Looking for experience with 3:70 gear.

[Bob Booth #33372]

Hello all,

Some of you may remember a previous post where I stated my desire to increase the acceleration in my '72 coupe by changing my r&p from a 3:08 to a 3:70. Well, having no experience with this particular ratio relative to mine, it dawned on me that maybe I should ask your opinion/experiences on this matter before I embark on what promises to be a lengthy and expensive project.

Is it a good compromise gear? How is it on the highway? And most importantly, how is the acceleration when compared to the 3:08?

In case it's pertinent, I have a w.r. Muncie box, 350/300 engine (not original).

Thanks, Bob Booth

[Duke W.]

Re: Looking for experience with 3:70 gear.

Your 350/300 is a relatively low rev engine and produces excellent torque from just off idle to about 4500 revs. Going back to the mid years, the 327/300 engine was typically equipped with a wide ratio four-speed (2.54 low) and a 3.36 axle. This was an excellent combination for all around driving, but certainly not as quick or visceral as a solid lifter engine. The solid lifter engines typically had a CR four-speed (2.20 low) and a 3.70 gear was typically standard. These engines had less torque than the hydraulic lifter engines below about 2500 to 3000 but then really began to scream at 4000. I think a 3.70 is too steep for your engine. It will accelerate a bit better, though maybe not as much as you think,but revs at 60 MPH will increase about 20 percent from around 2400 to 2900. For the time and money I think you're better off installing an L-82 cam. My SWC has a 327/340 Duntov-cammed engine, a CR four-speed and a 3.08 axle. The shift points at 6500 are about 75/105/130. It's like a five speed with no first gear. Yah - It's kind of a dog off the line, but once your up to 40 MPH it doesn't make any difference and the car accelerates like an F-16 on military power. A big block would accelerate like an F-16 on afterburner.

Duke

[Jerry Clark]

Re: Looking for experience with 3:70 gear.

Hi Bob:

I swapped my 323s for 370s in my 69 , 400 hp, with the tri power AC combination 323s were all the general would allow in 69 and the acceleration, while good, could always use improvement. The 373s are a great all around choice, won't spin everything to death on the road and vastly improved acceleration around town, much more derivable. Good Luck.

jerry

[Bob Booth #33372]

Re: Looking for experience with 3:70 gear. Duke,

Duke,

Sorry, I should have been clearer on the engine. It's a 350 (block of unknown origin) bored .030" over, heads (194's) decked approx. .020", Holley 600 on an Edelbrock Performer intake with a mild RV type cam (don't know the lift/duration specs.) built to an est. h.p. rating by the builder (local race car & street engine builder of good repute) of 320 h.p. (I'm calling it a conservative 300 h.p. to be safe), not a "350/300 Corvette" engine as I think you were describing. I don't know if this makes a difference, but I figured I would let you know anyway in case it does. I never thought about changing the cam, but I assume you mean to go to a cam suited more for low end power to make up for the low end lag. Is this right?

Bob Booth

[Duke W.]

...maybe it's not the gearing

Well, I think we're both lost now. A "mild RV cam" usually biases torque toward the low and mid range, so it should be making plenty of torque to pull a 3.08. An L-82 cam is more agressive. You can run into almost any problem when you start mixing all these aftermarket parts. The carburetor may not be set up right for the cam. You mentioned the block was decked.020", (which is a LOT) but didn't mention what the compression ratio might be. I doubt if your engine is anywhere near 300 horsepower. Maybe 250 net and 200 at the rear wheels. "Mild RV cams are designed to make as flat a torque curve as possible between about 1500 and 4000, then it falls off a cliff, so they don't make that much top end power. My recommendation at this point is try to establish a baseline of where you're at. What are all the engines specs? Make,model, and specifications of the cam? How is the carburetor jetted? What's the centrifugal and vacuum spark curve look like? What exhaust system are you running? Figure out which four-speed you've got so the real gearing can be determined. Then I'd find a good dyno shop that understands small blocks, make some power runs, then get the fuel and ignition curves working with the cam and fuel quality you're using. I think you've got an engineering development issue hear. Doing the development work might solve your problem for two or three hundred bucks. I could even be something real simple like a lazy ignition advance curve.

