Opinions needed, change the ignition timing? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Tom · 06-23-2003, 04:18 PM

On my 79, Ive been trying to get the most gas milage possable. I found that happend with ported vaccum, and 10* initial with the vaccum pluged off. I can manage 18-20mpg on the freeway if Im carefull and traffic allows. This is with the 3.07 rear I just found out I really have instead of the 3.42's I thought it had. I also really need 89octane with the summers heat even though this vortec motor is a 87octane motor. Now I know vortecs use less timing then normal heads because of the combustion chamber, and the stock vaccum advance can gives WAY too much advance, so I was thinking of messing with the timing using a crane vaccum can. Im hoping I can get it back to 87octane and maybe even more milage, but I REALLY dont want to kill the milage Im getting now. So what do you guys think, should I try the crane can and manifold vaccum, or just live with 89octane?

sbc350gearhead · 06-23-2003, 06:39 PM

You want to run the vacuum off a ported source. If you run it off full manifold vacuum, it can cause an off idle miss, due to too much constant vacuum. If you are wanting to decrease the advance of the vacuum can, then just use an adj. can or use a gm 10 degree can (stock is 18 if I remember right).

navybear70 · 06-23-2003, 06:50 PM

I'm looking for good gas mileage too and I'm having a vacuum advance and timing problem. Could anyone post a pic of their set-up and/or give me some help here? I posted the problem on the Engine board.

Tom · 06-23-2003, 07:03 PM

Quote:

Originally posted by sbc350gearhead
You want to run the vacuum off a ported source. If you run it off full manifold vacuum, it can cause an off idle miss, due to too much constant vacuum. If you are wanting to decrease the advance of the vacuum can, then just use an adj. can or use a gm 10 degree can (stock is 18 if I remember right).

Actually according to aftermarket ignition makers you need full manifold. The reason GM [and me right now] used ported vaccum was two fold: better idle emissions and because the stock can gives about 24* advance which is much too much at idle so they said use ported so you dont get all that advance

Thanks though.

sbc350gearhead · 06-23-2003, 07:13 PM

Each person's timing curve is adjusted to what they are looking for. It sounds to me as if you already answered your own question. I run 18 degrees initial advance on my Z, with an additional 20 in the distributor for 38 degrees total, all in by 3000 rpm. With that curve, I wouldn't think of using full manifold vacuum for my vacuum advance. But then again, I don't use that curve for fuel economy.....LOL

Tom · 06-23-2003, 07:21 PM

I know what the ignition is supposed to be at, but Im affraid of loosing the great gas milage I have now. I know I can put it to a 10* can, have 34* mechanical [what vortec like] and run full manifold vaccum with 8* initial [with the vaccum advance un-pluged] and run 87octane all day long, but I dont want to loose the 15-20MPG freeway Im getting right now. What do you think, should I try it?

sbc350gearhead · 06-23-2003, 09:29 PM

As I said.....it seems like you answered your own question. It is highly unlikely to find someone with the exact same engine combo, running the same curve in the same climate as you, who has attempted to do what you are contemplating doing. Try it and find out. At worst, it won't work and you will be out 25 bucks for an adj. can. At best, you will either improve fuel economy, or be able to run cheaper gas. Even if it doesn't work for you, you may find a use for the adj. can at a later date.

**swervin ervin** · 06-23-2003, 09:35 PM

I don't know why everyone thinks GM always ran the vacuum advance on a ported source. It just ain't true.

Even when my truck had the 305, it was hooked to full manifold vacuum, throught the TVS. Full manifold vacuum is the preferred place to hook it too. Never heard of any off idle miss because of it. Only reason you would ever want to run ported vacuum advance is because the distributor refuses to work with manifold vacuum. This is because it's not set up right in the first place.

A properly setup distributor will work wonders with mileage and power. Don't just limit vacuum advance, do the whole nine yards.

Check this article for some good advice.

navybear70 · 06-23-2003, 09:45 PM

Once again you have provided a wonderful source of info. Tanx again.
BTW... is your name a reference to Ernie Irvan?

sbc350gearhead · 06-23-2003, 10:09 PM

http://www.carcraft.com/howto/77958/index2.html
http://www.2quicknovas.com/happytiming.html
http://www.bzerob.com/bobsgarage/tec...ei_install.htm
http://www.l81vetteregistry.com/tech/hei.html

As the car craft link states......the ported vs. full manifold debate is never ending. Much like the ford vs. chevy debate. The other links are opposing expert views.

**swervin ervin** · 06-23-2003, 10:14 PM

Quote:

Originally posted by navybear70
BTW... is your name a reference to Ernie Irvan?

Yep, sure is. Several years ago, I helped a guy with his dirt track race car. He always called me swervin ervin, because of Ernie. Well it stuck and before long every one was calling me that. So when I started hanging out on message boards, it was the name I used.

Oh, I forgot to mention, my last name is Ervin.

