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Old 02-07-2011, 08:57 AM
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low vacuum at cruise good at idle

I have been working on getting the best timing for my pretty much stock 350 in my 72 chevy pickup. I was running ported vacuum with my quadrajet and idled at 19 and cruise was at around 15. I switched over to manifold vacuum and have a tee into the line for my vacuum gauge. I still have about 19 at idle but my cruise reading is 12. Timing is at 14 initial and 38 all in at 2500 like it was before. I recently installed a tach in the cab and very rarely go over 3000 rpm. I am just below 3000 cruising at 60 with a 350 turbo and 4.10 in the rear.

Should I try to set the timing for highest vacuum while holding the motor at 2500 rpm. I know it seems to run better with about 2 to 4 more degrees of timing but that put me very high total with vacuum at 56 and 58. I want to keep the initial and mechanical down to around 36 or 38. I could add iniial and change the stops to only add 18 mechanical.

Any idea why I would be loosing vacuum by switching from ported to manifold if all else is the same?

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Old 02-07-2011, 11:40 AM
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I want to see where this goes. I've got the same set up in my truck as far as gears, engine, and tranny but with a 600 Holley edel. intake , headers, and adj. vacuum adv dist. I'm only running 8* initiall, and my vacuum at idle is 19. I never tried to read it at cruise rpm.
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Old 02-07-2011, 12:10 PM
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IMO seems your vaccum advance amount is too high at approx 20 deg. Limit this to 12-14 max which will give you 50-52 total, don't go higher than this.

Not sure why you are trying to set the timing for max vacuum at 2500. Not saying that you are wrong in doing so but in my book the timing is set for either max performance or backed off a bit for a more conservative approach. Vacuum levels are secondary to getting the timing and curve correct, the timing will have an effect on the vacuum level but vacuum levels should not dictate timing settings unless you are dialing in your vaccum advance can (assuming adjustable type) with vacuum level numbers taken and recorded while driving.

Might be interesting to do as you say and record the vacuum level changes as you adjust the timing slightly while reving to 2500, I don't think you will see much change in vac levels unless you are really changing the dist postion quite a bit.

I would say that your initial timing of 14 is high enough for the stock cam, 36 is all you really need for total without vac advance. Maybe just set the total at 36 while reving at 3500 and call it a day (block the wheels) let the initial fall were it may after that, from the numbers you gave it should be around 12 deg. (define "pretty much stock" please) what is the cam in this mill?

The vacuum readings from ported to manifold should be the same while driving and the throttle plates are open, can't think of why you have a difference, one would assume that manifold vac would be higher than ported if anything if you are cruising with very little throttle opening,,,odd
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Old 02-07-2011, 12:24 PM
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Motor has an Edelbrock performer intake and ceramic headers with an MSD ignition. I also added roller tip rockers but the cam is stock as is everything else.

I like to monitor the vacuum while I drive. I figure the higher the vacuum the more efficient the motor is running. The lower vacuum at cruise would also limit the vacuum advance I get. With my current setting that is probably good. I get 16 on the can and it is not adjustable.

My timing is all in at 2500.
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Old 02-07-2011, 01:25 PM
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Assuming then that you have a stock HEI with non adjustable vac adv can. Most of these will have a vac adv can that is all in by about 11-12" hg. The starting point is about 5-6 " hg. You can test yours by applying a vacuum with the dist cap and rotor off and see were the can rod movement starts and ends, record the numbers, just good info to know.

You are correct that when the motor is pulling its highest vacuum level while freeway cruising it is going to net you the highest overall mileage in most cases.

Using the vacuum level procedure for timing adjustment while at a no load (driveway) rpm of 2500 is what I thought you were wanting to do, not sure there are real tangible benefits to that method.

Regardless getting the timing optimized is surely one of trial and error. I would still suggest to limit the vacuum advance so that you don't exceed approx 48 total. The stock HEI was most likely setup for a base timing of like 4 deg so moving it up to 14 base will shift everything up 10 Deg, take that amount out of the vac advance, http://www.summitracing.com/parts/CRN-99619-1/
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Old 02-07-2011, 01:49 PM
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I thought about adding one of those plates. My MSD comes with a vacuum can that puts in 16" of advance. That is part of the problem. I would like to limit overall total to 51 but getting initial high enough to get good power puts it way over.

