[Bronze66]

After reading another post that stated for max performance and gas mileage 52 degrees w/vac advance is best. I believe there is a correlation with higher vac advance and gas mileage. Am I thinking right? Any of you people getting good gas mileage, 15+ hwy, with 389/400 motors? If so what is your vacuum advance set at? In order to get around 52 at cruise then your idle will be around 32. Does that sound about right? I thought that was a tad to much. I have #96 heads with 9.2cr. I've read where the larger chamber heads liked less advance. I'm trying to up the gas mileage on my car and was wondering if having this much advance would help mpg or just generate more heat with that much advance. Before I go experimenting I thought I'd get some other opinions or actual facts. If there is no pinging how do you tell if you have to much advance to hurt?

[amcmike]

I think I am at about 50 at cruise. 36 mechanical at 2600rpm plus about 14 from the can (adjustable).

What you have at idle depends on your mechanical curve and whether you are hooked to manifold or ported vacuum.

[Z Code 400]

Bronze,

I drive my 1975 Formula 350 108 miles a day (round trip) to work and I found that it liked 44° total timing the best. Other than 1-3/4" 4 Tube Headers and 2.50" Exhaust, its a stock engine with a bothersome (1975 #7045568) Q-Jet for the time being.

The timing curve is 18° BTDC @ idle and it has 36° total (mechanical) with full advance taking place around 2800 rpm. I added 8° of ported vacuum advance.

The rear axle is a 2.73: or a 2.76:1 and I get around 16 to 17 mpg on the average with a best ever of 18 mpg a few months ago. Recently, that's driving in 90° to 100° heat too.

The last 9.00:1 SCR 400 I set up had a 220°/226° - 111.5° LSA hydraulic flat tappet and milled 6X-4 heads. Keep in mind, the big combustion chambers will tolerate more timing than the early, small chamber heads.

I set that engine up with 15° BTDC @ idle, 36° total and 8° of ported vacuum advance. It really seemed to run nicely at that setting and had no tendency to ping on available pump fuel. When I tried to push the vacuum advance up around 50° BTDC, I picked up a slight surge, so I backed it down to 44° total and locked it down.

Keep in mind, that with an HEI, changing the timing curve (unfortunately) means brazing (I prefer silver-solder) up the advance slot to limit the amount of total advance and strike a balance between timing at idle and total timing. It can be a real challenge without access to a distribitor machine.

I would suggest referring to Jim Hand's book on max Performance Pontiac V8's for additional information on this procedure.

I think Cliff posted once about a certain 1/8" or 3/16" screw equating to a certain number of degrees of vacuum advance, but I cannot recall for sure. Perhaps someone else can chime in on this point.

Also, keep in mind that different LSA's, intake closing points, engine temperatures and varying fuel quality will all have an impact on how much timing you should be running.

My 1975 350 should probably be getting more than 16-17 mpg, but I am convinced, the 1-3/4" Headers and 2.50" Exhaust (installed for the future 412 CID engine) are hurting me. The system is just overkill on a mild 350.

I realize the stock 350 settings may not be all that helpful to you, but as illustrated, they are very close to the timing curve I am using in 8.75:1 to 9.00:1 406's and 412's that I am tuning for people.

Hope this helps...Robert

[Motor Daddy]

Depends on all the typical variables (bore, stroke, compression, cam, A/F ratio, intake, exhaust, cylinder heads, air quality, mechanical curve, total gear ratio, load, etc) as to how much advance your motor will like. In my 400, I've found that a cruise A/F ratio of 13.5, with 35 total, plus 14 degrees vacuum advance is the sweet spot. If I lean it out anymore, the engine will have a slight lean misfire problem. I limited my vacuum can that normally has 16, down to 14 due to a slight surging problem under a few specific conditions, and the problem stopped. That puts me at 49 degrees of advance (total+vacuum), while cruising with 19" of vacuum. I recently did 149 miles on 9.9 gallons of gas (15.05 MPG), which consisted of 67 miles highway at 75-80 MPH, and the rest on secondary roads, stop and go traffic, and a few exhibitions of speed off the light (WOT bursts). The engine runs flawlessly at all RPM and throttle positions. I do have to admit that I have spent an extensive amount of time tuning the carb and distributor to arrive at this point, but it's time well spent.

