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Old 07-21-2013, 10:18 PM
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Vacuum gauge reading

I have a question concerning vacuum readings. Have a 327 with a mild cam (Corvette hydraulic), bored 30 over. Timing set at 10 BTDC. At idle the vacuum gauge reads between 15-17 inches of mercury, which I understand is good. I have increased the vacuum more by setting the timing higher, but trying to start it is almost impossible. When doing highway speeds (60mph), foot set just right to get this speed (also used cruise control) and I have a reading of 7-10 inches of mercury. I know that the vacuum does drop, but isn't this a little too much? Am I too far advanced or are the valves too tight or? Car runs well except when I shut it off and try to start it, the motor is firing against itself as if it is too far advanced. The only thing that was changed is that the carb (quadrajet) was rebuilt. Starting cold is no problem. Any suggestions?

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Old 07-21-2013, 11:16 PM
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Quote:
Originally Posted by 27Tall T View Post
I have a question concerning vacuum readings. Have a 327 with a mild cam (Corvette hydraulic), bored 30 over. Timing set at 10 BTDC. At idle the vacuum gauge reads between 15-17 inches of mercury, which I understand is good. I have increased the vacuum more by setting the timing higher, but trying to start it is almost impossible. When doing highway speeds (60mph), foot set just right to get this speed (also used cruise control) and I have a reading of 7-10 inches of mercury. I know that the vacuum does drop, but isn't this a little too much? Am I too far advanced or are the valves too tight or? Car runs well except when I shut it off and try to start it, the motor is firing against itself as if it is too far advanced. The only thing that was changed is that the carb (quadrajet) was rebuilt. Starting cold is no problem. Any suggestions?
What's the total? And 10* is too low as a base, if its hard starting hot at only 10* initial, look at all your elec for something amiss, or is the the original 40yr old starter? At only 10* you should never have hot start problems. Get the starter issue solved. Why are you chasing a vacuum number at cruise if the car runs right?
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Old 07-21-2013, 11:33 PM
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The total is 36 degrees and the starter was rebuilt 30,000 miles ago. Think it's time to replace it with a mini one. Not really chasing a vacuum, but my mileage should be a lot better than what it is (10 mpg). This thing is hooked up to a 200 4R trans as well, rear gear ratio 3.54 and tires 28". Hence my question.
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Old 07-22-2013, 07:01 AM
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Make sure the vacuum advance is hooked to a ported source. If it is hooked to a manifold vacuum, it will pull timing in as the motor is cranked.
That would advance the timing as you try to start. Just a thought
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Old 07-22-2013, 07:21 AM
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After sorting out the distributor advance curve, use an interrupter switch if you're still having trouble w/hot restarts.

Good luck!
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Old 07-22-2013, 05:13 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
To run agressive idle timing yet avoid excessive full advance recurve the distributor and restrickt the mechanical advance travel stop limit.
Can you provide some instructions or part numbers here? I have been trying to figure out the correct way to do this. It seems as if MSD has some bushing you can use...will those work on all HEI distributors MSD and OEM? Or MSD only?
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Old 07-22-2013, 07:15 PM
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For what its worth... How to Use and Interpret a Vacuum Gauge
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Old 07-22-2013, 10:19 PM
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Quote:
Originally Posted by Silver Surfer View Post
Can you provide some instructions or part numbers here? I have been trying to figure out the correct way to do this. It seems as if MSD has some bushing you can use...will those work on all HEI distributors MSD and OEM? Or MSD only?
Some info on timing curves, etc. here.

MSD supplies:
Red (allows the least amount of initial timing) - 28 degrees
Silver - 25 degrees
Blue - 21 degrees
Black (allows most initial timing) - 18 degrees

If you need even more initial timing you can use aftermarket 10 and 14 degree bushing$, found here.

You can also download the MSD instructions at the link below to look get a full explanation of the bushing and how it affects the total timing.
http://www.setyourtiming.com/

Last edited by cobalt327; 07-22-2013 at 10:35 PM.
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Old 07-23-2013, 12:18 PM
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Quote:
Originally Posted by cobalt327 View Post
Some info on timing curves, etc. here.

MSD supplies:
Red (allows the least amount of initial timing) - 28 degrees
Silver - 25 degrees
Blue - 21 degrees
Black (allows most initial timing) - 18 degrees

If you need even more initial timing you can use aftermarket 10 and 14 degree bushing$, found here.

You can also download the MSD instructions at the link below to look get a full explanation of the bushing and how it affects the total timing.
Set your timing - for more performance!
So those busing are MSD specific? What about limiting mechanical advance on an OEM HEI?
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Old 07-23-2013, 12:55 PM
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Quote:
Originally Posted by Silver Surfer View Post
So those busing are MSD specific?p
That's correct.

Quote:
What about limiting mechanical advance on an OEM HEI?
Refer to the link above in post #10 where it says "Some info on timing curves, etc." Down a ways from where the link first places you on the page, you'll see a photo and description on limiting the mechanical advance on the GM HEI distributor.
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Old 07-28-2013, 07:10 AM
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Have you verified the vacuum advance is working at cruise? One reason its there is to provide better fuel economy.

