Vacuum reading at idle 63-327-340 HP

[Richard M.]

Re: Vacuum reading at idle 63-327-340 HP

Sorry, Richard. That comment was really aimed at Jeff in post #5. I meant no offense to either of you, but Jeff said he didnt understand the Two-Inch Rule, which is explained in the pdf link along with how to set up an optimal spark advance map.

Thanks for spreading the word, but sometimes I get perplexed because guys say they read it, but don't seem to have any more understanding than someone who's clueless.

A 250/300 HP engine with manual transmission should idle at 18-19" at 500 in neutral. The '62-'63 distributors had a 15.5" VAC, which was fine, but maybe not for a Powerglide idling at 450-500 in Drive because that condition pulls less vacuum. How much less I'm not sure because no one has ever provided the data despite my several requests.

The VAC changed in '64 and again in '65, which has been discussed on the TDB. Finally the '67 300 HP got a 12" VAC probably because it worked with Powerglide, but I don't like to see a more aggressive VAC than what it takes to meet the Two-Inch rule. That's why I recommend a 15" B22 for a base type engine, and a 12" for L-79 because it meets the Two-Inch Rule since L-79s typically idle in neutral at 14-15" @ 750, so the OE 8" VAC is more aggressive than necessary.

A more aggressive than necessary VAC can cause part throttle or transient detonation, but has no offseting advantage. So among the three applicable VACs, 8, 12, and 16", always select the least aggressive VAC that meets the Two-Inch Rule for the normal idle configuation, which is in neutral with a manual transmission and Drive for an automatic.

Duke

Okay good, I understand, thanks. Sorry if I got a little postal.

Well, I guess we can have a little bit of "base-engine" intermission until we hear back from the OP. What you explained is just what I was thinking about the base setup. Makes sense. When I pulled the distributor for the intake manifold change and saw the B26 I was surprised. I expected a B22. Plus when I tested it it was all in around 10", even more aggressive I thought. But maybe my vacuum gauge has gone bad at higher readings, it's quite old. I haven't heard back from Lars yet about his B26 VAC choice. However, I got the numbers from the car owner's paperwork.....

"B26 Vacuum Advance"
"20* centrifugal advance"
"Set timing to 16* below 700 RPM, or set for 36* total"
"Vac Adv starts at 6", Full Vac Adv 13* @ 13"

What's very confusing to me is the fact that the base 250hp timing spec is 4*, and he's saying to set to 16*.

I set static(no VAC connected) to 4*, ran it up and it ran very well. Connected vac and saw about 20* on the dial back. Normalized the temperature and verified around 180* operating. Took the car for a ride and it was perfect. A few WOT bursts and it was good. Brought it back and idled for a long time cool and comfortable. Since he DRIVES his cars here in Florida heat, he added A/C, and even with that on the car still runs cool.

PB070214.jpgPB070215.jpgPB070217.jpgPB060207.jpg

This is my query to him.....

================================================== ================================================== =
Are you stating to set timing to 16* (with VAC can hose plugged)?
i.e. ...........
16*static + 13*(vac adv actual) + 20* (centrifugal) = 49* total timing

Or.....Are you stating to set timing to 16* (with VAC can hose connected)?
i.e. ...........
4*static + 13*(vac adv actual) + 20* (centrifugal) = apx 36*(actual 37*) total timing

Also, You used a B26 VAC. It's specs state starts at 6", all in at 12".
The B22 VAC starts at 8", all in at 15". Would that have been closer to get the "2 Inch Rule" since Frank's idle vacuum is around 20"?

================================================== ================================================== =

edit...........
I found a thread on CF where he explained why. The distributor had a hi-perf baseplate with the smaller slot for the VAC arm. Rather than opening the slot and using a B22, and since he wanted 36* total, he used the B26 (13* spec). Here's his explanation....

Frank's distributor had the stock, hi perf short advance slot in it, which resulted in a 16-degree initial spec to achieve 36 total. I could have machined the slot longer, but the 16 degree initial spec actually tends to work pretty well when combined with a 12-14 degree vacuum advance: This produces actual idle timing of about 30 degrees, which is about as perfect as you can get.

Yes, I replaced the vacuum advance control unit with a B26, limited to 13 degrees.

Mystery uncovered.....

