A vacuum advance should really be called a vacuum retard.
Tell me why you think a vacuum advance is the way to go on an engine that, if tuned correctly, will create virtually no vacuum at wide open throttle.
The literal only way you can make a vacuum advance work is if you were able to determine how much advance the engine can remove from itself (again, works like a timing retard) and still sustain acceleration without detonating itself to pieces at intermediate throttle settings. It would be very difficult to tune without the aid of an extremely adaptive ecu.
As the advance starts to drop when the engine comes up to wide open throttle (speaking strictly jet boat) the vacuum numbers waver as the timing goes advance and retard, this vacuum has a direct effect on timing and vice versa.
The vacuum advance is actually designed to optimize engine efficiency and performance at a cruise, in a car. The torque curve on a car, versus the curve on the jet pump is opposite. The jet is almost perfectly linear, increasing constantly with RPM. The car on the other hand, the torque completely falls off from extreme under acceleration, reducing the torque in the upper rpm ranges. This is called "pulling a gear". A jet has virtually no load until about 3,800 rpm. It's a constant torque vs variable torque scenerio.
I'll be up for a bit. Be more Than happy to debate this further and looking forward to being converted to vacuum advance systems.
Whatcha got?
GT
Sent from my SCH-I545 using SoCal Jet Boats mobile app
Hi GT , yea I'm still here. And I got something for you.
You still don't understand vacuum ADVANCE systems.
They don't RETARD timing, they ADVANCE timing over and above the mechanical advance curve of the distributor at times of high vacuum. At times of low vacuum the timing will fall back to the same mechanical advance curve you would have with a non vacuum advance distributor. Less vacuum = less timing ADDED above the base mechanical timing curve.
You set the mechanical advance curve to give the timing the engine needs/wants at WOT, just like you would if there was no vacuum advance because the vacuum system will not be active at WOT. Then you hook up the vacuum advance, It will add timing at times of increased vacuum/reduced load such as idle and cruise. With less load (idle/cruise) the engine can handle way more timing than it can get from the mechanical advance system that is set for proper timing at WOT.
A properly set up vacuum advance system will increase efficiency at idle/low load/part throttle situations on any engine no matter if its hooked to a transmission, a V-drive, a jet, or whatever as long as the engine sees a varying load. And even a jet has this. In your own words "A jet has virtually no load until about 3,800 rpm". At 3000 RPM cruise, you don't have the throttle wide open. The engine has vacuum, and the system will ADD timing over and above the base timing curve because this is a low load situation. If it wasn't a low load situation, you would need full throttle to maintain that RPM. The engine may only be able to handle 36 deg advance at WOT. But at cruise it may be able to handle 46 deg because of the reduced load. No mechanical advance system can change timing based on engine load, only a vacuum advance or computer controlled system can do that.
Don't believe me ? Go out and advance you initial (idle) timing 10 deg and see what it does to idle RPM. It will increase RPM, you will have to turn down the idle speed at the carb. So now the carb is supplying less air/fuel to hold the same RPM. Same thing will happed at cruise, it will need slightly less throttle to hold the same RPM, as long as there is enough vacuum to activate the vacuum advance system.
All of this is doubly true if you carb has power valves. At light load the power valves will be closed providing a lean mixture (cruise) vs the proper (WOT) mixture. This lean mixture will not bun properly, it actually burns slower increasing exhaust gas temps. The lean mixture at light load/cruise needs more ignition advance.
What are the down sides to vacuum advance ?
1. They are ugly.
2. They have no advantage on a race engine that spends its life at WOT. The system would only be active at idle, and nobody really cares how well their race engine idles.
3. Things can get complicated and most people don't take the time to learn how they work or take the time to find the right vacuum canister to work with their engines vacuum levels. A vacuum canister for a stock 454 truck engine will be useless on a performance boat engine, you wont have enough vacuum to fully advance the system. You need a vacuum canister that goes to full advance with slightly less vacuum than your engine makes at idle, and the vacuum advance needs to be hooked to manifold vacuum. If the canister doesn't have enough vacuum to hold it at full advance your idle timing will fluctuate and idle speed will be erratic. It also needs to be the right canister to give the right amount of added timing at cruise (based on load). If you get it wrong you can end up with to much advance for the load (detonation). Or not enough advance for the load (not taking full advantage of the system). If you take the time to get it right, advantage always goes to the vacuum advance system.
At idle = advantage vacuum advance. Timing will be advanced beyond base timing curve. Better idle quality, less throttle opening, same RPM. Increased efficiency
At cruise = advantage vacuum advance. Timing will be advanced beyond base timing curve. Less throttle opening to maintain the same RPM. Increased efficiency
At WOT = No advantage to either. Both will be following the same base timing curve.
