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Technically, based on your description of how your uncle will use the car most of the axle build up is probably way overkill but sometimes you just gotta do things the way you want to.. Your uncle has had that car for a long time and you guys both have history and memories with it.. If the 350 engine that is in it runs good but leaks a little oil that’s fixable.. You can even clean it up a bit and paint it if you want it to look better. The 3.42 gear up grade you are putting in the differential will make it feel a lot more powerful then it did with the 2.73 gears.. Just enjoy it..!

I have an uncle like that too. He has a 69 3/4 ton that was a bit tired and I found him a slightly used crate engine, front disc brakes, a 700r4,16” steely wheels and new tires.. He’s had the truck since 1979. It’s always ran it was just time to refresh and upgrade a bit.. It drives like a dream and stops really well too..

Keep us posted on your progress..
 

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Hey Phillip, no problem with going through your rear end guy...I was just pointing out alternatives in case you were pinching budget, as many guys aren't familair will some of the alternatives. Since you aren't doing the work and have a guy you trust, let him do his thing and maintain a friendly relationship with him and it should work out well.
 

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Discussion Starter · #43 ·
Pics after media blasting
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New 30 Spline Axle shafts
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Last pic we got (before final painting, we opted not to do Powder Coat)...Going to pick it up today and deliver it to my uncle. I'll post a pic after the grand reveal.
410947
 

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Discussion Starter · #46 ·
Well the Nova is finally back on the road. Inspection has been renewed also. Still needs some work but looking pretty sharp in my opinion. Not great videos but the best I was able to get so far.


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20201209_133035.jpg



 

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Discussion Starter · #48 ·
Nice.

Do you happen to know the source of the grill? Appears to be some sort of aftermarket bullet??
At first I thought it was a reworked '68-72, but it doesn't seem to be.....or maybe it is??
Yes, we ordered it from classic industries. I believe they are the only ones who carry it from what I've seen. They sell a few different variations depending on what cutouts you want/need. We ordered the one in the first link.



 

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Discussion Starter · #49 ·
@Redrider66SS @nova 71-436 @Ericnova @Nova Thug

Hey guys, I hope you don't mind me tagging you. You guys have been helpful thus far so just wanted to run something by you. My uncle is complaining that at highway speeds around 60-70 MPH that the engine sounds too loud but isn't moving fast. Like it is revved up too high. I told him he needs to let me know what RPM he is at when he is driving on the highway and what speed. Basically for him to drive 65MPH or so and tell me his RPM.

He is convinced the rear end guy "screwed him" and put racing gears in his rear end. He initially thought he needed a tranny with overdrive since someone told him that was what he needed (I told him it would help too). I told him if he tells me the RPM I can pretty much prove what gears he is running without even opening the diff.

My thinking is that going from a 2.73 rear gear to a 3.42 gear should only change the RPM's by about 600. Is this a big difference? Enough to feel like "it needs another gear" according to my uncles description of the way it feels?

See thumbnails below for RPM calculation. I am using the rear tire height size, they are 325/60/R15's. Should I use the front instead which are 305/60/R15 if I remember correctly.

2.73RPMCalc.jpg
3.42RPMCalc.jpg


While I am waiting for him to let me know the RPM's while at highway speeds he suggested I ask the "internet Nova experts", thus here I am :) .

Not that I can prove this is the actual diff that went into the car is there a way to tell the gear size from the picture? Look like a 3.42 or a "racing" rear end to you? Let me know lol.

Thanks in advance for putting up with our nonsense!

RearGear.jpg


P.S. Don't help your crazy uncle with anything...ever...JKJK
 

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If the number on the edge of the ring gear were visible we could tell you what ratio they are. I cannot tell just by looking at them. You can count the pinion rotations to one rotation of the wheel if you jack up the rear end. A 3.42 will be 3.42 pinion rotations to one rotatation of the wheel.

