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A buddy of mine is building a 36 Ford two door sedan with a 350 Chevy, 700R4 and an El Camino rear end with 2.41 gears. According to the RPM calculator, with 29" tires on the rear, it will be running 1385 rpms at 70mph. He asked me if that's too low. Will it run there or be a total dog? Will it get decent gas mileage at 1385rpms or will it dog down and keep down shifting out of overdrive?
I don't know much about the Chevy setup so I'd appreciate your thoughts.
Thanks.
At 70 mph with 2.41 rears he would be running about 1800 rpm and with 3.08 rears he would be around 2300 rpm if he is running a 1 to 1 final in the trans.
If it were me I would rather be around the 2300 rpm. Would not dog as bad when you stepped on the loud pedal.
My youngest son had a 81 Vette that ran around 1800 at 70 mph. It was fine but a Vette is lighter.
Thanks for the response Bug, but the 700R4 is an overdrive trans so the final drive is somewhere in the .68-.73 range I think. Thus, the lower 1385rpm at 70mph. I agree that it will probably dog quite a bit at that low rpm. He's trying to avoid messing with the rear end.Quote:
Originally Posted by Bug
I read where an 81 Corvette can weigh in at around 3300 pounds. Usually, these old Fords are less than 3K, so 1800rpms at 70 would probably work great.
With those 2:41 gears he is all set up for Bonneville. :LOL: With that OD transmission something in the 3:42-3:55 range would be ideal IMO.
Don
Don, I totally agree with you. I'm hoping someone will give some details as to why this combination would not be good, perhaps some specs or something that backs up personal opinion with actual examples. If this engine is running at 1300+ rpms at 70mph in overdrive, will it continually want to downshift to 3rd because the rpm's are too low?Quote:
Originally Posted by Itoldyouso
Not sure about the shifting up and down, but I think those low rpms might be lugging the engine somewhat. He will be running not only the overdrive ratio, but pretty tall tires and a less than aerodynamic body style. It just might overwork the engine pushing a 70 mph wind at those rpms. An engine should come into the range where it is running most efficiently, where the cam is starting to work . If he puts any kind of cam in that combo at all he will be operating below it's intended range.
None of the above is scientific, just my gut feelings. :):)
Don
He either needs shorter tires or more gear as noted. Even my diesel truck lugs at 1300 rpm and it is nearly at full torque 600 ft lbs. I'd shoot for about 1700-1800 rpm at cruise speed. This is a problem with OD's. If you have a vac gage it will tell the story very quickly. Low vac = lugging.
Everything I see says the 700R4 has a 0.70 OD ratio but some may have been different. Like Don says, he's got Bonneville gears now and he is going to be lugging at highway speeds and his mileage will suffer drastically because of that. Like bentwings says, he should shoot for something in the upper teens range for his cruise point, IMO. I assume his torque curve for the 350 is going to come up at about 2100 and be fairly flat through the normal rpm range?
Thanks for all the responses, guys!! I appreciate it! Will pass the info along and try to save him some disappointment. If he insists on using that rear end, he'd be better off without the overdrive transmission.
Thanks again for your input!
Smart move, all you can do is give him your best advice and let him make the final decisions. :) In something that weighs as much as his 36 I think he would be very disappointed with the performance off the line with those gears, and while 350 Chevies are great motors, they aren't exactly torque monsters.
In the old days we called rear ends with high ratio gears "Western" rear ends. The reason was that in the flat States you didn't need a lot of hill climbing torque, like you would back East, so the car manufacturers back then sold one ratio to the dealers in those flat States, and another, deeper ratio in the States with hills and mountains. A friend had a 50 Ford that came with a Western set of gears and overdrive 3 speed to boot. It was really dead on the hills, but once you got it going on the flat sections it would wind forever. I don't know if the auto makers still do that or not.
Don
the real problem is the El Cameo rear end---he really needs a Ranchero rear with around 3.50-3.90 gears---
And if he was to happen to use a F100-F150 rear, the bolt pattern for the wheels would be right
Go with a taller tire...
