My 1953 Chevy truck has a 350 engine with a mild cam (I bought the truck with the engine installed so I have no information on the cam) The rear differential is a1976 Camaro 10 Bolt with a 2:56 ratio. At low speed the carburetor (Holley 4150 670ccfm) loads up. Once you accelerate it is OK. I was told that I need to replace the torque converter to correct the problem. My question is how do I determine which torque converter to replace it with. I am running a Chevy turbo 350 transmission. All ideas will be greatly appreciated.
A 2:56 rear axle ratio! There is your problem!!!! Selecting a correct converter can be a little completed. I would talk with one of the suppliers and see what they recommend. It depends on car weight, axle ratio, engine torque (camshaft type), etc.
you might think about using a lower gear in the rear end.3:42 or maybe a little lower . with a 2:56 it is a wonder it gets out of it's own way.
A higher stall converter will help it get moving but at a cost. There will be a B U N C H of slipping and heat. Then when those 256 gears get up to speed the converter will still be slipping. The correct fix is a Low rpm torque cam or gears A N D a converter that suit the combo. Cam, converter, gears are a package that needs to work together.
The differential is a stock differential from a 1976 Camaro 10 Bolt– 2:56 Ratio PH G7240. It worked fine in the Camaro. It is not a drag truck and it is not as quick on the take off as if it had a 4:11 or a 4:56 but it cruises at 60 mph - 70 mph great. I reacts as if it has another gear once it gets up to speed.
As stated ..You don't want a higher stall converter with a 2.56.. ....And you don't want to replace it until you figure out what it is. If you've got an increased duration cam, it's going to hard to make it work with a 2.56 also. A 750 Chevy Q jet may help though.
I just got a 78 Nova rear axle for my 57 chevy. Its a 2.41 gear which seems insane but it worked for the Nova!
I've got an O/T car with a 301 Pontiac, 3.08's and a 200 4 R.. That's 2.05 in 4th..plus lock up. Works fine...but stock camshaft...That's why.
It worked great in the Camaro because everything was designed to work together. So unless you have the whole drivetrain from the same Camaro and your truck weighs about the same as the Camaro it's not going to work as good. What motor and transmission were in the Camaro?
Get a vacuum gauge on the intake and see what the idle vacuum is. A lot of speculation here on what to do. If the idle vacuum is too low the power valve will open and the engine will load up. If indeed it is a 'mild' cam, you can probably tune around the problem. Sure a lot easier (and cheaper) than replacing the torque converter.
Setting up a drive line is more like chess than checkers. The numerically low gears need an engine that makes low end torque, and lots of it. The cam decides where the torque curve's peak will be. The mid gas crunch era drivetrain parts were set towards economy. The engines were low on horsepower because of a early torque peak. They were done breathing, pulling and making power by 3000 rpms and the power fell off. Figure your cruising speed rpms with 2.41 or 2.56 gears- So here comes Johnny hot rod who wants his engine to have that sound. In goes a cam that just starts making power at 2500 rpm. These bleed off low rpm compression so not only is the car down on power where it needs it most, it only starts running good well beyond the speed limit. Not a street car, not a drag car, more of a salt flats car. Stick a stock 401 nailhead that makes well over 400 ft lbs of torque at 2800 rpms in front of that 2.41 and you'll be happy.
By coincidence, I have just gone through this with my 57 pickup(avatar). The truck was a fresh build when I bought it. It's a 327, mild cam, performer intake and carb, 350 turbo and a 2:68-10 bolt posi. I've had the truck almost 5 years and I have tried different options to improve the 8.5-10 mpg at highway speeds. I have been turning 3200-3350 rpm at 70 mph. I could not figure that out with a 2;68 gear. I had replaced an axle bearing so I then learned of the rear ratio. At a friends advice, I brake torque'd the truck and watched the tach. The tires started to brake loose between 2400-2600 rpm. A stock chevy converter locks up around 1395 rpm. By calculation, a 2:68 gear with 29" tires(255x70) should be turning 2200-2300 at 70 mph. Again, I was turning 3200-3350. Last weekend I changed out the trans and converter for another****umed used set up I had purchased from a friend. I now turn 2800-2900 rpm at 70mph. It is definitely improved but I'm still loosing 400-500 rpm somewhere. My electronic speedo is calibrated exactly to the 0.0 mile markers. Sam
2x...It sounds like your power valve is opening too soon due to a low vacuum signal. NOT a torque converter problem.
