Do I need vacuum advance? (Long Post)
I have an 86 chevy 1/2 ton 4x4. It has always been kind of sluggish down low. I know I have posted many times on here describing what I am working with, but just wanted to thorough.
Edelbrock performer intake
Edelbrock cam: 204/214 .420 lift intake/.442 lift exhaust
Stock exhaust manifolds
3.42 rear gear
I recently put a curve kit in the distributor with one heavy spring and one light spring. I bought a new vacuum pod since the old one didnt work. Hooked it up full manifold vacuum. Set timing at 12 BTDC as that is where it ran best. Any less and its very sluggish off line. I figured I would gain some throttle response, but didnt. I took it down to the local engine builder and he told me that I dont need to run any vacuum advance. He said I need to weld the advance tab in the distributor forward. He then went on talking about pulling the distributor and taking it apart and putting a different bushing on the shaft to lock out or minimize how much it can advance at idle. Ideally he said I would want to run 22 degrees at idle with a total of 36 to 38. He claims that this would really pep the motor up and be more responsive. He said that if this doesnt work then I need to resort back to a stock cam. I know a few very basic things about how the distributor works but he has totally got me lost. I don't doubt his knowledge, but I just dont know how to do it myself. I cant even get the truck to turn the tires over from a dead stop let alone pull anything with it. Is what my mechanic says the right thing to do or can someone elaborate on this? Thanks.
Hmmm, where to start. Vacuum advance conversations always seem to get way out of control but I'll throw in my 2 cents here. If the truck is really a total mut, there may be other things going on too.
We have about the exact same engine build and same gears too. I have a '77 chevy truck. I cannot spin my tires either, unless it's been raining outside. I have 31-10.5-15 tires. Actual diamter is about 29.5" sitting on the ground.
First off, let me say that whether you have the advance hooked to manifold or ported vacuum, it will not help the pepyness of the truck off the line. Once you step on the gas, the vacuum signal goes away, period! If you take off very very gently, you will retain "some" advance, but it goes away very quickly once the vacuum signal drops off as you get into it more.
Running either manifold OR ported will give you almost the exact same results when CRUISING at a steady speed. And again, when accelerating from a steady cruise, the advance goes away until you take your foot out some and the signal rises again.
Where manifold vacuum has the advantage is at idle. It will raise the idle speed and smooth out the idle some on big cam'd engines and will also help keep engine temps down some. But again, once you step on the gas, the signal goes away.
Personally, I think 22 degrees initial is way too much for a fairly mild engine. You can try it but it may be real hard to start when it's hot. You will also have to really limit the mechanical advance to keep it down to the 36 degree area.
Bottom line is, you just don't have enough engine and gears to make your truck a tire smoker. If the engine checks out ok all around and depending on tire size, making the next move to 3.73 gears would be the best move to really get it moving from a standstill.
What I"m trying as of 2 days ago on mine is 12 degrees initial, springs changed to get me another 22 on the mechanical advance and then another 10 degrees on the vac advance hooked to manifold vacuum. I know I'll have problems passing the emission test next year but for now, it's doing good and no pinging on 87 octane gas. I built it so I did not have to run the big dollar stuff because at todays prices and 9mpg, it's just not worth the extra money needed.
The purpose of vacuum advance is to allow more burn time when the mixture is not dense, the manifold vacuum will be high and the RPMs are fairly low. This occurs under part throttle from off idle through cruise RPMs. Advancing the timing in this area helps with fuel economy, helps a lot, without it you'd have to open the throttle more to increase mixture density. Making the timing vacuum sensitive at this point protects the engine from detonation because if it's working hard and you suddenly slam the throttle open, instead of the heads being blow off from the detonation, the timing falls back a little as the manifold vacuum drops, then the timing picks up from the centrifugal system, this guy wants to weld together, as the RPMs raise and the engine can handle the additional spark advance with the heavier mixtrure. The requirement for timing in a competition engine is a lot different, to start with the vehicles tend to have a much higher power to weight ratio, so the engine never really lugs at cruise as it does in a street machine, they don't have extended transition periods where the engine is up and down the power band. Stuff like that. At NASCAR the problem is really in reverse, at super high RPM is also where high loading on the engine actually occurs as it trys to shove those shoe boxes thru the air. At those speeds the timing is actually pulled back a bit, probably starting around 6000 RPM the curve begins to retard.
