Tommy, I'm not sure I understand you. At part throttle cruise, whether you are at 60 MPH or 75 MPH, the engine will develop a relatively high vacuum. Certainly enough to fully activate your vacuum advance. The stock cannister will supply between 14 and 18* additional timing. At that point it doesn't matter whether you are hooked to ported or manifold vacuum. The levels will be very close if not the same. And the stock setup certainly did have total timing at those levels and even higher. That chart I posted is right out of the 70 Buick chassis manual. The OEM guys will tell you to use ported vacuum, but performance shops usually recommend full manifold. Alot of the 60's Buicks used full manifold vacuum. I use a points distributor converted to MSD magnetic pick up design. It has about 16* of mechanical advance in it, all in at 2500 RPM. It has a Crane adjustable vacuum advance, limited to 10 degrees, run off manifold vacuum. I run between 30-34* initial and mechanical, depending on the octane thats available. I can trim off timing with the High speed retard on the MSD. The car is extremely responsive at all speeds, it never falls on it's face. I would say, if you notice an increase in performance with ported vacuum, you mechanical advance curve might not be optimized. Ported vacuum will go to 0 at wide open throttle same as full manifold vacuum
I guess we are both saying the same thing. Here is what i define as ported vacuum; An area above the throttle body of the carb. that creates a low pressure, or vacuum signal, based on the speed of the air passing through the venturi. The faster the air flow, the greater the negative pressure generated in this port. Yes, there would be some signal loss due to secondaries opening, but I doubt it would correspond to the amount of manifold vac. loss under the same conditions. What threw me off was the other 18* you said came in between 1800 and 4600 rpm. I attached a spec sheet that shows total vacuum on a 1970 Stage I and it dont get anywhere near 42 total. Also note that vac is @ 0 until 8" hg. Hope it gets there
Tommy, The stock cannister delivered between 14 and 18* at 16 inches of vacuum., at 8 inches of vacuum, it would deliver 0* If you are rolling down the interstate at a steady speed, your foot is barely on the gas, and the engine is developing about 16 inches of vacuum. The cannister is delivering very close to its maximum amount of degrees. That is added on to whatever your initial and mechanical total is. Doesn't matter, ported or manifold vacuum. The stock Stage1 distributor had 10 initial + whatever the weights were generating. In the chart I posted, it said 9-13* at 1800 RPM, I figure at 2600 RPM steady cruise, you are probably getting about 16-18*. out of the mechanical advance. That is 26-28* initial +mechanical. And at steady cruise, the engine is generating at least 16 inches of vacuum, so your getting another 14-18* vacuum advance. This is all at steady 60 MPH or light cruise. The engine is not accelerating and is at light load. At that point, a stock Stage1 was getting at least 40* of total timing. In order to hit 4600 RPM, your foot has to be in it, and vacuum levels in the engine are very low. At that point, you have no vacuum advance. All you have is your initial and mechanical, which in the stock motor topped out at 30-34* There is no difference between ported vacuum, and manifold vacuum at steady part throttle openings. If you have a dialback timing light, leave your vacuum advance hooked up, asnd measure the total advance at say 2500 RPM. I think you'll find it is in the area of 40 +degrees if you have a stock vacuum cannister. What timing are you running BTW. initial?, total/, and vacuum advance? Just curious.