BTW, That is with a Torker II, a 240/248@.050" Crower 60310 solid cam, and a Holley 750 DP with a Proform main body.

[TransAm525]

I have a stock (internally) 400/406. One a recent 1900 mile trip from Maryland to Texas I pulled 16 mpg at 70 mph (and that's at 3000 rpm with a TH400 transmission and the carb on the rich side). I recently spent two weeks of continuously adjusting the carb a little further and switched to a manifold vacuum source which seemed to add another 1-2 mpg and increased driveability. I'm running 13 initial, 22 mechanical, and 14 vacuum (49 total). I'm going to add another 2 degrees of vacuum advance (adjustable canister) and maybe 1 more degree of initial timing to see if I can pull a little more mileage out without pining (8.1:1 compression).

[66GTO Jim]

Here's the link to some great info I posted today on another post for vacuum advance. Should be required reading for everyone who doesn't already know this stuff. Author is Lars Grimsrud. Jim

http://forums.maxperformanceinc.com/...d.php?t=524623

[Bronze66]

Jim your post was the one I was referring to. I just upped my vac timing yesterday by 2. I have 15 initial w/12 vac =27 at idle and 36 total w12 vac = 48. Motor seems to like it better. I'll see if mileage improves any before I bump it more.
MD I would consider 13.5 way rich for cruise. I'm thinking 14.5-15 A/F. What gears and trans do you have?

[Motor Daddy]

Originally Posted by Bronze66

Jim your post was the one I was referring to. I just upped my vac timing yesterday by 2. I have 15 initial w/12 vac =27 at idle and 36 total w12 vac = 48. Motor seems to like it better. I'll see if mileage improves any before I bump it more.
MD I would consider 13.5 way rich for cruise. I'm thinking 14.5-15 A/F. What gears and trans do you have?

It's not rich, it's perfect. I have done extensive testing with a wide band O2 A/F ratio meter (in dash). I also have a in dash vacuum gauge. I can easily get it to cruise at 14.5-15.5, as I had it that way for quite a while. I tried everything to get rid of the slight surging, lean misfire, and couldn't. I gradually richened it up by .5 A/F ratio increments, and retested. It didn't run absolutely perfect until I got to 13.5.

It's not all about the A/F ratio, and being stuck on a specific number, as I had to learn for myself, it's about giving the engine what it wants, and my motor absolutely loves to cruise at 13.5. Any leaner and it lets me know!

Here is my complete combo:

79 Firebird, 3860 lb race weight
400 +.060"
Stock crank turned .020"/.020"
Stock rods
Cast pistons
#62 heads, intakes ported to 239 CFM @ .600"
9.33:1 compression ratio
Crower 60310 solid lifter cam, 240/248 @ .050", .477"/.501" lift, 112 LSA, 109 ICL
Crower solid lifters with coolface option
Comp 995 springs, 1.7" installed height
1.65 Scorpion roller rockers, .022" intake / .024 exhaust lash (hot)
Torker II, 1/4" heat insulator, 1/2" nitrous plate
Holley 750 DP (4 corner idle), with Proform main body.
Orange pump cams on both primary and secondary
31 primary shooter, 35 secondary shooter
69 primary IAB, 45 secondary IAB
36 primary HSB, 36 secondary HSB
Keep the secondary throttle blades just barely cracked, not exposing any transfer slot
.033" IFR's
Primary idle screws 5/8 turn from lightly seated
Secondary idle screws 5/8 turn from lightly seated
73 primary jet, 80 secondary jet
6.5 power valve in the primary only, no power valve in the secondary.
float level approximately 1/16" below sight plug
Mallory 140 fuel pump
6 PSI fuel pressure
AC R45S plugs gapped to .045"
HEI, MSD Digital 6 plus box
14 initial 35 total
VC1862 (NAPA part#) vacuum advance, starts at 2-4", has 8 DISTRIBUTOR degrees (16 at the

crank) @ 6-8", hooked to a manifold source and limited to 14 degrees.
Use a PCV valve.
1 5/8" Dynomax headers
3"x 2 1/2" Dr. Gas X pipe
Dual Dynomax 2 1/2" turbo mufflers
Flowmaster 2 1/2" tailpipes
Centerforce DF clutch
Tremec TKO 5 speed trans
8.5" posi rear with 3.42 gear
26.5" street radials


