Picking out new rear gears. Poll!

[TheSilverBuick]

In my unending quest for 30mpg I'm looking at changing out the rear gears. I seem to be stuck at 17mpg (still a few more long distance tests to do though) and am looking at dropping the cruising rpm some. So here are some specifics.

The tires are 26.1" tall (235/60R15), the trans is a TKO-600. 1st:2.87, 2nd:1.89, 3rd:1.28, 4th: 1, 5th: .64 I really have no idea what this car should mph in the 1/4 with traction and not drying out the carb, so I arbitrarily/optimistically threw a 115mph value on the list there to give you guys a ballpark idea of what rpm it would be going through the 1/4 mile traps at, adjust for your judgement for what you think my combination is probably going to do on your own experiences.

2.73 Gears
60mph...........1350rpm
70mph...........1600rpm
80mph...........1800rpm
115mph in 4th.........4000rpm
5500rpm in 4th........156mph

3.08 Gears
60mph...........1500rpm
70mph...........1800rpm
80mph...........2000rpm
115mph in 4th.........4550rpm
5500rpm in 4th........139mph

3.23 Gears
60mph...........1600rpm
70mph...........1850rpm
80mph...........2100rpm
115mph in 4th.........4800rpm
5500rpm in 4th........132mph

3.42 Gears
60mph...........1700rpm
70mph...........2000rpm
80mph...........2250rpm
115mph in 4th.........5000rpm
5500rpm in 4th........125mph

3.70 Gears (currently in there)
60mph...........1850rpm
70mph...........2150rpm
80mph...........2450rpm
115mph in 4th.........5500rpm
5500rpm in 4th........115mph

Pending the fuel mileage results I may change the cam to a smaller cam to see improvement. If I end up swapping the cam to get the mpg gains I want to see I'd be willing to switch back to the 3.70 gears as a test to see how critical the rpm's are with that cam.

A little more background, running my Centurion engine in there (~330hp) with a .454 lift cam and low duration and the 3.23 gears (7.5" rear) the car got 23mpg consistantly at 75-80mph. And I am leaning towards getting a set of 3.23's, but the 3.08's are kind of looking appealing too for mpg's.

Ladies and Gentlemen, and Chad, VOTE! ;D

[Scott Liggett]

Re: Picking out new rear gears. Poll!

What do you want? mpg, or go fast? That's the question you must answer.

I think you could still get good mpg with a lumpy cam who likes running at a higher rpm. My car's mpg has consistently improved with every larger cam, head and carb change.

[TheSilverBuick]

Re: Picking out new rear gears. Poll!

MPG for now, that is goal numero uno. The goal has been N/A 30mpg and 400Hp. My t-bird with bad rings get's 17mpg with no over drive and probably weighs 1,000lbs more, it has 3.00 gears.

[stage1scott]

Re: Picking out new rear gears. Poll!

hey Randall, have I ever told you about my dads 1976 Electra limited 4 door? that thing weighed about a billion pounds, hauled him, mom and my aunt an uncle all over the west coast with 120k on it. yosemite, tahoe, san fran, the works. trip average??16.1!! low comp, stock cam, QUADRAJET!! maybe you should try a QJ and adding about 2 thousand pounds! Of course, those old folks rarely went over 55 and never ever floored it!!

[squirrel]

Re: Picking out new rear gears. Poll!

what are the cam specs again for those of us with short memory?

and does the car weigh about 3600 lbs?

also rpm in 3rd and 4th at 100 mph would be helpful for figuring out the racing thing

[TheSilverBuick]

Re: Picking out new rear gears. Poll!

;D My Centurion gets about 15mpg with a tailwind with the points and Q-jet, at 75+mph though. What has shocked me is I've yet to best the Q-jet for highway mileage (Q-jet was on the engine for an 8,000 break in :P). The EFI is killing the Q-jet around town though. I'm almost tempted to do the cam first, but I'm having a hard time believing it's that inefficient....

