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Old 02-22-2003, 07:22 PM
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Arrow Rpm's to high on my 68 chevy pu doing 65mph

I drive a '68 Chevy PU with a 402 bb (400+ hp) th 400 with 2800 stall converter

348 dana positrack

when I'm driving around 65 mph I'm looking at 3800 rpm (normal size rims and tires)

to me this seems high
Can anybody shine a light on this please?

what could be wrong (converter?)

[ February 22, 2003: Message edited by: oosterhout ]

[ February 22, 2003: Message edited by: oosterhout ]

[ February 22, 2003: Message edited by: oosterhout ]</p>

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Old 02-22-2003, 08:10 PM
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What's 348 the rear gear ratio? Where did you come up with that figure?
It could be something in the converter but I doubt it, does it stall above the rated stall speed?
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Old 02-23-2003, 05:05 AM
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:o

At 65 MPH, with a 3.48 rear end gear (which I have no knowledge) and a 30 inch tall tire, you should be at 2500 RPMs.

Either the speedometer is off, the tachometer is off or you have a lot more gear than you think.

A 30 inch tall tire is a common 235/75/15 tire. Even with a 26 inch tall tire ( a common 14" tire), the RPMs should only be 2900.


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Old 02-23-2003, 06:04 AM
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Arrow

Thanks for the info so far guys

I'm new to this so sorry about the poor info

The info I've posted here is what the seller gave me.

Coverter seems to stall at right rpms

I jacked up the rear end and turned rear wheel 1 full turn
the drive shaft turned aprox 3.5 times

so 3.50 rear end.

So if rpm should be 2500 and I have a stall conv for 2800 rpms, that thing is slipping the whole time or is this not that easy?

[ February 23, 2003: Message edited by: oosterhout ]

[ February 23, 2003: Message edited by: oosterhout ]</p>
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Old 02-23-2003, 07:05 AM
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I would say low like 4.56 or 4.11 I had a 1970 truck I got from a man he said it had 3.07 but it had 3.55 V6 rear running 60 tacking 3500 might be a V6 rear
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Old 02-23-2003, 07:32 AM
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Arrow

When I put that in it would even increase my rpm's, wouldn't it?
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Old 02-23-2003, 10:16 AM
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The T400 you have may have been out of a truck with higher # 3rd gear. The higher the gear # the faster the engine turns at highway speed and engine overrevs through 1/4 mile. The only solution for your problem would be to go to lower # gears in the rear say 3:05 even 2:90 or use an overdrive trans. Running as big an engine as you have the lower gears would still do great off the line. Ray
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Old 02-23-2003, 10:20 AM
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[quote]Originally posted by snkbyt:
<strong>The T400 you have may have been out of a truck with higher # 3rd gear.</strong><hr></blockquote>
All factory th400's have the same gear ratio's in the transmission,,,unless you are talking about speedometer gears?

oosterhout, how do you check your mph? with the speedometer or?
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Old 02-23-2003, 10:37 AM
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street gears go with 3.08 good top end racing gears go with 1/8 or 1/4 mile gears 3.73 best all around gear.
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Old 02-24-2003, 04:23 AM
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Arrow

Thanks for all the help so far

I checked mph with a radar detector,

should be pretty accurate

I still have the same question though:


stall converter at 2800 rpm
65 mph should be around 2500 rpm

this don't add up for me.

according to the cam specs cruise rpm are
3500-3700 (crane energizer .544 lift)
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Old 02-28-2003, 07:49 AM
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Jasonís reply is inaccurate. Gm did make several versions of the TH400. Some for big blocks and some for small blocks. The ratios are different for the model that the transmission is in too. Gm didnít put the same transmission gears for all their vehicles. I know that the camaro TH 400 is different than a C20 TH400. Check your facts before you make assumptions. As for your torque converter, from what I understand, a 2800 stall means that the fluid isnít going to be transferred until the engine hits that 2800-rpm mark. It has very little to nothing to do with your top speed. It tells your transmission when to move power from the engine to the wheels. Here, ummÖ. See if this makes sense. Say your stall is 2800. And your peak horsepower is at 2800 rpm. Your torque converter is going push the power through your transmission when it is ideal for your power combo. Note this is only when you have an aftermarket torque converter. Stock converters do not make for good performance items. Hope this helps you out.
Silver
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Old 02-28-2003, 11:22 AM
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[quote]Originally posted by Silver:
<strong>Jasonís reply is inaccurate. Gm did make several versions of the TH400. The ratios are different for the model that the transmission is in too. Check your facts before you make assumptions.
Silver</strong><hr></blockquote>

Jason is correct. 3rd is 1 to 1 in all Turbo 400's.

Check here:
<a href="http://www.tciauto.com/tech_info/gear_ratios.htm" target="_blank">http://www.tciauto.com/tech_info/gear_ratios.htm</a>

There are some clutch pack, filter, and housing differences. Some seals also have different sizes. No gear ratio differences.

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Old 02-28-2003, 12:38 PM
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Thanks Deuce, I have no idea where Silver gets his information. It's true with manual transmissions that there are different gear ratios for different applications. There are also aftermarket gear sets for the th400 that vary but third is almost always 1:1.
So, Silver get your facts straight. And your a little off on converters too, but who cares thats your problem not mine.
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Old 02-28-2003, 03:25 PM
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I chuckle at such incorrect information that is given out like Silver is doing. Must be confusing some numbers from some where.

All OEM T-400's have the same gear ratios as pointed out. Even the T-475 motor home tranny has the same gear ratios, but the planetarys have straight cut gears.

I often see incorrect info out in cyber land. I try to gently correct it.

I even read a post on another bbs that a kid said automatic tranny rebuilding is easy, just very time consuming. That is why rebuilt trannys cost so much...... I about Pee'd my pants laughing.

I tell folks that replacing parts in a tranny is easy, diagnosing the problem and rebuilding a tranny correctly is a little tougher.
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Old 02-28-2003, 05:12 PM
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[quote]Originally posted by Crosley:
<strong>I chuckle at such incorrect information that is given out like Silver is doing.
All OEM T-400's have the same gear ratios as pointed out.
I often see incorrect info out in cyber land. I try to gently correct it.
I tell folks that replacing parts in a tranny is easy, diagnosing the problem and rebuilding a tranny correctly is a little tougher. </strong><hr></blockquote>

THE transmission MAN has spoken......

He does it for a living.
Check out his stuff: <a href="http://highperformancecars.com/crosley/" target="_blank">http://highperformancecars.com/crosley/</a>

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