12.84@110.68 (no nitrous)
Engine idles at 900 RPM, and pulls 12" of vacuum

[Cliff R]

We have found thru many years of testing, that most well prepared and thought out engines will like about 42-48 degrees timing at cruise. Really "low" compression engines may like a bit more, in contrast high compression engines with small cams typically like a bit less. My own engine wants right at 40 degrees. We use 10 degree initial timing, 30 degrees total and about 10 degrees from the vacuum unit. I've tried both ported and manifold vacuum as the source for the advance, no measurable difference noted in fuel economy either way. I simply leave it on a ported source, as the engine is more stable in and out of gear, dropping barely 50 rpm when placed in gear. When hooked to a manifold source it drops off closer to 150 rpms, but idles equally as well either way.

We agree that the engine will let you know when you have gone too far with total timing, and too lean with the cruise A/F ratio(s). Every combination of parts is slightly different, and it may take considerable testing to determine the exact A/F and how much timing to add from the vacuum unit.

The absolute best set-up we have found to quickly cut to the chase, is to use a late model q-jet with an APT (Adjustable Part Throttle) system, and an adjustable vacuum advance unit. We have customers using q-jets with APT and adj vacuum units acheiving near 20 mpg's with 455 engines. Our own 455 manages right at 15mpg's, with 3.42 gears, TH400 and 28" tall rear tires.

Back when we were doing fuel economy testing, we ran the same loop, filling at the same points. No problem at all getting 200-230 miles from 14 to about 14.4 gallons of fuel. The BIG problem was driving that distance without a few full throttle blasts. I simply don't have enough control these days with 514hp and near 600ft lbs torque lurking under the hood!....Cliff

[66GTO Jim]

Hey George (Bronze66),

Still working out what total timing and vacuum advance to run. Currently running 10 initial + 24 mechanical = 34 total with 16 in the vacuum can for 50 overall. Not sure where I'll end up but I have not had a lot of time to work on it this summer. Running the 389 + .030 with a +.500 stroker crank (447 Cubic Inches) with Tri-Power, 700R4 Trans and 3.23 rear gear. Street only.

Lot's of tuning to do still as I just got the Tri-Power where I want it. When I get the timing finalized, I'll update this post.

Thanks to Motor Daddy and Cliff R for the detailed "real world" Pontiac info! Most stuff we read is Chebby oriented and can only be used a starting point for our torquey Pontiacs. Jim

[Cliff R]

Thanks Jim. The numbers we came up with took decades of testing, on a variety of engine/drivetrain/vehicle combinations.

We always let the engine be the guide, it will tell you want it wants (and doesn't want). Tuning is always a total package, carb, distributor, cam timing, compression ratio, etc. Cylinder head efficiency another key player, as is the quench distance. The more efficient the engine, the LESS timing is required to get a clean burn at the proper time, less fuel required as well.

We continue to hear a lot of terms, such as rich/lean, used in conjunction with "efficient" or "efficiency". Keep in mind that "lean" mixtures are incredibly difficult to burn, requiring more heat, earlier ignition, etc. Experimenting with ignition timing alone may NOT yield the desired results, in almost all cases we've had to fine tune the fuel curve(s) in conjunction with timing changing, for the best overall results. The correct mixture(s) will always be the most efficient. When the A/F is on par for the engine speed/load, less throttle position is required to sustain the vehicle at speed, and typically less fuel is consumed, all else being equal......Cliff

[Half-Inch Stud]

Think I'm getting the 18MPG now while COMMUTING to work.
No Vac Advance//470/9:1/Q-Jet/Sw-Pi TH400/3.42:1/27.5"tire.

And I'm CERTAIN the 68GTO MPG goes down to ~12//14MPG for Highway because of the combinerd reasons of not using the Vac ADV & reving the engine too much.