I got a 440 Coronet with the rear two plugs on the drivers side bad, I can't change the plugs without lifting the engine, should I put new tires on it? First determine what your problem actually is. Once you have determined that your "mild" cam is not mild then worry about a torque converter. Sounds like you may need the proper power valve or have some other tuning issue to me.
No converter or gear change will fix a carb with idle circuit issues. As some have said, could be a low vacuum/cam issue but I would borrow a known good running carb and try that first.
Thanks guys. I have a lot of great information to process and will definitely let you know what we find out. keep the solutions coming because I enjoy reading through what you post. I keep everything in a folder because you never know when you will need it down the road. One other thing is I had another differential with a higher gear raatio (3:??) in it before I swapped it out for the Camaro differential and it worked fine so I never thought that the carb was the problem. Actually the Holley Street Avenger 80570 is a new carb and like I said it worked fine with the other differential. We determined that the original differential came from a 1955 -1957 Chevy. We do not know because like I said originally I bought the truck like it sat.
It sounds like your power valve is opening too soon due to a low vacuum signal. NOT a torque converter problem. Thanks Rick. This will be the first thing I will check out.
I got a 440 Coronet with the rear two plugs on the drivers side bad, I can't change the plugs without lifting the engine, should I put new tires on it? First determine what your problem actually is. Once you have determined that your "mild" cam is not mild then worry about a torque converter. Sounds like you may need the proper power valve or have some other tuning issue to me. Pork, a new set of tires is always a great option. I have had several of my friends listen and they too determined that this is a mild cam, but the fact is unless we actually see the cam and trace the numbers we have no way of knowing exactly what it is. I am thinking that you are correct on your last****umption. At least I hope it is. Like I said everything was fine until we swapped out differentials.
but I would borrow a known good running carb and try that first. Stock, the carb is new. When I bought the truck the guy told me that he had just put the new carb on. It worked fine with the other differential.
If the gears are too tall a converter won't help it at all. I would check for vacuum leaks, the gears are probably not related to the problem very much. if it does not have a vacuum leak I would check my vacuum at idle and adjust my power valve from there. Idle vac/2=power valve. I actually bought that Coronet in the '70s cheap and tires was the fella's solution for the engine with the major miss. I did something that a MOPAR purest would kill me for, I took a hole saw and cut two holes in the inner fender well. I still had to pull a tire to change the last two plugs as did the next owner but it beat lifting the engine like the book said to do.
Another vote for measuring your idle vacuum and then installing a power valve that has a lower opening value.
To check for the proper power valve, tape a vacuum gauge on the windshield with a long vac line, put the car in gear, tip in the throttle a bit and see what the vacuum is. It should be about 1 lb above what your valve is marked. This is for the old style Holley's, not sure about the Avengers. The old carbs came with 6.5's.
Thanks Pork. You did the same thing with the MOPAR that I did with my 1968 Camaro. I had to change the heater motor out and the fender had to be removed to swap it out. I decided that this was too much so I cut a hole in the fender well, replaced the motor, cover the hole up with sheet metal over caulk, screwed the sheet metal down with sheet metal screws and painted it black. You could hardly tell the patch was there. Thanks. That story made my day because it brought back a lot of memories of how we use to improvise to come up with simpler solutions.
LOL I had an OT small truck with a small block and a turbo 400 for a while back in the '90s. Everything was a tight fit and there was no way to pull the******* without sliding the engine forward, so I cut holes on the firewall to pass a socket through then covered them up with duct tape and the floor covering.
Pork, sounds like the first car I ever owned. A '50 Chevy. The transmission went out (one of five) and it took me all day to get the three top bolts lose (no room) and then I discovered the plate in the floor board which when removed the 3 bolts came out in 5 minutes. My guess you and I could tell these stories all day long. A friend had an S10 with a 4.3 engine. He had to drop the steering column down to be able to change the front driver side spark plug because no matter what the shaft was in the way and would not allow him to back the plug out. He said if they had only put a half moon shape in the steering shaft at the plug location it would have been easy.
With the facts given here so far ( there maybe more parts and pieces to come ) I think it's pretty simple explanation. At low road speed, the foot is too deep and the engine is trying hard but just can't rev. It's like starting out in second gear. Throttle plates open with sluggish rpm gain.
That is a ridicules rear end gear, that's all there is to it. And then the engine has a cam in it to kill what power/torque it did have. There could be a power valve issue, but that is not going to solve a cammed engine with a ridicules rear end gear. I have drive cars with 3:00 rear ends and couldn't stand them. The highest gear I have in anything right now is a 3:73. What is the point of having a car that is not fun to drive? Let along the strain you are putting on the engine. And why in the world would you have a cam in the engine to kill what power it did have?
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