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-   -   Question for the guys running 3.73's with a 6cyl and manual trans (https://67-72chevytrucks.com/vboard/showthread.php?t=676015)

curvedglass 07-08-2015 09:06 AM
Question for the guys running 3.73's with a 6cyl and manual trans
 
I bought my 63 last year for use as a daily driver. It has been awesome albeit with one caveat. The previous owner swapped out the original 3.73's in favor of 3.08's for highway driving. I don't do much highway driving and I average about 16.5-17.5 mpg. My problem comes when I go up steep hills. It is worse from a dead stop. I'm lucky if I can make it to 30-35 mph. On the move I need to build up speed so I don't lose momentum or slow down the traffic behind me. Engine has plenty of power. Even at highway speeds it just seems hard to keep it in the power band. 230 stock with Weber 2bbl replacement and an HEI, 3spd on the tree.

My question is for you guys running the original 3.73 rear. What is top speed on the highway and what rpm's are you seeing?

Back when I bought the weber I asked Tom over at Langdons about adding power and he recommended that I revert back to the original ratio and put in an overdrive trans as opposed to hopping up the motor.

I would like to start with changing out the gears but the trans will have to wait. I want to keep driving the truck daily and don't want the 3.73's to kill me too bad on mileage and wear at higher rpm's.

jayoldschool 07-08-2015 09:38 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Keep the gears, enjoy good mileage, downshift to 2nd on the big hills?

curvedglass 07-08-2015 09:47 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by jayoldschool (Post 7234273)
Keep the gears, enjoy good mileage, downshift to 2nd on the big hills?
That's ok for the side streets. On the major roads at cruising speed I do not want to drop down a gear. Most of the time I'm happy with the slow cruising but it is frustrating in traffic and I can't even pass a VW Bus :o)

vince1 07-08-2015 09:50 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
I think 3.08 is a car ratio, in the trucks it is 3.07.

My original diff in my 66 was trashed so I put in a 3.73 from a 64. Teaching my daughter how to drive a standard (4Sp), she asked where is the next gear? I said that's it, which was met with peals of laughter.

Now it has a 3.07 that I like a lot more. If I were to use the 3.73 again it would have to have an overdrive in front of it. In your case a 5sp? Or maybe there are overdrive units available for the 3 sp?

If you look up RPM calculator you can get all that info by putting in tire size and axle ratio.

curvedglass 07-08-2015 09:53 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by vince1 (Post 7234284)
I think 3.08 is a car ratio, in the trucks it is 3.07.

My original diff in my 66 was trashed so I put in a 3.73 from a 64. Teaching my daughter how to drive a standard (4Sp), she asked where is the next gear? I said that's it, which was met with peals of laughter.

Now it has a 3.07 that I like a lot more. If I were to use the 3.73 again it would have to have an overdrive in front of it. In your case a 5sp? Or maybe there are overdrive units available for the 3 sp?

If you look up RPM calculator you can get all that info by putting in tire size and axle ratio.
Copy that. I want to convert to a 700R4 at some point. One of the reasons for this truck was that you can swap in just about anything in to a Chevy. My knees are on the way out and the clutch leg is going first :o(

geezer#99 07-08-2015 09:57 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by curvedglass (Post 7234238)
I bought my 63 last year for use as a daily driver. It has been awesome albeit with one caveat. The previous owner swapped out the original 3.73's in favor of 3.08's for highway driving. I don't do much highway driving and I average about 16.5-17.5 mpg. My problem comes when I go up steep hills. It is worse from a dead stop. I'm lucky if I can make it to 30-35 mph. On the move I need to build up speed so I don't lose momentum or slow down the traffic behind me. Engine has plenty of power. Even at highway speeds it just seems hard to keep it in the power band. 230 stock with Weber 2bbl replacement and an HEI, 3spd on the tree.

My question is for you guys running the original 3.73 rear. What is top speed on the highway and what rpm's are you seeing?

Back when I bought the weber I asked Tom over at Langdons about adding power and he recommended that I revert back to the original ratio and put in an overdrive trans as opposed to hopping up the motor.

I would like to start with changing out the gears but the trans will have to wait. I want to keep driving the truck daily and don't want the 3.73's to kill me too bad on mileage and wear at higher rpm's.
Don't blame the gear ratio!
What size of tires are you using? Height??
What's your initial timing set at?

