Getting the Feel of My New 8.8 3.73's

Topic: Getting the Feel of My New 8.8 3.73's (Read 1664 times) previous topic - next topic

Getting the Feel of My New 8.8 3.73's

August 10, 2017, 12:15:53 AM

Driving Wonderbird home from the body shop gave me a chance to get the feel of my new rear end and tire combo. It was totally different than what I expected.

The 3.73 gears did NOTHING to improve the anemic acceleration provided by the stock 86 5.0 EFI. I realized that the taller tires would cut the gear down a bit, but honestly, it accelerates no better than it did with the 3.08's although timing equipment would likely say otherwise. I have no doubt I could beat this car in the quarter mile with my 2014 Focus 5 speed stick. If there is any improvement, it is throttle response when in OD. So much for the bad.

The 17" wheels and tall 2006 Mustang GT tires gave the car an impressive ride. Much better than with the old 14's. I expected just the opposite. Perhaps the new rear springs and Sachs shocks helped somewhat, but this car rides very well. My new SN95 springs and Sachs struts just arrived today for the front end (hoping for a 1.0" or 1.5" drop). The car sits a bit high in the front with the new tire/wheel combo, but the rear didn't change at all with the new rear springs, which is good. It STILL has a bit of the dreaded Thunderbird/Cougar lean, but not so pr0nounced as before.

Next, the BBK shorty headers, off road H pipe and 2.5" duals will be installed.

Getting the Feel of My New 8.8 3.73's

Reply #1 – August 10, 2017, 03:30:29 AM

Well to be fair, you didn't add any power, added probably 60lbs to the car and greatly increased the traction. And they your aren't impressed that it isn't faster then a car that has more power and almost 1klbs less weight. Im not trying to talk any or piss ya off, just trying to put it into perspective.

270lbs ft of torque x 3.08 x 1st gear ration of about 2.76 if i remeber right is just short of 2300lbs of force. 270 x 3.73 x 2.76 = around 2800lbs. If you moved up to a 27 or 28" tall tire with the 17" rims, you would be reducing torque by about 10% at the wheels compared to a stock sized 14" combo of 25.5" tall. 2800-280 = right around 2520.

When i went from a stock 87 bird with a 302 and aod with 2.73 gear, to a sn-95 t-5 the difference was amazing from a dead stop. 270lbs ft torque x 2.76 1st gear x 2.73 rear end was about 2000 ft lbs in first. With the 3.35 1st gear, the math becomes 270 x 3.35 x 2.73 = 2460 or so lbs. Almost a 450 ft lbs difference took the car from not being able to spin on wet grass to spinning all the way through first.

Even though the theoretically rear end ratio got better, you may have killed some of the perceived power from a dead stop with a limited slip and a taller tire plus the extra weight from the heavier rear end to lug around.

When you get more power into it, it will make a world of difference when you launch. Both moving more weight onto the axle itself and the better gear ratio. The stock 302 is also pretty lazy once you hit about 3000k rpms due to the insanely light springs on the valve train, anemic cam shaft profiles and intake mean the engine is about done making power before the shift from 1st to 2ng gear.

It feels like it has more punch in od because you are dramatically changine engine rpms and highway speed. Stock power levels, od ratio and rear end is as follows.

270 x 3.08 x .67 = 557 lbs ft theoretical in od.

Now with the new combo, assuming same sized tires.

270 x 3.73 x .67 = 674 lbs ft.

Basically, a 2300 ft lbs (stock in 1st gear) vs 2800 ft lbs (after the rear end swap - whatever the taller tires do) is only a 18% difference where the car should have the most acceleration in 1st.

But in od you get the same 18% difference where there is much less "power".

Once you add some rpms to the car, it will really wake things up. Im curious how tall of tires you went with.

