123 Install Today - GETTING CLOSER..., PROGRESS: BAD MPS |
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123 Install Today - GETTING CLOSER..., PROGRESS: BAD MPS |
FlacaProductions |
Dec 14 2023, 09:50 AM
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#1
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Senior Member Group: Members Posts: 1,733 Joined: 24-November 17 From: LA Member No.: 21,628 Region Association: Southern California |
74 2.0 D-Jet
Heading to the garage now to start the install and my main, unresolved-in-my-brain question is if i need to hook up the vacuum line that is on my current stock distributor, on the 123. |
technicalninja |
Dec 20 2023, 02:13 PM
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#2
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Senior Member Group: Members Posts: 1,827 Joined: 31-January 23 From: Granbury Texas Member No.: 27,135 Region Association: Southwest Region |
After thinking about how I'd try to "skin the cat" using ported vacuum...
Can't be done in my book. This is the scenario that kills it. Cruise at 70. manifold will be at 11-15 inches vacuum. Ported will also be close to that number as the throttle blade is open enough to expose the port. Cruise at 70 will require between 45 and 50 degrees total. We are at full centrifugal and full vacuum advance. Hammer it WOT. Manifold vacuum should instantly drop to below 2 inches. 1.5 inches of vacuum is "Perfect restriction" amount to make carburetors function. If your carb is too small you will have more than 2 inches and if your carb is too big you will have less than 1 inch. Manifold vacuum disappears quickly when you snap the throttle open. This is MORE pronounced with ITBs. Fuel injection can go lower restriction with no issues, this is one of its advantages over carburetors. A fact to keep straight... engines actually don't suck atmosphere in. They create a lower pressure atmosphere, and the ambient air pressure PUSHES the air/fuel mixture in. Less restriction is usually better. I believe ported will not change between cruise and WOT. It is dependent on airflow across the port in the carburetor and all you did was increase the volume of the airflow by opening the throttles. I will be testing this theory, once I get the 914 up. Never really wondered "what does ported vacuum do at high speed/high load?" I believe ported vacuum will INCREASE with load. There's a chance I'm wrong. Right now, I'd bet $100 that I'm correct. We need 33-39 degrees advance at high-speed full load. This number is dependent on cylinder head design and the use of quench. The more efficient the engine the less total advance we need. Super common is the number 37... I believe this is the red mark on most of the fans on T4s that you adjust timing too at 3K rpm. They're having you set MAX timing at MAX load this way. Don't the instructions tell you to disconnect vacuum hoses and plug? This cruise to WOT advance change is WHY vacuum advances were designed in the first place. The ONLY engines that need this are engines that are operated at varing load. Aircraft and boat engines do not need vacuum advance. Stationary engines that are operated at one RPM/Load range don't need it either (truth be told, a stationary engine might get away with a fixed distributor with no advance mechanisms at all.) Ported vacuum is NOT load based and we need timing control that is... Can't be done with ported IMO. Edit: I'm now planning on plotting both manifold and ported on L-Jet stock, the classic progressive two BBL and a set of both 40 and 44 IDAs on the 75 1.8L and later on the 2056-2300 cc engine I'll end up building. |
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