I had my stock 2:73 gear changed out to a 3:42 in my auto 305 Formula. When I get to high speed at about 70 plus it feels like the car is not in overdrive. It revs to engine too high and I feel as if I am pushing, what the hell is wrong with it? Any help please???? Andrew

 

That's what it's supposed to do. In fact, that's THE POINT of swapping the gears in the first place.... for any given wheel speed, the engine turns faster. Since the engine produces a certain amount of power in each revolution, increasing the gear ratio allows more engine revs (more power) at all vehicle speeds, resulting in faster acceleration.

2.73 gears are 15 teeth on the pinion and 41 teeth on the ring.

3.42 is 12 and 41.

Your engine RPM is now EXACTLY 20% higher at ALL road speeds than it used to be.

That's what gears do.

 

Thank you, I see you are very much valued on here. I asked because I had a stock 350 with the 3:42 and it was never that rough, it would easily climb to the 130 mph, this does not, 85 is like the top for it. It is a 305 stock engine and rear axle is 10 bolt with drums and posi.

What about the vibration? My center console rattles when I get up to 65 or more, is there something wrong there? My 2 options are, putting the 2:73 back in or going to trans am creations and getting a rear end complete, I do not know but I am so frustrated now.

Thanks, Andrew

 

TBI, I'm guessing? Stock exhaust? Stock cam? Stock heads?

Basically, the motor has been forced up into a range of RPMS by the gear change, that it can't run in, because it's so restricted.

What you've done is what we all advise against: "over-gearing" a low-RPM motor. You've kind of stepped onto the slippery slope of "one mod leads to another".

If I was you, I sure wouldn't put those grocery-cart gears back in there, though. The solution lies in attacking it from the opposite direction: open the motor up to where it can breathe, so that it is comfortable running at its new RPMs.

Step 1 would be, take EVERY SINGLE PIECE of the exhaust off, starting at the heads and proceeding to the back bumper. When I say EVERY SINGLE PIECE, I mean EVERY SINGLE PIECE: manifolds, Y-pipe, cat, exh pipe, muffler, and tailpipes. Leave nothing but the hangers. Replace it with good quality, chassis-specific headers, specifically built for one of these car, BUT NOT FOR THE TBI MOTOR. I'd recommend getting the stuff for a 350 TPI car. For headers, SLP would be great if you can find some, they'd have to be used; Edelbrock TES; or some of the Hooker ones. Whatever you get, make sure it comes with the corresponding Y-pipe from its mfr; and get them ceramic coated. Makes all the difference in the world. THen a high-flow cat such as a Catco, which you cna get from Summit and elsewhere for about $120; then a 3" cat-back of your choice, from Dynomax, Hooker, Blowmaster, Borla, Edlebrock, etc.

For a cam, the easiest and cheapest way to go, is a used LT1 cam. Look for one out of a Corvette or F-body, NOT a Roadmaster or Impala. You can pick em up on eBay for $50-75 all the time.

For the vibration, get a used aluminum drive shaft out of a 98-02 (LS1) F-body. Again, widely and easily available; about $150. The reason you have it now, is because the drive shaft is ALSO turning at a higher RPM than it used to; when you're doing 65, it's spinning at the RPM it used to be at almost 90. the actual vibes aren't new, they're just at a speed now where you can notice them. The newer design and lighter drive shaft will cure that.

 

Thank you so much! I just bought the aluminium drive shaft yesterday for 170 shipped from a 98 Camaro. I will have it installed as soon as I get it and test it. The rest will be done in reasonable time. Thanks! Andrew