The Rodding Roundtable
Motorhead Message Central => Rodder's Roundtable => Topic started by: oj on May 30, 2012, 08:20:13 AM
I'm finishing a '66 Chevelle and need input for rear gear. The car has 9" ford rear with tremec 6spd, 28" tall rear tire, 632 engine with 750-850lbs torque from 3500rpm up.
With all that motor i am thinking the first couple gears in the tranny will be about useless, i am also thinking that i don't want to be anywhere around when this thing is in 6th at about 5K rpm.
What do you guys think for the gear ratio?
Thanks, oj
What are your plans as far as driving? Local only or distance? By distance I mean putting some real miles on it in a day. 200-500mi +
Not anywhere near enough information. I've never had anywhere near 632 inches with that kind of ponies. Normally aspirated, one carb or two, fuel injected, estimated idle speed, etc.? If it has turbos or other centifical power adders the approach would probably be different too. What does your engine builder think about where the sweet spot might be?
Generally your gonna want 2300-2500 at 70 mph. Is the Tremec OD in 5th and 6th?
Read Glens thread about his recent big horsepower Chevy II build.
Charlie
Well, a little more info:
It has a single dominator, naturally asperated, i imagine it'll idle 800ish if they do a good job on the headers and the tremec is overdrive both in 5th and 6th.
With the 396 motor it would cruise 12-1400rpm at about 65mph, the 396 was about as gutless a bigblock as i have driven but it must've had some torque.
Sweetspot? Where this thing will want to run at? depending on the headers i would imagine you are about right 2500-2800ish but that just scares me silly to set this car up to run at that kind of rpm with this much torque, somebody will get hurt.
I am thinking 1st and 2nd will be near useless with the rear gearing that it now has.
Is this an April Fools Joke left over posting?????
Quote from: "Dusty"Is this an April Fools Joke left over posting?????
Starting to wonder
Nope, this be the real deal.
It is Blueprint Engines# PS6320CTC, ser# 290016
Holy S%@t Batman.
There are a number of calculators for gearing, engine speed and vehicle speed available by Google search. I believe I would want to run that engine in cruise at aroung 2300-2400. You just need to pick whether you want to do it in 5th or 6th.
If you set it up to cruise 70 in 5th then you alway have 6th for the BIG numbers.
Charlie
Is the trans and rear gear the same as you are using now? If so the rpm's and speed will be the same as you have now. What ratio do you have presently?
I don't have the tremecs' gear specs, i presume they are nearly the same as the muncie.
I expect without a rear gear change the motor will not be loaded properly and it'll be like pedaling a 10spd bike in 1st gear - its' legs will go like crazy but it won't go anywhere because there won't be a load on the engine.
Think of when you did a dyno test, the motor can't make power until they put a load on it. When they do you hear the motor respond, give a grunt to get its' shoulders squared away and then you throw the rpm to it.
So you have a Munice 4 sp, but you don't know your rearends gear ratio but it go 65 mph at just off idle with the 396. Does this about sum it up?
Quote from: "Mikej"So you have a Munice 4 sp, but you don't know your rearends gear ratio but it go 65 mph at just off idle with the 396. Does this about sum it up?
Forget it, sorry i asked the question.
oj, don't want to be a butt but to help you we do need a little more info then you have given. There are calculators you can put in different numbers so you can see the different effects for tire size, rpm and rear end ratio.
Try this site: http://www.project33.com/tiresize.cfm to calculate RPMs.
Here is what they say about their calculator...
"I hope some of you will find this RPM-finder useful. Just take the tire height information you learn above and use the drop-down menus below to calculate your RPM's for a desired speed. Keep in mind these figures will get you close but are not exact. Only OEM automatic transmissions are listed (manual transmissions have too many variables) and the differential ratio menu contains most of the common ratios. Hope you find this useful. Let me know if there's anything else I can program to make your project easier. The answer will appear above the form."
Hope this helps.
Quote from: "oj"Quote from: "Mikej"So you have a Munice 4 sp, but you don't know your rearends gear ratio but it go 65 mph at just off idle with the 396. Does this about sum it up?