What were dealing with here, is in effect a variable venturi. At least it is as far as ported vacuum goes. The variable venturi bit due to throttle blade position. I got curious about what Larry said concerning manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment today. The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, Edelbrock Performer intake, Carter 750 cfm competition carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. Its a smooth cam and the car when warm idles @ 19" vacuum. The dash carries a large (2 5/8") S-W vacuum gauge which compares favorably with the vacuum/pressure test gauge I have. Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV). The car runs very well on 89 octane in summer and winter and does not overheat in traffic. Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm. Ported Vacuum (PV) read 12" on startup. Once the engine was warmed up, MV reads 19" and PV reads zero at about 500-600 rpm. Cruise at 40 mph with a light throttle setting gives you 18.5 - 19" MV and 12" PV. Rolling the throttle in about half way shows 8 - 10" of PV during acceleration. (Figures from memory as I didnt take notes.) Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read 10". Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero. Not what I expected, but exactly what Larry said. The key thing is, at idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero. The lack of additional timing at idle is what creates an overheating problem in the GM engines. My understanding is that it takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating. Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level. Theres a lot of confusion out there about timing, both centrifugal (mechanical) and vacuum as well as the vacuum sources to use. The key thing to me is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load. To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister. Ive been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source. Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and instead prefer to throw money at the problem. As far as spinning up a little experiment, Im not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with.
Thanks for the experiment C9. I was a bit surprised tha PV went to zero @ WOT. I expect you would lose some due to loss of air velocity when secondaries open. I understand that a closed choke, by design, is to create vacuum above the throttle plates to draw fuel from the bowls when air velocity and cool temperature do not allow venturis to function. Just took a few minutes to look through my Petersen's Big Book of Auto Repair. 1967-1983 American cars. Prior to the the drop of compression no manufacturer cranked in more that 32 degrees total on their V8 motors. After 73 you begin to see the high #s Larry refers to. No GM V8 vacuum canister delivered above 12 degrees until 1976 and Buick V8s did not exceed 10 degrees. You can almost gage when they went from Ported to MV by the amounts of initial timing and allowable vac adv the specs show. My engine is on the stand w/ the paint drying so I will be conducting some experiments on my own to determine whether I want to rethink my choice of vacuum source for adv. It does appear that compression will be key to determining how much total you can crank in. I see 2 competing theories, both used by the manufacturers over time. Would be interested in seeing what timing is doing on these newer High horse motors I usually set my initial @ around 10 BTDC with a properly jetted Holley 650 DP. Don't have the equipment to read total but my parameters are startability when warm and pinging @ full throttle. My Olds had a 157k stock 72 455 (buick) motor with a Holley carb, HEI and 2 1/4" dual exhaust. Weighing in at 3500 lbs., this car was able to pull 3 or 4 mustang GTs in some pretty serious street races. In a 2400 pound car with a 455 I can't help but imagine that looking away from the road to see vac gages @ WOT was a bit hairy. Have enjoyed the dialogue with both of you, Tom.
Tom, Every vacuum advance originally installed on any Buick 8 cylinder was rated just about the same. 14-18* advance at 16 inches of vacuum. All my chassis manuals agree on that, and the Steven Dove book "Guide to Buick GS" also agrees. The chart you posted, where did it come from? I believe that the degree figures in that chart are in distributor degrees. Since the distributor rotates at 1/2 crank speed, you would need to double those numbers, for crank degrees. Your chart says 7-9*@16 inches of vacuum. Double that to 14-18 crank degrees. All GM vacuum cannisters were marked with 2 sets of numbers. The left set was the part #, the right set was the degrees available in the cannister, in distributor degrees. Again, you have to multiply that by 2 to get the crank degrees. BTW, it is easy to check your total advance with a regular timing light. On the Buick V8, 1 3/4" = 30* of crank timing. Measure from the balancer mark, clockwise around the balancer(looking from the front), exactly 1 3/4", and make a second mark. Then hook up your light, and rev the engine until your second mark moves up no further. Then read you total off the timing tab. If the mark stops at 4, you have 34* ect.
