3.08 - 3.55 Opinions Needed

[Fishman]

It appears after reading several threads about moving from a 3.08 to a 3.70 ring and pinion maybe a bit excessive for my needs, both in lost top-end speed and much lower mph. Any of you move from a 3.08 to a 3.55 and were REALLY happy or disappointed about the change? I was thinking that splitting the difference may be the best of both worlds, but want to bounce this off the group before I hit the 'buy' button. Obviously this will cost a few hundred bucks and want to be sure I get some good bang for the buck.

As always... any input appreciated.

 

[BeaterShark]

IMHO, going from 3.08 to 3.55 is not enough of a change to warrent the money and work involved. Also, the difference between the 3.55 and 3.70 is really small (only about 130 RPM diff @ 70mph).

If it were me, I'd go with the 3.70.

 

[Glensgages]

IMHO, going from 3.08 to 3.55 is not enough of a change to warrent the money and work involved.

I don't know about that...if the engine makes good upper-RPM power, it'd like the deeper gears.
My '79 Z28 went from 14.40s in the 1/4-mile to 14-ohs/teens going from 3.42s to 4.11s.
As a general rule-of-thumb, the 'worse' you are when you start (3.08, 2.87, 2.72, etc.), the more drastic the improvement will be changing a given (i.e.: .5:1) ratio.
{Going from 3.08s to 3.55s [.47:1] will make a MUCH bigger improvement then going from 4.10s to 4.56s [.46].}

Also, the difference between the 3.55 and 3.70 is really small (only about 130 RPM diff @ 70mph).
If it were me, I'd go with the 3.70.

I agree; if you're gonna go THAT far (3.55s), you might as well go for 3.70s/3.73s.
3.73s aren't all THAT drastic anyhow; 4-speed Z28s from '78-'80 had them as OEM; you won't twist your motor THAT hard.
I put 3.73s in my '82 last Fall, and even though my ETs dropped from 15.904 to 15.209 (what do you EXPECT from a lowly Cross-Fire...),
I wish I could've put-in 4.10s (or deeper!!!) with the original carrier;
at 90.9 MPH, I was still only turning 4300 RPM in 3rd Gear!!!

 

[Fishman]

Thanks for the replies... it's still going to be a tough decision. I may just leave as is.

This is the part of the drive train I understand the very least about. I'll keep reading... :confused

 

[black_81_vette]

gears

Im goin to 3.54s in my 81 with a 4 speed wich has 2.72s in it now il let you know how it goes.

 

[BeaterShark]

I don't know about that...if the engine makes good upper-RPM power, it'd like the deeper gears.
My '79 Z28 went from 14.40s in the 1/4-mile to 14-ohs/teens going from 3.42s to 4.11s.
As a general rule-of-thumb, the 'worse' you are when you start (3.08, 2.87, 2.72, etc.), the more drastic the improvement will be changing a given (i.e.: .5:1) ratio.
{Going from 3.08s to 3.55s [.47:1] will make a MUCH bigger improvement then going from 4.10s to 4.56s [.46].}

Instead of looking at the gearing difference, look at the percentage differences, since a 10% increase is a 10% increase regardless of what you are starting with:
3.08 -> 3.55 = 15% increase
3.08 -> 3.70 = 20% increase
3.42 -> 4.11 = 20% increase
4.10 -> 4.56 = 11% increase

I threw the last two in just to help prove your point. Also, this shows that 3.70 gears give an extra 5% increase while only giving up 130 RPM @70 mph over the 3.55's.

 

[Glensgages]

Instead of looking at the gearing difference, look at the percentage differences, since a 10% increase is a 10% increase regardless of what you are starting with.

You've explained it very well for me; when I speak to majority of 'racers' about things like 'percentages', I get blank stares!!!

P.S.: 'Fish' & I spoke by phone last evening, and he understands 'percentages' very well.

 

[Glensgages]

...a 10% increase is a 10% increase regardless of what you are starting with...

