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Old 08-12-2007, 12:11 PM
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Tuning SBC for fuel economy questions?

I am fine tuning my SBC (383) for fuel economy here are the stats:

Base timing 10 deg with no Vac
All in timing 34 deg @ 3200 with no Vac
Base timing 20 deg with Vac
All in timing 44 deg 3200 with vac
15" vac at idol
13-14" vac at 2200 RPM (65 MPH)

Plugs are dark brown a Little darker than should be but Car runs very nice response is good

I am confused about the vacuum not dropping out guess I don't know all I thought I did! It seems like I need more open butterfly on the carbs but still a little confused on how to get that with out a lean condition also not sure if I need to re curve the dist to bring the all in RPM down from 3200. The car is a rod not a racer do more cruising and the occasional burn out but very occasionaly just to prove I can still do it Any positive thoughts would be appreciated I am out in the hot sun trying to figure it out?

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Old 08-12-2007, 12:41 PM
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For fuel economy you could try a little more advance. My initial is at 16.
I curved mine to start advancing at about 1000 and all in at approx. 2700.
Total w/vac for me is 16+24+8=48 total at cruise.

Some here have said it could/should be advanced a little more for best economy, but my vac can is set to a max of 8. I'm pretty happy with the fuel economy. My problem is altitude and jetting. I run a little on the rich side sometimes but I think I've found the middle ground.

Try kickin it up and see what happens, you can always go back.
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Old 08-12-2007, 01:07 PM
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Connect the vacuum advance to a full time vacuum source. Every chevy I ever worked on had better throttle response, better mileage and ran cooler with it connected to a manifold vacuum source. Every time it was connected to a ported source, it was real sluggish, ran hotter and the mileage would go down. Part throttle mixtures are leaner so they burn slower, this is why the additional advance is needed at idle and off idle. I asked the same question many have asked because I didn`t believe it either, I would say "why do you need vacuum advance at idle?" But I also wondered why on GM catalysts it would say set the timing with the vacuum advance disconnected and plugged. Then I couldn`t stand it anymore that my car was a complete sluggish turd, it wouldn`t burn a tire to save it`s life. BOBCRMAN of this site that`s highly respected for his knowledge, told me to connect it to a manifold source, I did and it made a huge difference.
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Old 08-12-2007, 01:32 PM
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Quote:
Originally Posted by DoubleVision
Connect the vacuum advance to a full time vacuum source. Every chevy I ever worked on had better throttle response, better mileage and ran cooler with it connected to a manifold vacuum source. Every time it was connected to a ported source, it was real sluggish, ran hotter and the mileage would go down. Part throttle mixtures are leaner so they burn slower, this is why the additional advance is needed at idle and off idle. I asked the same question many have asked because I didn`t believe it either, I would say "why do you need vacuum advance at idle?" But I also wondered why on GM catalysts it would say set the timing with the vacuum advance disconnected and plugged. Then I couldn`t stand it anymore that my car was a complete sluggish turd, it wouldn`t burn a tire to save it`s life. BOBCRMAN of this site that`s highly respected for his knowledge, told me to connect it to a manifold source, I did and it made a huge difference.
Yes I have it hooked to manifold vac
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Old 08-13-2007, 08:10 AM
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Quote:
Originally Posted by Not A T 25
I am fine tuning my SBC (383) for fuel economy here are the stats:

Base timing 10 deg with no Vac
All in timing 34 deg @ 3200 with no Vac
Base timing 20 deg with Vac
All in timing 44 deg 3200 with vac
15" vac at idol
13-14" vac at 2200 RPM (65 MPH)

Plugs are dark brown a Little darker than should be but Car runs very nice response is good

I am confused about the vacuum not dropping out guess I don't know all I thought I did! It seems like I need more open butterfly on the carbs but still a little confused on how to get that with out a lean condition also not sure if I need to re curve the dist to bring the all in RPM down from 3200. The car is a rod not a racer do more cruising and the occasional burn out but very occasionaly just to prove I can still do it Any positive thoughts would be appreciated I am out in the hot sun trying to figure it out?
Change one or both advance springs to slightly lighter (less tension) to bring the "All In" or Total Mechanical timing down to the 2500-3000 RPM range.