Awesome tips, thanks a bunch. Well I dug into it this weekend and the main culprit appears to have been a BIG vacuum leak at the back of the carb. The Brake booster hookup had gone funky and the car was running lean with the vacuum leak. I also shielded the Headers a bit. and I am fabbing up a shroud to mount the electric fans to. I may throw the old 7 blade on and run it for a few days while I have the electric out... Right Now I've got it down to about 200* or so. My autometer guage still reads 210*-220* and I know it is wrong/high. I wonder If i need to try replacing the temp sensor or?? Thanks again, I'll keep ya'll posted! Shawn
GOOD Guages are a GOOD indication of how your motor is doing...... ....Bad Guages ?? I wouldn't let a Doctor examine me with a Toy Stethescope !!! No Guages ?? :blast: :blast:
Larry, No my #s are based on crank timing. It has been a few years since I had an after market curved, but the guys that did it for me were a Pro Stock racing performance shop in Reno, NV. and they advised me not to go much beyond 30 degrees total. I took their advice and have used it on buicks. In that the 2 motors, 460 and 455, have similarly shaped combustion chambers and rod lengths I applied the same principles. Isn't there an old Buick engineer that comes around here? Sure would like to get his input on this. I wouldn't doubt the veracity of what you say if my resource book I used didn't contain every American V8 from 1967 to 1986 and their statistics have never failed me. Gonna put it to bed until I can research and get 3rd party validation of what we have been discussing. Will let you know, either way, what I find.
Your Welcome Tom, And yes, 30* is usually all it takes. Was talking to John Osborne at the GS Nationals, and that is what he runs his engines at. 30* is all they want, he said.
Try double checking your gauge. There are glass coolant temp thermometers out there, used by radiator shops and mechanics. (My dad used to have one, but it got away somewhere.) A candy thermometer is a good substitute. Available at any cooking supply store for not too much. These come in both glass form and metal probe types. Engine cold, remove radiator cap, start engine and when it's up to temp as per the dash gauge stick the candy thermometer in the coolant and see what you get. An even better substitute is an unmounted S-W mechanical temp gauge. More than likely you can drop the capillary bulb into the coolant deep enough so the gauge will register properly.
This a reply to having vacuum at wot. That is also away of telling if you need a bigger carb. If the carb is to small you will still pull vacuum at wot. If you see 1" of vacuum or more get a bigger carb.
OK, my car runs great with 12 intial, and 34 total in at 2000, but after all this talk i am trying to hook up a adjustable vac advance, i think i am hooking the hose to the wrong spot i hooked to one of the tubes on the carb and i get way to much advance (50+) that wont drop off even at WOT, i run 11 to 1 comp. with a 113A cam with 332 intake dur. SP1 intake, Qjet, and i have 13 in vac at idle. like i said the distribitor is dialed in now, i was just looking for a little more cooling. my car will start to push 210 idleing in 90 degree + weather, no over heating problems other wise.
Kelly, you need to limit the amount of degrees in the cannister to 8-10* Which adjustable cannister do you have?
CRANE CAMS, the problem is the total timing wont come down to 34 degrees even at WOT with the canister hooked up no matter how i adjust it. it gives me another 10 at idle then it keeps climbing as i give it more throttle and the moter begines to misfire.when i unplug it ,all is fine agine.
I just talked to the Crane TEC. HE SAID I NEED TO HOOK TO MANIFOLD VAC. , werei have it now it will pull vac all the time. needs to be below the carb.
Yes ported vacuum is somewhat slower to drop when you punch the throttle. Also the Crane adjustment for the spring tension only goes so far. I've found that the NAPA VC1807 is a great vacuum cannister. The spring is very stiff, so it will remove the vacuum timing even faster. You just need to modify it for proper total degrees. It's very simple. Just make a plate like in the picture. Distance from the pull pin is .086 for 8 degrees, ans .104 for 10 degrees.
I stumbled across this thread and I'm glad I did. I was always told to plug the vac canister on the distributor for (better thottle responce), which is what I've been doing, and always fighting the warm temp. gremlin at idle. After reading this, it seems people where saying this because they had it hooked to the carb. PV instead of the manifold MV. My case is a little diffrent. My 455 has an ATI supercharger feeding it so at wot, I'm pressureizing the manifold making boost which means I'll have to put (another) check valve between the vac canister and the manifold port. Thanks for everyones input on this subject, I'm hoping this makes my motor run cooler in stop and go traffic. Rick:beer