Your sluggish truck is that you're not getting as much power out of this as you think. You've got a pretty good induction system on it and crappy exhaust. The cam is working on the long side of duration (not radical but starting to get up there) against a system that can't get the burnt stuff out, so you've got a lot of reverse flow in the intake at lower speeds so you're not getting the cam's potential and the mixture isn't right probably inspite of everything you've tried to do to fix it. First order of new parts business should be a set of equal length or Tri-Y headers and a set of duals.
If these are 86 era heads, it's time for Vortecs, these alone will pump this puppy up with 40 ponies. Every time I build an engine for somebody with old model heads, and this includes all the double camel humps, engine power just evaporates compared to the same configuration short block with Vortec or Fastburn type heads. All these 040, 041, 186, 291, 461, 462, 492, 916 heads have to be ported by somebody that really knows BIG TIME what they're doing to even come close to off the shelf and bolt 'em on Vortecs/Fastburns. This applies to the many aftermarket suppliers of vortec style heads, they work great compared to the pre vortec stuff, they are comparable or better than the GM Vortec/Fastburn and will allow you to keep your current intake.
Next is state of tune, you really need a dyno tune to get the carb and timing correct, anything else is simply hit or miss with the probability of a "miss" being the highest state of expectation; after all there's one right and an infinite set of wrongs. So what would you expect?
So depending on my budget, I'd have at it. The minimum build up would start with headers and duals, then a power tune on the dyno. For a few dollars more, (wouldn't that make a great title for a movie?), I put a set of new heads on it along with the headers and duals, then a power tune.
Both excellent posts. Thanks guys. Yep, headers are coming soon. I was going to go with some hedman headers, the ones with the thick 3/8 flanges and thermal coating. Still trying to decide between mufflers and glasspacks. Would the Tri-Y's or equal length headers suit me better? I know they are more expensive. I actually do have a set of rebuilt Vortecs with a fresh valve job and the heavy springs that have been sitting on the bench for 2 years now along with an RPM vortec intake. Currently running flattops with a "487" head with 2.02/1.60. I think the flattop piston would put the compression pretty high with the vortecs. I guess I'm saving them for a future build.
Did you set your timing with vac advance hooked up ? If so unhook it and plug it then reset timing. That cam should do fine with stock exhaust manifolds. Thats the whole reason it has a longer duration and higher lift for the exhaust. I run the same cam in a 72 gmc with 8 inch lift 38" tires with 3.73's and a 4speed stock intake and exhaust manifolds (ram horns) and glass paks Does it smoke the tires? No... But it does have plenty of grunt for what it is.
Performer intakes aren't much over a stock intake....
Set the initial timing so the total timing will go to 38 to 40 degrees (without the vacuum advance connected).
So. Get a timing light with that little knob on the back and rev the engine up to 4000 rpms and set the total timing to 38 or 40. Let the idle timing fall where it may. If it goes below 10 you may need to weld up so the the advance slot. However, it should fall around 15.
Then connect the vacuum advance to the manifold vacuum. The idle timing and idle rpms should jump up with the vacuum advance connected. You should use a Crane vacuum advance canister that has an adjustable vacuum advance so you can limit the amount of advance. If the total mechanical timing goes to 40 then you would limit the amount of vacuum advance to 10 to 15.
Idle timing will be 25 to 30 degrees with the vacuum advance connected to the manifold vacuum.
Adjust the idle screw to 750 rpm then adjust the idle mixture screws for best idle.
