attached chart I changed the 3.55 to a 3.70 so it is an 'apples to apples' comparison.
For a related question and given the above I am surprised GM mated close ratio transmissions to L76 an L84 motors as they have lower torque in the 1,500-3,500 RPM range. What was GMs rational for recommending close or wide ratio transmissions?

[Gene M.]

Re: Wide Vs. Close Ratio?

James,
From what I have noticed it is not the engine that determines the tranny. But the rear end ratio that determines the selection. A 3.55 and higher get the wide ratio. The close ratio was used with 3.70 and deeper gears.

[William C.]

Re: Wide Vs. Close Ratio?

Close ratio was for roadracing, wide ratio for street driving essentially.

[John H.]

Re: Wide Vs. Close Ratio?

Here's an article I wrote some time back on the M20/21 ratio story.


M20RatioTech.pdf

[Duke W.]

Re: Wide Vs. Close Ratio?

I prefer to use the concept I call "inter-gear ratio" to represent gear spacing, which is simply the ratio of the ratios. For example, the CR Muncie 1-2 spread is 2.20/1.64 = 1.34, and if you shift at 6500, revs drop to 6500/1.34 = 4850. Likewise for the 2-3 and 3-4 intergear ratio, which is about 1.28. (I've see third quoted as 1.27 or 1.28 depending on the source.)

On a mechanical lifter engine the power at 5000 is about the same as at 6500, so spacing of about 1.3 keeps the revs in the power sweet spot when you are asking for maximum acceleration. With a "327 LT-1" that has massaged heads and LT-1 cam shifting at about 7000 drops revs to 5500, and 5500-7000 is the power sweet spot for that configuration.

As Clup said, the CR four-speed is geared for racing, but that results in a relatively narrow total spread of only 2.2:1, so depending on the axle choice, it's either a five-speed with no first, or a five-speed with no fifth. Optimum axle ratio choice for road racing depends on how long a straight the course has, so it could be a 4.56 for a short course or 3.08 for Daytona. A serious drag racer would want a 4.56 or even shorter.

GM recognized the shorcomings of the CR four-speed and the need to increase the total ratio spread, which they did by the simple expedient of just making a different clutch gear and countershaft with greater multiplication. All other parts inside are the same. This method shortened first, second, and third, and maintained the 1-2 and 2-3 spread, but resulted in a HUGE GAP between third and fourth. This gave good acceleration with a 3-3.5 axle in most applications, then it fell on it's face with the shift into fourth, but by that time speed was about 100 MPH, so the WR worked well with typical muscle cars where anything beyond a quarter-mile was not important.

European tranmissions (and most modern transmissions) are set up with progressively closer spacing as you work through the gears. The widest spacing is between 1-2 and the closest is between the top two gears. This is the way to go for a road engine. As vehicle speed increases, more power is required to overcome total drag, so less is available for acceleration. For this reason a wide gap between 1-2 is less noticeable than a wide gap between the top two gears.

GM finally got it right with a Super T-10 that was available on some engines in the late C3 period. The ratios are 2.88, 1.91, 1.33, 1,00:1, and the intergear ratios are 1.51, 1.44, 1.33. This Super T-10 is still available from Richmond and would work very well on a SHP/FI engine with 3.08 or 3.36 axle and a medium performance engine (like the 327/300) with a 2.73 or 3.08. For example first gear with a 3.08 provides about the same total multiplication as a 2.20 first gear with a 4.11.

This ratio set is not ideal for road or drag racing, but very few of us race, and the wider total spread provides a good starting gear and a tall cruise gear for the freeway. Typical intergear ratios for a road car should be in the 1.5 -1.7 range for 1-2, and progressively narrow to 1.25 to 1.35 for the top two gears.

Small displacement engines need more total spread, which means wider spacing or more total gears than large displacement engines because the latter provide greater low end torque/power; and engines with broad torque bandwidth will work well with relatively wide intergear ratios. Peaky engines need close ratios, which is why the 7-speed boxes in F1 cars have intergear ratios of about 1.1 from second to seventh. Shift at 18,000 and revs only drop to about 16,000, and the power in this range is nearly constant so near maximum power is delivered at any road speed.

I recall talking to a 300 HP C2 owner who said he rarely used second gear and usually shifted 1-3-4. Because the 300 HP engine has such high low end torque, you can do this and still have good acceleration in normal road driving. Indeed, 1-3-4 of a WR four-speed are about the same as 1-2-3 on the Corvette three-speed transmission.