TJ's Chevy 07-08-2015 09:59 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
3.73's will not kill your top end to badly. 3.73's and 30 inch tires for me was 3000 rpm at 65-70 I had a 292/sm420. An OD trans which I'm putting in with my new 292 will give me about 2500 rpm at 70 mph which will be perfect. Your 230 is a smaller engine, though yes indeed it has great power and luckly plenty of torque at the lower range...you'd be making ANY engine struggle to be in 1:1 with 3.07's pulling grades. Heck, I get another axle from a 66 with 3.73's....swap it out...drive it and see how you like it...and if you don't...simply swap your old axle back in. I've done it and it takes very little time. :chevy:

curvedglass 07-08-2015 10:10 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by geezer#99 (Post 7234291)
Don't blame the gear ratio!
What size of tires are you using? Height??
What's your initial timing set at?
P235/70/15 BFG T/A's
No idea on the timing.

geezer#99 07-08-2015 10:19 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by curvedglass (Post 7234307)
P235/70/15 BFG T/A's
No idea on the timing.
Try a poor man's underdrive trick to see what a 3.73 gear will be like. Swap on some shorter tires. Like 205 60 15's.
Figure out what your initial timing is!
That's the key to your lack of power.

curvedglass 07-08-2015 10:44 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by geezer#99 (Post 7234320)
Try a poor man's underdrive trick to see what a 3.73 gear will be like. Swap on some shorter tires. Like 205 60 15's.
Figure out what your initial timing is!
That's the key to your lack of power.
Thanks Geez. I will throw the light on it. What should my timing be set to? I swapped out the stock dizzy for an HEI unit so I might not be spot on anymore. Although I had the same issues before the Weber and the HEI swap.

slepysal 07-08-2015 11:05 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
I have 3.73 in my 60 and am running p215/75r15 ( 27.7") and I have no "power issues" 70-75 at 3500 rpm In all truthfulness I have to stay in the slow lane 70-75 mph in the bay area is considered slow !

geezer#99 07-08-2015 11:21 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by curvedglass (Post 7234351)
Thanks Geez. I will throw the light on it. What should my timing be set to? I swapped out the stock dizzy for an HEI unit so I might not be spot on anymore. Although I had the same issues before the Weber and the HEI swap.
Try 14 initial.
Plug off the vac to the dizzy when you se it.
Did you run a dedicated 12v wire to your hei or hook it up to the stock wire?

geezer#99 07-08-2015 11:23 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by slepysal (Post 7234371)
I have 3.73 in my 60 and am running p215/75r15 ( 27.7") and I have no "power issues" 70-75 at 3500 rpm In all truthfulness I have to stay in the slow lane 70-75 mph in the bay area is considered slow !
It's that 'Canuck' valve cover that makes you go faster Sal!!

TJ's Chevy 07-08-2015 11:28 AM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
14 advanced without vacuum is a bit high. 10 is more like it. You may start having detonation problems at 14 advanced if your not running 92 octane. Try it for sure but Beware and listen hard..if it starts pinging stop quickly and back it off a wee bit. 8-10 is the spec, I ran mine at 10 for ease of cold and hot starts. You'll know if its to far advanced as it will "struggle" to start when hot. You don't want to get over the 36 mark or you could start having problems. The little timing tab on the engine will have an A on it(stands for Advanced) with several little marks...you'll see a 0 and each mark from there reads 2, 4, 6, 8 ,10.

curvedglass 07-08-2015 12:35 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by geezer#99 (Post 7234396)
Try 14 initial.
Plug off the vac to the dizzy when you se it.
Did you run a dedicated 12v wire to your hei or hook it up to the stock wire?
I used the stock wire. I only had three open circuits on the fuse box. One is for the phone/speedometer-app/itunes thingy, one where I hooked up the reverse lights I mounted to the bumper, and the last went to the aux fuel tank. P.O. had the tank out to paint the cab and the wire to the sending unit was never put back in so ran a wire to that connection.

curvedglass 07-08-2015 12:39 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by TJ's Chevy (Post 7234405)
14 advanced without vacuum is a bit high. 10 is more like it. You may start having detonation problems at 14 advanced if your not running 92 octane. Try it for sure but Beware and listen hard..if it starts pinging stop quickly and back it off a wee bit. 8-10 is the spec, I ran mine at 10 for ease of cold and hot starts. You'll know if its to far advanced as it will "struggle" to start when hot. You don't want to get over the 36 mark or you could start having problems. The little timing tab on the engine will have an A on it(stands for Advanced) with several little marks...you'll see a 0 and each mark from there reads 2, 4, 6, 8 ,10.
thanx for the tips. I definitely don't want to run ethyl. This is the only vehicle in my stable that runs regular gas. Before the winter set in I paid $1.69 here.

jayoldschool 07-08-2015 01:40 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by curvedglass (Post 7234466)
I used the stock wire. I only had three open circuits on the fuse box. One is for the phone/speedometer-app/itunes thingy, one where I hooked up the reverse lights I mounted to the bumper, and the last went to the aux fuel tank. P.O. had the tank out to paint the cab and the wire to the sending unit was never put back in so ran a wire to that connection.
There's the first problem. The stock wire is a resistance wire, and you are not giving the HEI the full voltage it needs. Run a new wire, ASAP, then check that timing.

