Would you notice the difference!!
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
Would you notice the difference!!
Hi Team,A lot of little things add up to bigger stuff...right? Here's what I am thinking,changing the diff from 3.08 to 3.36 and also changing the torque converter from stock to 2000/2200.... car is 77 L48 and has some engine improvements,bored .80 over,flat top pistons,Lunati mild cam,Alloy intake,rebuild Rochester 780 and duel cold air intake.
The exhaust manifolds are stock and so are the heads, but have had some work done to them,what I am after is an improvement off the line.
like always I'll be interested on your thoughts
The exhaust manifolds are stock and so are the heads, but have had some work done to them,what I am after is an improvement off the line.
like always I'll be interested on your thoughts
Last edited by nz corvette; Nov 13, 2019 at 03:47 AM.
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,986
From: Connecticut, USA
You will not notice much difference between a 308 & 336 gear, you would have to step up to a 373 to really feel it. I am not a big fan of 355's since there is not much difference between them and 373's and 373's work better with an OD trans if used. If you buy gears use US Gear, Tom's stocks them.
Racer
Joined: Oct 2001
Posts: 474
Likes: 64
From: Portage La Prairie Manitoba
Hi Team,A lot of little things add up to bigger stuff...right? Here's what I am thinking,changing the diff from 3.08 to 3.36 and also changing the torque converter from stock to 2000/2200.... car is 77 L48 and has some engine improvements,bored .80 over,flat top pistons,Lunati mild cam,Alloy intake,rebuild Rochester 780 and duel cold air intake.
The exhaust manifolds are stock and so are the heads, but have had some work done to them,what I am after is an improvement off the line.
like always I'll be interested on your thoughts
The exhaust manifolds are stock and so are the heads, but have had some work done to them,what I am after is an improvement off the line.
like always I'll be interested on your thoughts
Melting Slicks
Joined: Oct 2012
Posts: 2,156
Likes: 273
From: Dunedin NZ.
Living where you are starting with the torque converter would be my pick, this will allow you to still be running at reasonable revs on the long flat stretches, we are assuming the transmission is stock? Changes to the diff ratio will mean changes to the speedo gearing as well. Do some research before committing to any old TC because there are substantial differences in behaviour between designs.
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
Thanks,the OD trans is always an option and maybe the way to go....currently 308 is running 2500rpm at 60 so am thinking 336 will be around 2700 and 355/2900, not sure what 373 will be, but I would think away to high!!
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
FYI Right now your car is freeway ready. If you go to a different ratio without a overdrive trans, you'll be limited to in town driving.
Perhaps a trans change with more/lower gears and a overdrive?
Perhaps a trans change with more/lower gears and a overdrive?
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Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
I'm not sure why anyone thinks 3.73's are too tall. If you have 26" tall tires, going from 3.08 to 3.73 is 588 rpm more at 70mph.
If you're spending most of your driving on the highway then I'd agree that 3.73's without OD might be less enjoyable but I don't think we're talking hurting the Motor.
I have a 355 that I built decades ago with cast rods/pistons that I ran in a Jet Boat for 7-8 years using it some week nights and most weekends (granted Summers here are short but we used it into Oct. sometimes), it spent most of its time at 3500-5000 (better than 50% of the time a wot). The Motor is still fine.
If you're spending most of your driving on the highway then I'd agree that 3.73's without OD might be less enjoyable but I don't think we're talking hurting the Motor.
I have a 355 that I built decades ago with cast rods/pistons that I ran in a Jet Boat for 7-8 years using it some week nights and most weekends (granted Summers here are short but we used it into Oct. sometimes), it spent most of its time at 3500-5000 (better than 50% of the time a wot). The Motor is still fine.
Pro
Joined: Jul 2002
Posts: 514
Likes: 44
From: Toronto Ont
I would go with the OD transmission. That way you improve highway and get some get up and go as well. It is one of the best and cheapest upgrades I have done over the years on my autos. It really wakes up the car without sacrificing anything. I bet if you do it you won't change anything else.
Last edited by 79 Bullet; Nov 13, 2019 at 05:33 PM.
Melting Slicks
Joined: Apr 2001
Posts: 2,030
Likes: 363
From: Lubbock Texas
DC
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
You will not notice much difference between a 308 & 336 gear, you would have to step up to a 373 to really feel it. I am not a big fan of 355's since there is not much difference between them and 373's and 373's work better with an OD trans if used. If you buy gears use US Gear, Tom's stocks them.
