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Old 08-26-2007, 09:39 AM
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TH350; How much slip in 3rd is normal?

Stock TH350 in my 67 C-10 pickup behind rebuilt mild 350, I just installed 2.75s in my 9" rearend because highway driving with the old 3.50s required too many revs for my liking. So the truck drives nice with the 2.75s but I don't think it goes the speed in 3rd that the "gear speed calculator" says is should with my setup. According to the math with my 28.6" tall tire and 2.75 gear I should be going 77.5 mph at 2500rpms. My seat of the pants measurement says I am really only going about 65 or so... I need to get an accurate speed test but I am wondering how much slip is normal at cruising speed?

Also I don't know if its related or not, but at highway speeds the trucks seems to surge just slightly, not sure if the slipping and surging are related... thanks for any thoughts.

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Old 08-27-2007, 08:57 AM
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bump up...
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Old 08-27-2007, 11:26 AM
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th350

BIGSKY, I'm not sure I'm understanding the question exactly as you intended it but, you should not feel any "slip" in high gear at highway speeds, or any speed for that matter.

For example, if your crusin at say 60mph and you step on it, the vehicle should be responsive, there should be no lag. no slip, it should just go.

It has been said that a torque converter is only 90% efficient, unless it is a lockup converter.

But this 10% "slip" you will not feel, crusin down the road.

Please feel free to interject if my understanding of your question is not correct.
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Old 08-27-2007, 11:49 AM
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Thanks for replying Briscoe. I will try to explain better.

The truck seems to "surge" at highway speeds, like I am driving into gusting head winds. I know that sounds weird, but that is the best I can describe it. Also, my speed vs. rpm seems off. If the tranny was 100%, then for my setup 2,500rpm would be 77.57 mph, but its slower than that. If I figure a 10% loss though, then I would be going 69.8mph which sounds closer to my actual speed. I need to borrow a GPS and determine my exact speed at rpm.

Regarding the kickdown, if I punch it, the tranny shifts down like it should and the truck accelerates as it should but still with that surging.

Anyway because my truck seems to not be going as fast as it should per rpm, and it surges, I am just wondering if my tranny is slipping, maybe its a vacuum leak or something else.
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Old 08-27-2007, 12:27 PM
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th350

OK BIGSKY, a vacuum leak would cause the trans to upshift late, take a long time to go from 1-2 2-3, like instead of 2nd gear at 15mph, it may be more like 2nd at 25mph.

If the trans was slipping in high gear at say 50 mph you should be able to feel it slip in reverse. Put in on a hill and back up under a load, if it slips this way then yes, it may be slipping in high gear.

In reverse the high gear clutch drum is applied, so a problem in high should be a problem in reverse as well.

Perhaps the surging is the engine.?

Let's see what the gps tells us, ok?
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Old 08-27-2007, 04:04 PM
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Quote:
Originally Posted by BIGSKY
Stock TH350 in my 67 C-10 pickup behind rebuilt mild 350, I just installed 2.75s in my 9" rearend because highway driving with the old 3.50s required too many revs for my liking. So the truck drives nice with the 2.75s but I don't think it goes the speed in 3rd that the "gear speed calculator" says is should with my setup. According to the math with my 28.6" tall tire and 2.75 gear I should be going 77.5 mph at 2500rpms. My seat of the pants measurement says I am really only going about 65 or so... I need to get an accurate speed test but I am wondering how much slip is normal at cruising speed?

Also I don't know if its related or not, but at highway speeds the trucks seems to surge just slightly, not sure if the slipping and surging are related... thanks for any thoughts.
Being a long way away from my manuals let me ask if this is vacuum or mechanically modulated?

Reducing engine speed has changed several things, among them transmission line pressure and the relationship of throttle position to engine vacuum which will false signal the modulator valve whether it's linkage or vacuum. These will affect shift points and servo holding pressures.

There should be about 100 to 200 RPM differential between calculated and reality with a OEM torque converter. That could be another issue, the rev reduction on the engine could drop under the converters flash stall which would increase connective losses which you would see as increased engine RPM against road speed, also, tranny oil temp will go up.

Bogie
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Old 08-27-2007, 04:30 PM
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Quote:
Originally Posted by oldbogie
Being a long way away from my manuals let me ask if this is vacuum or mechanically modulated?

Reducing engine speed has changed several things, among them transmission line pressure and the relationship of throttle position to engine vacuum which will false signal the modulator valve whether it's linkage or vacuum. These will affect shift points and servo holding pressures.

There should be about 100 to 200 RPM differential between calculated and reality with a OEM torque converter. That could be another issue, the rev reduction on the engine could drop under the converters flash stall which would increase connective losses which you would see as increased engine RPM against road speed, also, tranny oil temp will go up.

Bogie
Mechanical or vacuum? Well I think vacuum because the tranny is hooked to a vacuum source at the manifold just behind the carburetor, as it was stock. There is also a kickdown cable at the carburetor.

Good point about throttle position vs. vacuum with the 2.75s, I hadn't considered that, but it surged with the 3.50s also.

100 - 200 differential; good to know. I will be borrowing a gps to find out my actual speed, to see if there is a problem there.

The surging is actually more noticeable at revs above 2,000, beyond where my stock converter should be hooking up I think. Maybe my converter is not healthy?

