-
-
Re: 1965 L79 Distributor Vacuum Advance Change
James,
I've also read about people changing from ported to full time vacuum (in addition to the vacuum canister change). Is full time vaccum something that L79s would benefit form as well? And would the answer be any different for a late '66 L79 than for a '65 L79? Aren't the '65 vs late '66 heads ever so slightly different, 461 vs 462?
Gary -
Re: 1965 L79 Distributor Vacuum Advance Change
Gary:
Only C2 small block cars that I know had ported advance were the California K19 emissions cars. And perhaps only the auto trans K19 cars and not the manual trans ones. Not certain about that last point. Otherwise they should all be full time manifold vacuum .
LarryComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
I agree with Larry's post, other than for some unknown reason '63FI engines got ported vacuum advance as did early '64 FI engines with the 375R FI system, but the 380 system was set up for full time advance. I believe the '66-'67 base with manual transmissions and AIR (K-19) got ported vacuum advance, too. I recall demonstrating how to take the spark advance map at a local SoCal Chapter meet a few years ago on an original owner, very original unrestored base engine manual trans '66 convertible. At first I thought the VAC was dead because there was no vacuum reading at idle. Then I noticed the air pump... DUH!
Of course, it's easy to check whether full time or ported. Just tee a vacuum gage into the VAC signal line and see if there is a significant vacuum reading at idle.
The latest information I have is that the black springs in the Mr. Gasket 928G spring kit might be the lightest. The OE L-79 centrifugal is very lazy, 30 @ 5100, so getting is in sooner will definitely improve low end torque and make the engine more responsive in normal driving. I started a thread back in Dec/2012 "L-79 distributor blueprint overhaul" which his worth reading. Set initial timing at 8-10 at below the engine speed where centrifugal starts, but my preferred method is to set total WOT advance a few hundred revs above the point of max centrifugal as high is the 36-40 range as the engine will tolerate without detonation, and even if it's an original Flint-built engine it will likely tolerate the upper range on 93 PON fuel
That simple 10-dollar VAC is very important, particularly having one that is the "best fit" with the Two-Inch Rule. You can't rely on parts books, either GM or aftermarket, to select the correct VAC due to both errors in the initial selection by GM and subsequent parts consolidations. The B1 is totally worthless. It takes up to 19" to pull to the limit, so it doesn't even pass the Two-Inch Rule for base engines with manual transmissions that idle at 500 @ 18-19".
There are only three single point/TI VACs that are "best fit" under the Two-Inch Rule for any stock or modified engine: the 8" B28, 12" B26, and 15" B22, so you just observe manifold vacuum and apply the Two-Inch Rule. It's that simple!
Most engines that have original ported vacuum advance will need a different VAC - one of the three listed above, because the original may not pass the Two-Inch Rule. Examples include the '63 L-84, and '67 L-71, and most, if not all, '68-up engines.
More information is in my 2012 San Diego National Convention Tuning Seminar, which is an easy Web search.
DukeComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
The 462 heads eliminated the small quench zone on the spark plug side of the chamber, which theoretically might need more spark advance, but practically it doesn't make enough difference to be noticeable, so all L-79s will run better with a 12" B26 VAC and the full 30 centrifugal in as soon as possible, 3000-3500 or lower. The only limitation is detonation, and actual CR, which is usually not very accurately known, altitude, available fuel octane, and ambient temperature range are variables, so the only way to determine how aggressive the spark advance map can be is road testing each individual car.
DukeComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
Thanks James, Duke and Larry. Your postings have been educational.
GaryComment
-
Comment
-
Re: 1965 L79 Distributor Vacuum Advance Change
James,
A posting on the Corvette Forum says:
"...a kit from Mr. Gasket, and at least for their kit, the color coding for the springs is:
Light = black
Medium = silver
Heavy = gold"
Gary
Comment
-
Comment
-
Re: 1965 L79 Distributor Vacuum Advance Change
It's curious that people say the black spring is the least still if it has the same wire diameter, and presumably the same OD, as the silver, but the silver has an additional coil. The spring rate is proportional to 1/n, where n is the number of active coils, so the 9-coil spring should be about 89% as stiff as the 8-coil spring, if I've done the simple math right.
GaryComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
The best test is on the car.
DukeComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
I believe Lars send me the attached graph of advance versus engine revs for a Delco single point distributor... bottom line is that the black set in the Mr. Gasket 928G spring kit appear to be the most aggressive by adding centrifugal advance the fastest even though the silver springs bring total centrifugal in at the same revs. Black, silver, and gold were tested as matched sets along with the silver/gold combination, so you can get an idea how you can fine tune the rate of centrifugal advance.