Speaking strictly from an 'acceleration/drag-strip' view-point, "10% = 10%" can be deceiving, especially when the vehicle in-question is "starting with" a gear ratio SO-FAR from the optimal gear. For instance:

My recently-sold Z28 ET'd best (12.70s) with 4.56:1 gears;
swapping to 10% taller gears (4.10s) resulted in just an ET-increase to 12-eighties {proven at the track};
going another 10% taller (3.73s) would most SURELY result in MORE than 2-tenths ET-increase than the 4.10s;
going another 10% taller (3.42) might result in .3-or-more increase from the 3.73s;
etc.
The motor just can't get "up" into the optimal power-making RPM during the initial 1320' with taller gears.

While ' Fishman's ' Corvette (with a ZZ4/5500 RPM/350+ HP crate motor) would surely benefit from a modest 10% increase in gear (3.08 x 1.1 = 3.38 {or 3.36s}) more than my old Z28 going from 4.10s to 4.56s, the 3.08s are SO-FAR from optimal that a 20% increase (to 3.70s) places him nearer to 'optimal'.

"IF" the ZZ4 can legitimately 'pull' to 6000 RPM, with 3.08s and his 255/60R15 tires, he 'could' (theoretically) reach 142+ MPH.....but it'd take Fishman forever to hit that speed, maybe 1/2-mile, maybe 3/4-mile.
(WHY you'd want to go 142+ MPH in a C3 is ANYBODY'S guess...!!!)
I'd "guesstimate" his 1/4-mile ET, w/ 3.08s, at mid/high 14's @ 90 MPH.

With 3.73 gears, the same 6000 RPM 'could' produce a speed of 117+ MPH
(MUCH more reasonable top speed),
AND he'd achieve that top speed MUCH sooner, too, probably running low-14s (maybe quicker!) @ 96-99 MPH, 'trapping' at approx 5000 RPM (closer to max HP-making RPM) in 3rd gear, compared to a trap-RPM of 'just' 4000 RPM (?) at 90 MPH w/ 3.08s.

Food-for-thought.....

 

[Fishman]

Man... you guys are GREAT! Was on the phone with Glensgages last night for almost an hour... THANKS BUDDY!

I put together a quick spreadsheet based upon the equations he gave me. We started with the confirmation that at 70 mph I was at 3000 rpm with the current 3.08s. I wanted to use 70 mph as my 'baseline' as that is the approximate speed I cruise on the freeway most of the time.

So based upon the calculations... if I installed the 3.55s I would then be at 3458 rpms at 70 mph (+458 revs), and if I installed the 3.70s I would be at 3604 rpms (+ 604 revs) at 70 mph. And then to take it to the extreme (for me)... if I installed 4.11s I would be at 4003 rpms (+1003 revs) at 70 mph.

As Chris pointed out, a very small difference in rpms between the 3.55 an 3.70.

Decision... the car will enjoy the new 3.70s! Can't wait to tell the wife! :L

 

[Glensgages]

...the car will enjoy the new 3.70s!

Your '78 will 'suck-up' to the Pace Car at T.M.S. much-better with the 3.70s...

Try going to 'Waco Willie's' Motorplex in Ennis for some 'before-and-after' testing, too;
I think you'll be pleasantly surprised!!!

 

[BeaterShark]

I put together a quick spreadsheet based upon the equations he gave me. We started with the confirmation that at 70 mph I was at 3000 rpm with the current 3.08s. I wanted to use 70 mph as my 'baseline' as that is the approximate speed I cruise on the freeway most of the time.

I'm now concerned about you really having 3.08 gears in there now. Where did the 3000 RPM @70mph come from? Was this a calculation or your tach reading at 70 mph?


According to my calculations, there is what I have for RPM @ MPH per gear with a 27" tire in 4'th gear (1:1):

	   RPM with gear:
Speed	   3.08	  3.55	 3.70
-------	 ------	------	-------
50	  1917	 2210	 2303
60	  2301	 2652	 2764	
70	  2684	 3094	 3224
80	  3068	 3536	 3685

I guess my formula cold be wrong (don't have time to double check it now), but it has served me well in the past.

