I've been having a problem with excessive speeding on highways... Mainly that I can't achieve excessive speeds...
Other than the obvious "I have the LG4" problem, I can't seem to break 100. I can't tell exactly how fast I'm going due to the 85 spedo, but from a bit of guesswork at 2500 rpm in 5th it tops out somewhere between 95-110. The issue is if I try to go any farther than that it feels like it losees power, I get a lot of vibration - it feels especially strong in the gas pedal, and it gets kinda loud - sounds a little like a semi engine breaking. I don't think it's the engine, since it runs fine at 2500+ in all other gears, so I think it's something with the tranny, drive shaft, or rear axle.
Any ideas?

 

although I won't condone the point of this thread....



Vibration eats a lot of power. You factory driveshaft is how old and was never balanced well, none-the-less balanced for high speeds. Start there. What rear gear ratio do you have? The comon 2.73 gear may honestly be running you out power trying to pull OD. The noise you're hearing could easily be spark knock due to the excessive load.

 

Well the shaft is stock, so 23 years old. My RPO says I've got 3.27 gearing, so that shouldn't be it.

What could be done about the spark knock? Adjust timing?

 

3.27...hmm, I didn't think they had the 9-bolt in '85 but I've never really looked into it. Anyhow, that's still a decent amount of gear to pull in OD with a stock/mild LG4.

Have the ds checked. Straightness and balance. Vibrations at that kind of speed can easily have disasterous results. As for the possible spark knock, yes, pulling timing should eliminate it, but it still may not have enough umph to run much faster and lesser timing will cost you some bottom-end power.

 

According to the RPO list in my console, and the decoder:
GW6 AXLE REAR, 3.27 RATIO Might as well get the shaft checked, I think I'm going to do some major drivetrain work as soon as I get some money. Rebuild carb, trans, rear axle, etc - just to keep everything in good shape.
It's only got 60K miles on it, but it hasn't had much work done in the past few years aside from oil changes. It's about time for a tune-up and FULL fluid replacement I suppose.

 

My 84 LG4 powered Firebird would do about 120-125, I dont remember what rear was in the car but I'm pretty sure it wasnt 2.73's. It didnt vibrate at all at that speed, so fix that first and see where you get from there. Old tired engines and transmissions dont help that any either, among mechanical problems as well.

As far as the rear in that 85, that was the first year the 9-bolt was used in thirdgens.

 

The stock LE9 305 in my fullsize van would do over 100 mph on the freeway. 100% stock at the time. 700r4 with 3.08s and 29" tall tires.

 

Try shifting down to fourth. Too tall of a gear is worse than too short, and 2500 rpm is past your torque peak.
To illustrate, I had a '79 firebird 301. Yeah, different engine. ultra-tiny intake ports. Rated 150 HP at 4000 rpm. All done by 4500. TH350, 2.73:1 axle, and 27.7" tires. I totally buried the speedometer, about 115 or 120 mph.
Had I had another gear in the trans to upshift into, yeah it woulda been closer to the torque peak, but the lesser torque at higher rpm was being multiplied by more gear than I woulda had with an overdrive.
Yes, your 3.27:1 is more than that car's 2.73:1, and likewise with the tires, but you don't have 400 rpm worth of torque converter slip. So, again, try it in fourth gear.