Could use some Advice on 86 - just added a 3.54

[adfac]

Hi. I just put in a 3.54 rear end with a stock 350 (replacing a 2.73). Seems to me the engine is working really hard at highway speeds. I love the way it drives between the red lights but hate it on the highway. The engine is doing about 3,000 rpm @ 73 mph and it's really working hard at 80-85. Right now, a good highway crusing speed is about 70. I'd like it to be about 80. I also noticed that when I hit overdrive on the highway, my tach doesn't redline but it's definitely up in the yellow area. My question is: What would I have to do to the engine to back off the rpms and smooth out the performance at highway speeds? Any advice would really be appreciated.

 

[vetteoz]

3.54 rear end with a stock 350 Seems to me the engine is working really hard at highway speeds.
The engine is doing about 3,000 rpm @ 73 mph
My question is:
What would I have to do to the engine to back off the rpms and smooth out the performance at highway speeds? .

Appear to be in top gear ( 1:1 ) so your trans may not be shifting into 4th? ( O/d)

70mph is 3200 with 1:1 ( D in a auto or 4th in a 4spd )
70mph should be about 2250 (.70 ) in O/d

 

[John Robinson]

Using my Auto Math Handbook here is what I came up with from what you indicate are the RPM's and speed with the new gears
Here is the formula for figuring the rpm for the different gear ratio

RPM=mph x final drive ratio x gear ratio x 336
________________________________________
26

So let’s use the final drive ratio and gear ratiofor my 93 for this example

75 x .70 x 2.59 x 336 = 456876 Divided by 26 = 1752 RPM

Now all you have to do is plug in whatever ratio you want

Using a 3.54 instead of a 2.73 will have the following results at 75 MPH theRPM will be 2401 or 549RPM more per mile. Based on your stock ratio giving an rpm of 1852 at 75MPH for the 2.73 gear.

Now if my calculations are right the difference between a 2.73 and 3.54 isabout 33% so if you did not change the speedometer drive gear your speedometeris reading slow so 3000 rpm should be about 99 MPH with the speedometerindicating 75. So I would suggest youdrive the car with the tach at 2400 RPM’s and note what the speedometer says isyour speed.

 

[Hib Halverson]

Hi. I just put in a 3.54 rear end with a stock 350 (replacing a 2.73). Seems to me the engine is working really hard at highway speeds. I love the way it drives between the red lights but hate it on the highway. The engine is doing about 3,000 rpm @ 73 mph and it's really working hard at 80-85. Right now, a good highway crusing speed is about 70. I'd like it to be about 80. I also noticed that when I hit overdrive on the highway, my tach doesn't redline but it's definitely up in the yellow area. My question is: What would I have to do to the engine to back off the rpms and smooth out the performance at highway speeds? Any advice would really be appreciated.

Change the axle ratio to 3.07.

 

[vigman]

I assume it's an automatic

I assume you didn't change the speedo drive gear...
3.54 ( in this car will make it a stump puller ) .. fast off the line.. nuttin past 5500 rpm...
So I hope this isn't a NEW rear end but exchanged used?
3.07 would be the best compromise between both worlds...

You don't get something for nuttin...
Mike

 

[Schrade]

Intake manifold of L98 is designed on flow characteristics of LB9 TPI manifold. It will start to turn blue at top end, from no air...

 

[adfac]

I'm sorry. I forgot to say it was an automatic. I did not change the driven gear but I did install a Dakota Digital SGI-5 Universal Speedometer Signal Inteface. I tested it with GPS and the speedo is right on the money. I also tried Eckler's Speedometer Calibrator which sold for a whopping $179.99. It came in DOA. I called the company that made the unit and they verified it but said I had to go to Ecklers for a refund or exchange. I had the Dakota unit and gave it a try and it worked just fine. I discussed the ratio change with the manager at my auto parts store and he said 3.54 was still on the top side of "stock". Looks like he was wrong and I'm feeling like "what was I thinking" ... after hearing from you guys it looks to me like I should have gone with a 3.07 (which I still might do) or go back to my old 2.73 where it was slow but everything worked. Sometimes "lessons learned" can cost alot of time and $$$. I still have the 'upshift' light coming on at highway speeds (doesn't come on around town) and I don't know how to describe it other than maybe it's not shifting into OD but when I lay off the gas, it feels like it's still in gear. It doesn't "release" until around 42 mph or so. So I guess the 3.54 has caused more problems than it's worth. Moral of the story, "if it ain't broke, don't fix it".

 

[vetteoz]

I did not change the driven gear but I did install a Dakota Digital SGI-5 Universal Speedometer Signal Inteface
I tested it with GPS and the speedo is right on the money.

Irrespective of whether the gears are right for your combo or not your speedo in reading right which means your indicated revs/ speed
(3,000 rpm @ 73 mph) as you posted are correct which would indicate you are not getting 4th which would give you approx
2250 @ 70 mph

You can confirm that here
Interactive RPM and MPH Computator Program

In Transmission Gearing Ratio use 0.70 for 4th gear and 1.0 for 3rd calcs
Use 26" for tire diameter

 

[adfac]

Irrespective of whether the gears are right for your combo or not your speedo in reading right which means your indicated revs/ speed
(3,000 rpm @ 73 mph) as you posted are correct which would indicate you are not getting 4th which would give you approx
2250 @ 70 mph

You can confirm that here
Interactive RPM and MPH Computator Program

In Transmission Gearing Ratio use 0.70 for 4th gear and 1.0 for 3rd calcs
Use 26" for tire diameter

I agree. What would be causing that? And how would I correct it?

 

[Schrade]

I'm sorry. I forgot to say it was an automatic. I did not change the driven gear but I did install a Dakota Digital SGI-5 Universal Speedometer Signal Inteface. I tested it with GPS and the speedo is right on the money. I also tried Eckler's Speedometer Calibrator which sold for a whopping $179.99. It came in DOA. I called the company that made the unit and they verified it but said I had to go to Ecklers for a refund or exchange. I had the Dakota unit and gave it a try and it worked just fine. I discussed the ratio change with the manager at my auto parts store and he said 3.54 was still on the top side of "stock". Looks like he was wrong and I'm feeling like "what was I thinking" ... after hearing from you guys it looks to me like I should have gone with a 3.07 (which I still might do) or go back to my old 2.73 where it was slow but everything worked. Sometimes "lessons learned" can cost alot of time and $$$. I still have the 'upshift' light coming on at highway speeds (doesn't come on around town) and I don't know how to describe it other than maybe it's not shifting into OD but when I lay off the gas, it feels like it's still in gear. It doesn't "release" until around 42 mph or so. So I guess the 3.54 has caused more problems than it's worth. Moral of the story, "if it ain't broke, don't fix it".

It still runs out of air at higher RPMs. LB9 that I referred to is TPI 305 motor.

~42 mph is where the RPMs drop, for shift into 4rth. Again, lower RPMs is where TPI is happy. With the 3.54, RPM's rise quick with vehicle speed, and it gets quickly to the choke point...

I bet even that your city MPGs will go up SLIGHTLY (unless you keep the hammer pegged )

 

[adfac]

It still runs out of air at higher RPMs. LB9 that I referred to is TPI 305 motor.

~42 mph is where the RPMs drop, for shift into 4rth. Again, lower RPMs is where TPI is happy. With the 3.54, RPM's rise quick with vehicle speed, and it gets quickly to the choke point...

I bet even that your city MPGs will go up SLIGHTLY (unless you keep the hammer pegged )

Absolutely right. That's it exactly.