Rear Gear too High...Suggestions for New Ratio

[kryptik]

Hello Guys,

I am using a Currie 9" Unit with 3.50:1 Gears, Tru-Trac, and 31-spline Axles. This was originally set up for an automatic transmission (.70 OD) 68 Camaro. Since that car is now gone (RIP) I have modified and installed this axle into my 1970 Camaro.

The problem I am having is that when getting into first gear, I have to slip the clutch a little too much for my liking. The motor is tuned properly, the timing is set, no issues there. Also, when in 5th on the highway, I have to be moving AT LEAST 68mph (already post-legal speeds ) to keep the RPM's up and the motor happy. The RPM's are right around 2100-2200 at 75mph...too low.

I am really considering swapping out the 3.50:1 gears to either 3.89:1 gears (roughly 200-220 RPM increase @2000) or 4.11:1 gears (roughly 350 RPM increase @2000). I just wanted to hear what y'all would recommend and any other suggestions you may have.

Some quick specs on the car:
'70 Camaro ~3100-3200lbs
GMPP zz383 Long-Block
222/230 @.050 | .509 intake / .528 exhaust lift cam
Fastburn 62cc Heads
Edelbrock RPM Air Gap
Quickfuel 680cfm SS Vacuum Secondary
Tri-Y 1-5/8 Headers into 2.5" x-pipe
TKO600
2.87:1 first gear
0.64:1 OD

Any help will be much appreciated



Cheers,
Matt

[sik68]

Hmm, how well is your carb tuned?

I have a 350 (less displacement), bigger cam (236/242 solid roller), bigger carb (Pro Systems 750cfm) and my engine doesn't like it below 1600rpm, but it's happy at 1800rpm (55-57mph) in 5th gear (TKO500, 3.73 rear).

I have a real steep first gear ratio which, like you said it great for not having to slip the clutch hardly at all. But as far as RPM on the freeway, you should be able to lug it down pretty far... it doesn't sound like this is necessarily a gearing issue so much as it is perhaps a fueling issue?

[dontlifttoshift]

Easy, Side step the clutch with the throttle mashed to get started and don't drive less than 75 mph in 5th gear :-)

How tall is your rear tire? Assuming 27", 4.11's will put yu at about 2500 at 75. 3.89's gets you 2350 at 75. Plus or minus about 100 rpm for a 1" difference in tire diameter. 4.11's will make for a very short first gear....I would go 3.89 tops maybe even just one step to 3.70

Donny

[Vegas69]

Something isn't right with your engine. I have a solid roller in my big block and can cruise in 5th comfortably at 1800-2000.(No steep grades) That little cam should be happy in the same range easily. Also, 1st gear in the 600 box is pretty deep. You shouldn't have much trouble taking off. I can take off with virtually no gas. I have a 600 and 3.73.

[Nessumsar]

I agree with Steven and Todd; you should be able to run near 2000rpm while cruising no problem.

What is the engine doing to tell you it doesn't like it?

[big gear head]

My 427 doesn't like to run below 2000 RPMs. It's got a big nasty solid cam in it. I've got 32 inch tall tires and 4.11 gears in the rear with a Muncie 4 speed and no overdrive. It likes this very well.

[Skip Fix]

Many of the old muscle cars came with 3.36 rear gears and a 2.52 first 4 speed and didn't require alot of clutch slippage. Light flywheel??

[kryptik]

Basically when I have the motor around or under 2000rpm on the highway it feels as if its struggling. Hard to describe but almost the same feeling as if you tried to start in second, or shifted into too high of a gear too early. It feels very flat and the exhaust noise is not smooth; it gets deeper (and has kind of an "air puffing" sound) as if the motor is really struggling.

I'm thinking a possible small vacuum leak, but I can't determine where. I have two carburetor spacers (a wilson 1" tapered with a 4-hole phenolic above it). I sprayed starting fluid around these spacers to no avail. The phenolic spacer was involved in a very intense head on collision, so I speculate a possible crack in the spacer, but I have my doubts. I also have the PCV line coming out the back of the carb running into a BRASS T, with one side going to the PCV and the other to the power brake booster. I have always questioned this but I do not see how it could cause this kind of issue.

I'm also almost certain the intake is properly sealed. I used OEM o-ring sealed gaskets, properly torqued in the correct sequence. I used 3 Felpro gaskets with gasgacinch for the carburetor and spacers.

It honestly feels like a tiny vacuum leak because the engine has a somewhat inconsistent idle, has a tendency to diesel, and also has the idle mixture screws a half-turn farther out than on my previous car (68 Camaro with the same engine and same carb, minus the Wilson spacer). Then again these issues could all be from the California mandate that requires all 91 gasoline to have 10% ethanol in it. What do you think?

Thanks in advance for the help guys.

Cheers,
Matt

P.S. It has a steel 11" flywheel.

[big gear head]

Have you changed the jets in the carb to make up for the alcohol in the fuel? You will have to increase the jet size for this, which could be why it seems to be running lean.

[kryptik]

No I haven't thought about that...that's a good point! What would you say jet it up 2 points to start?

[big gear head]

I'm not a carb expert, so I can't say for sure. That seems like a good place to start. I would increase jet size until it seems to smooth out. Then start checking the plugs.

[kryptik]

Alright I will try that tomorrow when it is not raining and report back.

Thanks!

[garys 68]

Before you do any jetting, what's your distributor timing curve look like? A little more initial timing can help your low end torque a lot. A $10 set of spings and stops is worth a shot.