I could be wrong but the tire size you have given (325/60/15) don’t seem likely to be the correct size as pictured on the car. The new 3.42 gears are going to be a pretty big difference from the 2.73 ratio. The car will accelerate faster and will be at a higher RPM when at highway speed. I don’t recall if we discussed the transmission that is in his 74 but I will assume it is a non-overdrive automatic most likely a TH350.

Another thing to keep in mind is that the rear gear change will also effect the speedometer reading. The speedometer cable is connected to the transmission and the reading will be off and appear as a higher rate then the actual speed being driven. This can be corrected by switching the speedometer gear in the transmission. You can also use a app on your cell phone to check the actual speed through the GPS system and see how far off tha speedometer is.

I have posted a transmission gear ratio chart so you can run some scenarios and calculations as accurately as possible.
F0A89D54-063E-42D3-B1C5-E3102836835A.png
 

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Discussion Starter · #51 · (Edited)
If the number on the edge of the ring gear were visible we could tell you what ratio they are. I cannot tell just by looking at them. You can count the pinion rotations to one rotation of the wheel if you jack up the rear end. A 3.42 will be 3.42 pinion rotations to one rotatation of the wheel.

I could be wrong but the tire size you have given (325/60/15) don’t seem likely to be the correct size as pictured on the car. The new 3.42 gears are going to be a pretty big difference from the 2.73 ratio. The car will accelerate faster and will be at a higher RPM when at highway speed. I don’t recall if we discussed the transmission that is in his 74 but I will assume it is a non-overdrive automatic most likely a TH350.

Another thing to keep in mind is that the rear gear change will also effect the speedometer reading. The speedometer cable is connected to the transmission and the reading will be off and appear as a higher rate then the actual speed being driven. This can be corrected by switching the speedometer gear in the transmission. You can also use a app on your cell phone to check the actual speed through the GPS system and see how far off tha speedometer is.

I have posted a transmission gear ratio chart so you can run some scenarios and calculations as accurately as possible. View attachment 413275
On the rear tire size I meant to put 235/60R15, must have fat fingered it.

Also yes, he has a non-overdrive 3 speed. I can't say for sure but pretty sure it is a TH350 also.

So my uncle just called me and he said when the speedometer reads 60MPH, his tach (aftermarket), is reading 5,000 RPM and at around 70/75 it is reading 6,000RPM!!!

Good point on switching the speedometer gear, but even with that...these RPM's seem alarmingly high no?

I may be able to go for a ride with him tomorrow and use a GPS app on my phone to verify his actual speed, as my uncle is not technology inclined.

What would be the most likely scenario for having 5 to 6K RPM on the highway? He also says he feels the car shift into 3rd...Has to be something wonky with the rear end? I am thinking maybe there is a Nova at the drag strip as we speak going 55 down the strip, while we have a drag strip setup under ours o_O.

Any input is appreciated.
 

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What would be the most likely scenario for having 5 to 6K RPM on the highway? He also says he feels the car shift into 3rd...Has to be something wonky with the rear end? I am thinking maybe there is a Nova at the drag strip as we speak going 55 down the strip, while we have a drag strip setup under ours o_O.
Too much axle gear..
Really tiny tires..
Not shifting to top gear..
 

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Discussion Starter · #55 ·
Those rpms don’t seem right.. Is the tach set for a 4,6, or 8 cylinder engine?
Dunno, he's had it for years and no idea if it was ever set up correctly. Based on his description of what the car is doing I believe they are right. He says the car is revved up high on the highway and he barely moving. We are going to jack it up tomorrow and see how many times the drive-shaft spins through one tire revolution. Our guess at the moment is the diff guy put the biggest gear he could in it.
 

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Dunno, he's had it for years and no idea if it was ever set up correctly. Based on his description of what the car is doing I believe they are right. He says the car is revved up high on the highway and he barely moving. We are going to jack it up tomorrow and see how many times the drive-shaft spins through one tire revolution. Our guess at the moment is the diff guy put the biggest gear he could in it.
I've set up a lot of gears, I can tell you that is not a racing gear in that rear pictured.... it is something between 3.23 and 3.55 just based off the visual diameter of the pinion gear and the approximate number of teeth on it.
The higher the ratio, the smaller in diameter the pinion head gets, and the thicker the ring gear gets to compensate.

