'66 427/425 UPDATE for Duke Williams

[R N.]

Back in early Sept. (approx. 9th) I posted some questions about some cooling problems I was having with the L-72 in my car. Duke Williams sugested a five step process, in which I've completed most, but still have some questions.

I have since connected the vacuum line from the vacuum can to the other vacuum line on the carb (non-ported side) and now have vacuum at the vacuum can at idle. This has resulted in excelent idle and response. Some how I fell I still don't have enough vacuum. (see details below)

Now the questions. Would I have more vacuum by installing an adapter in the manifold plug direct vs: tapping into the existing vacuum line on the carb?

I have not changed the can as of yet, (as recommended) but would like to know does this NAPA one (Echlin/NAPA # VC1810) have lighter springs to give more advance than what I have now? (I don't know which vacuum can I have on the car now, but I assume it is the correct replacement for the L-72.)

Here is the current readings from my car:
Test # 1 - Hooked to ported vacuum
Base timming 6 @ 850RPM (no vacuum at can when hooked to ported vacuum)

8 @ 1000 RPM

17 @ 1500 RPM

37 @ 2000 RPM

45 @ 2500 RPM

Test # 2 - Mechaincal advance only, vacuum line disconnected.
7 @ 1000 RPM
17 @ 1500 RPM
24 @ 2000 RPM
30 @ 2500 RPM

Test # 3 - Vacuum advance only.
5" = 6 = base timing
8" = 12 - 6 (base timing) =6
10" = 18 - 6 = 12
15" =26 - 6 = 20

And the last question, the factory calls for timimg of 8 degrees BTDC. Is this a good starting base?

As usual thank you for your help.

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

Based on conversations and data from other L-72 owners the OEM vacuum signal line on the carb. is "ported" and does not provide full manifold vacuum to the vacuum can at idle and off idle operation.

A source of full manifold vacuum is the choke vacuum break line, so in order to provide full manifold vacuum to the vacuum can, you could tee into this line, or as an alternative you could probably find a 1/8" vacuum nipple pipe plug adaptor that will thread into the manifold in place of the plug. I don't think there would be a functional difference between these two sources.

Beyond this, the OEM L-72 vacuum can does not have suitable specs for a SHP engine, and this is why I recommend the NAPA/Echlin VC1810 which is equivalent to the OEM Delco can on '65 SHP/FI engines. This can starts at about 4" provides a maximum of 16 degrees of advance at 8" manifold vacuum. The OEM L-72 can starts at about 6" and delivers a maximum of 15 degrees at 15.5". The engine will not achieve 15.5" of vacuum at any reasonable idle speed (say up to 1000 RPM), no matter what you do. The OEM L-72 vacuum can will not allow the engine to achieve optimum idle timing of 25 to 30 degrees, which will minimize idle fuel consumption and EGT. Minumum EGT will minimize the amount of heat rejected to the cooling system, which should keep idle coolant temperature within reasonable bounds assuming the radiator is in good condition.

Being as how the L-72 was the only Corvette that did NOT have AIR as part of the California emissions package, it appears that CA emissions were met with ported vacuum and a "lazy" vacuum can, and, for whatever reason, Chevrolet Engineering elected to sell this single configuration nationwide.

Mechanical lifter SHP engines, both big block and small block have very similar vacuum characteristics. The '65 SHP/FI vacuum can has proved its worth on SB SHP engines, and I highly recommend it for SHP BB engines. The basic rule of thumb for vacuum advance - absent emissions requirements - is to have full vacuum advance delivered at least two inches Hg. below the engine's idle vacuum,. so if the engine idles at 10-12" you need the 8" can.

The centrifugal spec for the L-72 is 0@900, no intermediate spec, 30@5000. The initial timing is 8 degrees with a range of 8-14. These specs are from Corvette News.

From what you have reported, your centrifugal is faster than OEM. This is fine and is a good way to gain low and mid-range torque. In general it is best to get the centrifugal in as soon as possible, but the limiting factor is usually low rev detonation. In this case you must either slow down the centrifugal, decrease the initial, or come up with a higher octane fuel blend.

Initial of 8 degrees is conservative, so you can experiment with higher values up to 14. Keep in mind that if timing is varied two to three degrees from optimum it will only reduce power about one percent, so it's tough to determine optimum total WOT timing (the sum of initial and centrifugal), either SOTP or even with dyno testing.

My recommendation would be to cut to the chase, spend the twenty bucks on the VC1810 can, install it, and rig up a signal line with full manifold vacuum to the can. One L-72 owner said he can swap cans without removing and disassembling the mag pulse distributor.

Set the initial at 10-12 then connect the can and go through the idle speed mixture adjustement procedure. Set the idle speed at the lowest possible, commensurate with acceptable idle quality, AND at least 10" manifold vacuum. This will probably be in the range of 800-900.

Now take it out for a test drive. Load it up at low revs - WOT from 1000 in top gear and listen for detonation.

Duke

[John L.]

Re: '66 427/425 UPDATE for Duke Williams

Kurt

Here are the vacuum readings from my L-72 from the normal source.