Duke.... Yes i'm picking up what your throwing down! Don't get your feathers all ruffled!! I still want the vacuum lines connected to the carb base if possible. I need to examine this base plate to see if i can drill a passage for this to work. Has anyone done this before?

Well, I guess we can have a little bit of intermission until we hear back from the OP. Plus it keeps his thread up on top of the heap.
Yes it's a better method to start another thread with your question, and we've discussed that at length many times. Copy/Paste this string in the Advanced Search for lots of info. holley AND ported AND vacuum AND ( OR L71* OR L79* OR L68* 435 OR 400 )

I know how to drill out the Holley L71 center carb thanks to others here, but not sure about your L79 unit. It may be similar. If you start another thread and I bet those-in-the-know will jump in.

You should start another thread to discuss whatever issues/questions you have with a '68 L-79. This one is about a '63 L-76 even though the OP seems to have gone AWOL. That seems to be happening a lot, lately.
Duke

Hmmm, I get frustrated getting involved in threads and never get replies from the OP's. I have a list of about 5 or 6 threads on here and the CF where they never return. Hopefully not for serious reasons.

Richard, the OP......where are you? We're all waiting. Sorry we got a little sidelined with unrelated jargon(me especially), but we're all here to help you get your answers. Plus it helps others in the future when they search the archives in here.

Richard, My '63 360 HP with '097 cam, NOS 201 VA puts out 16" of vacuum at 900 RPM. Same engine as yours except for the FI. John

John, did you ever test your MS 201-16. I tested one once and it was way out of spec. It was a used one and likely just a defective or worn out spring, but NOS has old rubber diaphragms inside and wondering if yours is still good. Too bad they're sealed units, so we could change diaphragms and springs and still retain the external appearance. Hmmm, Frank A. can do that to CSV's, I wonder if he can do that the VACs?

Rich

[Duke W.]

Re: Vacuum reading at idle 63-327-340 HP

It's not clear to me why your friend's 250 HP distributor is set up that way (and Lars and I are usually on the same page), but here's how and why it was set up by Chevrolet. All '63 engines including FI have the same OE spark advance map except FI had ported vacuum advance for unknown reasons, which was a one year deal only.

The centrifugal is 0 @ 700, 24 @ 4600 and the 201 15 VAC is 0 @ 8", 15 @ 15.5". Initial timing recommendation is 4 deg. for 250 HP, 8 for 300HP and 10 for 340/360 HP. These are very conservative spark advance maps and the reasons for this are in the San Diego presentation.

The reason 250 HP has less recommended initial timing is probably detonation. The small port heads generate higher VE at low revs, which makes it more suspetible to detonation than the 300 HP engine, so the initial timing recommendation is very conservative, and total WOT advance at that timing setting is a mere 28 degrees, which is quite short the the nominal ideal of 38 (36-40 range).

SHP engines need more total idle and low load advance and will tolerate it without detonation - reasons in the San Diego presentation - so less centrifugal and more initial works best. For base engines more centrifugal and less initial is ideal since they need less total idle and low load advance and actually have more low rev detonation tendency due to the early closing inlet valve and high low rev VE.

The '66-'67 300 HP setup is best with 30 centrifugal and nominally 8 initial although the centrifugal is not all in until 5100, which is lazy and can usually be made more aggressive without detonation. Then the optimal VAC should be the least aggressive to meet the Two-Inch Rule and is likely different for manuals idling in neutral at 500 and Powerglides idling at 450-500 in Drive.

I'm surprised your friend's 250 HP engine does not detonate on the current spark advance map, but I suspect it has lower than OE compression. Measure the head gasket thickness. OE is .018", so if it has a typical .038" composition gasket the actual CR is likely no more than 9.5, and with a proper spark advance map may run detonation free on unleaded regular or mid grade fuel.

My recommendation for base engines that have less than 28-30 centrifugal is to grind the slot to bring it 28-30, then set initial to achieve nominally 38 and use lighter springs to get it all in by 3000-3500.

At the other end of the spectrum is the '64-'65 SHP/FI setup where 24 centrifugal is all in at 2350 with a 8" VAC. Because of the high exhaust gas dilution due to the very high overlap 30-30 cam and late closing inlet valve, these engines, and most other SHP/FI engines need and can tolerate a lot more idle and low load advance.