For him to be actually turning 5000 rpm and only running 60 mph it would need a gear in the high 5.xx's or low 6.xx's ratio, and that pinion would be 1/3 the size shown and have just 7 or 8 teeth on it.
As far as I know, 5.38 is the deepest gear anyone makes for the 8.5/8.6"" 10-bolt, and the pinion gear for that set is about as big around as a golf ball, and the ring is about 2" thick in order to reach over far enough to touch the pinion teeth.
As a matter of fact, the ring gear is so thick it nterferes with assembling the spider cross shaft back into the center of the diff, and requires having a modified pin to fit it..

Either your uncle is exaggerating, the tach is on the wrong cylinder count setting, or the trans isn't shifting to 3rd.
Obviously the gear change is going to make the speedo read faster than the car is actually moving since the driveshaft is now spinning faster for any given MPH, and the speedo reads driveshaft/output shaft rpm..
.
2.73 gear and 26" tire, 2250 rpm @ 60 mph actual ground speed.
3.42 gear and 26" tire, 2800 rpm @ 60 mph actual ground speed.
Roughly 550 rpm difference at 60 mph cruise...
(which may feel lke a missing gear upshift to someone not educated about the amount of rpm change that comes from that rear gear ratio change, or someone who is used to low rpms due to overdrive in everything modern.....it does freak some people out.)

Use your GPS to prove to him what is really happening.
 

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Discussion Starter · #57 ·
I've set up a lot of gears, I can tell you that is not a racing gear in that rear pictured.... it is something between 3.23 and 3.55 just based off the visual diameter of the pinion gear and the approximate number of teeth on it.
The higher the ratio, the smaller in diameter the pinion head gets, and the thicker the ring gear gets to compensate.

For him to be actually turning 5000 rpm and only running 60 mph it would need a gear in the high 5.xx's or low 6.xx's ratio, and that pinion would be 1/3 the size shown and have just 7 or 8 teeth on it.
As far as I know, 5.38 is the deepest gear anyone makes for the 8.5/8.6"" 10-bolt, and the pinion gear for that set is about as big around as a golf ball, and the ring is about 2" thick in order to reach over far enough to touch the pinion teeth.
As a matter of fact, the ring gear is so thick it nterferes with assembling the spider cross shaft back into the center of the diff, and requires having a modified pin to fit it..

Either your uncle is exaggerating, the tach is on the wrong cylinder count setting, or the trans isn't shifting to 3rd.
Obviously the gear change is going to make the speedo read faster than the car is actually moving since the driveshaft is now spinning faster for any given MPH, and the speedo reads driveshaft/output shaft rpm..
.
2.73 gear and 26" tire, 2250 rpm @ 60 mph actual ground speed.
3.42 gear and 26" tire, 2800 rpm @ 60 mph actual ground speed.
Roughly 550 rpm difference at 60 mph cruise...
(which may feel lke a missing gear upshift to someone not educated about the amount of rpm change that comes from that rear gear ratio change, or someone who is used to low rpms due to overdrive in everything modern.....it does freak some people out.)

Use your GPS to prove to him what is really happening.
Haven't got a chance to verify gear size in person yet but I talked to our rear end guy and made a good point. It would be physically impossible to have a gear in the rear end that would make the car run at 6K RPM @ ~70MPH. It would have to be a 6.xx or so and that doesn't exist. He was very helpful and seemed genuinely concerned and offered to do whatever was needed to ensure we had peace of mind in the rear diff, even if that meant opening her up and going over the parts with us. He said there is no way he put a huge gear in the diff. He said maybe if they sent him a 3.73 instead of a 3.42 or something similar, it's possible that he would miss the difference in gear size but no way he would miss that big of a change. He suggested maybe the torque converter isn't engaging properly or something similar. I expect that we will find the rear gear is indeed 3.42 and we are having another issue related to tranny or torque converter but I will let you know what we find.
 