So the end result should be around 50 to 55 degrees of advance when you rev the engine up in neutral. Don't worry about 50 to 55 degrees of timing. When the engine is under a load, the vacuum advance will go away. 50 to 55 of timing going down the highway will be good for 2 more mpgs for your truck.
Q-Jets are the best carb ever made, but tuning them is a science. The right setup will make all the difference. Call Chiffs high performances for some tuning recommendations (nice guy that likes to talk about Q-jets).
The timing your mechanic recommends is good for the drag strip. However, you need to use the vacuum advance on the street for throttle response and MPG.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
Yea what he said!
good explanation of ported vs manifold vacuum. Even I understand now
your kind of between a rock and a hard place due to the vehicle weight
Jmark gave the best reponse....deeper gears to multiply the available 350ft/lbs TQ to overcome the 4,500+ lbs of weight....= acceleration power at street rpms...
"but" to gain enough TQ to overcome the weight will be 4.56 or lower gears and they suck for hwy rpms
your motor is making 350ft/lbs at around 2800 rpms....but from a stop sign at say 1500rpms there is only roughly 200 ft/lbs
200ft/lbs x 3.50 gears = 700ft/lbs of force twisting the tires
200ft/lbs x 4.56 gears = 912 ft/lbs 0f force twisting the tires
the only real cure for a vehicle weight problem is a big cube motor....for big TQ
350ft/lbs (350cube motor) x 3.50 gears =1225ft/lbs
500ft/lb (454cube motor) x 3.50 gears =1750ft/lbs twisting the hides!!!!
4500lbs/1225ftlbs=3.6 lbs of weight per ft/lb of force
4500lbs/1750ftlbs=2.5 lbs of weight per ft/lb of force= 0-60mph plenty quick
do try bumping up the base to 14-15*....often a small change does make a big difference for bottem end power (timing chain stretch/cam advance degrees/worn dist/etc "slop").....
headers add next to nothing for HP and TQ at street rpms and can't add enough to help much at HP peak rpms (it will be a tiny bit quicker passing a truck)...if you gotta have headers then definitely tri-y's for more bottem end TQ
better heads will definitely improve mid range and top end...but you still will have roughly 200ft/lbs from a stop sign
RE: manifold versus ported vacuum
USE THE PORT THE DISTRIBUTOR MANUFACTURER (or OEM) SAYS TO USE IN THEIR INSTRUCTIONS....it's that simple....(ex: MSD does say hook to ported on page one of their instructions for the Chevy dist)
your timing 101 post does apply "IF" you are dealing with a early 70's "smog" motor which it references...the change to 87 octane unleaded and crude smog controls motors needed all the timing help they could get...
(drives me nuts when people mis-represent things and state it as "gospel")
from the 101 article:
"the lean idle mix needs more timing"....duh....the idle screws need adjusting for richer
"makes heads hotter on ported"...yes...about 10* coolant temp "if" it is a 98*+ summer day and your cooling system can't handle it ....otherwise it is just wasted heat...not lost power
Don't think there is much of a difference between a 1975 350 and a 1986 350, especialy one that has a mild aftermarket cam, aftermarket intake, and carb. A 86 350 is not a high tech LS series engine, you know that right?
I have used both ported and manifold vacuum on my engines and I prefer the the manifold vacuum. Especially with 87 octane. More timing allows the throttle blades to be closed more and can burn a leaner mixture. This greatly reduces the amount on gas in the cylinder when the engine is shut down which prevents run on. Besides having more throttle responce off idle with the manifold vacuum since there is no waiting for the timing to come up. And the exhaust isn't as stinky and vacuum is higher with more timing.
Ported vaccum and manifold vacuum becomes the same thing once the throttle is moved off off idle. So is there a difference only during idle and right off idle.
65redmustang....Not trying to hijack this thread but i'm curious about upgrading to headers on a stock/very mild engine and the benefits/drawbacks. Will they really help?
