Author Topic: new old truck questions  (Read 6356 times)

Offline oldnslo

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new old truck questions
« on: April 22, 2003, 01:31:00 AM »
Hi guys--I'm new here, just bought a '79 Scottsdale 2500.  Anybody know the hp of a 350 4-bbl of that vintage?  Also, it has 4:11 rear end I don't think I can live with on the freeway, so will be looking for something in the 3:23 range.  Probably will have to count the bolts to determine what brand it is--seems several were used.  Also, will going to this low a ratio take away ALL the power from this engine?  I'm not looking for neck-snapping power, but would like to drive on the freeway without running at 4000rpm.  Thanks for any advice.......


Offline 73K20

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Re: new old truck questions
« Reply #1 on: April 22, 2003, 09:02:00 AM »
Exactly how fast are you running on the highway?  I have 4.10's and can do 70mph and still not hit 3000 rpms.  If you bought a '79 3/4 ton pickup and expect to get great gas milage with good road manners then your mistaken.  Your 350 (based on no idea of milage or history) probably has about 160-170 hp.  The 3.23's will just about make it gutless.  You didn't mention what you plan to do with the truck, any towing?  Is the truck 2wd or 4wd.  That is something to consider when regearing becuase you will have to gear 2 diffs, not one.  When considering a gear swap to save gas you need to figure out how much the swap will cost vs. how much you will save in gas.  What transmission do you have?  A more cost effective change maybe to switch to an overdrive transmission.  If you have a TH350 then a 700R4 is almost a direct replacement.

1973 K20 - 350/350/205


Offline ChevyGuy

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gears
« Reply #2 on: April 22, 2003, 09:36:00 AM »
try 3.43 i think thats what they are... they will work well and still have alittle launch. Those 4.10's are for pulling and suck anywhere you go anyways. Also other things consider speed and RPM's. Like tire size.... tranny. How heavy the thing is like add on stuff like big bumpers and stuff will kill gas milage. The less wieght you can take off the better it can get. Just play around and spend a few dollars on small performance stuff will help gas milage. A MSD box would help alittle but that is costly. Good luck


Offline oldnslo

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new old truck questions
« Reply #3 on: April 22, 2003, 06:01:00 PM »
As usual, I am probably getting ahead of myself--I will pick up the truck tomorrow, then can present the question of rpm vs speed more intelligently.  Previous owner says that 2500 rpm is good for @ 58-60 mph.  I don't plan to spend a lot of time on 70 mph freeways, but 60 is too slow on the occasions I will .  Truck has 235 tires and 16" wheels, don't know what profile the tires are.  I understand enough @ gears to know that 3:23 will make it gutless with only 170 hp on tap,and a compromise will be in order.  Don't plan any towing or heavy pulling, but want it to get out of own way and not rev the snot out of it.  My last truck was a Ford F250 with 460 and a 3:66, and it seemed about right for the hiway, and pulled like it had 500 hp.  A different animal, I know.  Maybe a better question might be, what is a safe rpm range to run this thing on the highway?  I had a 350 engine in a boat and it would run 3600 all day without complaint.


Offline EFF 83

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Re: new old truck questions
« Reply #4 on: April 22, 2003, 11:02:00 PM »
Ive been doing some thinking about the drive train in my truck. It has a TH350C tranny and 3.08 rear end with what I believe to be a limited slip because when I spin I sometimes leave 2 skid marks and I some times leave 1, and sometimes 1 is longer than the other one. But anyways, I was running numbers around in my calculator, actually made a program, to find the speed of your rear tires depending on RPM, tire height, rear end, and transmission gear. My numbers, IF I REMEMBER CORRECTLY, is that with a 700R4 and 3.73 gears, and 28 in tall tires, you will turn 2800 rpm @ 70 MPH. and with the same combo but with 4.11 gears it would turn 3200 RPM @ 70 MPH. That does not take into effect wind drag and other forces, so I beleive them to be within 150-200 RPM of accurate. I will have to redo these numbers because, this is form memory, but I belive they are correct.