Set your Total Timing to 36-38 degrees with the vacuum advance dis-connected and plugged. If you have Vortec heads you will find that the Total Timing will work better around 32 degrees.

Your idle RPM should be below 800. If automatic this would be while in DRIVE. If manual this would be in Neutral.

Your vacuum at idle seems a little low; but this could be due to whatever cam you are running. The dark brown plugs would possibly indicate a overly rich gas/air mix. Hard to say because it all depends on how you tested to check the plugs. Most folks do not know how to test for that. You can not just run the engine and pull over and shut the engine off and then look at the plugs. The correct way is to run the vehicle up in speed, shut the engine off while putting the trans in neutral at the same time and coming to a complete stop. Then pull the plugs to check for "color". Also with todays unleaded gas it becomes increasingly difficult to "read" the plugs.

The vacuum will not "Drop out" except when the engine is under a load.

If the vehicle is stationary and the RPM's are increased the vacuum will remain.

If you are driving the vehicle and are at a "cruise" or fairly steady speed the vacuum will still be there. This is how you get the "economy" as the timing is increased at cruise. If you were to drive your vehicle with the vacuum gauge hooked up, you would find that the needle would fluctuate under different engine loads. You would also see that when cruising the needle would be in the green colored section (which is good) and that vacuum would be present.

Your vacuum at speed looks good.

Basically, timing and fuel delivery that yield the best performance will also yield the best economy as the engine will be running near it's optimum. How you drive (fast starts, etc.) will also change the economy.
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Old 08-13-2007, 08:37 AM
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Frisco:
Thanks for the feedback! I did change the springs yesterday to drop the all in point to 2400 RPM. The total timing is at 38 deg at 2400 with 14 initial and 24 mec. looks like I need to make a road trip and check the mileage before I do any other modifications. I appreciate your conformation of what I have and what I needed to do.
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Old 08-13-2007, 08:43 AM
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Quote:
Originally Posted by Not A T 25
Frisco:
looks like I need to make a road trip and check the mileage before I do any other modifications.
Yeah! Road trips are the best part of doing a tune-up!!!

ENJOY!
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Old 08-13-2007, 09:00 AM
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Quote:
Originally Posted by Frisco
Yeah! Road trips are the best part of doing a tune-up!!!

ENJOY!
Hey Frisco:
Is the 5 window in your photo album your latest project it looks awsome and sounds great. I was looking to sell my 25 this spring to get the cash to build a 5 window much like that! The American Graffiti style coupe is still my dream car.
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Old 08-13-2007, 02:03 PM
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I have my 350 in my 72 Chevy pickup timed about the same. I pull a 5000 lb trailer with it once or twice a year. This year it was knocking while pulling up a grade. Outside temperatures were close to 100. I put in 92 octane gas so with the regular in the tank it was about 87. The store were we go fishing only sells 92 so I filled it with that for the rest of the trip. It solved the knocking problem and I was getting 15.5 mpg. It runs fine on regular under normal conditions but I have never had as good mpg with pure regular. Part of it might be the fact that I was only going in the 35 to 45 mph range most of the time.

Vacuum is ported. I tried manifold but it won't start at times. It just locks up. I read 16" at cruise on the level (no trailer) and 18" at idle. Downhill I can get it to 21-22".
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Old 08-14-2007, 07:41 AM
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Quote:
Originally Posted by Not A T 25
Hey Frisco:
Is the 5 window in your photo album your latest project it looks awsome and sounds great. I was looking to sell my 25 this spring to get the cash to build a 5 window much like that! The American Graffiti style coupe is still my dream car.
Yes, the five window has been my latest project for the past three years. Very slow going due to some health problems (myself and my wife); but I continue to plod along. Just got the engine running a few weeks ago. I still need to run exhaust pipes and mufflers from the lake style headers. I have had it registered and insured all the while and here in N.C. I can run YOM (Year Of Manufacturer) plates as long as I have the actual registered plates with me. I want to change the rims and tires as I don't like the look of the larger front tires. Steering is a Vega box and is as easy as power steering on this light car. Four wheel disc brakes with power assist all around. Mild 350 engine. 350 trans with B&M Transpak. 9" Ford rear with 3.50 posi and Moser axles.