RPM In/Vac

800 5
900 6.5
1000 7
1300 9
1500 11
2000 15
2200 16
2500 18
2700 18.5

The VC1810 can cost me $8.99 at my local NAPA store. It hits the stop at 8" of vacuum as verified by my Mity Vac. I am idling at 950 rpm. That seems to give the motor enough vacuum and advance to run well. Granted it is not the full 8" however darn close to full advance. I really did not want to tap into the choke cut off. The modified plug in the manifold sounds like a good idea. I would be really curious to see what your vacuum readings are at that location. I am running 9-10 degress initial and stock mech advance specs on the distributor. The motor ran very well after switching to the 1810 can. Althought my overtemp probems did not go away completely, it has sure helped. I use a 50/50 mix of racing fuel (110 or 112 both leaded) and pump 93 unladed. I do not have detonation probems.

I was not able to see a way to swap the cans without removing the distributor. I have also read the post where someone did however it was not me and for the life of me I can't see how they did it.

Good luck with your L-72. Please keep us posted as I am interested to see what the net effect is on your temp situation.

john lolli

[Everett Ogilvie]

Re: '66 427/425 UPDATE for Duke Williams

You can remove the vacuum can with the dist in the car if your shielding is removed. Mine is removed b/c I have been working on my L72 timing/vacuum (with Duke's help) for some time now. If you are in the same boat, remove the shielding now and leave it off until all your testing is complete.

Remove the two screws from the vacuum can. The inner one is tight, but if you use a small screwdriver it helps with the slight angle. Rotate the stationary pole unit (which is no longer stationary b/c the screws are removed) counterclockwise and slide the vacuum unit along (out) while rotating. At the end of the travel lift the vacuum can up, twist, etc. to disengage the hook from the stationary pole unit. Installation is the reverse - it is a bit tricky to engage the hook but it can be done.

If your shielding is off, it is simpler to just pull the dist to remove the vacuum can, but you never have to disassemble the dist to remove the can.

8 degrees initial is very conservative for this motor. I run 14 and may go to 16 or higher (to try to get 36 or more total mechanical at WOT). I buy good fuel so I don't have detonation problems. Set your total mech advance where you want it and let the initial fall where it may. Also, to see your TRUE initial without any component of centrifugal, you have to reduce your idle to 550 or 600. For example, if your idle is 800 to 1000, your centrifugal is adding 2 to 4 degrees to your initial. If you set your initial up at 10 degrees, at idle speed of 1000, your true initial is only about 6 - this is not enough and you are limiting your total mechanical advance at WOT.

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

Clarification, John. What is your 950 RPM idle vacuum with the 8" can connected to a full manifold vacuum source?

Duke

[R N.]

Great help

In response to Duke Williams, John Lolli and Everett Ogilvie, Thank you.

I have put a tee into the choke vacuum break line and connected the vacuum can there and WOW, what a difference in idle. Currently it's idling @ 950. As my car has been restored back to orininal I do not what to make too many modifications, splicing into the choke vacuum break to me if the easiest to switch back for judging. However, in an earlier post from Duke he did state "If I owned a L-72 Corvette I would probably figure out how to modify the carb internally to provide full manifold vacuum to the vacuum can signal line at idle. This would be undetectable during judging and I doubt if it could be caught during a PV either" This to for me and others would be the best solution. Duke, do you or any others know who could to this?

Monday I will order a new can (Echlin/NAPA # VC1810) to have installed on the car, which leads me another question. Will the NAPA can pass judging or do I need to purchase the correct replacement?

Once I have the new can I will have it installed and set the intial timing as recommended, and then check all my readings again.

John, thank you for the readings from your L-72. They will be good reference for me to compare to. Once I have my new readings I will let you know my results. Regarding your over-temp problems I replace the original clutch fan (just until I have the orinal rebuilt) with GM # 3916141 and that has made a difference in the reading I get in my temperature gauge, much cooling running at lower speeds and idle. Perhaps check your fan.
Also, as Duke asked I too would like to know what your vacuum is at 950 idle connected to full manifold vacuum. You also mentioned you use a fuel mixture of racing fuel and 93 unleaded. Where can one purchase racing fuel?

I will take Everett's advise and remove the shelding first (and keep it off until everything has been done)and attemp to replace the can w/o removing the distributor. I will take your suggestion about intial timing and set it with a little more, than less.

Regarding detonation, I currently don't have a problem, but my intial timing is only at 6 degrees and I have been using Sunoco 94 and I add a lead additive.

I'll keep you posted after I can get the next step done.

[Clem Z.]

question for Duke Williams

duke why do these cars now have this heating problem as i drove my new BB corvettes everywhere and never had a heating problem, even to south carolina in the heat of the summer. i also always took off my front bumpers just for the looks and better weight transfer when street racing(i know i be bad),would this allow better air flow? could it be the temp gages are off now? can it be the fuel? my BB never had the vacuum advance connected because i had full mechanical advance in at 2500/3000RPM with a total of 38 degrees.

[Duke W.]

