Stall speed rpm

[arrowheadracing]

Another ?. I ve read the alternator is what gives the engine rpm. If so, when replacing the alternator do you pull and use your pully ? My alt was replaced at the dealer prior to my purchasing the truck. At idle I am reading 750 rpm and steady. The truck shifts by itself at 3700-3800 on full throttle. From a dead stand still, 750 rpm , I can hammer the pedal and have the convertor stall at 2200 rpm. Is this normal. On the highway ( I have to recal my vssb ) my 45 mph rpm is 1500 and when I hit the throttle it will spin to 2000 area and take off. Do these trucks have that much stall speed ( or slippage ) in the convertor, or is mine sick ? It does lock up on the highway , but it also only locks up at 50 mph ( roughly 2000 rpm in 4 th gear ).

What is the best way to check your rpm on the motor other then through the alternator ? I have yet to pick up a service manual. Is this in the trouble shooting section ?

Thanks and sorry about all the long wind, but thats me.

Todd

[rjwest]

White chalk mark on crank pully.

shine a floresent shop light on it when at Idle.

Mark should appear to stand still,
If idle at or close to 600 rpm
also same for 1200, 1800 rpm.

600 is my warm idle, per laptop read out...

My stall speed is abiut 1800 rpm...

[JohnC]

The correct pulley is 2.6" OD (largest point).

A Tech II will give you accurate RPM from the crank sensor.

If you have a laptop with a microphone, Go here (http://www.tunelab-world.com/rpmsound.html) for a software package that will figure RPM from engine sound.

BTW, if you're seeing 2200 RPM stall speed I'd venture that either your tach is wrong, your Torque converter is wrong, your trans is slipping, or you've got about 350 hp...

[ 11-02-2004, 09:51 AM: Message edited by: JohnC ]

[cruzer]

The pulley dia. is 62mm where the belt rides or 2.44".

I believe using a scan tool will also give the info on RPM. There are other members who know more about this than me. Hopefully they will chime in, or you can use the search function. I know this info is out there.

[DA BIG ONE]

A stock torque convertor will always stall at higher rpms when extra torque from motor is applied. A diesel motor that is modified and using a stock convertor the stall speed rises, a crappy running motor putting out less than stock torque the stall speed will be lower.

Then there is the torque convertor ratio that must be thrown into the whole mix too.

If you know your hp/torq out put then a custom billet converter can be had for say $1,000.+ USD.

[kowsoc]

Another thing is that when checking stall speed you must ensure that a rear wheel is not starting to spin as that will raise the RPM somewhat. When you hammer it off the line there may be some wheel spin that you are unaware of. smile.gif

[Hye]

JC,
Thanks for the link to the sound tach. Can't wait to try it.

DA BIG ONE, where did you find 3.42 gears for a 10.5"? full floater?

Arrowheadracing,
Yes, the tachometer gets its info from the alternator. For your tire size, axle ratio, Paul Rowland's RPM/Gearing Calculator Excel Spreadsheet in the Member's area says you should show 2000rpm at 70mph if you are in 4th gear (OD) and TC is locked. The pulley diameter and the number of poles in the alternator will both affect the tachometer reading. I always keep the old pulley, unless its damaged, but that doesn't gaurantee tack accuracy. The numbers you give make me think that changing your alternator is the only way to get your tach to read correctly. The PCM uses the crank position sensor for the rpm signal, so the Tech II will always be accurate.
Got any pictures of your I/C and intake box?

[DA BIG ONE]

Originally posted by Hye:
JC,

DA BIG ONE, where did you find 3.42 gears for a 10.5"? full floater?

Got lucky w/3.42:1 GM CORP gears from Broward Power Train in Ft. Lauderdale Fla., uses the same carrier. Most of the larger gear vendors have em too.

Best mod I have done, and w/o a timer one would be hard pressed to see, or feel and loss in performance because of the diesels massive torque, add to that torque there are performance gains.

This link will help ya some,
http://www.pirate4x4.com/tech/billavista/14b_bible/index.html