330 V8/TH350 in a 1968 Cutlass (cont.)

Looks like my issue regarding seemingly poor performance out of this engine/trans combination from my previous post had been resolved!


I had the tachometer installed by my mechanic with the intent of doing a stall test (and look at other small issues such as a faulty door switch causing battery drain). While my mechanic was taking a look he noticed a loose fan belt leading to the alternator and tightened it. He also mentioned to me when I picked it up that he thought it shifted normally on his test run. I took it out on the open road and true enough, the high pitched whirring noises that would appear at 55 and 60 miles per hour disappeared! Not only that, but with a tachometer I found that even doing 90mph (on a deserted freeway at 1AM just to avoid any cops or other motorists for about a minute or so necessary to accelerate to that speed) the RPMs hovered around 2,500, which all appears normal!

My guess is that the noises kept me from accelerating past that point due to fear of an exploding engine, giving me the impression of poor performance. I?m just happy to reach freeway passing speeds again and know that the loud engine and transmission noises aren?t due to insanely high revving; it?s just naturally loud!

I want to thank kieth, pleasedodgevan2, Caddyman, oldschool, ok4450, TwinTurbo and FoDaddy for all their helpful suggestions, and maybe if someone else with a similar issue comes across this post they can try this simple suggestion and avoid a whole lot of headaches later on!

Thanks again guys!

What’s the rear end ratio? Without overdrive 90 MPH @ 2500 RPM is really low. A really tall rear end gear would explain a lot with regards to tepid acceleration.

I believe it is the original stock type-O rear differential since I know for a fact we never switched it out in the 18 years my family had it, and it has a 12 bolt housing (which I counted myself one night), which according to the following link is more like a Chevy 10 bolt rear end:


Not sure personally what the ratio is but the above site lists the ratios as being between 2.56 and 3.91.

90mph is also a guess as my speedometer is more skewed the faster I go (listed as 60mph when really doing 45mph and listed as 100mph when doing maybe 65mph to 75mph. As my speedometer stated I was doing 120mph at about 2500 RPM I figured it was in actuality about 85 ? 90mph, but the only way to know for sure, outside of pulling the speedometer from the dash and sending it out to be recalibrated (a project later on!), is to shadow another car when no one else is on the road and match speeds, using a cell phone to keep track of what speeds I hit and when.

I can understand what you mean, as my mother?s 2002 Hyundai Sonata V6 also has some (electrical) speedometer problems and she uses the tachometer to track speeds when it?s acting up, of which 2500 RPM in her car is about 75mph if I remember correctly; much lower speed vs. RPM than my results.

Is a higher or lower ratio considered a ?tall rear end??

I’ve always understood “tall” to be a numerically lower rear end ratio. Like a 3.08 rear end is “taller” than a 4.11 rear end.