This spark advance map would not be suitable for an OE engine, even with the biggest OE cams unless the CR was well below optimum, but Lars has found it works well for big aftermarket cams and moderate compression ratio.
All were tested on the same distributor with "OE weights" though it's not specified which OE distributor they come from. Centrifugal is limited to only 18 degrees with 18 initial, and all have constant 10 degrees vacuum advance, so subtract this out to get total WOT advance. Ignore the "#3 spring" because I don't know what it is, and note that it uses 16 initial rather than 18 in all the other tests.
Since the only difference is the springs I can't account for why the gold springs only allow 16 instead of 18 deg. centrifugal other than measurement error. I believe Lars has a Sun distributor machine, which you are probably aware is graduated in one degree distributor increments, so multiply by two for crank degrees, and if the pointers aren't exactly on a graduation marks you have to interpolate.
You need to spend a little time studying this graph because it isn't quite an apples to apples comparison, but close.
My approach to spark advance map tuning, which I've been doing for over 50 years, is to convert all ported vacuum advance applications to full time (as long as field emission testing is not required) and install the least aggressive VAC that meets the Two-Inch Rule. Then I install the lightest available springs and set total WOT advance in the 38-39 degrees range. Then I do a detonation test. Drive up a steep grade at 1000-1200 revs in second gear and then accelerate, and then slog up a modest grade at 1000-1200 in top gear and accelerate. This is not how most of us drive our cars, but it's a severe detonation test, and that's the point of the test. Try to get the engine to detonate under worst case conditions. If it passes these tests up to about 3000-3500 then you are unlikely to experience detonation in normal driving, and you're good to go.
If there is detonation, slow the centrifugal curves using the smallest increment possible as many times as necessary until you run out of options. At that point you have to reduce total WOT advance, but if you have to go below 36, you have too much compression ratio for the fuel octane and the only choices are to run suboptimal total WOT advance or make a blend of the highest available pump octane and high octane race gas or avgas and figure out the minimum blend ratio of the high octane gas that is required to quell any detonation.
A little transient detonation when you open the throttle or upshift is okay as long as it doesn't last more than a second.
Notice that I didn't say anything about "initial timing" Why? Because I don't care. I want total WOT advance in the optimum range, which is why I use the total WOT advance method to set the distributor. Actually, I usually do check initial timing as a reference point, but it can be tough to get engine revs below the point were centrifugal starts, especially on mechanical lifter engines.
I also check total idle advance (VAC connected), which is the sum of initial and full vacuum, plus maybe 2-3 degrees of centrifugal if it starts below typical idle speed, like 30-30 cam engines. Total idle advance should be low twenties to low thirties depending on valve overlap. Low overlap cams, like base engine cams should be in the low to mid twenties, mid to high twenties for medium overlap cams, like the L-79 cam, and high twenties to low thirties for mechanical lifter cams, which have the highest overlap of all OE cams.
DukeComment
-
Re: 1965 L79 Distributor Vacuum Advance Change
Duke,
In mapping your advance curve on a stock L79 distributor, you'd have to rev it past 5000 to see what you've really got since stock centrifugal doesn't come all the way in until 5100. Is there a danger of damaging engine when you rev one way up with no load?Comment
-
Re: 1965 L79 Distributor Vacuum Advance Change
Probably not..........although growing up my father (a Ford Dealership mechanic) always told me not to rev up too high with no load on the engine. I have gone to 4000 RPM with no issues, and made a graph of RPM vs Timing from 700 RPM to 4000 RPM. You can then extrapolate to 5000 to see what it does.........although there should be a hard stop at some point which will limit all further advance.
Bigger issue may be the chance for engine at 5000 RPM to throw something back at the person with the timing gun. I know I wore monogoggles during this time and double checked the fan and blades for any looseness. Problems at this RPM with you leaning over the engine fan to check the timing marks are unforgiving if a problem arises.
I have heard of persons removing both springs from the distributor weights and plugging the vacuum advance........and then reving up to 3500 or 4000 RPM. Without springs, the advance should quickly go to the max stop, which you can see. I do not think there will be noticeable detonation during this time, with the vacuum can disconnected. You might even dial back the initial advance beforehand to TDC before doing this.....or perhaps just remove the heavier spring (one spring only) and try it. That should bring in the max advance stop point pretty early in the RPM.
OR, take it to a guy with a SUN Distributor Machine.
LarryComment
Comment