Question for the guys running 3.73's with a 6cyl and manual trans

[sambrutay]

Thank you sir!

[slepysal]

Originally Posted by Lugnutz65

I will send you a PM with tips on how I find them.

I want tips too (please)

[curvedglass]

I was so stoked to try setting the timing that I gave it a little advance just by ear for my drive home yesterday. (I carry a distributor wrench!) I listened for detonation and nothing even with my foot to the floor. Felt way better on the throttle and I did much better on my least favorite hill. I wouldn't call it a miracle but much improvement.

Got home and after the motor cooled I got to work. Found the timing indicator. It was hidden so well by 52 years of grime that I had to search for a photo just to locate it. Found the mark on the pulley and marked it with paint. Plugged the vac line and fired it up. The timing was so far off that the timing mark was at almost 12 o'clock under the water pump pulley. No idea what degree that is. I bumped it down to 10. I installed a dedicated wire too.

Drove it in this morning and it ran fine but just doesn't feel like it did on the way home with all that advance. Maybe 14-16 degrees isn't that far off for my motor then. How far can I advance?

Thoughts!!??!!

[TJ's Chevy]

Originally Posted by curvedglass

I was so stoked to try setting the timing that I gave it a little advance just by ear for my drive home yesterday. (I carry a distributor wrench!) I listened for detonation and nothing even with my foot to the floor. Felt way better on the throttle and I did much better on my least favorite hill. I wouldn't call it a miracle but much improvement.

Got home and after the motor cooled I got to work. Found the timing indicator. It was hidden so well by 52 years of grime that I had to search for a photo just to locate it. Found the mark on the pulley and marked it with paint. Plugged the vac line and fired it up. The timing was so far off that the timing mark was at almost 12 o'clock under the water pump pulley. No idea what degree that is. I bumped it down to 10. I installed a dedicated wire too.

Drove it in this morning and it ran fine but just doesn't feel like it did on the way home with all that advance. Maybe 14-16 degrees isn't that far off for my motor then. How far can I advance?

Thoughts!!??!!

You want no more than 36 degrees no load so I'd bump it up to 12 and see what happens, the engine may be low enough on compression that 14-16 may not be an issue. Try 12 then 14 and if 14 is good I'd stop there and be sure to run high octane. lol

[curvedglass]

Originally Posted by TJ's Chevy

You want no more than 36 degrees no load so I'd bump it up to 12 and see what happens, the engine may be low enough on compression that 14-16 may not be an issue. Try 12 then 14 and if 14 is good I'd stop there and be sure to run high octane. lol

I ran out to the parking garage at lunchtime. I set it to just about 14. Seeing how much it was off yesterday I don't think I have to worry about pings. I drove it around the garage one time and goosed it pretty good. No pinging. Felt just a least bit better at the pedal. Have to see what the ride home brings. It is great having the corporate parking garage for lunch forays. I have done detailing prior to cruise nights, tire rotations, wiring. Everything but an oil change. Security did come out one day - I had my Buick wagon to work and was adjusting the vacuum switch for my 200-R4 kick down. Ended up frying the meats with a little too much smoke. Said he heard the squealing!!

[66Submarine]

Originally Posted by curvedglass

P235/70/15 BFG T/A's
No idea on the timing.

235/70r15 @ 745 revs per mile with a 3.07 rear=2,475RPM@65MPH.
Same tire with 3.73's=3,010RPM@65MPH.

in other words, you would be turning roughly the same RPM you do now at 85MPH with the 3.07's @ 65MPH with 3.73's.

Originally Posted by TJ's Chevy

You want no more than 36 degrees no load so I'd bump it up to 12 and see what happens, the engine may be low enough on compression that 14-16 may not be an issue. Try 12 then 14 and if 14 is good I'd stop there and be sure to run high octane. lol

36º is only a very rough "rule of thumb" and does not include the extra provided by vacuum advance under low load conditions (and some stuff will make max power with timing in the low 30's, while other engines will want 40+). There are also different mechanical advance curves for different years/applications, so you can't really make too much of an assumption based purely on initial timing.