????????? That would only make the problem worse. :confused::)Quote:
Originally Posted by moter
Don
I agree the El Camino rear end is the weak link. Perhaps he'll see the light once I present him with all these responses. The F100 rear wouldn't match the front wheel bolt pattern because he converted to a Mustang II front end.Quote:
Originally Posted by jerry clayton
The other problem with that low an rpm is if he has any kind of a stall convertor at all there won't be enough torque to keep it locked up, it'll be slipping and the trans temp will rise. Bad juju.
Drive 100mph.... Problem solved :D
-Chris
I would also worry about stall speed and trans temp......the mid 90s camaro had a 350 with a six speed manual and would cruze on the highway at about 1200 rpms a 60 mph....but that is a different situation all together from his.
My opinion for what it's worth.
Drive much faster ..
Let me guess that he doesn't had a external tranny cooler either.Those 700's do not like heat.
I would guess this car is 3500lbs to 3800lbs too.That comes into play as well.
The 700r's I think get a bad wrap.Yrs ago Art Carr build a bunch of them for the local police force.All that abuse and mileages and when the cars where retired,not a single failure.
In my 70 pick up I have a fuel injected 302 with a AOD trany and 3.25 rear end gear.
It does that jumping up and down in gears at about 50 to 55 MPH.
It's a dog all the way around in the performance class, so I keep it in drive till about 60 MPH then I pop it up to over drive. Now keep in mind this truck weighs over 5 thousand the way it sit's. Now this thing can run top end pretty good as long as there is no hill or a strong head wind. If there is a hill or a wind it will get to about 75 MPH and won't pick up any more speed. So yes this is a big dog, but it was this way when I bought it.
The motor does not have enough torque with those gears and trany to pull strong.
The only good thing is the 30 plus miles to the gallon it gets.
Kurt
I know your right Louey.
My motor and trany came out of a 79 licoln cont.
The rear was already in the truck, it's a bad combo
is all I was trying to show. I would put a C - 6 in it
but the guy did such a good job on the install I have left it alone.
Kurt
I am running 2.41 gears in my 54----
it has a turbo 350 trans--so final ratio is 1 to 1----
at 70 Im running around 1800-----Im running a comp cam with a power band of around 1800 to 5400----while the low rpm is good for gas mileage, Im running the bottom of the band and it is just a little doggie-----
However, Im 65 yrs old, Im a little doggie too!!!!! For a cruiser it does ok, as a ground pounder------it wont!!
Well I'm 66 years old and I love my 4:56 gears and a Muncie 4 speed ,Quote:
Originally Posted by katie
Forget about mileage have fun
How in the world are you getting 30mpg out of a 302/AOD in a 5 thousand pound Ford pickup????Quote:
Originally Posted by vara4
With those gears he`d be better off with a th350... or change the gears.
I don't know how the guy did it he is a buddy of mine and is 76 years young.
All he has done his whole life is electronic's and fuel injection system's.
He hates carb's and switches carbs over to fuel injection all the time.
He's got 3 older Ford he's working on right now, a 65 a 69 and a 70.
He does the same thing with motor homes.
And his son is some kind of deisel god, he heard of a tug boat sinking with two brand new diesel motors on it, he went out and salvaged it for about 20,000 went thru both motor's then turned around and sold them for 40,000 a piece. Not a bad deal. These guys are good at what they do. I had my old bronco in the shop about 5 time's to the tune of about $2500 and they couldn't fix it. I took it to this guy one time and he fixed it. That's how we met and we have been friends ever since.
Kurt
Interesting! And of course, he couldn't be farther away from me! LOL! I would love to try to get better mpg's from my 66 F100 352, automatic. It currently gets about 10mpg.:CRY::CRY:Quote:
Originally Posted by vara4
Randy
Kurt has hit on the secret.......fuel injection and computers. Even modern full size cars are getting great gas mileage compared to the older ones. On the highway my Northstar gets almost 30 mpg.........around town is another matter altogether. :(
Don
You should take the camshaft into consideration, too. There is an operating range listed with most cams. In my case it is 1800-6000 rpm. I have a 700 R4 with a 3.89:1 rear gear and 30 inch rear tire. I cruise at 2200 rpm in overdrive at 70 mph. Around town, I drive in D rather than O. It will do okay at 1500 rpm in slow traffic, but regular driving, it likes 1800 rpm and above.