A local chapter member recently had a detonation problem on a '67 300 HP engine, and when he checked initial timing it was 12 degrees and total WOT advance was about 42, all in at 4000. It was simply a matter of dialing back the initial to 8, which yielded 38 total, all in by 4000, and the detonation was quelled on California 91 PON fuel.

The onset of detonation can be very sudden as initial is advanced, and often as little at two degrees less initial timing is all that's necessary, but if total WOT advance drops below 36 you may be giving up some power and fuel economy. In the above case the initial timing was clearly set improperly as it yielded more than the nominal 36-40 degree Total WOT range, which caused moderate detonation, but dialing the inital back to achieve 38 total WOT advance was all that was needed.

Most engines operate most efficiently at all speeds and loads on the ragged edge of detonation. This is how most modern engines are set up, but they have detonation sensors and software that will dial back advance to keep things under control. The crude, non- real time adjustable spark advance controls of our vintage engines had to be set up conservatively for worst case field conditions, but since most of us drive our cars in a much narrower range of operating conditions, there is almost always significant room for improvement. I give general guidelines for the various engine famileis in the San Diego presentation, and beyond that experimentation will often yield further optimization.

Duke

[Timothy B.]

Re: Vacuum reading at idle 63-327-340 HP

Okay good, I understand, thanks. Sorry if I got a little postal.

Well, I guess we can have a little bit of "base-engine" intermission until we hear back from the OP. What you explained is just what I was thinking about the base setup. Makes sense. When I pulled the distributor for the intake manifold change and saw the B26 I was surprised. I expected a B22. Plus when I tested it it was all in around 10", even more aggressive I thought. But maybe my vacuum gauge has gone bad at higher readings, it's quite old. I haven't heard back from Lars yet about his B26 VAC choice. However, I got the numbers from the car owner's paperwork.....

"B26 Vacuum Advance"
"20* centrifugal advance"
"Set timing to 16* below 700 RPM, or set for 36* total"
"Vac Adv starts at 6", Full Vac Adv 13* @ 13"

What's very confusing to me is the fact that the base 250hp timing spec is 4*, and he's saying to set to 16*.

I set static(no VAC connected) to 4*, ran it up and it ran very well. Connected vac and saw about 20* on the dial back. Normalized the temperature and verified around 180* operating. Took the car for a ride and it was perfect. A few WOT bursts and it was good. Brought it back and idled for a long time cool and comfortable. Since he DRIVES his cars here in Florida heat, he added A/C, and even with that on the car still runs cool.

[ATTACH=CONFIG]56087[/ATTACH][ATTACH=CONFIG]56088[/ATTACH][ATTACH=CONFIG]56089[/ATTACH][ATTACH=CONFIG]56090[/ATTACH]

This is my query to him.....

================================================== ================================================== =
Are you stating to set timing to 16* (with VAC can hose plugged)?
i.e. ...........
16*static + 13*(vac adv actual) + 20* (centrifugal) = 49* total timing

Or.....Are you stating to set timing to 16* (with VAC can hose connected)?
i.e. ...........
4*static + 13*(vac adv actual) + 20* (centrifugal) = apx 36*(actual 37*) total timing

Also, You used a B26 VAC. It's specs state starts at 6", all in at 12".
The B22 VAC starts at 8", all in at 15". Would that have been closer to get the "2 Inch Rule" since Frank's idle vacuum is around 20"?

================================================== ================================================== =

edit...........
I found a thread on CF where he explained why. The distributor had a hi-perf baseplate with the smaller slot for the VAC arm. Rather than opening the slot and using a B22, and since he wanted 36* total, he used the B26 (13* spec). Here's his explanation....

Frank's distributor had the stock, hi perf short advance slot in it, which resulted in a 16-degree initial spec to achieve 36 total. I could have machined the slot longer, but the 16 degree initial spec actually tends to work pretty well when combined with a 12-14 degree vacuum advance: This produces actual idle timing of about 30 degrees, which is about as perfect as you can get.

Yes, I replaced the vacuum advance control unit with a B26, limited to 13 degrees.

Mystery uncovered.....