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Discussion Starter · #58 ·
I've set up a lot of gears, I can tell you that is not a racing gear in that rear pictured.... it is something between 3.23 and 3.55 just based off the visual diameter of the pinion gear and the approximate number of teeth on it.
The higher the ratio, the smaller in diameter the pinion head gets, and the thicker the ring gear gets to compensate.

For him to be actually turning 5000 rpm and only running 60 mph it would need a gear in the high 5.xx's or low 6.xx's ratio, and that pinion would be 1/3 the size shown and have just 7 or 8 teeth on it.
As far as I know, 5.38 is the deepest gear anyone makes for the 8.5/8.6"" 10-bolt, and the pinion gear for that set is about as big around as a golf ball, and the ring is about 2" thick in order to reach over far enough to touch the pinion teeth.
As a matter of fact, the ring gear is so thick it nterferes with assembling the spider cross shaft back into the center of the diff, and requires having a modified pin to fit it..

Either your uncle is exaggerating, the tach is on the wrong cylinder count setting, or the trans isn't shifting to 3rd.
Obviously the gear change is going to make the speedo read faster than the car is actually moving since the driveshaft is now spinning faster for any given MPH, and the speedo reads driveshaft/output shaft rpm..
.
2.73 gear and 26" tire, 2250 rpm @ 60 mph actual ground speed.
3.42 gear and 26" tire, 2800 rpm @ 60 mph actual ground speed.
Roughly 550 rpm difference at 60 mph cruise...
(which may feel lke a missing gear upshift to someone not educated about the amount of rpm change that comes from that rear gear ratio change, or someone who is used to low rpms due to overdrive in everything modern.....it does freak some people out.)

Use your GPS to prove to him what is really happening.
Well my uncle is attempting to "test" to verify what the gear ratio is in the Nova. We are using the link below but when we turn the wheel one time we get somewhere in the neighborhood of 1.4 to 2 turns. I tell him, hmm that can't be right. So I was reading that a torsion LSD will act like an open diff and you have to turn the tires twice and count the driveshaft rotations. He says the driveshaft is turning 3 times on the money when spinning his passenger side tire 2 times. I'm thinking it may be a little more than 3 but he is just not measuring his tire/driveshaft rotations very accurately.

Ok so kind of a normal result if you factor in the possible human error lol. But he says when he turns the passenger side wheel, the wheel will turn like half a rotation before the driveshaft even starts moving. Is this amount of play normal? And if not what could be the cause here? Sorry for all the questions but we are out of our element here and figured you guys may know. We plan to talk to our rear end guy Monday but we are just trying to figure out if we need him to look at the rear end or if we need to be looking somewhere else.

 

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The 1/2 turn free play between one side and the other is just gear lash take-up in all the workings of the helical diff.
Sounds like a gear ratio close to the low to mid 3's to me.

You need to be looking elsewhere....either in your uncle's head(LOL), wrong tach setting,..... or trans not upshifting if rpm really is overly high.
Verify the tach is set for the correct number of engine cylinders, and not on the 4 or 6 setting.
Then take it out on the road, use your GPS and tell us what the tach reads for rpm @ GPS indicated 60 mph.
 

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Discussion Starter · #60 ·
The 1/2 turn free play between one side and the other is just gear lash take-up in all the workings of the helical diff.
Sounds like a gear ratio close to the low to mid 3's to me.

You need to be looking elsewhere....either in your uncle's head(LOL), wrong tach setting,..... or trans not upshifting if rpm really is overly high.
Verify the tach is set for the correct number of engine cylinders, and not on the 4 or 6 setting.
Then take it out on the road, use your GPS and tell us what the tach reads for rpm @ GPS indicated 60 mph.
Ok, my uncle is now saying (before I read the message) that the issue is trans related. He doesn't believe it is shifting into 3rd. May be a while before we verify but wanted to say thanks for the tips and info. Good to know the backlash is normal. I'll update this post once the car has a new transmission and what not. I think he said he wants a TH750 or something like that.
 
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