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Pretty High Rpm's At 65 Mph, Torque converter clutch bad?
Vintage 465
post Feb 8 2018, 09:26 PM
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I've had a 1999 Suburban 4x4 with trailer towing package for about 17 years. As far as I know it has a 3:73 rear axle. I will check tomorrow. At 65 mph it's always spun right at 1600-1700 rpm's. I test drove a real nice 2007 Tahoe and it ran beautifully. Has a tow package and a 3:73 rear axle. Equipped with 4 speed auto that I assume is a 4L60E, but I've not really verified that. 5.3 engine. When I was cruising down the road at 65 mph it was spinning 1900-2000 rpm's. This seems high to me. Does anyone out there have a Tahoe or Burb with 4 speed auto and 3:73's and recall what they're spinning at 65 mph?
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carguru
post Feb 9 2018, 08:13 AM
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That is the right range for that generation truck. 2007's have the 4L60E. The 5.3 is a higher revving engine as compared to the good ol 5.7. Torque band is a little higher in the 5.3.


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Vintage 465
post Feb 9 2018, 09:59 AM
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But isn’t the rpm at cruising based on the gear ratios in the transmission and the final drive ratios? They both are 3:73 and the both have 4L60e tranny’s and stock tires though the ‘07 has 17” tires. But if anything that should slow things down a bit.
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carguru
post Feb 9 2018, 11:13 AM
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Bottom line, it is how much torque is required to carry the load. For the same amount of torque from the 5.7, 5.3 delivers at a higher rpm. What tire size were you running in the 99? Most of those stock were running 245/70-16. '07 would normally run 265/65-17. And depending on the option packages, the '07 would also weigh in heavier than the '99.


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Vintage 465
post Feb 9 2018, 11:36 AM
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I’m now thinking my logic isn’t sound. If it had a 4 cylinder engine it would spin a ton more rpm to make it to 65mph. But 400 rpm difference still seems like a lot for “similar “ engines
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carguru
post Feb 9 2018, 05:19 PM
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Not that far fetched. Looking at the hp/torque curves between a ‘99 5.7 and ‘07 5.3, torque of the 5.7 at 1500 rpm is the same as the torque of the 5.3 at—-1800 rpm. The old 5.7 just has more low end grunt than the 5.3.

Pretty much goes along with the old saying of there is no replacement for displacement.


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