I think I will just bump the timing 1 degree at a time and see how it drives. I know at 16 in the driveway it sounded a lot better and seem to have more power when I power brake it. I think if that doesn't work I will change the stop in the distributor so the mechancial is at 18 and give it more initial. I figure 18 plus 18 plus 16 will get me 36 and 52.
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Old 02-07-2011, 02:16 PM
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OK so you have an HEI type MSD dist with a non adjustable vacuum advance can that adds 16 deg vac advance right now?, please clarify for me thanks I guess I should have asked off the get go.

Also I don't think that 18 deg BTDC is nessesary with a stock engine but give it what it wants. Did you verify TDC and damper pointer/mark?
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Old 02-07-2011, 02:20 PM
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I verified the TDC when I installed the roller tip rockers but it would not hurt to check it again for more accuracy.

The MSD is a ready to run multiple spark that doesn't need a 6A box and it does supply 16" from the vacuum can which is not adjustable.

Everyone always says give it what it wants and that is what I intend to do by making some adjustments and driving it up a steep grade nearby.
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Old 02-07-2011, 02:39 PM
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First rule of setting the timing on an engine. Do not let it knock!

After verifying that the timing pointers are correct, maybe play with the timing a little.

With a pretty much stock engine, start at what the factory recommends. Take the truck for a test drive, and record what the vacuum is.
If it does not knock, try advancing the timing about two degrees. See if it knocks. Take the truck for a test drive, again, and record what the vacuum is. If the vacuum goes up, advance the timing two degrees again. Listen for knock, if it knocks, retard it!

If you set the timing to the max vacuum without a load on the engine, you might end up with the timing too far advanced, because when you put a load on the engine, (drive it) the throttle is opened more, and the timing might end up being too much.

When you are all done. check the timing again. If it seems way out of factory specs, I would find out why.
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Old 02-08-2011, 03:29 AM
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First of all, I don't see how your getting away with 14* initial timing. I get mine up around 12* and the starter want's to drag when the engine is warm.
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Old 02-08-2011, 05:12 AM
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On my truck ('75) the factory timing is 8 degrees BTDC. However, I have the GMPP 350/290 HP and it seems to work better with initial timing of about 15-16 degrees. However, the 222/222 @.050 cam in my engine is quite a bit more duration than the stock 350 truck cam (maybe 184/194?), and the extra initial advance might not be worth it with the stock cam.

I get another 18 degrees from mechanical advance, and the vacuum can adds about 18. I use manifold vacuum, and don't have problems with pinging. I am getting about 15" vacuum at idle, and about 10-12" at cruise of 3000 rpm.

I recently added an AFR gauge and found my cruise mixture with the stock calibration Edelbrock 1406 was much too lean. I changed rods and was able to get it to an AFR of about 15.5-16 at cruise. As I gradually accelerate it pulls the rods into power mode and AFR transitions to about 12.2-12.5. Overall drive-ability is much smoother now.

Bruce
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Old 02-08-2011, 07:21 AM
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To limit the total amount of vacuum advance w/an HEI-type distributor, you will need to physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619 Instructions.
Or you can easily make one.

An adjustable can might be a good addition, one that allows you to alter the tip in point, etc. Crane has an adjustable vacuum advance can kit- Crane #99600-1, 99600-1 Instructions.

DETERMINING TDC will allow you to be sure the timing tab and damper are correct for TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light.
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Old 02-08-2011, 08:57 AM
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Actually I have had it as much as 18 and it still does not kickback against the starter even with manifold vacuum.
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Old 02-08-2011, 12:21 PM
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Quote:
Originally Posted by Mertz
Actually I have had it as much as 18 and it still does not kickback against the starter even with manifold vacuum.
Have you tried an advace curve weight and spring kit in the dist? You could set the initial lower which would subtract from the total yet still bring it all in sooner.
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Old 02-09-2011, 09:08 AM
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I plan on trying it with 16 and 18 initial before I recurve it. The recurve will be with 18 initial and an 18* stop which will put me at 36 and 52. The instructions on the MSD distributor say to put in as much initial as possible and I think that will work for my motor. The curve will also get me to all in closer to 3000 than the 2500 I have now which seems to work better for a truck according to some. I am also going to check the timing reading at highest vacuum just to see where it is.

I also will be confirming TDC. I have an idea to use an old spark plug to make an easy to read device to tell me when the piston is at TDC. If that doesn't work I will see if I can borrow a camera.
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