simpleold

Hi there, looking for some advice I have a 1972 cutlass I upgraded the engine to a 455 from the original 350, I put a cam in it specs follow
.50 duration 232 232, cam lift 296"/.296", valve lift 474"/.474", lobe separation 107 119

I kept the original th350 transmission in it,

with the stock 8.2 rear end I was doing 4000 RPM at 60 miles per hour it was pretty high , I have a 2000 2200 stall torque converter,

I upgraded to a 12 bolt Chevy rear end 3.73 and I knew it was going to get worst on the RPM, and it did of course now I'm doing 4000 at 43 miles per hour, however the vehicle gets up to speed very nice, but the rpms of course are pretty bad now with the 12 bolt Chevy 3.73,

My th350 has always shift pretty quick, before 20 mph it has always shift into third gear,

What would be your recommendation on lowering the RPMs, I have always had the high rpms on this car even with the stock rear end, Any advice? I was think of putting a th 200r4,,

or even going the manual t56 route which I know it will be very complex but very cool,

anyway what can I do, I really want to keep that 3.73, this car sees some highway miles and of course as it is right now it will not even be possible, thank you

Fun71

Something doesn't add up. 4000 RPM at 43 MPH with 3.73 rear gears means your tires are 13.5" tall, which is impossible on a 14 or 15 inch wheel.

Even your original numbers don't add up. 4000 RPM at 60 MPH with 26" tall rear tires calculates to a 4.91:1 rear gear ratio.

Are you certain it's shifting into third gear?
Are you certain your tach and speedometer are correct?

What rear tire size do you have?

simpleold

Tires on back are 275 60 15, and on front 245 60 15,

I know RPMs are very high but I even tried manual shifting from 1st, 2nd, and drive and in all shifts RPM went down but still at 43 mph, 4000 RPMs, so I assume third gear is working as I manually tried it and in all shifta RPMs went down

And yes tachometer seems right,

Fun71

OK, the calculator shows that with a 28" tall rear tire, 3.73 gears, and 43 MPH the engine should be turning around 2000 RPM.

I recommend you get a second opinion on your tach accuracy.

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

oldcutlass

I agree, time for a new tach.

edzolz

Does your tach have a cylinder setting? Sounds like your tach is set for a four cylinder engine.

simpleold

Let me take a look at the tachometer and I'll keep you posted thanks again for the responses

76olds

I realise, you mentioned its shifting. If the tack is in good working order and calibrated for a V8, then , I would have to look at the vacuum line coming off the modulator to engine, along with the modulator as well . It may feel as thou its shifting but the RPM's dictate otherwise.

Eric

oldcutlass

Doubt its the modulator, if it doesn't shift it would never make it to 60mph.

simpleold

Well i just double checked the tachometer and the little adjustment switch somehow move to 4 cylinder, bummer!!

Thanks now RPM at 60 mph show 2900,

what I see different now is the speedometer I know for a fact it was accurate with the stock rear end, because I always compare it to a speedometer app I have on my phone,

now with this 12 bolt Chevy posi 3.73 swap, the car speedometer is about 13 miles above actual speed, compared to the GPS phone app why is this? any pointers on how to fix this?

Fun71

With the lower rear gears, the driveshaft is spinning faster than it used to for a given vehicle speed. The speedometer is driven from the drivehsaft in the transmission tail housing and is calibrated for a particular RPM to equal a particular MPH. So you need to change the speedometer gears in the transmission to match the rear gears in the differential.

simpleold

Great thank you will do thanks for all the help!!!!!!

svnt442

The be clear...as you go faster the speed differential increases. It's a percentage, not a direct mile for mile correlation.

simpleold

Yes indeed i encounter this as speed was increasing thank you for sharing,

4speed455