Doug1964

Did orig 1960 low hp corvette distributors have vacuum advance cans on them or were they mechanical advance only?

Frankie the Fink

Yes - and the vacuum was derived from a hollow carburetor mounting stud at the driver's side rear of the carb (as shown in picture). Higher horsepower motors had dual point, mechanical only distributors.

This pic is a '61 but '60 is the same.

Doug1964

Do you happen to know how much initial advance and how much vacuum advance they ran? I am tuning to 4 initial plus about 12 vacuum. However with only 4 initial I seem to be too little advance at WOT. However if I run more initial, then with the vacuum advance I have too much advance at idle

Vetterodder

It's very likely that a 50 year old distributor has different specs than original. Rebuild, modifications, replacement, etc.. I'd start by setting total to something reasonable like 36º. If that results in too little/too much initial, mechanical advance will need to be changed. As for "too much advance at idle", how much do you have? Keep in mind that there's no load at idle so lots of advance usually isn't a problem. A lot of us with modded engines idle with 30º or more (initial + vacuum).

Doug1964

I think your right. I am not running a completely stock engine. Has performer manifolfd and edelbrock 500 cfm carb. Not sure what cam is in it. The cam seems tame. However, the engine does not like running at the initial 4 degrees because I do not get enough total timing at WOT. I am going to forget about trying to set to the initial low hp spec of 4 and just adjust to the timing that gives me the best performance.

wmf62

Bill

Doug1964

what idle rpm are you tuning to. The spec 475 seems awfully low. Are you tuning idle closer to 750 rpm?

simard

I like the 36 degree answer. Remove the vacuum advance and rev the engine until the advance stops increasing. Then set the advance to 36 degrees at or above that rpm. Done.

Doug1964

I have steady dwell at 31 degrees. I set initia timing to between 10 -12. With Centrifugal on top, I am at about 36 total plus or minus one degree at about 2500 - 3000 rpm. There was a 3/8" spacer (four hole) between edelbrock carb and intake. I do not think the four hole was required because I was going from edelbrock performer to edelbrock performer. I removed that four hole gasket and used the open paper gasket that came with the new carb. I was able to get the rpms down to an acceptable 600-700 and acceptable quality idle with only very, very slight shaking. I believe the four hole spacer may have been blocking some internal passage ways under the carb and causing an internal vacuum leak in the carb. That seems to have solved my high rpm problem. Before, if I advanced initial plus vacuum, I could not get the rpms to drop to below 1200 and the idle mixture screws had little effect. Now the IMS seems to do their intended job. I also have 19 inches of vacuum and it is steady. So I think I am good to go. I will give it a run tomorrow around town and try a little wide open throttle action and see if it misses or pings above 3000 rpms. If carb overheating becomes a problem, I will try to find a 3/8" open spacer in lieu of the four hole spacer. Thanks for the help.

wmf62

my .02...

first, i don't know how a carb spacer could block any carb functions; and second, i don't see the 4 holed spacer would make a difference even if the manifold has a full opening. however, using a open spacer between a 4 hole carb and a 4 hole manifold would certainly seem like a no-no...

someone please explain.
Bill

Frankie the Fink

This is why I like the Pertronix billet distributor with an adjustable vacuum advance (which I always add separately). I run my 283ci solid lifter motor right at 12* initial, and, with the Pertronix default mechanical advance (without limiter tabs) of 24* I get a nice 36* (initial + mechanical)...I use one medium and one light spring and get that to kick in at about 2500RPM (perfect with a Duntov 097 cam). Then I crank in as much vacuum advance as the motor can handle (I'm prob around 50* with EVERYthing 'all in' now). This is all running off FULL manifold vacuum of course. I don't have a SUN machine but a dial back timing light is nearly as good.

I run 87 octane all day long with no pinging, no surging at cruise speeds, engine runs nice and cool and is as peppy as a stock 283 gets.

Frankie the Fink

Really not the optimum way to do it (per Lars, Hinkley and many others). This way you are going for the max advance at the default manufacturer's RPM which is always conservative....could be as high as 2900RPM-3300RPM.

You really want to vary the RPM to get the optimal curve for YOUR SPECIFIC ENGINE (using various advance springs) and THEN shoot for the best advance (be it 36*, 38*, or whatever). E.g. 36* 'all in' may be optimal at 2400RPM for some motors and 2700RPM for others.