I rebuilt the engine in this truck about 5 years ago and it ran fine after that, same tranny as now, both stock from factory. This surging is a new problem since my restoration, so its something that has happened with my reinstallation of the engine/tranny, or something has gone bad since when I took the truck apart and put it back together. The idle isn't perfectly smooth either [cam is mild], maybe I have an electrial problem... I installed new wires and plugs, but used the same cap from before the restoration. Maybe I should try a new cap?

The surging is driving me a little nuts, so thanks for any suggestions.
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Old 08-27-2007, 09:09 PM
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Sounds lean to me.
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Old 08-28-2007, 08:11 AM
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It is my jetting!

Quote:
Originally Posted by Greg T
Sounds lean to me.
I have been asking everyone I know for their suggestions and a coworker mentioned this yesterday. So last night I richened my Edelbrock as much as I could with the jets I could get at my local parts store. I am not rich enough yet but the surging is going away! So I will stop picking on the tranny for now and concentrate on jetting my #1406.

It didn't dawn on me that this could be the problem, because the truck ran fine before.. but I did put on different headers and much shorter exhaust, same mufflers. Plus I added one of those black plastic type plates under the carb [4 hole version] to stop the vapor locking. I didn't think my changes would lean it out because I went from long 1 -7/8" tube 3" collector headers to 1 5/8" 2.5" collector shorty headers. I guess the mild motor is much happier with this setup and runs enough better to actually require more fuel? Is that possible?

Still will check tranny slip with GPS, but at least the "lean" surge is diagnosed! Thanks for the help!
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Old 08-28-2007, 08:34 AM
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There is no way you'll get 100% out of an automatic, unless you have a lockup converter. The converter will always have a bit of "slip" to it.
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Old 08-28-2007, 08:58 AM
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And throw that edelbroke away and get a good Holley or Q-jet (preferably a Holley) edelbrock is great for a cruiser SB that rarely sees over 3,000 rpm, but I have seen them destroy motors.

I was at the track one night and I heard that there was a mustang that was "popping and cracking" out on the top end. I asked what they ment and one guy said that some hot shot had a stang with a mildly built motor with a 750 edel on top, and the carb was leaning out in the high RPMs. So if course this was the highlight of the evening, everyone was betting when the motor popped or burned a piston all thru the test and tune and up into the qualifying. SEVERAL ppl tryed to tell him that he would hurt the motor if he didn't do something about the carb so somewhere thru test and tune he had someone run out and get a tune kit for it, and they put LARGE jets and rods in it, to no avail it would tear you up idling and detonate out on the top end. Needless to say halfway down the track on his first race a piston let go.

After that I'v had a bad taste in my mouth about edelbrock carbs, and if you look close even vic uses holleys on their dragsters.

Of course this is the same guy that caught his nitrous system on fire because the solenoids and switches weren't on a fused circuit.

Last edited by Holder350; 08-28-2007 at 08:58 AM. Reason: Spelling
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Old 08-28-2007, 01:04 PM
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Quote:
Originally Posted by Holder350
edelbrock is great for a cruiser SB that rarely sees over 3,000 rpm
Well my air bagged C-10 pretty much is just a cruiser. And I agree, that carb doesn't really work above 3k, at least in my application, even when richened up.
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Old 08-28-2007, 04:51 PM
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Quote:
Originally Posted by Holder350
And throw that edelbroke away and get a good Holley or Q-jet (preferably a Holley) edelbrock is great for a cruiser SB that rarely sees over 3,000 rpm, but I have seen them destroy motors.

I was at the track one night and I heard that there was a mustang that was "popping and cracking" out on the top end. I asked what they ment and one guy said that some hot shot had a stang with a mildly built motor with a 750 edel on top, and the carb was leaning out in the high RPMs. So if course this was the highlight of the evening, everyone was betting when the motor popped or burned a piston all thru the test and tune and up into the qualifying. SEVERAL ppl tryed to tell him that he would hurt the motor if he didn't do something about the carb so somewhere thru test and tune he had someone run out and get a tune kit for it, and they put LARGE jets and rods in it, to no avail it would tear you up idling and detonate out on the top end. Needless to say halfway down the track on his first race a piston let go.

After that I'v had a bad taste in my mouth about edelbrock carbs, and if you look close even vic uses holleys on their dragsters.

Of course this is the same guy that caught his nitrous system on fire because the solenoids and switches weren't on a fused circuit.
There is a very good chance that the mustang was leaning out at the top end due to the fuel bowl running low on fuel. A properly fed and set up Edelbrock carb will perform quite well actually.
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Old 08-29-2007, 08:55 AM
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Ok, fixed a vacuum leak at the carb base, and have re-jetted and the carb seems good, surging nearly gone.

Checked speed with a GPS; 2,500 rpms is 68.6 mph, if the converter was locked up I would be going 77.5 according the to math so I am slipping nearly 9mph or 11.6%...

The truck still doesn't feel right in 3rd, so I drove the truck in 2nd at 2,500 rpms and it feels good, no surging in 2nd. Also, as suggested I drove backwards up hill from a stop to test 3rd, I couldn't feel a slip and I can get the tires to spin doing that drill too, so maybe its not slipping enough to feel, but slipping some...
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Old 08-29-2007, 09:50 AM
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Overdriv, that could have been it but the car was useing the factory fuel pump with a regulator for fuel and if I had a carb that wouldnt take the gas when I got it up there, it would be in the trash.

Last edel I had I got on a motor, carb was BRAND new it got ripped of and sold.

I got $125 out of it tho.
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