06-23-2003, 04:18 PM	#1
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,711	Opinions needed, change the ignition timing? On my 79, Ive been trying to get the most gas milage possable. I found that happend with ported vaccum, and 10* initial with the vaccum pluged off. I can manage 18-20mpg on the freeway if Im carefull and traffic allows. This is with the 3.07 rear I just found out I really have instead of the 3.42's I thought it had. I also really need 89octane with the summers heat even though this vortec motor is a 87octane motor. Now I know vortecs use less timing then normal heads because of the combustion chamber, and the stock vaccum advance can gives WAY too much advance, so I was thinking of messing with the timing using a crane vaccum can. Im hoping I can get it back to 87octane and maybe even more milage, but I REALLY dont want to kill the milage Im getting now. So what do you guys think, should I try the crane can and manifold vaccum, or just live with 89octane? __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

06-23-2003, 06:50 PM	#3
navybear70 hometown heroes! Join Date: Jan 2003 Location: Ewa Beach, HI Posts: 487	I'm looking for good gas mileage too and I'm having a vacuum advance and timing problem. Could anyone post a pic of their set-up and/or give me some help here? I posted the problem on the Engine board. __________________ 2009 Silverado 2wd, 121k as of 04APR13. I love my truck 29 days out of the month. The payment is due on the 30th... Do your duty in all things. You cannot do more, you should never wish to do less. -Robert E. Lee "Never give up ground you've already taken." -Unknown

06-23-2003, 07:21 PM	#6
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,711	I know what the ignition is supposed to be at, but Im affraid of loosing the great gas milage I have now. I know I can put it to a 10* can, have 34* mechanical [what vortec like] and run full manifold vaccum with 8* initial [with the vaccum advance un-pluged] and run 87octane all day long, but I dont want to loose the 15-20MPG freeway Im getting right now. What do you think, should I try it? __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

06-23-2003, 09:45 PM	#9
navybear70 hometown heroes! Join Date: Jan 2003 Location: Ewa Beach, HI Posts: 487	Once again you have provided a wonderful source of info. Tanx again. BTW... is your name a reference to Ernie Irvan? __________________ 2009 Silverado 2wd, 121k as of 04APR13. I love my truck 29 days out of the month. The payment is due on the 30th... Do your duty in all things. You cannot do more, you should never wish to do less. -Robert E. Lee "Never give up ground you've already taken." -Unknown

06-23-2003, 06:39 PM	#2
sbc350gearhead Registered User Join Date: Mar 2003 Location: Columbus Ohio Posts: 1,413	You want to run the vacuum off a ported source. If you run it off full manifold vacuum, it can cause an off idle miss, due to too much constant vacuum. If you are wanting to decrease the advance of the vacuum can, then just use an adj. can or use a gm 10 degree can (stock is 18 if I remember right).

06-23-2003, 07:13 PM	#5
sbc350gearhead Registered User Join Date: Mar 2003 Location: Columbus Ohio Posts: 1,413	Each person's timing curve is adjusted to what they are looking for. It sounds to me as if you already answered your own question. I run 18 degrees initial advance on my Z, with an additional 20 in the distributor for 38 degrees total, all in by 3000 rpm. With that curve, I wouldn't think of using full manifold vacuum for my vacuum advance. But then again, I don't use that curve for fuel economy.....LOL

06-23-2003, 09:29 PM	#7
sbc350gearhead Registered User Join Date: Mar 2003 Location: Columbus Ohio Posts: 1,413	As I said.....it seems like you answered your own question. It is highly unlikely to find someone with the exact same engine combo, running the same curve in the same climate as you, who has attempted to do what you are contemplating doing. Try it and find out. At worst, it won't work and you will be out 25 bucks for an adj. can. At best, you will either improve fuel economy, or be able to run cheaper gas. Even if it doesn't work for you, you may find a use for the adj. can at a later date.

06-23-2003, 09:35 PM	#8
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	I don't know why everyone thinks GM always ran the vacuum advance on a ported source. It just ain't true. Even when my truck had the 305, it was hooked to full manifold vacuum, throught the TVS. Full manifold vacuum is the preferred place to hook it too. Never heard of any off idle miss because of it. Only reason you would ever want to run ported vacuum advance is because the distributor refuses to work with manifold vacuum. This is because it's not set up right in the first place. A properly setup distributor will work wonders with mileage and power. Don't just limit vacuum advance, do the whole nine yards. Check this article for some good advice.

06-23-2003, 10:09 PM	#10
sbc350gearhead Registered User Join Date: Mar 2003 Location: Columbus Ohio Posts: 1,413	http://www.carcraft.com/howto/77958/index2.html http://www.2quicknovas.com/happytiming.html http://www.bzerob.com/bobsgarage/tec...ei_install.htm http://www.l81vetteregistry.com/tech/hei.html As the car craft link states......the ported vs. full manifold debate is never ending. Much like the ford vs. chevy debate. The other links are opposing expert views.



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