Originally Posted by curvedglass

I ran out to the parking garage at lunchtime. I set it to just about 14. Seeing how much it was off yesterday I don't think I have to worry about pings. I drove it around the garage one time and goosed it pretty good. No pinging. Felt just a least bit better at the pedal. Have to see what the ride home brings. It is great having the corporate parking garage for lunch forays. I have done detailing prior to cruise nights, tire rotations, wiring. Everything but an oil change. Security did come out one day - I had my Buick wagon to work and was adjusting the vacuum switch for my 200-R4 kick down. Ended up frying the meats with a little too much smoke. Said he heard the squealing!!

I can't speak for your exact combo, but this is what I did the 250 in the truck I'm currently driving. I only run 87 octane.

I had an HEI I put in that was frozen solid with rust, so the timing was locked at wherever you set it to all the time. I tried 36º and it did fine most of the time, but would knock with a lot of load and throttle opening at lower speeds. I backed it down to 32º and it only did it at very low speeds and WOT under heavy load. It was early in the year and cooler outside at the time.

I finally replaced that with a working points distributor a little bit later. I found a good one in my pile that had 24º of mechanical advance and some typically stiff springs that made the curve lazy. I found some very light aftermarket springs in a SBC distributor I had laying around and installed those.

I wanted to start out at 36º total, so I set the initial to 12º and checked the curve. It came in quickly and already had 28º by 1,800RPM (all in at 3,000).

IIRC this did pretty good overall (can't remember if I could make it ping or not, though).

Point wear eventually increased the timing to something like 38-40º and it would ping under heavy load going up highway hills. It was around 95º at the time and the engine runs somewhat warm at highway speed (something like 190º instead of 180º). The engine was also rebuilt at some point in time, so any pitiful quench it once had has no doubt been taken away by rebuilder pistons and a thick composite head gasket.

I backed it down to 8º initial/32º total as a very safe tune that I can run WOT under any condition and not worry about detonation. Fuel economy didn't appear to change, although I think it made a little more power with more timing.

I don't know what the curve for your HEI is, but IIRC most have 20º of mechanical advance, so 12-16º initial is probably about right. If the curve is slow you still might not have very much advance where you are running it, so check it and see what it's like there. An advance spring kit is only a few $ from Summit or whoever.

Hope this helps.

[jayoldschool]

One more thought... have you popped the cap and rotor off to make sure the advance mechanism isn't sticking? This would certainly contribute to your issues.

[curvedglass]

Thanks to all for the useful information. The ride home at 14 was better than the ride in at 10. Truck is home in the shop today. Weather was too nice to take my ragtop to work. The beauty of owning a small fleet of classics / modified cars. The hour+ ride to work is never dull.

I checked the advance and it is moving fine. Over the weekend I plan to tweak around a bit more. Based on the the adjustment I made before I started to use the light etc, the truck seems to run the best. No idea what that timing is due to the fact that the mark on the pulley is so far off from the indicator on the timing cover but the power was waay better than ever and there wasn't a ping to be heard. I have stock type exhaust so it is nice and quiet. The thing idles like a timex watch.

I will report back next week.

Beers to all for help!

[66Submarine]

Originally Posted by curvedglass

...No idea what that timing is due to the fact that the mark on the pulley is so far off from the indicator on the timing cover...

Measure the diameter of the balancer and calculate the circumference. There are 360º in the circumference (obviously), so 1/10th of that is 36º. Make a mark there and use that to check total timing. 2º ATDC on the tab with the 36º balancer mark = 34º, etc.

[curvedglass]

Originally Posted by 66Submarine

Measure the diameter of the balancer and calculate the circumference. There are 360º in the circumference (obviously), so 1/10th of that is 36º. Make a mark there and use that to check total timing. 2º ATDC on the tab with the 36º balancer mark = 34º, etc.

cool.

Thanks again for all the help.

I checked out your Impala build thread. Looks cool. My first car was a 67 Bel Air 2 door 250 PG. Solid but worn. Girlfriends Grandpas car. 34k original miles when I got it. Drove it from 1980-83. Would love to have it now of course. Would look good parked next to my 63. (Although my 61 Coupe DeVille looks good there too)

[curvedglass]

Originally Posted by geezer#99

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.