I don't mean to knock ya, Bob, but the Chevy engineers and technicians spent hundreds if not thousands of man hours developing their engines, but every car guy out there knows more than they do, so they start replacing parts. It usually starts our with too big a cam and an off the shelf carburetor that isn't anywhere near right, and most don't even think about the spark curve much less actually detemine what it is and then make sensible adjustments. Usually they end up with a stone. I've been watching guys do this for over 30 years. The cloud in the silver lining is that there are good shops that can probably do the tuning required to get everything working together.Ask around - drag racers, vintage racers, SCCA guys. Somebody should be able to set you up with the right shop. Once the engine is set up right I don't think you'll need to worry about the gears.

Duke

[Bill Becker]

In My Opinion...

Bob,

In my opinion, the answer lies in what transmission you are running. A long time ago I had a 72 LT1 with a Wide Ratio with a 3:55 rear end. Personally I think this is the best overall combination for a small block. That gets my vote with your wide ratio. If you had a close ratio, then I might say go with the 3:70.

To compare, here are some first gear final ratios:

Wide ratio 2.53 (or is it 2.52?) Close ratio 2:20 2.53 x 3.55 = 8.98 2.20 x 3.55 = 7.81 2.53 x 3.70 = 9.36 2.20 x 3.70 = 8.14

2.20 x 4.11 = 9.04

The 3.55 rear end will still give you pretty much the same hole shot as close ratio with a 4.11! And you will still have a great highway gear.

Bill Becker

[Marty]

Re: Looking for experience with 3:70 gear.

BOB:

Have a 3:70 in my beater '63, and i feel it is too much gear if you do a LOT of interstate cruising as we do. I am taching well over 3000 at western interstate traffic speeds. Lousy gas mileage, and probably shorter engine life. I have a '64 yanker with 3:08's and it's a dog off the line, but will do 90+ in 2nd, and cruises at 80 in the low 2k's. I guess you should pick a gear for the way you plan to use the car. Incidentally I don't think the 3:70's help the acceleration that much after you get the car rolling, as using the 3:08's give you more gears to chose from. (IE: 2nd instead of 3rd.) With 3:08's I can cruise @ 3K in 2nd, and then it's off to the races.

[Rob Brainard]

Re: Looking for experience with 3:70 gear.

If you are looking for economy stay with the 3:08s. But I don't think that is what you are looking for in a Vette. 3:08s started being available in 62 and my 57 has 3:70s. I realize our cars are "diiferent", but performance is performance. With 270 HP and 4spd my goes pretty good and the highway mileage is about 16 mpg [with no vaccuum advance]. 3:70s were the standard gear for a long time and I doubt anyone will drive their car today as it would have been driven when new so the "high rev" issue is really not an issue. Go for the 3:70s and have fun.

[John H.]

Re: 3.70 Gears - The Numbers

Bob -

I think you'll find the 3.70 is too much gear for today's driving, assuming you spend some time on the freeways at 70+. In 4th gear, with 205/75R15 or 6.70x15 tires (27.7" diameter, 87" circumference), a 3.70 axle will give you 22.23 mph per 1000 rpm, or almost 3200 rpm at 70 mph. Fun around town, but gets to you after a while on the Interstate (I have a '57 with the standard 3.70). I'd leave the axle alone and focus on optimizing what's under the hood once you figure out what's really there.

John

[Duke W.]

Re: Looking for experience with 3:70 gear.

Economy was a secondary consideration, but what I was really looking for was top speed, though the handful of times I had my SWC up to 150 MPH were probably enough to last a lifetime in that car given it's aerostability problems. Having driven two Silver State Classic Challenges, the Nevada open road race (a Merc 190E2.6 at 130 and a Saleen Mustang at 160), I can attest that modern cars are much more stable at extremely high speeds - thrilling, but not scary.

