The Stovebolt Site Forums The Shop Area Driveline new transmission, same problem...doh!

i broke down and bought a new TH350 for my truck, the th350 that was in it had been leaking for sometime, poor shifts, bogs down when put into gear, and would not get me over 50 mph.

this was my first trans swap and took about 7 hours from jack stands to driving again lol.

unfortunatley not all the problems went away. the biggest one is the 50mph max speed! before i pulled the trigger on the transmission i went to several online RPM calculators.
with my set up: 350/350 w/ 3.90 gears
most of them say i should be at 3k rpm @60 mph, but im at 3500 rpm @50
am i missing something?
is there a ajustment on the transmission that regulates rpm range?
this is a budget build, so im trying to throw money at the right parts.
thanks for any advice.

Throw tire size into the equation. To tall of a tire or to short of a tire will effect rear end ratio. You will need actual height of the tires. If your running a taller than original tire,
formula is orig tire height devided by new tire height times rear end ratio. That gives you your effective ratio.
Although with the higher RPM you describe i am inclined to think your tires are not tall enough. Not sure if you considered this, hope it will help you out.
Stranger

Oh yeah, sorry. Tires are 27" tall wide whites.
I'm looking into rear ends now.
I have a eaton, and I think 3.90 and 4.11 is all
That was ever offered for the 1/2 ton trucks.

Hy elsangre, did you install a new torque convertor with the transmission, or use the old one? Is the transmission shifting twice? Have you verified the linkage adjustments?

Did you have a manual transmission before the first TH350? Did it only run about 50mph before you changed over to the first TH350? If so then you may not have a 3:90 ratio after all. Manual transmission or automatic will have 1:1 in high gear unless it has an overdrive.

So the TH350 wouldn't get you down the road any faster than the manual 3 or 4 speed transmission that may have originally been in your truck to start with. Especially if you have 4:11 or other lower geared rear axle. The only two ways to lower rpms for higher road speeds is either with an over drive transmission or better gearing in the rear end. Or both.

Sounds more like an engine problem, like it's out of time or something,
Scott

Not if his rpms are 500 faster while the speed is ten mph slower.

3:90 with the TH350 should give him 60mph fairly easily. 4:11 may not.

But he does need to make sure the engine is properly tuned and running right first. Then hunt the transmission problems if there is one. If the engine is running right, then he has a gearing problem.

I think if the transmission were not shifting into high gear his rpms would be much higher at 50mph than they are now. Course I could be all wet too. LOL

the way I see it, if it has tall gears it, wouldn't bog out, it would be a stump puller,
Scott

If he has stump pulling gears, he won't get down the road very fast without an over drive.

With my 58 with 3:90 gears in 3rd about 60mph is a fairly comfortable speed. Any more the engine sounds like it is getting the rpms on up there.

With the 50 I had with the same engine and transmission but with the 4:11 gearing about 50mph was the comfortable speed. Above that the engine felt like it was revving pretty high.

In either case he does need to make sure the engine is tuned right and running right before worrying about what the transmission is or isn't doing. With the TH350 if the vacuum source isn't good, or the engine is running poorly it would mess with the shifting of the transmission. So the engine has to run right first. Then see if he really has a gearing issue or not.

thanks y'all!

@3B yes brand new torque converter, i asked for as close to stock as possible. it should have 1600 stall

@roy yeah i pulled out a TH350 and the speed limit was the same
i did go ahead and pull the diff cover, they are 3.90 gears.
im with you though, i feel like i should be able to hit 60 with reasonable rpms. i know 3.90 gears are not for the highway flyers, but 45/50 is tops and i wouldnt want to drive at 50 for too long.

i will work on the engine as you and scott have suggested. the plugs and wires are a bit worn. i also picked up a vacuum gage, i should have enough line to run it into the cab with me.

thanks again y'all.

3000rpm with 27" tall tires and a 3.90 rear is 61.75mph discounting any torque converter slippage. A breeze for a bone stock 350 that's running just fairly well. Lousy gas mileage set up. If it doesn't crank out 3000rpm the engine needs attention bad.

You're in 2nd. gear. Figure out why it's not making the 2-3 shift.
Jerry

Jerry, I did have a mid 70's Blazer not go to third. The high clutch piston seal was not properly installed during it's rebuild. Don't know what the chances of it doing it from the first trans to the next tho.....
Scott

A vacuum gauge is good.
Check your vacuum at idle then watch it as you slowly increase RPM in park, no load. Vacuum should be very close to what it was at idle (assuming no EGR, variable valve timing,etc) as RPM goes up. Your not doing throttle snaps to high RPMs just slowly increase engine speed and let it stablize. IF vacuum is dropping off as RPM increases may have restricted exhaust.
Soung like something is keeping this engine from pulling enough vacuum on the modulator at road speed to shift into 3rd perhaps.

thanks again for the help!

i changed the plugs/wires and put new vacuum line to modulator.
also rewired tach just to be on the safe side.
here are the new numbers:

test 1
38mph = 2000rpm and 15 vac
40mph = 2000rpm and 14-16 vac
45mph = 2100rpm and 13 vac
50mph = 2400rpm and 14 vac

test 2
40mph = 2200rpm and 15 vac
44mph = 2300rpm and 15 vac
50mph = 2400rpm and 14 vac
53mph = 2600rpm and 13 vac

@idle
800rpm 8-12 vac
1100rpm 14 vac
1100rpm 12 vac
600rpm 5-6 vac

i'm sorry if these numbers are irrelevant, we are trying to wrap our heads around it as well.

i did not get a chance to get to the highway for more speed.
it does seem like we are in the ball park for the set up i have.
the timing seems to be off a bit and at least one lifter is tapping.

again thanks for everyone's input, it is much appreciated!
els

Hy elsangre, those vacuum readings are definately low, this could indicate a timing problem, or a restricted exhaust. Time the engine to spec. and recheck your vacuum readings.

All those vacuum readings are WAAAAAYYYY low! Create an exhaust leak by loosening a pipe at the manifold on one side or the other, and see if the vacuum comes up. It looks like you're getting an exhaust restriction and the modulator isn't allowing the trans to go into 3rd. gear.
Jerry

By my math, these RPM / MPH numbers are close to where they are supposed to be for 3.90 gears, certainly within the margin of error for tire size variance and torque converter slip.

Here are the numbers for 3.90 gears.

Road Speed vs RPM 3.90

Here are the numbers for 4.11's

Road Speed Vs RPM 4.11

As others have mentioned, ANY 350 in good working order should easily spin beyond the 3000 RPM mark. Sounds like you certainly have a problem of some sorts that is not in the tranny.