The new C7 achieves top speed in fifth gear and the total 1-5 spread is 2.97/0.71 = 4.18, which is nearly double the total spread of the old CR four-speed. Through fifth gear spacing is nearly constant in the range of 1.41 to 1.45, which is fairly wide for the upper gears, but given the LT1's prodigious torque bandwidth (up to 4000 it makes about the same torque as the LS7.), the relatively wide spacing does not detract materially from performance. Only five speeds are necessary for maximum performance. Sixth and seventh of the new seven-speed transmission are strictly fuel economy gears for cruising at highway/freeway speed.

Years ago I set up an Excel spreadsheet with inputs of tire revs/mile, transmission ratios, axle ratio, and shift revs. The outputs were inter-gear ratio, rev drop, and maximum speed in each gear, and I could then plot the data in graphical form with lines showing speeds in each gear from 1000 to maximum revs. The graphical output easily showed the rev drop shifting at any engine speed, and then I could superimpose a simulated or tested power curve to see how appropriate the gearing was to the predicted or actual power curve and ultimately choose the best off-the-shelf transmission and axle ratios for the specific application be it racing or road use. This is all part of the engine system engieering.

Duke

[Robert M.]

Re: Wide Vs. Close Ratio?

I agree with Duke's analysis. I have a 67 427/390 with a wide ratio box and a 3.08 rear. It is a pleasure. With all the torque there is I almost never use 2nd gear. As a matter of fact a shift from 1st to 4th is completely fine for normal driving. I have had cars with 3.70 and higher rears with a CR box and for normal dribing I did not like it. On a hill, just pulling out in 1st gear often required slipping the clutch.

[Timothy B.]

Re: Wide Vs. Close Ratio?

"I recall talking to a 300 HP C2 owner who said he rarely used second gear and usually shifted 1-3-4. Because the 300 HP engine has such high low end torque, you can do this and still have good acceleration in normal road driving. Indeed, 1-3-4 of a WR four-speed are about the same as 1-2-3 on the Corvette three-speed transmission."

I do this all the time with my 1967 300hp motor and 3.36, don't downshift much either as the car will motor around most corners in fourth gear.

[Duke W.]

Re: Wide Vs. Close Ratio?

With the 2.88 first Super T-10 and a 3.36 axle (about equivalent to a 2.54 first and a 3.70 axle or a 2.20 first and a 4.56) a 327/300 could do very impressive burnouts.

Duke

[James B.]

Re: Wide Vs. Close Ratio?

Very interesting and educational discussion (particular thanks to Duke). I'm starting restoration of my '67 L79; you all have convinced me to keep the WR/3.55 it was born with,despite my comments starting this thread I will admit that package is great at clutch engagement (a lot better than CR/3.70) and 1st and 2nd gear are strong! Thanks to all, Jim B

[Robert M.]

Re: Wide Vs. Close Ratio?

Jim
I think you are making an excellent choice. Back in the day we all thought it was the best and cool thing to have a CR four speed. Today I would look for the car that has the WR vs the CR when buying.

[Gene M.]

Re: Wide Vs. Close Ratio?

Here's an article I wrote some time back on the M20/21 ratio story.


[ATTACH]48214[/ATTACH]

John,
Was a 67 L79 and 3.70 rear gear available with a wide ratio? I have not come across this combination in anything other than a close ratio. Also any 67 L79 and 3.36 or 3.55 rear gear has always had a wide ratio tranny (again from my findings). What are your thoughts? thank you

[Timothy B.]

Re: Wide Vs. Close Ratio?

Very interesting and educational discussion (particular thanks to Duke). I'm starting restoration of my '67 L79; you all have convinced me to keep the WR/3.55 it was born with,despite my comments starting this thread I will admit that package is great at clutch engagement (a lot better than CR/3.70) and 1st and 2nd gear are strong! Thanks to all, Jim B

James,

I think your combination is great for a fast street car..

[Daniel Y.]

Re: Wide Vs. Close Ratio?

Duke, If you wouldn't mind - would you share that spreadsheet with me? bowtie66@ptd.net

Thank you!

[Joe R.]

Re: Wide Vs. Close Ratio?

Good idea Dan, Duke how bout sending one my way?

Thanks!
Joe

[John H.]

Re: Wide Vs. Close Ratio?

John,
Was a 67 L79 and 3.70 rear gear available with a wide ratio? I have not come across this combination in anything other than a close ratio. Also any 67 L79 and 3.36 or 3.55 rear gear has always had a wide ratio tranny (again from my findings). What are your thoughts? thank you

Gene -

The axle ratio you picked with a '66 or '67 L-79 dictated the transmission ratios; with a wide-ratio 4-speed, the 3.36 was standard, and the 3.55 was optional. With a close-ratio 4-speed, the 3.70 was standard and the 4.11 was optional. 3.08 and 4.56 weren't available with the L-79.