Oh I got ya! Around my area in the northeast outside Philly nothing is $40 though.

After all the help I got from you guys I am sticking to troubleshooting the power side.

[jayoldschool]

Fresh fuel filter? No cracks in any of the rubber fuel lines?

[curvedglass]

New filter and lines. Fresh new weber two barrel three months ago.
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[joedoh]

I am running a similar engine, a 120hp but only 140lb ft tq, with a 3.73 rear and 0.73 overdrive. my engine makes peak torque at 2900 rpm and with my current 31" rear tires I spend a lot of time in 4th, less in 5th. its interesting to note that a 3.73 in 5th would behave very similarly to a 3.08 in 4th (1:1)

I am installing a higher numerical rear end, 4.56 (4.30 would have been ideal but they are 3x the cost) to get my engine back in its peak torque band and allow me to use my 5th gear. it may seem like the best idea would be to just use 4th, right? after all, 4th with 3.73 will be similar to 5th with 4.30. except, like you have noted, there is a big rpm difference between 4th and 3rd, that would go away if I had the opportunity to have the same speed and rpm in 5th because 4th is much closer to 5th numerically.

but I dont think thats what your engine needs. aerodynamic load is exponential as speed increases, and these trucks are less aerodynamic than a brick. for me, I am reducing the gear to get closer to peak torque (at 2900 rpm, currently I am at 1800 or so in 5th), which will allow the truck to achieve its drag limited top speed (I am guessing around 90mph with my 140lb ft on a windless day and flat road).

for you, you are already well PAST your peak torque, which is in the teens, 1600 I think, so you should consider running a test with a TALLER tire to see if the additional torque around 1600rpm will let you have a higher top speed. telling a fat guy, used to walking, to run faster is the analogy here, you should instead see if he can pull more weight at a slower speed, and he probably can.

anyway, thats my stab at it. shorter tires may accomplish the same thing because of torque multiplication, but its not going to help mileage or engine longevity. my two pennies.

[Lugnutz65]

This chart is in my files and is either for a 230ci or 250ci. The numbers changed a bit during the late 1960's to early 1970's but you get the idea. Cruising in the 1800 - 2000 RPM range is ideal for a 230ci or 250ci.
Of course, this assumes you have good compression X 6 cylinders. That's something that hasn't entered the conversation yet. Have you ever check the compression?

[1976gmc20]

I can't remember ever having any power limitations with the 230/420 in my 1965 short step-side.

Of course, that was back in the days of 55mph speed limits, and most of my driving was on forest roads anyway.

No clue what the axle ratio was but probably 3.73 if that was stock. I think that I was running 7.00x15 and/or 6.50x16 light truck tires. Seems like the gas mileage was around 15 but I didn't calculate it very often because of all the short haul driving. Anyway it was way better than the 1971 K10 350/350 that we had at the same time.

I've always had pretty good luck setting timing by ear. Always seems to run better than setting to some spec with a timing light.

[Keith Seymore]

Originally Posted by Lugnutz65

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.

Originally Posted by sambrutay

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.

The New Process A833 (four speed manual; third gear 1:1 and 4th gear .73:1) was available as a regular production option during the squarebody era.

http://www.classicjunkyard.com/new_process_a833.htm

It's perfect with a 3.73 rear axle ratio.

K

[66Submarine]

Originally Posted by curvedglass

cool.

Thanks again for all the help.

I checked out your Impala build thread. Looks cool. My first car was a 67 Bel Air 2 door 250 PG. Solid but worn. Girlfriends Grandpas car. 34k original miles when I got it. Drove it from 1980-83. Would love to have it now of course. Would look good parked next to my 63. (Although my 61 Coupe DeVille looks good there too)

Thanks, no problem.