If you're changing gears, go with the 3.73. This will actually get you a noticeable change in torque and off-line performance. We run the 3.73 in a lot of setups we do, and everyone loves the performance and the cruise speed rpm compromise.
FYI, there are no 780 Rochesters. Q-Jets were built in 750 and 800 cfm versions. See my tech paper for all the breakdowns and facts.
Lars
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Higher stall. I run 3000 stall and love it. Also go at least 3.55 rear.
also running 77 L48.
the stock manifolds and heads are both performance killers.
Really good performance is in the heads.
also running 77 L48.
the stock manifolds and heads are both performance killers.
Really good performance is in the heads.
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
If you're changing gears, go with the 3.73. This will actually get you a noticeable change in torque and off-line performance. We run the 3.73 in a lot of setups we do, and everyone loves the performance and the cruise speed rpm compromise.
FYI, there are no 780 Rochesters. Q-Jets were built in 750 and 800 cfm versions. See my tech paper for all the breakdowns and facts.
Lars
Last edited by nz corvette; Nov 13, 2019 at 11:59 PM.
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
Thanks for all the great feed back,sounds like I need to do the o/d trans and diff at the same time.....but as I have just come back from a month of touring around your great country (USA) funds are a little short.
Engine and trans(turbo 350) where built last year..so with less than 2000 miles on them,I might hold off for a bit,so that leaves the torque converter....how do I work out what I need?
Engine and trans(turbo 350) where built last year..so with less than 2000 miles on them,I might hold off for a bit,so that leaves the torque converter....how do I work out what I need?
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I feel a 3.36 is the most you will want on the street with a 1 to 1 3rd gear.......here in Texas with 75MPH speed limits, doing 60 is downright dangerous and doing 70-75 with anything more than 3.36 just winds the motor too high. I drive my cars down the interstate......I am not that guy in the slow lane with the old car......at 70MPH the 66' in my Avatar was taching 2800 rpm in 4th......that is almost too high for me but the engine was in a sweet spot there so I left it alone......
I am going to tell you that if you want more snap......put long tube headers on, curve the distributor, and install some real cylinder heads on.......there is 70 horsepower right there......power you can feel.....the 2400 convertor is an option.......but it would put the engine right at the meat of the torque curve right away.
Jebby
I am going to tell you that if you want more snap......put long tube headers on, curve the distributor, and install some real cylinder heads on.......there is 70 horsepower right there......power you can feel.....the 2400 convertor is an option.......but it would put the engine right at the meat of the torque curve right away.
Jebby
Race Director
Joined: Mar 2016
Posts: 14,402
Likes: 8,209
From: Napa Valley California
Do you know for a fact that you have a 3.08 rear gear.
If you have a 3.08 rear gear, and assuming you are running a 27" tall tire, my gear calculator says with a non OD auto trans (TH350 or TH400) and a 3.08 rear gear you should be turning about 2290 rpm's at 60 mph
With a 3.36 rear gear you should be turning 2500 rpm's at 60 mph.
With a 3.55 rear gear you should be turning 2640 rpm's at 60 mph.
With a 3.73 rear gear you should be turning 2775 rpm's at 60 mph.
My 73 has 245-60-15 tires (26.5" tall), with a TH400 and 3.36 gears.
At 70 mph on California highways, I'm in the middle to slow lanes turning between 2900 and 3000 rpm's.
To check what rear gears you actually have:
Lift both rear tires off the ground.
Mark the inside tire with a chalk line from the rim to the tread.
Mark the drive shaft with a line along the bottom of the shaft.
Turn the rear tire in the forward direction one complete turn and count the rotations of the drive shaft.
Approximately 3.0 turns = 3.08 rear gears, 3 1/3 turns = 3.36 gears, 3 1/2 turns = 3.55 gears and 3 3/4 turns = 3.73 gears.
If most of your cruising and driving is at 60 mph then find the rear gearing that best fits your driving style and needs.
If you have a 3.08 rear gear, and assuming you are running a 27" tall tire, my gear calculator says with a non OD auto trans (TH350 or TH400) and a 3.08 rear gear you should be turning about 2290 rpm's at 60 mph
With a 3.36 rear gear you should be turning 2500 rpm's at 60 mph.