Seems I Must re-re-re-re-re-consider the Vac-ADV, now that the Q-Jet is dialed-in. I'm wishing there was a solenoid-active VAC-leak to kill the VAC feed to the HEI for PROMPT VAC- ADV KILL.

ALSO, thinking to re-visit the Direct-Pack pressure Switch in the TH400, but how to get that signal out with a Switch Pitch & Kick-down already wired up?

[Hawk397]

I have the luxury of running computer controlled fuel injection and timing on my 71 with a wide-band kit. Her "sweet spot" at cruise is AFR of 14.2 with timing at 44. I was able to test AFR in increments of .1 and timing increments of 1 degree at a time.

This is a 60 over 400 with 9.5:1 compression and a moderatley aggressive roller cam and ported/polished 6x-4 heads with tri-y headers.

Mark

[Cliff R]

"I have the luxury of running computer controlled fuel injection and timing on my 71 with a wide-band kit. Her "sweet spot" at cruise is AFR of 14.2 with timing at 44. I was able to test AFR in increments of .1 and timing increments of 1 degree at a time."

Gotta LOVE modern technology. It took me several years to figure out how to do the same thing by changing the amount of timing added by the vacuum advance and adjusting the APT in the q-jet!

Going back over 20 years, I had a GMC 3/4 ton truck with 4.10 gears, 350 engine. I used to frequently make 600 mile trips from Ohio to Virginia. Armed with an adjustable vacuum advance unit, and q-jet with an APT system, I tried every possible amount of total timing, and A/F setting. That particular engine also wanted 44 degrees at cruise, and the APT set for a very slight 50 rpm increase if we "tipped" in the choke flap with the carbs fast idle cam set at 2000rpm's. No LM1's available at that time, but the AFR was probably in the 14-15 range.

Interesting to me at the time, was that trying to run a lean mixture, resulted in a very slight increase in fuel economy for driving around locally. However, when I hit the road for the 600 mile run, the truck consumed MORE fuel. The increased load placed on the engine for driving sustained high speeds, climbing steep grades, etc, required more throttle opening, and used more fuel.

I didn't give up, played around with manifold and ported vacuum to the advance, and dozens of different spring weight combinations in the distributor. Learning all the time. The final settings I ended up with for that engine was 10 degrees initial timing, 22 degrees from the mechanical advance, and 12 degrees from the vacuum advance. It wanted all the mechanical advance in by 2800rpm's. The carb (early Chevy with APT), ended up with 73 jets and 44 rods.

The fuel economy was decent for the application, typcally between 13.5 to as high as 15.5. Never could hit 16mpg's, I wished the 700-R4's had been around at the time.

A few years later I restored a 67 Impala SS, an replaced the PG with a 4L60 overdrive transmission. Mileage went from 13-17 to 18-25. Did tons of testing with that vehicle as well, but that's another story.....Cliff

[Bronze66]

That's where I'm at right now. Trying to figure out what timing and A/F the motor likes. I have #96 heads and thought they didn't need/want a lot of timing. My thinking is changing. I just upped the vac advance to 12 degrees. I did not readjust the timing for the added 2 degrees it adds to the mechanical, Crane vac can. It now has 15 initial, 22 mechanical, all in by 26/2800. The motor woke up it seems. Very very light throttle from start the car moves effortlessly now. Seems to be getting better gas mileage. Can't really calculate accurately because speedo is off 5mph. Which should be fixed soon. I'm going to a dyno to get A/F set. Not really sure what ratios and timing will give me best mpg and not really hurt power. It's a Tri-power set with a 1 5/16" venturi 2bbl from a 72 455 I modified to work. I just jetted down to .063 prior to timing change. My cruise at 70mph should be 22/2300 rpms in 4th and around town 3rd gear is 20/2200rpms. That's where I have to tune for. I'm thinking 14.5- 15 as an A/F for cruise and 12.5-13 for wot. Don't have much experience with this yet.

[Rocky Rotella]

MD, did your manifold vacuum reading at idle increase, decrease, or remain the same when you richened to 13.5 from 14.5?