Originally Posted by joedoh

I am running a similar engine, a 120hp but only 140lb ft tq, with a 3.73 rear and 0.73 overdrive. my engine makes peak torque at 2900 rpm and with my current 31" rear tires I spend a lot of time in 4th, less in 5th. its interesting to note that a 3.73 in 5th would behave very similarly to a 3.08 in 4th (1:1)

I am installing a higher numerical rear end, 4.56 (4.30 would have been ideal but they are 3x the cost) to get my engine back in its peak torque band and allow me to use my 5th gear. it may seem like the best idea would be to just use 4th, right? after all, 4th with 3.73 will be similar to 5th with 4.30. except, like you have noted, there is a big rpm difference between 4th and 3rd, that would go away if I had the opportunity to have the same speed and rpm in 5th because 4th is much closer to 5th numerically.

but I dont think thats what your engine needs. aerodynamic load is exponential as speed increases, and these trucks are less aerodynamic than a brick. for me, I am reducing the gear to get closer to peak torque (at 2900 rpm, currently I am at 1800 or so in 5th), which will allow the truck to achieve its drag limited top speed (I am guessing around 90mph with my 140lb ft on a windless day and flat road).

for you, you are already well PAST your peak torque, which is in the teens, 1600 I think, so you should consider running a test with a TALLER tire to see if the additional torque around 1600rpm will let you have a higher top speed. telling a fat guy, used to walking, to run faster is the analogy here, you should instead see if he can pull more weight at a slower speed, and he probably can.

anyway, thats my stab at it. shorter tires may accomplish the same thing because of torque multiplication, but its not going to help mileage or engine longevity. my two pennies.

The problem is that HP moves the truck, not torque. Example:

70MPH with 3.07 gears will be 2,668RPM. Going by the GM chart that Jay posted, net torque will be around 210FTLBS and net HP will be around 105HP.

70MPH with 3.73's (and a 745 rev per mile 235/70r15) will be 3,242RPM. Net torque is lower at 195FTLBS, but HP is up to 120.

With 3.07 gears you will have 3.07x210= 644.7FTLBS at the tire. With a 3.73 you will have 3.73x195= 727.4FTLBS.

A taller tire will further reduce HP and make it even weaker.

Originally Posted by Keith Seymore

The New Process A833 (four speed manual; third gear 1:1 and 4th gear .73:1) was available as a regular production option during the squarebody era.

http://www.classicjunkyard.com/new_process_a833.htm

It's perfect with a 3.73 rear axle ratio.

K

X2. An OD trans and deeper rear gears would give you the best of both worlds. With a common 3.73 rear and the trans Keith mentioned, you'd have the 3.73 final drive ratio in 3rd (direct/1:1) for hills and a 2.72 final in the .73 OD 4th for cruising.

[curvedglass]

Update:
things have been better. I have been able to get some power out of my third gear now and my last tank of fuel was over 18mpg.

New question ... re-gapping spark plugs.

When I added the carb and HEI I never changed the plug gap. Actually never thought of it. Friend told me I can't take full advantage of the HEI without re-gapping the plugs.

What should the gap be for HEI?

[TJ's Chevy]

Originally Posted by curvedglass

Update:
things have been better. I have been able to get some power out of my third gear now and my last tank of fuel was over 18mpg.

New question ... re-gapping spark plugs.

When I added the carb and HEI I never changed the plug gap. Actually never thought of it. Friend told me I can't take full advantage of the HEI without re-gapping the plugs.

What should the gap be for HEI?

.045-.055 pending on how hot the HEI is. For a stock GM HEI I would do .045.

[1976gmc20]

For a 350 v8 with electronic ignition, I usually gap around 0.050"

[curvedglass]

Thanks for the info. Baby steps - I will start at .045

Just gassed up at lunch 18.01 this tank - and regular is $2.19 here.

[TJ's Chevy]

wow....ain't seen it that cheap in years!

[72step]

My 65 C10 230 4spd 3.73 truck runs great. Around 65mph is where is feels like it could use another gear. That NP A833 swap sure sounds interesting. Does anyone know if it is a direct bolt in for one of our trucks or does it require lots of modifying? I'm not sure I'd want to get rid of my tall 4spd truck shifter. I wonder what 3.42 or 3.55 gears would do? I feel like 2nd and 3rd now are almost too low geared and fourth feels awesome until about 65mph.

[1976gmc20]

Originally Posted by 72step

My 65 C10 230 4spd 3.73 truck runs great. ...

Yeah, mine did too. Of course the speed limit was 55 back then.


You pickup isn't a short bed stepside that came out of Colorado by any chance, is it ???