Forget it, sorry i asked the question.
You're getting po'd because no one can answer your question? You asked a question without information on what rpm you want the motor to be turning. To be cruising at 1200-1400 at 65 mph that motor must have been lugging like crazy. Don't expect this new motor to do that. As far as trans gear ratio do some research, they are listed on the net
The engine will be most fuel efficient at about the torque peak for rpm.RPM/trans gear ratio/diff ratio x tire dia x 3.14/12/5280x60=mph. with that much torque and that big a carb ,if the engine doesnt see some rpm the carb may be running in the idle/off idle circuit and fuel economy will suffer. What type of dominator are you running? Some dominators have little in the way of off idle fuel metering causing problems with part throttle operation. Sounds like a fun car,should be a beast when you crack the throttle!
Looking at the ratios for the 6 speed, I find 5 gear sets, depending on the application. But, first gear is around 3.00 on all of them, and 6th gear is about 0.50.
From your description, I suspect the current rear gear is likely a 2.75 or 3.00. That would make a final drive ratio in 4th gear on the Muncie of 3.00 to one. The 6 speed would be the same in 4th gear, but in 6th gear the final ratio would be 1.50. With a 28" tire, that is 300 mph territory.
In 1st gear with a close ratio Muncie, a 3.00 gear gives a 6.6 starting ratio. That may explain why it feels doggy with the 396. But first gear with the 6 speed gives a starting ratio of 9 to one, which is more in line with 600 hp.
So as I see it, a good all round ratio would be about a 3.50, or a 3.75. In both cases, with that much engine, first gear, and sixth gear are going to be pretty much useless on the street. but it gives you decent choices if you want to install a bigger tire and race it.
Quote from: "enjenjo"Looking at the ratios for the 6 speed, I find 5 gear sets, depending on the application. But, first gear is around 3.00 on all of them, and 6th gear is about 0.50.
From your description, I suspect the current rear gear is likely a 2.75 or 3.00. That would make a final drive ratio in 4th gear on the Muncie of 3.00 to one. The 6 speed would be the same in 4th gear, but in 6th gear the final ratio would be 1.50. With a 28" tire, that is 300 mph territory.
In 1st gear with a close ratio Muncie, a 3.00 gear gives a 6.6 starting ratio. That may explain why it feels doggy with the 396. But first gear with the 6 speed gives a starting ratio of 9 to one, which is more in line with 600 hp.
So as I see it, a good all round ratio would be about a 3.50, or a 3.75. In both cases, with that much engine, first gear, and sixth gear are going to be pretty much useless on the street. but it gives you decent choices if you want to install a bigger tire and race it.
Thanks, i believe the current rear gear to be about 3.77ish. I suspect that you are right - 1st & 6th will be useless.
I believe i'll leave the rear gears as is and when after the headers are made i can provide 'real' info and then you guys can give informed advice.
We shall see what we see, thanks again oj
Quote from: "chimp koose",if the engine doesnt see some rpm the carb may be running in the idle/off idle circuit and fuel economy will suffer.
Good advice there
Its a personal preference. I would want to be at 72-75 MPH cruising at the perfect RPM in 5th gear.....Then use 6th gear for 80 MPH plus. Thats my personal preference. since its not EFI, the carb needs to see a certain RPM to work properly at a cruising speed. I would also want my cruise RPM to be at a point the cam is happy and not choppy.
A brief update to the Chevelle. After stuffing the engine in i boxed it up and sent it down to Kooks' Headers in NC. They did an incredible job, 2 1/4 primaries, 4" collectors, 3" thru the removable X member into the muffler and out to the rear bumper, all stainless. I'll get some pics, the work is beautiful.
Anyway, i drove it yesterday for the first time and have some carb problems to deal with but the gearing isn't as i thought. 1st gear is managable - you just don't put your foot down unless you have lots of room - but i never got out of 4th gear. It was a brief shake-down run and i didn't go onto 4 lane highways, i drove it a couple miles out of town, turned and came back and never took it above 60. When i get it running better i'll see about getting it into 5th.