SpeedMetalT/A

I have an 87 Trans Am with a 350 and a th350 trans. I want to go with 3.73 or 4.10 gears but I don't have an overdrive like a 700r4 car would have. This car is driven on the highway and I'm looking for some opinions from people who have similar cars. What rpm are you at around 65-80mph? Is something like 3.23's or 3.42's a better choice? or should I just go with low gears and scrap the idea of driving this car on the highway?

TKOPerformance

I have 3.55s in my '67 Mustang with a C4 (3 speed auto no OD), and it still turns faster than I'd like it to at highway speeds. At 80MPH it's turning 3,500 RPM, which is way too fast. I can run slower and turn fewer RPM, but then I become a hindrance to traffic, which I do not like at all. Basically I keep the car off the highway and am waiting for funds to free up to complete the Tremec swap I've been waiting for.

My advice would be to ditch the TH350 and get a 700R4. Then run 3.73s. You'll be able to drive the car wherever you want, and still have great punch off the line (a 700R4 has a 3.06:1 first gear, which is steeper than a TH350). If you feel you must stick with the TH350 then I would suggest a gearset no higher than 3.00.

bluegrassz

iTrader: (3)

Im running a TH350, 3.73 gears and 26" tall tires. At 60 mph, I am turning 3000 rpm. I could swap to a 28" tall tire and it would take some rpm out, but for now it is street/strip.

My guess is that 3.23's or 2.73's would be just fine with the TH350 on the highway.

It all depends on your use of the car. If it is you r DD or just a weekend warrior.

elano

iTrader: (2)

I have a TH350 with 3.23 gears. with the stock size tire (245/50/16) that have 3 years of wear on them, I turn the motor around 3000rpm @ 65mph. This is completely bearable for me however it isn't very plesant. I really like to cruise around 2600 rpm which puts me at 55mph. Sometimes when I have to go to the big city and merge onto the interstate, I will be around 3500rpm @ 75mph. It kind of sucks and gas mileage goes out the window.

I have a torsen and 3.73 gears I'm gong to put in very soon. I figure if it becomes a problem, I will just throw some bigger tires on there and deal with it. I drive the car 90% city, 9% highway, and 1% track.

I would go with the 3.73 if I were you if you wanted the posobility to even drive it on the highway. 4.10 is not a gear you want to drive on the interstate for more than a few miles. 3.73 will have you going 60mph@3000 rpm. If you pm me, I will let you know how It feel when I swap gears. Hope this helps.

TKOPerformance

The problem isn't just gas milage and annoyance though. Engine wear becomes a problem from sustained high RPM use. A stock motor like your L98 was designed to run at minimal RPM 90% of the time, and make a few trips into the upper RPM zone 10% of the time. Manufacturers design their cars so that cruise RPM at 65 MPH is only about 30-35% of the engines max operating RPM. An L98's max RPM is about 5,200, so at cruise speed you want it to turn about 1,800 RPM. At 3,500RPM you are turning it 67% of max RPM!

Engines can be designed for sustained high RPM, just look at NASCAR. They turn 9,000 RPM for hours on end and rarely fail. The key is that the oiling system is a very sophisticated dry sump unit, the bearing clearances are perfect, the machine work is perfect, and the parts are the best that money can buy.

An engine that you built specifically for racing or street/strip use can probably take that kind of abuse as well. It isn't as complicated or expensive as a NASCAR engine, but it doesn't have to turn 9,000 RPM either. It's probably designed to turn 6,000 to 6,500, and running it 3,500 is closer to 50% of max RPM. The motor's tougher and built with better parts, and should be fine.

However, a street engine that is asked to turn this kind of RPM on a daily basis, for a prolonged period of time is living on borrowed time. I agree that is the car is a toy and for street/strip action then it should be fine. If it's a daily driver you are sacrificing longevity and reliability in the name of a moderate performance gain. Stick with 3.23s and a 26" tire and you should get the best that is realistically possible from both worlds.

BTW, running a 3.73 gear and a 28" tire is the same as running a 3.46 gear with a 26" tire. If you want to know your cruise RPM just plug your info into this formula:

RPM = (MPH*gear ratio*336) / tire diameter