Well, I guess we can have a little bit of intermission until we hear back from the OP. Plus it keeps his thread up on top of the heap.
Yes it's a better method to start another thread with your question, and we've discussed that at length many times. Copy/Paste this string in the Advanced Search for lots of info. holley AND ported AND vacuum AND ( OR L71* OR L79* OR L68* 435 OR 400 )

I know how to drill out the Holley L71 center carb thanks to others here, but not sure about your L79 unit. It may be similar. If you start another thread and I bet those-in-the-know will jump in.



Hmmm, I get frustrated getting involved in threads and never get replies from the OP's. I have a list of about 5 or 6 threads on here and the CF where they never return. Hopefully not for serious reasons.

Richard, the OP......where are you? We're all waiting. Sorry we got a little sidelined with unrelated jargon(me especially), but we're all here to help you get your answers. Plus it helps others in the future when they search the archives in here.



John, did you ever test your MS 201-16. I tested one once and it was way out of spec. It was a used one and likely just a defective or worn out spring, but NOS has old rubber diaphragms inside and wondering if yours is still good. Too bad they're sealed units, so we could change diaphragms and springs and still retain the external appearance. Hmmm, Frank A. can do that to CSV's, I wonder if he can do that the VACs?

Rich

Rich,

I think the initial at 16* will be fine with the 20* centrifugal advance, the only thing I would have done different is use the B-22 control which will retard quicker. Frank's car will run fine with 30* idle timing.

[Stuart F.]

Re: Vacuum reading at idle 63-327-340 HP

I used to run what I thought was 12 degrees initial with my SHP 63, until I learned on this forum that our tab marks are not each 2 degrees, rather more like 1.3 degrees each. As I recall, a full 12 degrees (maybe it was 14) would put you up off the end of the tab. That's why I conservatively set mine at 10 degrees. I'm not certain about this and am away from home and my notes. Maybe someone can confirm this.

Not sure if this would be a concern with the OP.

Stu Fox

[Duke W.]

Re: Vacuum reading at idle 63-327-340 HP

It sounds like you have it backwards. The tab readings are not accurate with the 8" SHP torsional balancer, but the inaccuracy is to the low side, so if you set it at 12 on the tab, you're closer to 10 true. Two degrees on the tab is about 1.6 actual.

That's why setting total WOT advance with a dial back light at an engine speed above where the centrifugal is all in is a better way to set timing, and if you have light springs, it shouldn't be more than 3000-3500 and can be as low as 2500 with the 365/375 HP weights and springs in a 24 degree distributor.

If you set total WOT advance at 38, which is 14 true initial on a 24 degree distributor, actual initial reading will be about 16 - pretty much at the end of the tab, but you have to get the engine down to 700 or less or you may be picking some centrifugal. The no-brainer way to set timing on SHP engines with quick centrifugal curves is to use the above method.

Back in the day I used to eyeball my initial at about 18 that I thought was 42 total, but it was actually about 39-40. I figured out the inaccuracy after wrapping my balancer with timing tape. With the timing tape I could set total WOT advance with a simple (non-dial back) timing light, and accurately read initial timing.

Some guys would just bump the initial in two degree increments until they got a little detonation, then back off a couple of degrees, and they were probably in the 40-42 range, but I certainly don't recommend that today when you can buy a decent dial-back timing light for about 50 bucks.

If you set your initial timing at ten on the tab, it's actually only about eight, 32 total WOT advance with a 24 degee distributor, and you're giving up measureable power and fuel economy. If we say ideal is 38, 35 will lose about one percent peak power, 32 2-3 percent, and fuel economy about 5 percent.

Duke

[Stuart F.]

Re: Vacuum reading at idle 63-327-340 HP

Duke;

Thanks for the clarification, I.e. 1.6 degrees not 1.3. What I believe I did was take 10 x 1.6 = 16 degree marks on the tab in order to arrive at an actual 10 degrees initial. That's why, as I recall, if I wanted to set it at an actual 12 initial, it would put me at the top or nearly off the tab. Does that make sense? Again, I'm not at home so I can't check my notes or my Vette.

The "blue dot" springs I use are rated to be all in @3000 RPM. The weights are from a 64/65 SHP, but are ground off on the little toe so they will pull all in on the "football" (cam). That was something we did back in the day, but I don't recall exactly why

[Duke W.]