A really fun ride was taken recently. The town I live closest to was having a local fair of sorts ('Mater' Fest) and they had a "Classic car show". I was out with the coupe to calibrate the speedometer. Got that done and told my son we should take a spin thru town. Probably the one time of the year I could do that with no mufflers without getting a ticket. Cruised on thru at parade speeds due to traffic. That was a memorable day. Lots of folks waving. LOUD pipes (barely above idle) and my Hot Rod coupe.

I've still got a ton of work to do; but the milestone of being able to drive it has been achieved.
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Old 08-14-2007, 08:27 AM
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Not A T,
confirm.....
your reading 13-14Hg from a vac guage "in the car" at 60/2200rpms cruisin' on a level road?
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Old 08-14-2007, 08:45 AM
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Quote:
Originally Posted by red65mustang
Not A T,
confirm.....
your reading 13-14Hg from a vac guage "in the car" at 60/2200rpms cruisin' on a level road?
Yes 65 MPH 2200 RPM 13 - 14"
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Old 08-14-2007, 11:25 AM
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Not A T,
thought this was going to be a easy one till I looked in your gallery!

3 2V's!

with the new timing.... 14base/24cent/10vac= 48* total just cruisin' 65mph....Hg should read 18"+...the 383 is totally loafing in your light car....
at 13-14Hg you are "tipping into" a richer mix/lousy burn...due to the 3 dueces????
"if" it is now 18"/65mph, and your vacuum can is adjustable....I would use 12/24/14Vac for 50 total cruisin'

depending on your cam and CR....I would be a little careful....WOT with 38* can be ping/knock
what does the Hg read on a mild downshift engine at temp?...
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Old 08-14-2007, 11:38 AM
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Quote:
Originally Posted by red65mustang
Not A T,
thought this was going to be a easy one till I looked in your gallery!

3 2V's!

with the new timing.... 14base/24cent/10vac= 48* total just cruisin' 65mph....Hg should read 18"+...the 383 is totally loafing in your light car....
at 13-14Hg you are "tipping into" a richer mix/lousy burn...due to the 3 dueces????
"if" it is now 18"/65mph, and your vacuum can is adjustable....I would use 12/24/14Vac for 50 total cruisin'

depending on your cam and CR....I would be a little careful....WOT with 38* can be ping/knock
what does the Hg read on a mild downshift engine at temp?...
Since this post I have recurved the distributor to be all in at 2400 RPM with 48 deg total. I did not record the vac at a kick down but it does drop to near nothing. I have a pretty good feeling that I am just getting into the secondary carbs at cruse since if I let off slightly it goes up to around 22" and if I accelerate slightly it starts to drop off significantly. my progressive linkage is adjusted all the way out and the only thoughts I had was increasing the jetting in the middle carb to get me all the way off of the secondary carbs but I am concerned that I may run fat at that point. The carbs in the picture are the old set up I have the demon six shooter on now it runs very nice just trying to get a little better mileage out of them.
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Old 08-14-2007, 12:26 PM
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Not A T',
"actual" cfm on/thru a 383 cube motor (changes some but not alot with cam/etc)....

2,000rpm=163cfm
2500rpm=223cfm
3000rpm=286cfm

......5500=540cfm

I agree, time to try just the center carb...with 48* cruisin...for higher Hg

comment?:
at 13Hg, a carb is at the borderline for the gas not atomized by enough velocity, not suspended in the air for a good mix...
you will gain ALOT of velocity using only the center carb
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