Re: question for Duke Williams

My recollection is that BBs always ran a little warm compared to SBs, especially in hot weather, low speed driving. No doubt not having the front bumpers on, especially the front license plate improved air flow into the radiator, but this wouldn't have had much effect at low speed.

You aggressive centrifugal advance curve probably kept your BB from running hot at highway speed, but, again, it wouldn't have had much effect in low speed traffic driving.

The slow clogging of radiators probably has some effect, and maybe our memories are starting to get cobwebs, too.

Duke

[Duke W.]

Re: Great help

Not being a Holley carb expert I don't know how to modify the L-72 carb. to achieve full vacuum on the vacuum signal line though I expect it could be done in an undetectable manner.

A Holley carb expert like Clem Zahrobsky could probably figure out a way.

You'll have to consult your '66 JG and perhaps the Team Leader to see what, if any deduction, would apply toward having an incorrect vacuum can. As far as I know the NAPA/Echlin cans have the same visual appearance as an original Delco can, but NAPA/Echlins all have different codes stamped on the mounting bracket and the code is visible with the shielding off. At Monterey this year I OJed on '63s and noticed a car with an incorrect can, but the '63 JG does not mention the markings on the correct can and the Team Leader told me if it wasn't in the book, he would not deduct.

In any event, swapping back and forth between configurations shouldn't be too much work, especially once you've done it a couple of times.

Duke

[Clem Z.]

Re: question for Duke Williams

maybe the reason is that to me 200 degrees is not hot as i know from dyno testing that race engines make the most HP at 190/200. the new LS-1/LS-6 corvette engines make the most power at this temp also. i must admit i never had a BB corvette ever puke water even when shut off after a hard run. i think it may something to due with the rads as my 88 C-2500 ext cab with 100,000+ miles now runs hotter than before while pulling a trailer but the red on the gauge does not start till 240 degrees and i have seen it at 230 pulling a trailer. as long as it cools down after pulling the hill or moving faster i do not think anything is wrong. i have see my C-5 corvettes run as high a 216 degrees in traffic.

[Duke W.]

Re: question for Duke Williams

Aluminum engines in particular seem to like it hot! I remember back in '85 I was running the Cosworth Vega in a Porsche Owners Club event on the long course at Riverside. I was July - a little over 100 in the afternoon - and the radiator was getting clogged up with all the inorganic salts from the traditional green anti-freeze. It was running about 230, but had never run stronger. Down back straight it was pullling 6200 in fifth at the breaking point for Turn 9 - about 124 MPH, and it was still pulling.

About two years ago when I was driving back from the Western Regional at Lake Tahoe in May in a C5 Coupe I had from the GM press fleet I got stuck in bumper to bumper traffic climing up the Grapevine on I5 due to construction. It was unusually warm - about 100 degrees - and the coolant temp rose rapidly. I even turned off the a/c (dumb move) and started to worry as the temp exceeded 225. Then I said to myself: I'm in a brand new Corvette that's been tested in Death Valley in the middle of summer! Shortly thereafter, the fans came on and the temp dropped from about 228 to 215. I turned the a/c back on and slogged up the hill with the temp cycling between about 215 and 225. GM likes to run them hot for emissions, and I think aluminum engines, in particular will make best power at high temps because the higher the coolant temp, the less heat transfer to the coolant.

I just shake my head in amazement at the C5 guys who install 160 thermostats.

Duke

[Clem Z.]

full vacuum at idle with a holley

if your distributor vacuum advance goes into the side of the metering body there is a grooved passage on the main body side of the metering body that leads to a hole in the lower part passenger side of the main body which connects to the vacuum port in the throttle base plate.if the vacuum source feed hole in the throttle base plate is above the butterflys you will only have vacuum advance once you crack the throttle,so just drill a connecting hole to below the butterflys and fill the orignal hole above the butterflys with epoxy. e mail if you have any questions

[Clem Z.]

Re: question for Duke Williams

with the A/C off the second fan does not come on till 230+ degrees but with the air on the second fan is running all the time. this deal with low temp thermo came from when engines have aluminum intake manifold that picks up engine heat and heats up the fuel charge. the throttle body water bypass came from when corvettes had a aluminum FI dog house which picked up heat from the the TB. on a C-5 with the composite intake there is very little heat transfer from the TB of the engine itself so there is reason to lower the engine temp. it will take a while for old habits to die.

[John L.]

Re: Great help

I am fortunate to live in an area where muscle cars, street rods, drag racing etc are very popular. I buy either Torco or Cam2 fuels. I personally do not like VP fuels. I get the gas either at a speed shop near my home or a gas station nearby that actually sells it out of a pump. It is between $4.50-5.00 per gallon depending on current gas prices. I buy it in 5 or 10 gal increments.

Regarding the vacuum can -- the 66 JG does not specify any markings on the can. It only qualifies the approx size and finish so, in theory, there should be no deduction.

[R N.]

Re: full vacuum at idle with a holley

Thanks Clem. I'm sure Ill be in touch once I get every thing running correctly. You guys have been such a great help, Thnaks!