I'm not so concerned about the racing thing, it'll do what it'll do. Maybe I'll add it tomorrow. *I'm Guessing the car weighs 3,800lbs, nothing I own seems to be light :P T-bird was 4,800lbs at Fontana*

VALVE LIFT

STOCK 1.55 RATIO:
IN: .478?
EXH: .475?

AT .050
IN: 228
EXH: 247
ADVERTISED
IN: 276
EXH: 295
LOBE CENTER: 118

[squirrel]

Re: Picking out new rear gears. Poll!

that's a big honkin cam to try to get mileage out of. Give up and just try to go fast...put some 4.11s in it!

[stage1scott]

Re: Picking out new rear gears. Poll!

who was it said "Fuel injection will never work better than a well tuned carburetor"

Smokey Yunick, of course

[TheSilverBuick]

Re: Picking out new rear gears. Poll!

Jim, you should hear how the EFI has tamed the idle on that cam. The exhaust is a ton quieter and there is pretty much no chop to the idle at 800rpm.

[Scott Liggett]

Re: Picking out new rear gears. Poll!

Not sure I understand such a big difference in duration from intake to exhaust. Are the exhaust ports that ineffecient?

[squirrel]

Re: Picking out new rear gears. Poll!

Interesting Randal...I guess a big part of the problem with idle quality with a cam and a carb is getting the carb to work right! so efi fixed that. Still doesn't make the cam work well (efficiently) at low rpm apparently....

How much are hydraulic rollers for them buicks? can you get one? might help out to give you more "area under the curve" while keeping overlap down.

[TheSilverBuick]

Re: Picking out new rear gears. Poll!

I don't say I understand the reasoning, it was designed a long while back. The appeal to me in picking it was it's closeness to being like OEM cams in LCA and almost no overlap (half degree of overlap). Which I hoped would play well with the EFI, it does have vacuum though :P

All the roller stuff for Buick's are $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$. I'll stick to my low lift, low spring pressure cams for longevity.

[squirrel]

Re: Picking out new rear gears. Poll!

Ok, I was just thinking about how GM gets so much mileage and performance from the newer V8s (starting early 90s?) and I think having a hydraulic roller is a significant part of it. maintains low rpm efficiency and still lets it make some power higher up in the rpm band.

[Scott Liggett]

Re: Picking out new rear gears. Poll!

I forget where you got this cam. Also, what was the static compression again?

I'm a fan of Comp, but I don't like what I see for them. Here's a Lunati that I like. Wider lobe separation, tiny bit more dur on the intake, less on the exhaust. More lift. Should be a torque monster and still have good vacuum.

http://www.summitracing.com/parts/LUN-33304/

Their Voodoo cams look interesting. Your 455 with it's good heads and decent compression produces a ton of torque all by it's lonesome; so bleeding some off at the low rpm's may pay off big.

http://www.lunatipower.com/ProductGroup.aspx?id=287&cid=52

choking off a big engine to try to get higher mpg seems to me would cause just the opposite.

[TheSilverBuick]

Re: Picking out new rear gears. Poll!

Static compression if all my measurements added up was 9.8:1, could be as low I suppose as 9.6:1. I did get it to ping in Fontana (the air is too thin here, and I've never tried loading the engine up enough with high advance).

I got the cam from T/A Performance. I don't really trust Buick grinds from other manufactures to be anything other than Chevy profile grinds on a Buick core. Different bore, stroke, rod length, etc. I'll go with the people who specialize in Buicks.

At cruising the throttle blades choke the engine a ton more than the cam can, it's the trade off between efficient low rpm flow and efficient high rpm flow. Admittedly this cam is designed for higher rpms. A smaller cam should do better at lower rpms. I may consider seeing if I can advance/retard the cam (which ever one lowers the power band rpm) to see if it suits my application better as well.

The CC article's small T/A cam with my intake (albeit wet flow) made 500ftlbs and 443Hp on the engine dyno with electric water pump and long tube headers. My car does have nearly a point more compression than their engine.