Re: Vacuum reading at idle 63-327-340 HP

What you say makes sense, but you're guessing, and now you've established that your distributor may no longer have the OE 24 degrees centrifugal due to grinding the weights. Rather than guessing at what the initial timing should be use a dial back timing light, rev the engine about 500 beyond where you verify that the centrifugal stops, set the dial to 38-39, and rotate the dist. as necessary to bring the balancer notch back to zero on the tab. After the distributor is snugged down run the test again to check accuracy, then blip the throttle hard to 6000 or so to verify that timing does not overadvance at high revs due to a deteriorated limit bushing or some other distributor problem

Then if you want, bring engine revs down to below where the centrifugal starts - probably no more than 700 - read the initial timing on the tab with the dial at zero, then bring the balancer notch back to zero with the dial and compare the dial reading with the tab reading, and that will yield the inaccuracy on the tab at that setting, and with a little simple math you can compute what two degrees on the tab actually is. This can be tough to do because mechanical lifter cam engines will barely run stably long enough at 700 or less to get accurate readings, which is I why I prefer the method I outlined if you want an accurate, specific setting.

It takes more time to R&I the distributor shield and set up/tear down the light and dwell/tach then it does to do the actual work. What could be simpler or more accurate?

Duke

[William F.]

Re: Vacuum reading at idle 63-327-340 HP

So, Duke, does the inaccuracy of the marks on the tab also apply to my '62 340 hp 327?
Thanks in "advance"

[Richard M.]

Re: Vacuum reading at idle 63-327-340 HP

Hmmm, still no OP activity? :toetap He's going to have alot of catching up to do......

It's not clear to me why your friend's 250 HP distributor is set up that way (and Lars and I are usually on the same page), but here's how and why it was set up by Chevrolet......

........I'm surprised your friend's 250 HP engine does not detonate on the current spark advance map, but I suspect it has lower than OE compression. Measure the head gasket thickness. OE is .018", so if it has a typical .038" composition gasket the actual CR is likely no more than 9.5, and with a proper spark advance map may run detonation free on unleaded regular or mid grade fuel.

My recommendation for base engines that have less than 28-30 centrifugal is to grind the slot to bring it 28-30, then set initial to achieve nominally 38 and use lighter springs to get it all in by 3000-3500.

Duke

Thanks for that explanation. More to comprehend and I understand better now. When I had the car here it appeared the heads were off and had thicker gaskets, I eyballed them as thick composition Felpros, which have a CT of around 0.038".

Here is the reply from Lars......

Rich -
Timing is always set with vacuum advance hose disconnected. Set timing to 16 degrees with hose unplugged. Once done, re-connect the hose. This will produce a near-perfect timing setup. If you set his timing at 4 degrees, it will be grossly retarded.
The 2" rule is a minimum. The B26 will work just fine on Frank's car.
Lars

Rich,

I think the initial at 16* will be fine with the 20* centrifugal advance, the only thing I would have done different is use the B-22 control which will retard quicker. Frank's car will run fine with 30* idle timing.

And that it does. It's running fine at those numbers.

[Timothy B.]

Re: Vacuum reading at idle 63-327-340 HP

My 67 300hp distributor has a 355 V/A control and it's specs are 15*@12" so the B-26 is probably the aftermarket replacement.

[Richard M.]

Re: Vacuum reading at idle 63-327-340 HP

We all obviously got into a discussion well beyond the OP's initial question. All great information. I think however he's gone AWOL.

So, since this has gotten totally off from a simple "what's my vacuum on a 340" question, I must express a issue with the modifications done to the distributor on the '63 I worked on and the subject of my initial input in this thread. In a way I feel I've been hoodwinked with the distributor issue in this car.

Case in point, I get a car in the shop from someone that asks to have his engine worked on. I change his valve seals due to a occasional start-up puff of smoke, swap the incorrect Edelbrock intake and AFB to the stock cast iron intake and WCFB. Get that ugly pipe-nippled bypass aftermarket water pump out of there and install a rebuilt '608. Actually 2 of them because the POS Goat Hill rebuilt '608 I had shipped from my supplier had a loose bearing that screeched like a Red Tail Hawk, so I rebuilt my own '608 core I got from a buddy........ on a Sunday night no less.