Doug - I think you are very close to the optimal setup on a slightly modified 283. If it runs good prob time to leave it be. You reach the point of 'diminishing returns' trying to over-tweak things to get that last micron of performance.

Matt Gruber

If a carb has a problem, say a clogged idle circuit, using an open gasket allows the 1 good circuit to feed the v8.
the 4 holer divides it into 2 V4's. If 1 idle screw is closed, those 4 cyls' shut off. The major advantage to a fully divided dual plane is 1 plane can not pull in exhaust during overlap from the OTHER plane. This makes a huge difference with a performance cam. It also changes timing needs. Exhaust diluted air needs more advance because it burns slower. My 61 needed 17 base timing w/open gasket, just 12 w/4 holer. Idle speed was 800-900 open, is now 625 4 hole. Mixture screws were 3 turns out, now 1.5T.

Doug1964

It appears as if there are some passage ways formed into the bottom of the edelbrock that would be blocked by the four hole spacer and that would not be blocked by the open spacer. Do not know enough about the carb to know what purpose the passages serve. All I know is that when I changed the four hole for the open spacer I was able to increase the timing to where it needed to be and still keep the rpms low for idle. It was not that it would idle rough but rather that it literally would not idle below 1000 no matter how far the idle adjustment screw was backed out. I suspect that with the passages blocked, the manifold would pull from both sides of the carb instead of just alternating sides. This then I beleive creates a vacuum in the throat that pulls from the throat beside it from the blocked passage way. Instead of the passage ways being open to the plenum, they were closed off and created a tunnel between the spacer and the bottom of the carb going left to right. Without the spacer, I now have the initial advance set to around 12 plus vacuum advance and I can pull the idle to 600 if I want. As far as the performance, the increase in advance completely woke up the engine. Pulls much harder and does not miss at WOT now.

wmf62

that's not the gasket or the spacer's fault; it's a carb problem...

the disadvantage to an open plenum is loss of low end torque...
Bill

Matt Gruber

Doug
Excellent description. The engine was so much more efficient 4-hole, it needed the throttle closed more, but it would not idle below 1000.
I'd be looking for a vacuum leak, or the throttle is not centered in the bore, or the SECONDARY is cracked open (very common). Needing more advance and liking it is exactly what happens when exhaust is drawn in. But less advance and less exhaust is more efficient. If it pulls harder it was probably too rich 4 hole. But open needs a richer mixture.
Like Bill says, watch out for a loss of low end torque! Does the link in my sig./profile still work? Related story there. Let me know if it is still working. Thanks guys!

