Inliners International Forums General Discussion Drive Train 5 speed update

Hello. It's been nearly 2 months since I put the 5 speed in my Nova with the 194. I actually LOST mileage. I went down from roughly 13mpg in the city to 11. I don't have the numbers but highway was definitely less. I went with a T5 from an '85 Camaro 4cyl. My rear gears are 3:36, tires are 14/70s. I could have put a Saginaw 4 speed in for a lot less.

strummin,
your rear end ratio is to high for the OD ratio you probably have. my guess is .73. multiply that times 3.36 & you get a 2.45 (!) final drive ratio. kinda like double overdrive & it's lugging your engine down. if you want to stick with those rear gears the 4 speed probably would have been a better choice.
robert

Strummin, I don't doubt that you're getting that kind of gas mileage, but I'm sort of curious. I had a 64 Chevelle wagon with a 194 and a 3 speed manual trans in it, and I got 19 mpg as an overall average. How the heck do you wind up with 11-13 mpg with either a 4 of 5 speed in a Nova body? Around town, you'd never need to shift up to 5th with 3:36 gears in the rear end. 5th should be a freeway only gear. If you only use the first 4 gears, it should come out about the same as a Saginaw or other 4 speed as far as your gas mileage goes. Robert's right about the double overdrive effect in top gear.

I don't use the OD around town at all, or even the freeway now. And yes the mileage was a little better 15-16mpg with the original non syncro 3 speed.
Putting lower rear gears will eliminate the need for 1st gear. The other option I see is going to a lower OD gear like an .86.
Since I plan to get a tach, where should my RPMs be at 65mph? Seems like it screams on the highway.

Martin

assuming those are 195/70-14 tires, your RPM's in 4th gear (1:1 ratio) should be just shy of 3000 at 65. Not bad, but yeah it'll be loud.

Personally, I think your carb tuning is probably too rich at the load/speed combo you are cruising at in OD. 3G Camaros came stock with 2.73's and a TH700-4R, giving a final drive of less than 2! You're talking 70mph at under 2000 rpm there.
Now, accounting for the Nova body being less aerodynamic, and the smaller engine, your current combo (assuming a 0.73 OD) still puts you at about 2175 rpm at 65, which shouldn't be too much for the engine.

What I expect is happening is that you have to have a fairly high throttle angle to cruise that way, and that is dropping your vacuum down low enough to trigger the carbs power circuit. Some carb retuning with a stiffer power piston spring ought to get that mileage back, but you'd really want to have an A/F gauge to feel safe.
Another thought: is the vacuum advance on ported or full manifold? If you're not getting vcuum advance at cruising speed, that would cut your mileage down.

i second what blue eyes said. what is the engine built like?

It's a stock 194 that I drive everyday.

I'll bet you can get some mileage by leaning out the power circuit a bit. But, before you go leaning out the mixture like that, get an A/F gauge so you know where you are and don't fry a piston!

Where do you have your vacuum advance hooked to - ported, or full manifold?

Are you sure the speedo is reading correctly after changing out the tranny?

Gearhead makes a good point. A speedo that only shows 7 miles travelled over 10 true miles will give you a 30% error in gas mileage when you calculate it. Easy enough to figure out. Find a spot on a highway that has those small little signs with mile and 1/10 miles numbers on them. Actual road speed doesn't matter here. Catch the odometer reading at one sign and when you pass the next sign that reads one exact mile difference then glance at your odometer. Probably won't be exact but within 1/2 a number as it rolls into view will tell you the speedo gear is close enough. Anything over a 1/2 number can be corrected by a speedo gear change at the tranny end of the cable.

Of course, was the speedo accurate before? Do you do a routine commute over the same path that you know the distance of from before the trans swap?

Ok to answer your questions: The vacuum advance is connected to a port near the bottom of the carb. Not the manifold. I'm pretty sure the speedo is a little off. I think it turns a little slower now. I'll check those mile markers next time I see them.

Sorry, by full manifold vacuum I meant full-time vacuum, as opposed to ported vacuum. At idle, is the vacuum cannister recieving vacuum?

I have fuull time vacuum. I played with the car a little bit today. I made sure the car wasn't running too rich, turned the advance way up and fixed some minor driving issues. The car runs a little quieter now so we'll see if my tweeks make a difference.

Martin

Well, I can honestly say my tweaks have made a noticeable difference. The car doesn't have to work as hard to keep up with traffic anymore and so far I've gained a day between tank fils.

Martin

Update: I put a tach in the car and here some numbers:
4th gear at 65mph RPMs are 3300
5th gear at 65mph RPMs are 2250

Gas mileage is stil hovering around 12mpg in the city and that's driving conservatively.

What level of accuracy are you using for yout testing?

I know a lot of people simply don't go to any trouble, but they work out their figures as if they're facts.

You need to check your odometer over a decent distance, 10 miles is good. You need to fill the tank right to the top and record the mileage to the tenth.

And then, if you have mile pegs on the highway, sit on a steady 60mph by the speedometer and time the mile with a stopwatch. Divide the number of seconds into 3600 and that will give you your exact speed.