Axle ratio with 521

[cwmcobra]

Hey guys,

I'm looking to tap your experience and recommendations for an ideal axle ratio for a West Coast Cobra with a 521 mated to a Richmond Gear Street 5-speed. I built the engine from a Ford Racing 514 short block with aluminum CJ heads, hydraulic roller cam with 0.600" valve lift, roller rocker arms, Edelbrock Torker II intake (hood height issue) and fuel with a Pro Systems custom carb. The car weighs about 2700-2900 lbs. and currently has a 9" solid axle with 2.75:1 gears and open differential. I've just acquired a Detroit True Trac differential and want to upgrade the ring & pinion when I do the install. This is strictly a road car! With the 1:1 ration in 5th gear, the engine currently turns 2200 RPM at 70 MPH. The engine is a torque monster, but I'm thinking of waking up the acceleration a bit with either a 3.10 or 3.25 gear ratio. With all the experience with this engine on the forum, what are your recommendations for ideal gear ratio?

I appreciate your comments.

Chuck

[Tom Wells]

Chuck,

Mine weighs ~3100 and has a 2.88 with 335-35-17 back tires. If I could have a 2.75, I'd do it in a Noo Yawk second! Just my $0.02

It can still light'em at almost any speed....

Tom

[Snake Psychosis]

On the same subject, would 3.55's be overkill?......I mean, regardless of the plant you put in the car (barring any 6 cylinder oddballs that might exist), the combination of low weight and torque and power of a good V8 would seem to suggest that anything at 3.55 or over would possibly just satisfy the urge to buy new tires every couple weeks. Am I way off base here?

I'd think that unless someone was looking to launch on a regular basis for 1/4 mile runs, 3.55, 3.73, 4.10, and higher would seem to be a bit much for such a light car. I think 3.55 might be about the cutoff between straight up launchability (that's not even a word, but I'm a goober like that), and better top end.

When that day comes, I may opt for a longer gear like 3.08, 3.25, or 3.31. Somewhere around 3.25 to 3.55 would seem to be a good combination of acceleration and top end, at least to me. Anything longer, and you have to find a nice road where cops are infrequent to really make use of them, and anything shorter, and you're just burning up tires unless you can make it hook.

Then again, maybe the combination of cleaning solvents I've been using today have clouded my ability to rationalize properly.

Anyway, just my $.02 worth of blathering.

[LENSJADE]

My self iam also building a 32 with a 502 and richmond 6 speed i went with 3.25 with the richmond and 15 inch tires the combo makes 1st gear like you have 411 gears so stay around 3.00 and 3.25

len

[wtm442]

Originally Posted by cwmcobra

Hey guys,
I'm thinking of waking up the acceleration a bit with either a 3.10 or 3.25 gear ratio.
Chuck

The best way to do that is to change the tires to a stickier compound. With that biggggggggg block, the lower gears are relatively "useless" if you are "boiling the hides".

I have a 3.73 rear and 5 speed but 5th is 0.68, so I'm doing only 3000 RPM at 100 MPH

If you do not take long highway trips, then go for the most gear. Figure out what rpm you want at 70 MPH and work backwards.

Everyone says you cannot have enouigh horsepower, and if that is true, you cannot have too tall a rear gear! Go for the 3.25 to 3.50 range and have some fun

[Rich A]

This is strictly a road car!

Road car as in street & highway only?
Or road car as not used at a track?

Changing your rearend to a lower gear (3.10, 3.25) is going to make that engine rev higher and have a louder exhaust noise at 70 mph.

I would go with some sticker tires like wmt442 mentioned.

[cwmcobra]

Lot's of different ideas I see.

Tom, I believe you have an automatic tranny, right? Maybe I'm not as fast on the clutch as you are with the automatic, but I can spin the rear tire as long as I want in first gear (the Richmond tranny has tall gears), chirp the tire in second, but no other wheel spin. My rear tires are 345-35-18 oversize Viper tires. Maybe they stick better than I think. I'm sure they will do even better with a limited slip diff. I'm thinking of trying either a 3.10 or 3.25 ratio with the LSD. I can always go back and put the 2.75 gears in if I don't like it I guess.

When I said "road car", I meant no drag racing or autocross. Strictly a cruiser that will see some expressway driving, but I typically go out of my way to find back roads to my destination. I've never cruised for more than 100 miles at a time, so the highway cruise RPM is not that big a deal.

Thanks again!

Chuck

[Tom Wells]

Chuck,

C-6, correct.

Mine gets 400 plus hp to the back tires from about 2600 rpm up to the 5700 rpm redline. Torque doesn't seem to drop below 460 ft-lb or so all the way up, with lots more at lower engine speeds.

With a stick and a live rear axle you should be getting a much bigger percentage of your engine's output to the back tires than I do. Automatics & Jag rears are known for eating some hp.

This leads to the question if a better tune would help, or if there is a restriction to output such as mis-matched engine parts, exhaust or air filter.

I can put Victoracer slicks on and still spin the tires at takeoff. Street tires, no problem

Maybe you are already using slicks?

Tom

[cwmcobra]

I guess I'll have to check some things. I just installed a new custom carb from Pro-Systems and am in the process of tuning it. I can tell it has better top end, and haven't got the low transition tuned right yet, so will wait and see how that turns out. I think I will also make a run with no air cleaner to see how the torque feel compares. I have a 14" air cleaner with 2.5" tall K&N element, but what I did to get it to fit under the hood might be causing more restriction than I thought. Exhaust can't be a problem. 2" header pipes to the sidepipes with nothing but spiral turbo baffles, no mufflers. I'm pretty sure I didn't mismatch the parts in the engine; the only internal possibility I can think of is that the timing set was installed in a retarded position and I'm not getting the spark advance that I think I am. Guess I need to read up on how to degree the cam to check this. I know it was installed in the "standard" position, so unless the sprocket was mismarked, that shouldn't be a problem either.

Food for thought. Thanks for that!

Chuck

[greg schroeder]

I'd figure out how much wheel HP the car has, estimate trap speed in the 1/4, use tire height and rear gear to calculate and have 4th gear a couple hundred RPM lower than your shift point at the end of the 1/4 mile.

[Tony Aprile]

CWM...do your calculations first....with your tire and wheel size it looks like you'll be doing a whopping 57 mph at 2200 with 3.08's compared to the 70 your doing now with 2.75's... give or take a mph... with your 1 to 1 final drive 5th gear. I know you want more acceleration but you also have to watch your cruising rpm's on the highway.

Let the torque of the big block do it's thing and keep the gears, just my opinion of course.

The bottom line is... at what rpm's do you feel most comfortable on the highyway and at what speed do you want to maintain those rpm's...then gear accordingly.

Tony

[jdean]

Like Tom, I have a 521 with a C6 (1:1 final drive), but with 3.27 gears. I have a set for 3.08s that I have never installed. No more miles than I put on the car, the 3.27s are fun. Most of the cruising I have been doing is country backroads where the speed varies a great deal, so no need for reducing cruise RPMs. Even when highway cruising at 70-ish it is not too bad, but I bet overdrive (or 2.75s) would help the hearing a bit.