Lugnutz65 07-08-2015 02:05 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by jayoldschool (Post 7234529)
There's the first problem. The stock wire is a resistance wire, and you are not giving the HEI the full voltage it needs. Run a new wire, ASAP, then check that timing.
I have read all the above and agree with the following:
Here's what I think you should do in the following order.

1. You need to run a dedicated full 12 volts to the HEI dizzy. Use a 10G wire or bigger.

2. Now, set the timing to 10 degrees with all vac lines plugged. You can go up to 14 degrees later, but see what kind of power you make at 10 degrees first. You can fine tune the timing using a vacuum gauge.

3. Your 235/70/15 tire is a 28" diameter tire and should be fine if the above issues are addressed first. A smaller tire will treat the symptoms but it's not the cause of your power issues. Also a smaller tire might look too small on a truck. It's a matter of personal taste, but I wouldn't go smaller for that reason.

4. Swap the rear to a 3.73. remember that changing the rear affects EVERY gear. A lower rear end (i.e. 3.42 or 3.73) will increase the power you have in EVERY gear. It will also reduce your MPG.

5. Add an OD transmission if you want to keep the 3.73 and want better MPG. An OD tranny will not affect performance in the first several gears but it WILL reduce your RPMs in OD and thus give better MPG with highway driving. A nice AT for a 3.73 rear diff is the 700R4.

curvedglass 07-08-2015 02:18 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by Lugnutz65 (Post 7234546)
I have read all the above and agree with the following:
Here's what I think you should do in the following order.

1. You need to run a dedicated full 12 volts to the HEI dizzy. Use a 10G wire or bigger.

2. Now, set the timing to 10 degrees with all vac lines plugged. You can go up to 14 degrees later, but see what kind of power you make at 10 degrees first. You can fine tune the timing using a vacuum gauge.

3. Your 235/70/15 tire is a 28" diameter tire and should be fine if the above issues are addressed first. A smaller tire will treat the symptoms but it's not the cause of your power issues. Also a smaller tire might look too small on a truck. It's a matter of personal taste, but I wouldn't go smaller for that reason.

4. Swap the rear to a 3.73. remember that changing the rear affects EVERY gear. A lower rear end (i.e. 3.42 or 3.73) will increase the power you have in EVERY gear. It will also reduce your MPG.

5. Add an OD transmission if you want to keep the 3.73 and want better MPG. An OD tranny will not affect performance in the first several gears but it WILL reduce your RPMs in OD and thus give better MPG with highway driving. A nice AT for a 3.73 rear diff is the 700R4.
Notes taken. I will try to give this a shot tonight or tomorrow. Tires are new so there isn't any change going to happen there. I have some wire at home so I will rewire the HEI and set the timing. Items 4 & 5 are on my wish list.

I really want to keep the six. A trans and gear change are cheaper than a 350 goodwrench swap (once you add in intake, carb, dizzy, headers, misc brackets, mounts, exhaust system.

geezer#99 07-08-2015 02:32 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
I didn't mean swapping tires permenantly.
Just some shorter test tires.
A 40 buck pair of test tires is a lot cheaper than a 1200 buck gear change that you might regret after the install.

slepysal 07-08-2015 05:03 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by geezer#99 (Post 7234398)
It's that 'Canuck' valve cover that makes you go faster Sal!!
:lol: you know it !!!

sambrutay 07-08-2015 07:10 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
l6 230 3ott 235/75/15 373 gears. 65 mph @ 3000rpm. traffic gets pissed on trips up and down I65. LOL

Lugnutz65 07-08-2015 07:14 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by sambrutay (Post 7234864)
l6 230 3ott 235/75/15 373 gears. 65 mph @ 3000rpm. traffic gets pissed on trips up and down I65. LOL
Install an OD tranny with a 0.73 5th gear and the RPMs will drop to 2000 - 2100 at 65 MPH with those same tires. Better MPG too.

sambrutay 07-08-2015 07:21 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
lugnutz, I really want to. would love to do a factory style 3 speed od. Have been looking for 5 speed s-10 trans but hard to find with no electric speedo.

Lugnutz65 07-08-2015 07:26 PM
Re: Question for the guys running 3.73's with a 6cyl and manual trans
 
Quote:
Originally Posted by sambrutay (Post 7234873)
lugnutz, I really want to. would love to do a factory style 3 speed od. Have been looking for 5 speed s-10 trans but hard to find with no electric speedo.
I will send you a PM with tips on how I find them.


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