I'd like more on your statement on headers.
headers add next to nothing for HP and TQ at street rpms and can't add enough to help much at HP peak rpms (it will be a tiny bit quicker passing a truck)...if you gotta have headers then definitely tri-y's for more bottem end TQ
I'm still seriously toying with pulling my Hookers OFF my truck and going back to stock manifolds and heat shields. I"m sick of burned wires, fried starters and blown gaskets. (so is my son and about everyone else I know that is running headers on a mild street/driver!)
Also, I have an '86 3/4 ton 350/400/4.10 camper special. It has A.I.R. and some factory manifolds that look more like extruded tube headers. Are they any better then cast? They look cool! LOL
Granted, Tri-Y (Thorley?) are very good headers but i'd like to hear more on your post above. My engine/gears/truck specs are almost identical to the original poster and some say i'll loose "XXX" hp and "XXX" torque and "XXX" mpg. What are your thoughts on headers on street vehicles like ours?
If this subject gets some GOOD, CREDIBLE replies, I may split it off on it's own for some good conversations.
Last edited by Jmark; 10-12-2007 at 02:20 PM.
I plugged in Chevy 21's motor spec into DD...9/1, 487 heads, 600cfm carb, 204/214 cam, typical dual plane intake,
stock log manifolds with 400cfm (stock?) exhaust:
500 56 5.4
1000 178 34
1500 252 72
2000 298 113
2500 320 153
3000 330 190
3500 334 223 TQ peak
4000 327 249
4500 298 255 HP peak
1.625"D x 34"L x 25" collector headers with 400 cfm exhaust
500 66 6.3
1000 186 35
1500 299 74
2000 305 116
2500 333 159
3000 344 196
3500 347 231 TQ peak
4000 344 262
4500 315 270 HP peak
same headers with 600 cfm exhaust (excellent duals and mufflers)
500 66 6.3
1000 187 35
1500 260 74
2000 306 117
2500 336 160
3000 348 199
3500 352 235 TQ peak
4000 352 268
4500 325 279 HP peak
so...picking just one "street" rpm, say 2500 (so there is "some" cfm thru the motor to make a pulse and some timing in the motor)....from bone stock to super duper headers and exhaust you gained a whooping big 7HP and 16ft/lbs at the flywheel (16ft/lbs on your torque wrench is for sure not alot of force)
7hp/4500lbs=.0015% hp/weight performance gain
320ft/lbs x 3.50 gears =1120ft/lbs at the tires with stock exhaust at 2500rpms
336ft/lbs x 3.50 gears = 1176ft/lbs at the tires with headers and duals at 2500rpms
56ft/lbs gained / 4500lbs = .012% TQ force to weight performance gain
headers and good pipes gained 24HP over the stock exhaust at the peak (4500rpms for passing) which looks like alot till you divide by 4500 lbs....
24hp/4500lbs=.0053% power gain (one half of one percent gain)
Thanks for posting that.
Guess I don't feel so bad about getting serious on changing back to stock manifolds! LOL It's all free so just my time is all it will cost me.
Anyone have any ideas on my '86 tube manifolds?
only to illustrate "why" (to avoid the heavy duty thermal math) and why your cool looking tubes will be fine here's "the rest of the story"
actual CFM thru Chevy21's carb intake 350 motor with headers and duals
(sticking with the 2500 rpm example)
221cfm/8 cylinders=each cylinder inhaled 27cfm....
let's say the "hot" exhaust air is expanded 4 times the volume 27cfm x 4=110cfm to push and scavange out of the cylinder....
just a one inch diameter pipe (.78 sq in area) flows 115cfm with virtually no restriction....
typical (?) stock manifold is 1.5 sq inch area (1.5"x1"?)...your using 50%(?) of the available sq in flow capacity
headers DO help at the strip...Chevy21's 4500lb truck is 6 tenths quicker (!!!) and 3mph faster with headers...using Bill Shoppes calculator...
tiny correction: you gained 7 FWHP with headers....about 6 RWHP
got a question....what was the typical stock 86' 350 CR and HP and TQ
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