I am having trouble with debating what is the optimal rear end gear ration to use on a 68 RS that is going to have a 454cc w/ edlebrock performer RPM cam/ intake kit with dual 600 cfm carbs. I am think'n of not too high of gears,,,so i can have optimum take off and highway range, someth'n along the lines of 3:55 or 3:30 gears. What do you guys recommend?

I would recommend not going with dual quads - more trouble than its worth especially with numerically low gears.

I run 3.55 gears in my 68 and like them. With a 26.6" tire I run just over 3000 RPM at 60 mph.

What do you plan on doing with the car? Daily driver, race only, weekend warrior, freeway use, etc. That makes a big diff in gear selection IMO.

I don't know what you want from the car but if you know where youwant to cruse at here is a formula to figure out rpm vs mph
mph * gear ratio * 336= ? div. by tire dia. will give you the rpm for that mph, also if you have overdrive you must mult. the gear ratio by the overdrive ratio (700r4 = .8) and insert that # for gear ratio. hope that will help deside aslo remeber that BB like to be loaded

I plan on using this car for cruising, both highway and daily, but i do not have a rear end and wanted to get anyone's opionion on a limited slip 3:55. I do like Eric's 3:55 running over 3000 RPM at 60 mph. Also, why is the daul quad more hastle than it is worth on a 454cc? I plan on bringing it to a performance place to fine tune everything.

What do you mean by "loading" a BB? <BLOCKQUOTE>quote:</font><HR>Originally posted by drpm:
I don't know what you want from the car but if you know where youwant to cruse at here is a formula to figure out rpm vs mph
mph * gear ratio * 336= ? div. by tire dia. will give you the rpm for that mph, also if you have overdrive you must mult. the gear ratio by the overdrive ratio (700r4 = .8) and insert that # for gear ratio. hope that will help deside aslo remeber that BB like to be loaded <HR></BLOCKQUOTE>

You won't need that much airflow even for your BBC. If you used two Holley 600's, that would be 1200 cfm. 1200 cfm + freeway gears = bog city. Plus bad gas mileage, slow throttle response and the extra expense of a second carb and all the linkage.

You can make more power with half the expense and half the hassle with a single 750 or 800 cfm carb. There are some great single carb dual plane intakes out there that have a power band that would fit the rest of your combo real nicely.

The only reason I can think of going with dual quads is "looks". There is something kind of cool looking about dual quads. There may be some power to be had with the right tunnel ram combo, but then you'll need a big hole in your hood and gears to make it work. There was an article in either Hot Rod or Chevy High Perf a few months ago that tested 8 or 10 different dual quad setups. Most of them were "terds" compared to a single 4 barrel, but there were a couple setups that actually did marginally better on the dyno.

ok, the cons have me convice to go wit a edlebrock 800cfm. Don't need the bog or bad gas mileage. Thanks.
<BLOCKQUOTE>quote:</font><HR>Originally posted by Eric68:
You won't need that much airflow even for your BBC. If you used two Holley 600's, that would be 1200 cfm. 1200 cfm + freeway gears = bog city. Plus bad gas mileage, slow throttle response and the extra expense of a second carb and all the linkage.

You can make more power with half the expense and half the hassle with a single 750 or 800 cfm carb. There are some great single carb dual plane intakes out there that have a power band that would fit the rest of your combo real nicely.

