Where does the line to the distributor vacuum advance hook up to on a blown engine? Does it go before the blower at the carb bases, or after the blower at the intake? Same question for a Vacuum Gauge. Thanks, guys...
I agree with Kevin....I dont use it on any of my rides (blown and unblown) and have'nt noticed any difference other than less complexity/more simplicity. Theory says vac advance is good for economy at part throttle cruise - I dont really care about economy of a perf ride, esp on a blown one. Strangely enough the fuel economy is npretty good if I keep my foot out of it, esp when at cruise the engine is showing 12 in hg of vac. Not even under boost at highway RPM, so economy is not adversely affected. Mechanical advance is simple, works well and is easily adjustable at the distributor with advance springs or bushings and inital adv at the crank. Simple and effective is good for me. Rat
Consider what it is that the blower is doing. It's ****ing air in through the carbs (vacuum) and pushing it into the engine (pressure). Like ajmopar said, vacuum above the blower and boost below.
I second what Kevin and Rat said. If you feel you need to hook it up it should be connected BELOW the blower in the intake manifold, same for the boost gauge.
Most blower motors have such a small advance window that a vac*** advance isn't nessessary. My blown Pontiac runs 26 degrees at idle and 38 total. The mechanical timing is all in by 2200 rpm, so a vac*** pot is unnessessary. Your combination might like something diffrent, of course, but generally speaking, your blower will need LOTS of initial and not too much total. Good luck, -Abone.
First off im a little scared that someone thinks there is vacuum below a blower, but anyway we are here to share and learn. Yes you really dont need vacuum advance on a blown engine. You must be carefull when setting your timing because too much total can ruin your motor. Pre ignition is bad for blower motors.
On an Early Hemi, limit the total advance to 36. In the old days we found this number the hard way. And, if you choose, vacuum ***isted dizzys want a ported, not manifold source. ,
Under most conditions, there is low pressure in the manifold. In fact, the only time you will get high pressure is under a load and a large amount of throttle.
im a little scared that someone thinks there is vacuum below a blower I'm scared that you think there isn't. If there were always pressure below the blower, the RPM would go off-scale the first time the engine was started. Carburetors do not admit air - they prevent air, that's why it's called a "throttle".
Every blown engine I have ever run, including my current nitro engine, has a vac*** below about 2500 rpm or higher. It depneds on the size of the engine - the size of the blower and the condition of the blower. With 331 Ci, a 671 at 1 to 1 drive and a good blower, you should have vac*** until around 2500 or so. Last week at Bakersfield I talked to guys who are running 1071's on 450 CI engines at 45% OD with hi-helix blowers and they still have a vac*** below 1500.
of course there's vacuum below the blower, until you open up the throttle enough to start making boost. 99.99% of the time a street blower motor will have manifold vacuum, not boost I put vac advance on my 55 because I drive it a lot....getting one more mpg on a 5000 mile trip is money in the bank. don't hurt nothing, either, if you do it right
My favorite gauge (that I don't own) is a McCulloch super charger factory pressure gauge. one needle with zero in the middle. Vacuum in inches to the left of zero and pressure in lbs PSI to the right. Start it up and it reads vacuum and drops toward zero as the Rs come up until it starts reading the boost in lbs PSI. A very rare and expensive gauge.
If you are building a mild street motor that you'll drive a lot, then connect the dist above the blower, but limit the amount of vacuum advance. If you won't put tens of thousands of miles on the the thing, then don't put vacuum advance on it.
I'd just say that you need to be a bit more precise when you say "above the blower". When talking about the top side and vac*** I think you need to say which side of the throttle plate. The bottom side of the throttle plates will always see vac*** created by the blower intake - that's why we boost reference the carb power valves to make them work correctly. So for a vac*** dist advance that leaves ported vac*** above the throttle plates - and that works for some dist setups but not all. On a mild street blower engine I'd skip the advance. On a street engine with more boost, I'd run a MSD box that will take timing out at higher boost. Just my opinion.
The vac advance = fuel milage is just your basic myth. Maybe someone can prove it by some theory in a book but in the real world it is just a myth. I've run mechanical advance distributers since gawd knows when. I've never gotten any better milage with a vac advance. I personally think it is because my mechanical advance distributers are more effecient than most Vac advance distributers. Most recently I was getting 20-22 consistantly with the Pusher and I'm not the only one who knows that. I prefer mech advance on a naturaly aspirated motor and would insist on it with a blower motor. But unlike a lot of the fellas around I don't think a power adder is for looks, well that and I still think that if you are running less than 12 PSI your fooling yourself. But thats an entirely different story and someone is liable to be offended by that, I hope.
OK, I don't know about those other folks recommending where to hook up a vacuum advance. Most times, this is the correct place to connect a vacuum advance. Vacuum advance at idle should be near zero. As engine rpm increases, so should the vacuum advance. That being said, the vacuum advance should be connected to VENTURI VACUUM. Get a vacuum gauge and throttle a motor with it connected to different ports on the carb to find the one with venturi vacuum that increases as rpm increases.
I very much disagree with you on this, I have run cars both ways for a lot of miles, and I will not run one on the street without a vacuum advance. For the extra effort of setting it up correctly, it makes a palpable difference in mileage, and so doing will make the engine last longer. Just another area to help you tune your engine for maximum efficiency. If it's a Saturday Night Special that sees few miles and is mostly run wide open, yes, vacuum advance is not necessary. Obviously you don't want your blown motor to pick up extra advance while under boost, but there won't be much vac*** under the carbs when the throttle is wide open, so the vacuum canister will relax and the extra timing is gone.
The only equation I applied was saving $50 on a long trip after I put it on the car. You don't have to believe it.
I've run some on the street tests with two vacuum gauges in operation. One to manifold vacuum and the other to ported vacuum - which you've called venturi vacuum. Here's a quote from an article I wrote: "I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment. The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm compe***ion 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 indicates the same as my vacuum/pressure test gauge. 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 87 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 warmed up, MV reads 18.5 - 19" and PV reads zero at about 500-600 rpm. Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV. Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration. 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. At idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero." I'm wondering if the blown engines not running vacuum advance sourced to manifold vacuum overheat at idle and slow traffic. Most naturally aspirated engines do overheat without vacuum advance. I'm curious too, are your blown engines running rich on the low RPM end of the scale?
My blown engine runs 190 all the time when its real hot out. Thats with the sensor in the head. No vacuum advance. Mechanincal only.
Use the vacuum advance, and source it from the manifold, or as you say "below" the blower. This is the vacuum that the engine (ie cylinders) sees, the cylinders have no idea whether there is high or low vacuum at the carbs, as the blower in between changes the numbers. Vacuum advance at idle and no-boost conditions WILL make the engine run cooler, and WILL increase economy. You may have to limit the vacuum advance some. At zero vacuum or any boost, it's all up to the mechanical advance. The best of both, in my opinion.
This is correct. There is always that chance that when you are cruising the vacuum advance will have everthing nice and advanced with high vacuum, then when you give it a little gas everything goes to pinging beacause the advance has the timing so jacked up and the mechanism doesn't react fast enough to lower the advance.
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