 

[Glensgages]

I guess my formula cold be wrong (don't have time to double check it now), but it has served me well in the past.

BeaterShark:
the formula I was taught and used in the early '80s is:
RPM x .9 (factoring-in a 10% slippage of the converter in a non-lockup automatic) x.006 (a 'factor', calculating 'pi', as well as inches/feet per mile, etc.) x 1/2 TH (tire height) div/by GR (gear ratio) = MPH

Using this, I calculated approx 3000 RPM (3000 x .9 x .006 x 13.5" {27" tire} div/by 3.08:1 = 71.006xx MPH) for Fishman (technically, 2957 RPM) @ 70 MPH, which he 'thinks' is about correct from his memory.

Seeing as you calculate 2684 RPM @ 70 MPH, is it possible the approx 10% slippage in the converter has been neglected?
(2684 RPM x 1.1 = 2952 RPM)
:confused

 

[Grasshopper]

Hey Fishman, thanks for starting this thread.Ive been following along since I am thinking about changing my gears as well. I am running about 3500 rpm at 70 mph, I think thats to high, maybe I am wrong just worried about burnning up a new motor.Let me know if your interested in selling that 308 or maybe trading? I'm running a 1977.

 

[BeaterShark]

Seeing as you calculate 2684 RPM @ 70 MPH, is it possible the approx 10% slippage in the converter has been neglected?
(2684 RPM x 1.1 = 2952 RPM)
:confused

Yes, I've never dealt with an auto before and all of my calculations are theoretical, although with my 3.36 gears and my Muncie, they are dead on.


I was just being cautios as I'd hate to have him crack open the pumpkin just to see that someone already put a 3.70 R&P in there.

 

[Glensgages]

Yes, I've never dealt with an auto before and all of my calculations are theoretical, although with my 3.36 gears and my Muncie, they are dead on.

When I began fooling around at the strip, being a 'semi-engineer', my friends 'elected' me to 'do-the-math' on all calculations.

My automatic '79 Z28 came with 3.42s, while 4-speed Z-cars came with 3.73s (almost exactly a 10% increase in gear), while being EPA-rated at the same MPG-figures.
I thought "Hmmmmm!", and several aftermarket converter manufacturers confirmed to me that a figure of 10% slippage for a non-locking converter was 'typical'.

If we had been discussing a 1981 Corvette, with a THM-350-C (lock-up converter) transmission, we'd not have used the ' x .9 ' quotient in our figures.

If we'd been talking about the 700R4, with a .7:1 OD ratio AND a lock-up converter, we'd disregard the ' x .9 ', AND divided the final MPH figures by .7 to account for the OD.

I was just being cautious as I'd hate to have him crack open the pumpkin just to see that someone already put a 3.70 R&P in there.

I agree with you; I questioned the accuracy of his tachometer.
As a final check, FISHMAN can always jack his '78 off the ground, and compare drive-shaft revolutions to wheel/tires revolutions before ordering ANY parts!

 

[Fishman]

This weekend I will... (1) check and confirm that 70 mph is 3000 rpm, and (2) get the car hooked-up to a diagnostic machine to make sure the tach is operating correctly.

Grasshopper... last time we checked the 3.08s were in great shape. Not exactly sure when I can get the gears changed but I'll e-mail you when I get them out in case you may still be interested.

 

[reegeemon]

went from 3:36 to 4:11 was HUGE difference. (454/2004r car)

in different car went from 3:70 to 3:36 (same gear as removed) (l48/th400 car)was large difference hovever not huge, bottom end was all but gone.

 

[Fishman]

Confirmed... at 70 mph the tach reads right before 3000 rpm and the tach is accurate.

Also noticed my tach does not go over 3300 rpms anymore.... guess it's time for another circuit board! These C3 are so high tech and difficult to trouble-shoot. :L

 

[Glensgages]

Confirmed... at 70 mph the tach reads right before 3000 rpm and the tach is accurate.

Fishman:
As we had discussed, I think technically, 70 MPH calculates 2955/2960 RPM...

Your tach'll get a work-out with those 3.73s!!!