From what I recall, this is the typical starting point recommendation for a stock GM HEI if you are interested in economy.
-- 8-18 degrees initial - you should verify TDC manually to make sure you get the right reading. More cam usually requires more initial advance to help low end. Most stock engines will take about 12-14 degrees or more initial without any problem. Adjust by turning the distributor.
-- 18-20 degrees mechanical. Works best if its all in by about 2000-2500 rpm. Adjust total advance using a mechanical stop; adjust rate of advance by changing springs.
-- 15-20 degrees vacuum. Stock GM canisters come in different ratings (both max increase of timing and minimum vacuum to activate) . Aftermarket vacuum canisters are often adjustable. You can run without vacuum advance, but cruising fuel mileage will suffer.
-- Initial plus mechanical should not exceed 36 degrees.
-- Initial plus mechanical plus vacuum should not exceed 54 degrees.

Economy cams usually run a duration of about 195-205 at .050, or for a little more power you can use an RV cam that runs up to about 210-214 at .050. Most of the GM cams will be dual pattern (e.g., 204/214) is a common RV cam.
If you want a wider power band you usually want a cam with a larger LSA (112-114). A lower LSA (108-110) may work fine for a particular application, but will be generally more RPM & speed sensitive.

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Old 07-28-2013, 11:33 AM
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You said you set the timing to raise the vacuum, did you actually put a light on it to see what the initial is. You cant set timing with a vacuum gauge. Sounds like your initial timing is too advanced. Anything past 12-14 BTDC can give you potential hot cranking issues.
is this vehicle a t-bucket, just a guess considering your screen name, if it is that tall flat windshield is your poor mileage issue if you have one. I have the same issue.
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Old 07-28-2013, 10:40 PM
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My Initial timing is set at 11 degrees; centrifugal is 21 degrees, at 2800 rpm. Vacuum is still at 15" at 700rpm and even lower at cruise speed. My next step is to change the vacuum advance (replace it with an AR12 or AR10). Hopefully this will make a change. Even at cold temp. the starter (at times) sounds like it is firing against itself. Right now it is on manifold vacuum, before on ported vacuum.
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Old 07-29-2013, 09:40 AM
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Quote:
Originally Posted by 27Tall T View Post
My Initial timing is set at 11 degrees; centrifugal is 21 degrees, at 2800 rpm. Vacuum is still at 15" at 700rpm and even lower at cruise speed. My next step is to change the vacuum advance (replace it with an AR12 or AR10). Hopefully this will make a change. Even at cold temp. the starter (at times) sounds like it is firing against itself. Right now it is on manifold vacuum, before on ported vacuum.
May be the old GM distributor hang. Check all the bushings, springs and pivot points in the distributor for wear.
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Old 08-30-2013, 04:46 PM
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Quote:
Originally Posted by 27Tall T View Post
I have a question concerning vacuum readings. Have a 327 with a mild cam (Corvette hydraulic), bored 30 over. Timing set at 10 BTDC. At idle the vacuum gauge reads between 15-17 inches of mercury, which I understand is good. I have increased the vacuum more by setting the timing higher, but trying to start it is almost impossible. When doing highway speeds (60mph), foot set just right to get this speed (also used cruise control) and I have a reading of 7-10 inches of mercury. I know that the vacuum does drop, but isn't this a little too much? Am I too far advanced or are the valves too tight or? Car runs well except when I shut it off and try to start it, the motor is firing against itself as if it is too far advanced. The only thing that was changed is that the carb (quadrajet) was rebuilt. Starting cold is no problem. Any suggestions?
Do you know what the cam is either by part number of by duration, lift and lobe seperation angle? As well as what the compression ratio is, if not that the casting number of the head and if you know what pistons are in there?

Cam timing and compression ratios are partners both in power output and efficiency so when you stated this is a 327 that only gets 10 mpg with a 700R4 and 3.56 gears my eyebrows raised. But all this begs some other questions as well.

- Is the lock up converter (TCC) operational?
- What is the vehicle?
- 10 mpg in Imperial or US gallons? Imperial would only point to an even bigger problem.
- What is the ignition? As engine RPMs are reduced as with overdrive it takes more spark energy to get a timely burn, especially if the compression is low, the cam closes the intake late and the mixture of to either lean of rich side of correct. These are things that require a modern high energy ignition to insure spark energy is sufficient to get the burn going in a timely fashion. Slow or miss fires as well as late timing will cost a lot of fuel.
- Cam, the 15-17 inches at idle speak to a cam larger than say the classic 300 horse 327 cam of 195/202 degrees with .390/.410 lift on a 112 LSA. When you get above this cam you can start to see the effects of late intake closing and incresed overlap. When this starts having higher compression is important to recover lower RPM torque. Without this the engine will want excessive spark advance and will use more throttle opening which will reduce manifold vacuum which in turn may turn on the power enrichment system of the carb. All of these will contribute to increased fuel consumption. Modern cam's tend to reduce overall timing by eliminating quite a bit of ramp timing that is a feature of older designs. These older cams crack the valves open over a considerable duration which costs quite a bit of fuel by loosing a lot of mixture at lower RPMs to reverse pumping of the piston rising on the compression stroke into an unseated valve. Modern thinking is to shoten the ramps thus the total duration, then kick the valve open quickly from .050 lift. The reduction in ramp duration get the intake closed sooner in the compression cycle so there is more mixture in the cylinder to compress which times the ratio increases the pressure which then extracts more work for the fuel being burnt. Bascally that means more power with less throttle opening. This also means whose cam you're using is important. A modern cam like the Comp Xtreme Energy series develops a stronger bottom end torque at a lower RPM with no loss of top end power compred to older designs of the same .050 duration. The torque peak to horsepower peak of the newer cams is also further apart in RPMs than older designs which gives the engine a much wider and useful power band on the street. This is important for an automatic where the RPM jumps between gears can be considerable.

Bogie
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