Matt Gruber

i guess the sig. doesn't work.
here it is.
carb

4bbl vs. Individual Throttles? Is there any comparison?
I've always been fascinated by ONE throttle feeding ONE cylinder. Even back in the late 60's, Hillborn injected road race 350's, put out 650 hp and had a very wide power band.
I might have tried EFI "Hillborn" on my car if it didn't cost so much. But, with a TRUE divided dual plane, I found I don't need it to get many of the benefits of independant runners. http://images50.fotki.com/v1525/phot...G_6596a-vi.jpg
A totally divided dual plane is a clever way to get a wide power band, like with independant runners, at very low cost. Come out of a corner at a lower rpm without downshifting as much. But, any leakage between the 2 planes draws in exhaust during idle, cruise and part throttle, anytime there is high vacuum. The greater this internal leakage, the worse the street manners. The carb base has passages that connect the 2 planes. I blocked the passages and used a 4 hole gasket.
Bigger cams usually have higher overlap that kill the low end, giving it poor street manners. The full separation solves this common problem. Street manners have improved lots!.Climbs hills easily with light throttle and no need to downshift. A lot less downshifting in slow traffic. It pulls strong from 1000 rpm with light throttle just above idle! Was 2400 before with more throttle. Much higher mpg! Updates: Since 10-07, the 61 has never backfired, despite 45F cold starts, with no warmup, and no choke. Tried a 4011 Holley on my 72 but it would stall on 45F starts(no choke) Blocked the carb base passage(secondary only) 11-10, for 60% less cross flow, and on a 38F start it didn't stall! Quickie mpg check, was up to 18.3 from 14.2..
proper sealing raised mpg to 14.5-18.3 from ~11.
I thought my 61 got poor mpg due to the 4.11 rear and high overlap cam (224/224 110lsa). I've owned my 61 since '75. On trips in the 70's it got 20 mpg(218/218/114lsa). In '94 I added a performer 2101, AFR 190's and Magnum 270H cam. Mileage dropped to 14 w/3.36 rear. I was stumped on this, blaming it on the cam. ( A cam change to a wimpy cam would fix the mpg, but I wanted the HP and eventually found another cure.)
I jumped at the chance to swap my 3.36 peg leg for a 4.11 posi. Mileage averaged ~11 after that. Having fun, 11 isn't real bad, but that is awful at cruise.
If you look on the bottom of any carb, there are passages that connect the left with the right, usually 3. I plugged them to match the Performer 2101 dual plane, MPG went to 14.5-18.3 from ~11-14. Cost $2 for JB weld epoxy, but a TIG weld would be permanent. When I studied my spare carb, I realized a 4 hole thick gasket would seal it easier, using a lot less epoxy, I made my own gasket out of 2/10" plywood, extending it 2" under the Holley bowls, to act as a heat shield. Hot soak(engine off 20 min) dropped 10F. It raised the carb less than 3/16 inch.(old open gasket removed).
Why did my mpg go up? Less downshifting in traffic due more torque at lower revs, is my short answer. Here are my thoughts on why the engine became more efficient at low rpm:
The 4 hole gasket divides the V8 into 2, V4 engines, as far as what suction the carb sees. This is how factory engineers got good HP, strong torque, and good mpg when using a carb, they specified a dual plane with 4 holes.
The aftermarket 2 hole manifold largely defeated the intent of the factory design, wasting gas for no reason.
>>>>>>>>So without 4 holes cast into the manifold like the original, air leaks were created at the center divider thru passages in the carb base.. Not knowing this, I used the open gasket that came with the manifold. This open gasket allowed much more of the very bad cross plane airflow that sucks in exhaust fumes when both valves are open, during overlap. This combo wasted gas, made it run hotter, and gave it worse street manners with the CompCam! Guys often match the runners to the head, while the carb/flange interface is ignored, and this proved to be most important. With my Holley spread bore 4165, I should have used Holley 4 hole, 1/4" carburetor flange gasket #108-21.
So with the 4 hole gasket the carb sees a much stronger individual suction and gives far superior atomization, allowing it to waste less gas. The main circuit engages sooner, lowering economical cruising speed. There is little or no exhaust contamination from the high overlap, a trait performance cams are infamous for. So the intake charge is not as diluted. Less reversion may play a factor here too, reducing fuel flow.
Idle uses less fuel too. Went from 3 turns to ~1.5. From 2.1 gph to 0.9 gph.
(from 2007-11 on the corvetteforum)
Q. Why do some Qjets come with an open gasket?
A. Pollution control. EGR (exhaust gas recirculation) cuts pollution. The open center allows exhaust to enter the intake manifold when both valves are open, during overlap. Vacuum from one plane crosses over and reaches the open valves in the other plane and draws in exhaust. If your car flunks a pollution test, try an open gasket! (Cars in the 70's had many items that pleased the EPA, but often cut torque, power, mpg, and throttle response).
Q: I don't have 1/4" of height for the gasket. What can I do?
A: Take an aluminum plate out of the Mr. Gasket #86 spread bore or #97 square bore heat dissipator kit. Summit $15 plus $4.99 to ship. The thin aluminum plate will work just as well separating the high & low plane. A thin paper or rubber gasket would not have any metal to support it, as around the 4 holes on an original manifold. It could be destroyed by the pulses and get eaten by the engine, so don't try that! A gasket designed to withstand the pulses has to be pretty thick, about 1/4 inch in paper composite or 2/10" in wood. Aluminum plate, no problem.