Getting the Feel of My New 8.8 3.73's

Reply #2 – August 10, 2017, 11:23:01 AM

I've probably changed gear ratio's 10 times or more in my lifetime, so my comments are a bit tongue in cheek regarding the change in acceleration. I was just surprised that it didn't feel more responsive on the bottom end, as I have come to expect. The unsprung change in weight not withstanding. That stock 86 5.0/AOD is a real pooch compared to other combo's where I have experimented with gear ratio changes.

The tires are over 27" in diameter, so that is what cut the effective gear ratio change down somewhat. I went with a larger tire specifically to reduce the effective ratio change, as I had originally planned to go with a low boost turbo/347. When I bought the rear end, I really didn't want the 3.73 ratio, but as always happens, the deal couldn't be passed up.

Getting the Feel of My New 8.8 3.73's

Reply #3 – August 10, 2017, 02:42:06 PM

Quote from: rotorr22;462102

The tires are over 27" in diameter, so that is what cut the effective gear ratio change down somewhat. I went with a larger tire specifically to reduce the effective ratio change, as I had originally planned to go with a low boost turbo/347. When I bought the rear end, I really didn't want the 3.73 ratio, but as always happens, the deal couldn't be passed up.

I LUV my 3:73 with 25" tires & even more with 24", looked a little funny tho... Cruzed nice at 65-70 MPH with the AOD in O/D...

Off the line a 2400 stall converter will do more to wake up that paltry 5.0 than probably even 4.56 gears...

Getting the Feel of My New 8.8 3.73's

Reply #4 – August 10, 2017, 04:17:10 PM

I agree, Tom. I've seen a higher stall converter drop an ET well beyond what one might expect. I'm saving a converter install for my 4R70W install. The 3.73 install is just one component of my plans for the car. It was easy to do while the car was in a shop that had a lift. Had I had the stock rubber on the car, I'm confidence the difference would have been more profound.

Getting the Feel of My New 8.8 3.73's

Reply #5 – August 10, 2017, 04:17:34 PM

My NA 306 likes 3.73s and a 2800 stall converter with 26" tall tires, although it's far from stock. Probably should have gone with 4.10s when I rebuilt the 8.8 rear two years ago. As it is the car is traction limited. If you stomp the gas a 20-30 mph it lights the tires.

Getting the Feel of My New 8.8 3.73's

Reply #6 – August 10, 2017, 08:13:07 PM

I just swapped out my 8.8 with 3.08's for a turbocoupe one with 3.55's and I was surprised at lack of change. For seat-of-the-pants acceleration, It feels about the same. I haven't been back to the track to see if there is an ET difference tho. I realize that its not a big of a change as your 3.73's, but my case was 8.8 to 8.8 so that rules out added weight and drive train losses and all that. If the swap didn't give me rear discs, I'd have really been upset, because gears alone was not worth the trouble.

Getting the Feel of My New 8.8 3.73's

Reply #7 – August 10, 2017, 08:24:41 PM

Quote from: rotorr22;462108

I agree, Tom. I've seen a higher stall converter drop an ET well beyond what one might expect. I'm saving a converter install for my 4R70W install. The 3.73 install is just one component of my plans for the car. It was easy to do while the car was in a shop that had a lift. Had I had the stock rubber on the car, I'm confidence the difference would have been more profound.

Yeah Ed, no doubt you've had more experience with jacked up stall converters than I'll ever have... Difference is truly amazing...

Getting the Feel of My New 8.8 3.73's

Reply #8 – August 10, 2017, 10:25:03 PM

Quote from: grutinator;462112

I just swapped out my 8.8 with 3.08's for a turbocoupe one with 3.55's and I was surprised at lack of change. For seat-of-the-pants acceleration, It feels about the same. I haven't been back to the track to see if there is an ET difference tho. I realize that its not a big of a change as your 3.73's, but my case was 8.8 to 8.8 so that rules out added weight and drive train losses and all that. If the swap didn't give me rear discs, I'd have really been upset, because gears alone was not worth the trouble.

My ratio change would have been far more significant had I not gone to a taller tire and yes, the discs are a plus. Like you, I would have been disappointed had I not planned to improve this powertrain and ultimately replace the engine in it's entirety.