The only reason I can think of going with dual quads is "looks". There is something kind of cool looking about dual quads. There may be some power to be had with the right tunnel ram combo, but then you'll need a big hole in your hood and gears to make it work. There was an article in either Hot Rod or Chevy High Perf a few months ago that tested 8 or 10 different dual quad setups. Most of them were "terds" compared to a single 4 barrel, but there were a couple setups that actually did marginally better on the dyno.<HR></BLOCKQUOTE>

by loading the engine i mean that too much gear is bad with a bb it's got lots of tq. and if you use to much gear your just wasting it in drag racing bb fans use taller tires to kill the tq. at starting line so not to spin them sb fans should uses shorter tires so not to kill the tq. at the start by useing the forula i gave you you can figure out gear to tire ratio that means that if you run a 4.11 gear /w a 28 tall tire the rpm comes out to a certen rpm and if you run a 3.73 gear /w a 26.5 tall tire the rpm will be the same (just a example not accurit) you can count on a 200 rpm differance for every inch of tire. shorter tire more rpm taller tire less rpm so figure out where you want your rpm to run and ajust gear and tire to match also your running street so match you parts for low end build grunt. i had a 69 camaro ( 3100 lbs) bb 454 back half for drag 11:1 closed oval port heads tqer manifold 750 dd pumper all stock comp cam (mechanical cam) 4.88 gear 31 tall tire w/ glide 4000 stall ran 10.56 @ 124 mph now the same car a tube chassis 2700 with me same engine 4.56 gears 9.62 @ 140 mph oh ya for the last five years in a row with min. maint.

I ran an Offenhauser medium rise intake topped with a pair of Carter Competition Series 500 cfm carbs on a 350 in high school. As long as you have a manual tranny and use a progressive linkage, none of the above is really true.

Keep the carbs small, 500 - 600 cfm tops. Edelbrock makes a matched set of 550 cfm carbs specifically tuned for dual quad applications that sounds good. A progressive linkage will keep the bog away. 95% of the time, you will be driving on the rear carb only anyway, so gas mileage isn't as bad as you would think.

Also, a dual 4 bbl set-up is not really that hard to tune, especially now that there seems to by a chassis dyno on every street corner. I was able to tune mine by myself.

Everyone does stuff to their car that is just for looks. I always got a lot of looks with my setup. I had a pair of small 6-inch diameter chrome air cleaners with K&N filters. I will agree that a single 4 bbl will almost always outperform a dual 4 bbl set-up on the street. But, since you are already running a 454, you could afford to lose a little bit of power. If you really want dual 4 bbls and you trust the performance place to tune the set-up properly, then go ahead and do it.

------------------
Pearl blue & white 69 Camaro with supercharged 350, Tremec TKO, and 3.73 12-bolt

See my website updated 6/10/02 at:

www.geocities.com/gheatly (http://www.geocities.com/gheatly)

Ok, thanks for the advice. I am thinking about putting a limited slip 3:55, I'm not going to street race with it, so posi doesn't seem the best fit for my needs.

<BLOCKQUOTE>quote:</font><HR>Originally posted by gheatly:
I ran an Offenhauser medium rise intake topped with a pair of Carter Competition Series 500 cfm carbs on a 350 in high school. As long as you have a manual tranny and use a progressive linkage, none of the above is really true.

Keep the carbs small, 500 - 600 cfm tops. Edelbrock makes a matched set of 550 cfm carbs specifically tuned for dual quad applications that sounds good. A progressive linkage will keep the bog away. 95% of the time, you will be driving on the rear carb only anyway, so gas mileage isn't as bad as you would think.

Also, a dual 4 bbl set-up is not really that hard to tune, especially now that there seems to by a chassis dyno on every street corner. I was able to tune mine by myself.

Everyone does stuff to their car that is just for looks. I always got a lot of looks with my setup. I had a pair of small 6-inch diameter chrome air cleaners with K&N filters. I will agree that a single 4 bbl will almost always outperform a dual 4 bbl set-up on the street. But, since you are already running a 454, you could afford to lose a little bit of power. If you really want dual 4 bbls and you trust the performance place to tune the set-up properly, then go ahead and do it.

<HR></BLOCKQUOTE>

Ok, thanks for the advice. I am thinking about putting a limited slip 3:55, I'm not going to street race with it, so posi doesn't seem the best fit for my needs.