Someone else made a good point that played on my statement that gearing doesn't make much difference once your over about 40 MPH, and I'll give another example comparing a mid year with a CR gearbox and a 3.08 or 4.11 diff. If you run the overall gearing numbers, first, second, and third with the 3.08 are almost identical to second, third, and fourth with the 4.11.

I used this to my advantage back in my youth when I used to go trolling for a little weekend fun. I have a vivid recollection of beating a fuelie and a 440 Charger, but it took that 130 MPH shift into fourth gear and another ten to fifteen seconds on the power before the other guy ran out or revs or courage. I never was much into drag racing - either the quarter mile or stop light type as I thought it was just a quick way to tear up the clutch and drivetrain, so I wasn't that concerned about hole shots, but I still love to go fast.

[Bob Booth #33372]

Duke, Rob, Jerry, Bill , Marty, John, Thank you...

...for taking the time to help me with this. It sounds as if the answer is part subjective, part engine. I don't do alot of highway driving (maybe 10 miles at a time) so that's not much of an issue for me, though it may be sometime if I decide to travel more, like to NCRS meets , etc. Fuel mileage isn't much of a concern either as this thing isn't exactly a miser in that respect anyway (though I definitely think that it's running too rich at the moment).

It definitely sounds like I have some engine issues to deal with before I do anything. I trusted the guy who built the engine because he has a good rep. locally. When I asked him to build it, he asked me what I was looking for and I told him, "about 300 streetable horsepower". When he told me what he did to the engine, it sounded somewhat questionable that it could have that much horsepower but with my limited knowledge and no way to prove otherwise (other than taking it to a dyno shop for about 700 bucks), I just assumed it was true. I mean, it goes like stink once you get it rolling, so it seemed ok to me. It all kind of makes sense now because I've been after him for two years now for the build sheet with no luck. Hmmm...wonder why?

Anyway, it seems I have some tuning to do. I was also thinking of starting from scratch and getting one of those "carb, intake, heads and cam" kits from Edelbrock or the like that's supposedly rated at X amount of horsepower and engineered to all work together with your block. Maybe this is the way to go. Then at least I will have a definite baseline to start from.

Thanks again for helping out. I really appreciate it! Bob Booth

[Jerry Clark]

Re: 3.70 Gears - The Numbers

HI Guys:

My original response took Bob's post into consideration. He wrote: "And most importantly, how is the acceleration when compared to the 3:08", which gave me the impression , by the use of "most importantly", that performance was his most important concern, I stand by that answer.

Respectfully

jerry

[Bob Booth #33372]

Re: 3.70 Gears - The Numbers. YES Jerry.....

....performance IS the key! POWER! I NEED MORE POWER! But seriously, I think performance is why alot of us are doing this and to me it is extremely important. And once I get the engine straightened out I think a gear change may almost certainly be in my future.

Thanks again,

Bob Booth

[Jerry Clark]

Re: Duke, Rob, Jerry, Bill , Marty, John, Thank yo

Hey Bob:

Unless you are dying to buy something for your toy, those already installed components can be made to work together. If you are not comfortable in your ability to do the tuning ask around and find someone capable of dialing in the ignition and carb although, as Duke mentioned, a cam change might be in order regardless of your gear choice. It can almost certainly be done and at a cost less than the total of your new parts.

jerry

[Bob Booth #33372]

Re: ...maybe it's not the gearing.Cam suggestion?

Hello again,

Using the info in my previous post where I describe my engine configuration, does anyone have a suggestion on a cam that may be appropriate? The other stuff I forgot to mention was that I believe the builder told me the compression was in the 10.5:1 range although not absolutely positive. Also, I am using a Mallory dual point distributor (soon to change to Mallory electronic conversion unit using the same distributor body). I am looking more for low end torque as opposed to revving ability. Any suggestion would be greatly appreciated.

Regards,

Bob Booth