Hodge

Firss off, I have an A/F gauge in my car running off an O2 sensor in my headers. It's not a wideband so I know it's only accurate around 14.7

When I dyno tuned my car using a wideband O2 I found it to make best power when it ran rich (around 12.0) so that is the jetting I left in the carb. Tuning it to 13.0 made less power.

At part throttle cruise my gauge always shows a little rich(as I would expect it to) and during highway cruising it always shows a little rich too.

I'm getting ready to take the car on a 700 mile trip and I was wondering if it would be safe to jet the primaries down a tad since I'm already running rich and it would just bring me closer to stoich?

Would dropping from 72 in front to 70 or 68 save me much in terms of getting better gas mileage?

Thanks
Hodge

F-BIRD'88

In a word....yes.

best power.. 12.5:1+/- best emissions with catalytic converter and computer engine management 14.7:1

best hiway mileage efficency.. about 15 to 17:1 afr.

Wether your motor will surge if you jet this lean depends on how even the afr is cylinder to cylinder.
The lean(est) cylinder may missfire or ping if its infact too lean. You have to tune for the leanest cylinder
or stagger jet the carb to get it even.

The motor will also want more ignition timing at cruise when set lean like this. (vacuum advance at hiway speed)

A rough way to find this point is to try smaller primary jets until the motor surges while cruising on the hiway.
then go up 1 to 2 sizes. On a dual plane manifold this may require staggered main jetting to tune it to a finer edge.
Be real sure the motor is completely warmed up before you evaluate the tune. The leaner the jetting, the more critical the engines operating temp.
Once you've found the best lean primary jetting, and seen a actual mileage improvment. You could restore the best power jetting that you got @ WOT with the bigger jets on the dyno by enlarging the primary power valve channel restrictions in the metering block to compensate for the *jet area* lost to the leaner/ smaller main jets.
This requires carefull jet area calculation and precision numbered drill bits.

As you lean-out the primary jetting to achieve a more efficient cruise Afr (leaner than 14.7:1) you may have to use the choke for acceptable cold engine operation till the motor warms up. You may also need a larger accelerator pump shot for good drivability.
May also want to tweek the powervalve opening point.
the lean(er AFr for best mileage will cause the throttle responce and drivability (especialy when cold) to suffer without careful tuning to compenate.
Exhaust emissions will generally be (a little) higher too.
remember a O2 sensor inthe exhaust pipe only reads a average AFR of all the cylinders. you would need a O2 sensor and/or EGT sensor in each cylinders exhaust to find the leanest cylinder and get a really sharp tune.

Your limitiations are going to be lean surge and ping @
cruise.
Let us know how lean you are able to jet the carb.
I'd recomend going "way too lean" and work up (richer in jetting) start with a #63 main jet ( it will most likely stumble with this jet) and work up till the motor will run at speed. You'll probabily end up around a 66 to 68 jet.
Do not operate the motor @WOT while doing this as a lean AFR at WOT will damage the motor in a hurry.
start with new spark plugs.
What rpm does the tach indicate when you are cruiseing @hi-way speed? (in 5th gear)

Hodge

Thanks for the great reply. First to answer a few questions.

75-80 MPH is between 2300-2500 RPMs

I have a Holley S/A 770 carb so I have an electronic choke and vaccuum secondaries. Also, I run an RPM airgap manifold (dual plane)

I have a vaccuum advance distributor. I'm not sure where full advance comes in but I set full mechanical advance to 36* with the motor at about 3000-3200 RPM's. The Vortec heads I've heard, like a few degrees less total timing than a conventional head due to the fast-burn chamber design.


If I bring the primary jets down a few sizes for the trip should I increase the secondary jets a few sizes to avoid going lean during heavy acceleration?

I'll just be going back to the best power jetting (72,72) once I get where I'm going. I'm just trying to get a few miles to the gallon improvement while on the highway. I'll be paying $2.30/gallon for premium by the time I get to NY so a few extra MPG's would be nice.

Hodge

F-BIRD'88

yes you can bump up the secondary jetting to compensate. but the front of the motor may still be lean.
best to enlargen the primary power valve channel restrictions. Watch for pinging and overheated sparkplugs when tuning.
At 10.5:1 you'll be right on the edge detonation wise
watch you don't go too lean. Not worth trying to squeeze the last bit of MPG out of it just to burn a hole in a piston or crack a ring land. Make sure you test the motor HOT!!!! takes a good 15minutes hiway driving to get it HOT.
I bet you'll end up with a 68 69-70 primary jet and 80-82sec jetting. or real close to this.
Getting the vacuum advance right is critical.
If you cruise at 75-80MPH all the way there you're going to use some gas and get some tickets too.
last time I checked, New York state's freeways were 55MPH. And the troopers don't have that much of a sence of humour. Good Luck

Hodge

Well I'm going to Western NY. I'll be on the NYS thruway(I-90) but only after taking 71 from Louisville all the way to Cleveland. The entire route is 65MPH with the exception of going through cities.

I've made the trip several times (origianally from Rochester, NY) but this will be the first time since the 305 TPI has been replaced with this larger carbed motor. I pulled 18MPG on a 3 hour trip a few weeks ago without mesing with the jets but this will be a 10 hour drive and a few extra MPG's will go a long way.

On the dyno I had 68's in front and 69's in the rear on one run and I still had an A/F ratio of 12.8 - 13.2 at WOT


Will switching the front jetting from 72 down to say 70 or 69 make enough of a difference over a 650 mile trip for me to worry about it?

Thanks again for the replies.

F-BIRD'88

yes..... Sounds reasonable to me. I use 70's in my 750 Holley. Seems to work well overall.