Q: What should I look for when buying a manifold?
A: Take very seriously the rpm range. Idle to XXXX rpm should mean it has a full height center divider. The 1500-XXXX rpm is sometimes actually a race manifold. Race manifolds have a cut down center divider that can impair normal low speed street use. Good for high rpm power when racing, but bad mpg at 35-65 mph cruise. Comparativly poor street manners. But it is easy to convert these cut-down to more friendly street use; simply weld up the center divider to fully separate the high & low plane. Or take the above thin aluminum plate and weld a divider filler to it. Then you can take it out as often as you like(for drag racing). Make sure it is thick and strong. see posts #2 and 3 http://speedtalk.com/forum/viewtopic.php?t=19990
Or just sell it and buy one like the Weiand 8150 $140 or their raised plenum 8501 $216 for summer use only..
Q: Why do so many manifolds have a cut down center divider?
A: They often get dyno tested and manifold makers want to win business! Nobody tests MPG, or normal driving back and forth to work.. Who cares if it gets 11 mpg instead of 18? Certainly not the car magazines trying to sell horsepower. They sure don't often drive it around and compare street manners. Who drives off 5 seconds after a cold start and rates drivability? I sure don't want to wait around for it to warm up! This is VERY important on a real street car.
Q: Is the epoxy necessary? Will a 4 hole thick gasket be effective?
A: Keep it simple and skip the epoxy. Thick 4 hole gaskets that separate the high plane from the low plane, and can maintain good road manners & mpg have been available for decades. I just didn't understand how important the base gasket is. And lots of more popular Holleys have tiny cross passages. Don't waste any time filling them in.
Buy a 4 hole base gasket and try it: First thing you will notice is those idle mixture screws will need to be turned in, and it may need less base timing.
FELPRO #1905 1/4" heat insulator gasket. fits QJET
Square bore Holley 108-51 0.26" or 108-58 0.24", 108-21 1/4" fits 4165 spread bore.
Q: What about high rpm power?
A. I,ve read posts by Joe Sherman that say an RPM dual plane can beat a single plane to 6300 rpm and produce 525-550 HP if an open spacer is used (at the track).
Check the vacuum at WOT at MAX rpm with and without.. If over 2", the carb is too small.
Q. Use a single plane on the street like LS1's?
A. Works well with a stock weenie cam as LS1's prove. But LS1's corrected the early Gen1 design flaw and now use a superior firing order. Early SBC's fire 5 then 7, creating an imbalance . The dual plane corrects that mistake.
Use a lumpy cam on an early SBC under 383 with a SP only if you like steep gears, lots of shifting or a high stall. With a stick it may have less power off idle than a 4 cyl VW. Some like the cool sound from an engine inhaling lots of exhaust. It lumps, and rumps and shakes the fenders as it chokes on exhaust. Once you realize the cause of the bad idle, why would you want to feed it more exhaust than you have to?
Q. Isn't a dual plane obsolete? What new cars use a dual plane?
A. Ferrari V12, Jaguar V12, Viper V10 all use dual planes (look for dual plenums).Dual planes cost more, way more with EFI so GM and Ford bean counters eliminated them.
See Vizards cam article, has nice pic showing overlap sucking in exhaust, and lots more cam info! http:http://www.popularhotrodding.com/tec...ics/index.html
Q: Could just turning in the idle mixture screws help mpg?
A: Sure!
Just turn in the screws 1/8 turn per week and check mpg. The leaner mixture will use less gas. When you go too far it will stall often. Then go 1/16 turn out. There abouts you will find the sweet spot.
Next lower the idle speed screw to minimum, a little each trip until it stalls too often, then back up a bit. The lower idle speed will use less gas. My 61 was 2.1 GPH, now it is 0.9.
9-12 Update. Note how different the tune is without excessive exhaust dilution.
BEFORE............................ .AFTER 4 Hole gasket
Base Timing 17* ......... ........ ..12-13*
Pri acc pump nozzle .037...........026"
Sec acc pump size ..50cc...........30cc
Power Valve 13" custom ..........8.5"
Idle mixture 3 turns out...............1.5 turns
Idle speed 800-900.....................600-625
4.11 rear helps............................3.08 is just fine!
A clue for your car is to look at the maximum base timing it can take without pinging. If it is happy near stock, I would not expect gains with a 4 holer. If it is high and cranking compression is 175+, look for significant improvements in street manners with a true dual plane.

wmf62

the Man-A-Fre was 4 2bbls on a FI-like adapter plate manifold. each carb fed a separate pair of cylinders. to be short and sweet, it was a fantastic setup...

Bill

Matt Gruber

Bill
It would sure be a head turner at a car show. Did u ever drive one? It connects each pair, the dual plane divides the pair so they are totally isolated thanks for the pic!

wmf62

i had one on one of my 65 coupes, it was responsive and impressive; the next best thing to FI if ya gotta have something with carburetors.

i almost bought one on ebay a couple of years ago but one of the carbs was incomplete.
Bill