[Motor Daddy]

Originally Posted by Rocky Rotella

MD, did your manifold vacuum reading at idle increase, decrease, or remain the same when you richened to 13.5 from 14.5?

It dropped about .5". It actually has the best vacuum at approximately 14.0, but when the car gets up to normal operating temp on a hot day, it idles just a tad leaner (14.3), and the idle quality suffers, the vacuum drops to 10", and the idle RPM drops about 100 RPM. I find my carb very sensitive to air quality and carb temp. I pulled my hair out for a while getting the perfect compromise. Shoot, I would tune it perfect on a 60 degree day and go to sleep thinking I finally got it perfect, and then the next day I wake up and the temps would be in the 80's-90's, and the car would run lean! I actually lost sleep over this crap, staying awake dreaming up different approaches. LOL I finally found the ultimate compromise, and the car runs flawless in all temps. I almost regret buying that stupid wide band meter, ALMOST!

[amcmike]

I was thinking on hotter days a carb would run richer. The air would be less dense, and therefore with the same fuel the mixture gets richer. So what is really going on?

Is it that it produces a weaker signal for pulling fuel, and therefore leans it out? Or perhaps his engine wants even more timing to get a good burn and therefore doesn't idle as well.

[Motor Daddy]

Originally Posted by amcmike

I was thinking on hotter days a carb would run richer. The air would be less dense, and therefore with the same fuel the mixture gets richer. So what is really going on?

Is it that it produces a weaker signal for pulling fuel, and therefore leans it out? Or perhaps his engine wants even more timing to get a good burn and therefore doesn't idle as well.

I'm not 100% sure what's going on, but I can tell you that my engine runs leaner when it gets hotter for a fact. Maybe it has to do with the intake temp. I noticed that when I start it first thing in the morning (no choke) on a 50-70 degree day, the engine runs a little lean for the first minute or so. Then the engine starts to warm up a little and it gradually gets richer. As the engine gets to normal operating temperature, the A/F ratio gets slightly leaner and doesn't really deviate to much at idle and cruise. After driving for a few miles, it gets just a tad more lean, and cruises and idles where I normally had it set. If it's 85-90 degrees outside, it idles and runs even leaner (possibly cause by hotter intake temps due to hotter air temps?? Maybe because the hotter atmospheric air is less dense, the cylinder pressure is lower on the compression stroke, which results in a slight loss of power at idle, which lowers the RPM by about 100 RPM. That would cause a lower "draw" on the carb (less vacuum), meaning less fuel from the idle and transfer circuits. Just guessing, but I can tell you for sure, my engine runs leaner when it gets hotter. If it's real hot outside, and I drive for about an hour in traffic and such, if I shut the car off, go into a store and return 10-15 minutes later, when I start the car, the initial A/F ratio will be extremely lean for a few seconds. After the engine is running for about 30 seconds, the A/F ratio will slowly return to normal. Is this caused by the intake and carb getting hotter when I shut it off and return a few minutes later? I'm leaning towards that theory, just like coolant temps will climb when you shut it off for a few minutes, so will the intake and carb due to heat soak.

[amcmike]

Originally Posted by motor-daddy

Maybe because the hotter atmospheric air is less dense, the cylinder pressure is lower on the compression stroke, which results in a slight loss of power at idle, which lowers the RPM by about 100 RPM. That would cause a lower "draw" on the carb (less vacuum), meaning less fuel from the idle and transfer circuits. Just guessing, but I can tell you for sure, my engine runs leaner when it gets hotter. If it's real hot outside, and I drive for about an hour in traffic and such, if I shut the car off, go into a store and return 10-15 minutes later, when I start the car, the initial A/F ratio will be extremely lean for a few seconds. After the engine is running for about 30 seconds, the A/F ratio will slowly return to normal. Is this caused by the intake and carb getting hotter when I shut it off and return a few minutes later? I'm leaning towards that theory, just like coolant temps will climb when you shut it off for a few minutes, so will the intake and carb due to heat soak.

That's another possbility I was thinking. Anyone else with a wideband (and a carb) experience this?