<BLOCKQUOTE>quote:</font><HR>Originally posted by drpm:
by loading the engine i mean that too much gear is bad with a bb it's got lots of tq. and if you use to much gear your just wasting it in drag racing bb fans use taller tires to kill the tq. at starting line so not to spin them sb fans should uses shorter tires so not to kill the tq. at the start by useing the forula i gave you you can figure out gear to tire ratio that means that if you run a 4.11 gear /w a 28 tall tire the rpm comes out to a certen rpm and if you run a 3.73 gear /w a 26.5 tall tire the rpm will be the same (just a example not accurit) you can count on a 200 rpm differance for every inch of tire. shorter tire more rpm taller tire less rpm so figure out where you want your rpm to run and ajust gear and tire to match also your running street so match you parts for low end build grunt. i had a 69 camaro ( 3100 lbs) bb 454 back half for drag 11:1 closed oval port heads tqer manifold 750 dd pumper all stock comp cam (mechanical cam) 4.88 gear 31 tall tire w/ glide 4000 stall ran 10.56 @ 124 mph now the same car a tube chassis 2700 with me same engine 4.56 gears 9.62 @ 140 mph oh ya for the last five years in a row with min. maint. <HR></BLOCKQUOTE>

A "posi" is a limited slip differential.

So posi is a limited slip. So realy it would not make a difference if i get a limited slip or posi? Would it put alot of stress on the rear end when taking corners? I figured limited slip would be best if there is alot of stress back there w/ the BB. <BLOCKQUOTE>quote:</font><HR>Originally posted by gheatly:
A "posi" is a limited slip differential.<HR></BLOCKQUOTE>

A Limited Slip (posi) is better for the street because is uses clutches and is less finickey on the corners than a gear type locker like a Detroit Locker. The Detroit Locker has gears that have to separate to let the wheels rotate at different speeds on corners. Lockers tend to push the car mke popping sounds and are loud. I also consider the powertrax to be a locker and would not recommend it for the street. Get a Limited Slip and use the GM Additive to keep it quiet. You will be happy with it.



------------------
Kent
67 SS 427

"Posi-trac" is GM's trademarked name for a limited slip differential. Chrysler had the "Sure Grip" and Ford had the..., oh well can't remember. They are all do basically the same thing, just a little differently.

As for the stress, I don't think having a posi impacts the amount of power that a rear end can handle. That's were the 10 and 12 bolt deal comes in.

If you have any kind of performance engine at all, you want a limilted slip differential, or posi. GM has been putting posi-trac rear ends in production cars for over 40 years, so I can't imagine you will have a problem with one.

gheatly,

Sorry to disagree, but a posi a.k.a. limited slip differential will hold up better than an open differential (same type rearend) especially if the tires get spun repeatedly. Equal power to both wheels under load will help the axles and bearings live longer. Put lots of TQ through one axle for a long time and that sucker is toast . . .

So posi-trac is GM's version of limited slip. Well that clears alot of questions i had previously. I just wanted a rea end that would'nt lock up when taking corners. in that case, I will definitely get a GM posi 3:55.1 I think that with R15 245 60 would go great. Thanks guys for all the help. I will send pictures when the car is fully restored..hopefully within a yr.

<BLOCKQUOTE>quote:</font><HR>Originally posted by gheatly:
"Posi-trac" is GM's trademarked name for a limited slip differential. Chrysler had the "Sure Grip" and Ford had the..., oh well can't remember. They are all do basically the same thing, just a little differently.

As for the stress, I don't think having a posi impacts the amount of power that a rear end can handle. That's were the 10 and 12 bolt deal comes in.

If you have any kind of performance engine at all, you want a limilted slip differential, or posi. GM has been putting posi-trac rear ends in production cars for over 40 years, so I can't imagine you will have a problem with one.<HR></BLOCKQUOTE>

Eric, you are probably correct. I was thinking more along the lines of actually breaking the case or the differential unit. I've never had an axle fail, but I have cracked a case.