In the process of all this fun of taking things apart and putting them back together.....of course I remove the distributor and table it during the process. I do all of my work, clean and detail the engine. I remove oil laden sand from every crevice you could find on the engine due to some bonehead body painter that didn't mask the engine bay when he sandblasted the body. I then repaint the engine by hand with a 1" artist fine detail brush because I didn't want overspray anywhere, etc, etc.

Then refinish and reassemble all of the pieces of the puzzle.

It's all finally back together and looking good. I go to doublecheck my AMA specs from the GM Heritage site, and read 250HP timing spec at 4*. I set my HB mark at 4*, pop in the distributor to line up at the #1, etc, etc. I run the engine, check basic timing, reconnect the VAC hose, adjust carb settings, take it for a ride and all appears good. I feel I'm done.

Then I find out that the timing should be at 16* static.......a miniscule 12* more than spec.

My point is, this distributor was worked on and designed to be tuned out of "spec" due to internal modifications. Unless you have a data card after it's been worked on, expect to perform a detailed timing and verification adventure when reinstalling it. I certainly wish I had the data card on this one before reassembly and startup.

A label affixed inside the distributor would have given me a better understanding of it's "uniqueness".

Rich

[Timothy B.]

Re: Vacuum reading at idle 63-327-340 HP

I agree with you, Rich, a tag showing the curve would have been helpful. Lars was probably working with what was sent to him because someone has been there before.

[Stuart F.]

Re: Vacuum reading at idle 63-327-340 HP

Since we're in the explanation mode now; let me further describe my distributor weight modification. The grinding is done on the outside of the small toe only to allow the weights to both close in tight to the football (cam). That should not affect the total mechanical advance. My distributor was set up on a distributor machine many years ago and I used to set the initial @ 10 degrees to give me the desired total of 38 degrees. Then I learned about the timing tab issue here on the board and reset my initial to a calculated 10 x 1.6 = 16 (8 degree marks on the tab) instead of the 5 marks position I used before. The results seem to be very positive with improved performance.

I believe I will follow Duke's instructions primarily to verify my "guessing". About the only better way would be to do like we used to do using a chassis dynamometer, I.e. Pull #1 = Engine accessories set for normal street driving (air cleaners on, etc.), locate initial setting for best power, apply a white tick mark, then Pull #2 = Engine set up for racing (air cleaners off, lake plugs open, etc.), locate initial for best power, apply second tick mark, pay the tech man and peel out!

If I recall right, there was another OP who left for a three week trip with out us threshing out his problem. Where's he at?

Stu Fox

[Duke W.]

Re: Vacuum reading at idle 63-327-340 HP

The grinding is done on the outside of the small toe only to allow the weights to both close in tight to the football (cam). That should not affect the total mechanical advance. My distributor was set up on a distributor machine many years ago and I used to set the initial @ 10 degrees to give me the desired total of 38 degrees.

On some distributors the weights do not retract all the way to the football, but hit each other. If you grind away this interference to allow the weights to retract further - all the way to the football, total centrifugal advance is increased.

You said your setup requires 10 initial to achieve 38 total WOT advance, which implies that total centrifugal is 28. The OE '63 centrifugal setup is 24 for all engines, which means 14 initial (about 16 on the tab for SHP/FI engines with the 8" balancer) is required to achieve 38 degrees total WOT advance.

You can't beat setting "timing" by the method I described on SHP/FI engines. It's very accurate and almost idiot proof. On base engines that still have the OE centrifugal that maxes out as hilgh as 5100, I set initial timing using the traditional method because the centrifugal usually doesn't start until about 700. So I can set initial with the engine idling stabily at about 500, but before doing so I always verify that the centrifugal starts above the engine speed that I set initial timing.

Over on the "other forum" the standard advice is to set total WOT advance at 3000 without regard to were the centrifugal advance maxes out. Totally clueless! On most OE engines centrifugal maxes out in the 4000-5100 range, so setting it at 3000 will result in too much advance and likely detonation.

Duke

[Stuart F.]

Re: Vacuum reading at idle 63-327-340 HP

Thanks Duke. Believe we are in tune on this, so to speak. I'm anxious to get back home to check it out. Leaving Chicago tomorrow for Cleveland to wrap up this medical trip, then home on Saturday if all goes well.

Stu Fox