As for it getting out of it's own way, the TH350 has a first gear of 2.52:1 while the 700R4 has a first gear of 3.06:1. The 700R4 is an overdrive unit with a .70:1 last gear. The overdrive is what lets you run a lower (numerically higher) rear end and still have it managable on the highway. Some (most) 700R4 have lock up torque converters, the lock up torque converter on my TH350C lowers my RPM by about 300 when I reach cruising speeds. IDK how much a lock up converter on a 700R4 will drop the RPMs. In my application I would have a 6.79:1 (3.06+3.73) gear reduction, and ill be running 27" tires so that should be enough to send my head through the back glass when I stomp it, compared to my stock 5.60:1 that leaves me begging for more.

To make a long story short keep your 4.11 rear end, and upgrade to a 700r4 tranny. With 170 HP you will need the gearing.

I will post the formula I used to get my numbers on here after I find it.


Offline Sierra4x4

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Re: new old truck questions
« Reply #5 on: April 22, 2003, 11:11:00 PM »
what are you gonna do with your 4.10's? im thinking of adding some to my truck w/35's. ive got some small gears like 3.03 3.31 or something and my truck does 55 @ 1600 rpm's (4 spd manual) is it 4x4?


Offline bored and stroked

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« Reply #6 on: April 23, 2003, 05:44:00 PM »
I had a th350, 3.42 gears, and stock 235/75/15 tires and I pulled 3000rpm at 70mph. With a 700R4 that would go down to about 2000rpm.

Running the numbers threw a gear calculator, 28" tires with a 700r4 [.76 overdrive] and 3.73 gears at 70mph= 2,381RPM. Change the gears to 4.10's and you get 2,617RPM.
You can check all the numbers you want, change em around, whatever at www.prestage.com
Just click on the car math link and scroll down for the link to the gear calculator.
A stock 79 350 *should* have 190hp if you get the HD version, 180hp with california emissions.


Offline oldnslo

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new old truck questions
« Reply #7 on: April 23, 2003, 08:15:00 PM »
Thanks for the input---just picked up the truck,  with camper attached, and the 4:11 seems about right.  But.....I am selling the camper and putting a topper on, so will be looking for a different ratio, and I appreciate your advice.  


Offline RSBAD454

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Re: new old truck questions
« Reply #8 on: April 24, 2003, 10:44:00 AM »
go 3.73 good all around gear.

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Offline oldnslo

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new old truck questions
« Reply #9 on: April 25, 2003, 01:50:00 AM »
Thanks--I was thinking of 3:73 too, will get at it.  Thanks to all.


Offline phaon1

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Re: new old truck questions
« Reply #10 on: April 25, 2003, 10:08:00 AM »
3.73's are good gears, but I don't know that switching from 4.11 to 3.73 is worth it.  Going from 4.11 to 3.73 is only a 10% drop in RPM's.  Going from 3000 to 2700 on the highway really won't do much for your mileage (once you get above 2500, 350's suck gas) and still cost $$ to do the gear swap.  An overdrive tranny like a 700R4 gives you a 30% RPM reduction (35% with the lockup torque converter).  To get that kind of mileage improvement from gears, you'd have to go to something a little above 2.73's (GUTLESS WONDER!!!!!!!!).
The only way to get accel. and mileage is by adding a fourth gear to the tranny.  It took my highway mileage from 10 to 15 mpg, and I come off the line faster.  With a stock engine, you could buy a rebuilt - stock 700R4.  If you put the tranny in yourself, it wouldn't cost much more than the gear swap and would do MUCH more for your mileage and power.

Chris

1981 GMC K10 – 75K miles on original parts (what few are left)
Exterior Mods –
- Royal Blue chameleon paint
- 15X11 Craiger chrome wheels
Interior Mods –
- B&M Light Truck Mega Shifter
- Grant steering wheel
- 6 Auto Meter Ultra Light gauges in dash, 2 on a-pillar
- Keyless entry
- Billet dash and door trim
- Solid headboard with four built in speakers
- Sony Mobil ES CD Deck, MTX truck boxes, Jensen AMP
Drivetrain Mods –
- GM Racing ZZ4 Engine with H.O.T. Cam kit, 150 amp alternator, PeteJackson Gear Drive
- Hooker SS headers, 2 1/2" SS exhaust, Dr. Gas x-pipe, Flowmaster 2 chamber mufflers
- K&N X-Stream lid and 14" air cleaner.
- Coan 3000RPM custom built torque converter
- Built-up 700R4 transmission
- Yukon 4.11 gears with titanium pins in stock 10 bolt front and 12 bolt rear.
- Eaton heavy duty locker.