With a 3.55 rear gear you should be turning 2640 rpm's at 60 mph.
With a 3.73 rear gear you should be turning 2775 rpm's at 60 mph.
My 73 has 245-60-15 tires (26.5" tall), with a TH400 and 3.36 gears.
At 70 mph on California highways, I'm in the middle to slow lanes turning between 2900 and 3000 rpm's.
To check what rear gears you actually have:
Lift both rear tires off the ground.
Mark the inside tire with a chalk line from the rim to the tread.
Mark the drive shaft with a line along the bottom of the shaft.
Turn the rear tire in the forward direction one complete turn and count the rotations of the drive shaft.
Approximately 3.0 turns = 3.08 rear gears, 3 1/3 turns = 3.36 gears, 3 1/2 turns = 3.55 gears and 3 3/4 turns = 3.73 gears.
If most of your cruising and driving is at 60 mph then find the rear gearing that best fits your driving style and needs.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
I agree with this comment, Getting the engine into its powerband with a good stall speed will be the best band for the buck. If the engine hits the power band wit the correct stall speed all gears will feel pretty good. I think the 3.08 gear is a good gear for a non-overdrive trans. If you go to a overdrive trans a gear would be nice added next step.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Consider that the motor is a 350, not a 406, 427 or 454 etc, so torque is lower than those motors. If you want performance it's in the RPM not the torque vs bigger displacement engines.
Higher stall is going to be better. I feel that 2400 (having had one) is not good enough. Get the RPM up in the launch so you can have some HP for that launch.
Same goes for cruising. Yeah my RPM is 3000 ish @ 60 and 3800 or so @ 80 MPH, but that puts it above the stall of the torque converter and in an RPM range that produces plenty of HP. I get 19 MPG @ 80 MPH with my Q-jet a 3.55 rear and the TH350. That's pretty good IMO.
The engine doesn't care, I feel it's more of a driver comfort issue cruising @ 3000 to 3800 RPMvs wear issue. Given I don't daily drive my vette and annual mileage is maybe 2000 miles the engine will last a good long time even at those RPM's. Use full synthetic oil to help it out with cylinder/ring wear if that's a concern..
Not uncommon for a modern small 4 cylinder car to cruise @ 3000 ish RPM and higher especially with 80 MPH speed limits popping up.
Higher stall is going to be better. I feel that 2400 (having had one) is not good enough. Get the RPM up in the launch so you can have some HP for that launch.
Same goes for cruising. Yeah my RPM is 3000 ish @ 60 and 3800 or so @ 80 MPH, but that puts it above the stall of the torque converter and in an RPM range that produces plenty of HP. I get 19 MPG @ 80 MPH with my Q-jet a 3.55 rear and the TH350. That's pretty good IMO.
The engine doesn't care, I feel it's more of a driver comfort issue cruising @ 3000 to 3800 RPMvs wear issue. Given I don't daily drive my vette and annual mileage is maybe 2000 miles the engine will last a good long time even at those RPM's. Use full synthetic oil to help it out with cylinder/ring wear if that's a concern..
Not uncommon for a modern small 4 cylinder car to cruise @ 3000 ish RPM and higher especially with 80 MPH speed limits popping up.
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
I feel a 3.36 is the most you will want on the street with a 1 to 1 3rd gear.......here in Texas with 75MPH speed limits, doing 60 is downright dangerous and doing 70-75 with anything more than 3.36 just winds the motor too high. I drive my cars down the interstate......I am not that guy in the slow lane with the old car......at 70MPH the 66' in my Avatar was taching 2800 rpm in 4th......that is almost too high for me but the engine was in a sweet spot there so I left it alone......
I am going to tell you that if you want more snap......put long tube headers on, curve the distributor, and install some real cylinder heads on.......there is 70 horsepower right there......power you can feel.....the 2400 convertor is an option.......but it would put the engine right at the meat of the torque curve right away.
Jebby
I am going to tell you that if you want more snap......put long tube headers on, curve the distributor, and install some real cylinder heads on.......there is 70 horsepower right there......power you can feel.....the 2400 convertor is an option.......but it would put the engine right at the meat of the torque curve right away.
Jebby