Understanding Engine Vacuum and MAP - What is Vacuum? Why engines have vacuum & why it's important

[@DadsGarageDiagnosticSpecialist]

This is premium content. A goldmine of knowledge.

[@mikebruegger8654]

Great in depth training for lots of us. Thnx!

[@ThunderbirdRocket]

Extraordinary detail ! Super helpful !! I learned lots !!! Thanks mate. 👊🏼 🔥 🔑

[@bitsofwisdom460]

Thank you, glad it helps!

[@budsodalsky]

Thank you for this - when i was a kid i wondered why people had accesory vacuum gauges installed on their dash. Lol "What's it good for" in pirate voice.

[@bitsofwisdom460]

Yes, that is why. They were popular, especially when gas was in short supply.

[@TaylorJensen-ys2cv]

Man I hate it when people know what they are talking about, leaves nothing for me to criticize........... this is one of those over look thing today, I think if more people really understood how vacuum is used in Modern EFI and it's affects on intake draw and Injectors, there would be a lot of really good tuners out there.

[@bitsofwisdom460]

Thank you, and I agree that vacuum or manifold pressure is often not understood. When I was working in the hybrid vehicle world, our calibrators had to watch manifold pressure very carefully. A particular example was that if you wanted a smooth restart while you had been running in a hybrid mode with the engine off, it was best to spin the engine with the throttle closed and the fuel off until the intake manifold pressure pumped down to the right pressure / vacuum. Then you could fuel the engine and not have a big spike in torque due to all the air in the intake. It made for very smooth restarts.

[@doglegjake6788]

Excellent video

[@PhaseConverterampV]

Be more intuitive to lose the entire vacuum nomenclature altogether. Calculations don’t like negative numbers, use the aviation convention of manifold pressure , positive units are better. Plus, put emphasis that no engine, ever , pulled in air. The atmosphere always pushes itself into cylinder p1v1=p2v2. You know all that jazz better than most. Excellent video, thanks so much.

[@bitsofwisdom460]

Yes, I agree that dealing in absolute pressure in the engine is probably better. But, I approached this from a traditional automotive direction to try to bring those viewers to a better understanding. Also, while this video was focused on the engines, it ties in to some other topics. I have had some previous topics dealing with vacuum because in the automotive world there are a number of systems where vacuum or low absolute pressure is the force making things operate-- purge / cruise / brakes / hvac. I think that measuring vacuum can be easier to handle when working on those systems.

[@jimknowlton342]

Thank you so much!

[@bitsofwisdom460]

You're welcome!

[@marck3122]

Thanks for sharing Sir

[@bitsofwisdom460]

My pleasure

[@EricKiltie]

Great video - thanks.👍

[@bitsofwisdom460]

You're welcome.

[@ernestdamo677]

Thanks for the knowledge you have provided, please what does late ignition timing means

[@bitsofwisdom460]

Late ignition timing is a little variable. In general, engines like to have a base timing several degrees before top dead center. Depending on the design it can be from perhaps 5 degrees before TDC up to as much as 15 degrees. If the timing is much lower than that, then vacuum would be lower and would be considered late timing. It might even be AFTER TDC. When I say it is variable, if an engine should be set to 10 BTDC and it's running at 2 BTDC, it would be late.

[@ernestdamo677]

@@bitsofwisdom460 thanks

[@bryanst.martin7134]

It used to puzzle me when pilots talk about manifold pressure in inches. Vacuum is half of the equation of a boosted engine. So how do you reference between vacuum and pressure? Inches of Mercury. No conversion, quick and concise. I rebuilt Pratt and Whitney R2800 radials and they had 2 stage superchargers. Along with T56 turbines, and that is a whole new story.

[@bitsofwisdom460]

Yes, and I expect that dealing with manifold absolute pressure rather than vacuum / relative pressure can really be a help in aircraft, because it isn't affected by altitude. A specific manifold pressure should result in equivalent power at any altitude or barometric pressure. Granted it may be harder to get a boost pressure at higher altitude. And gas turbines are a different animal for sure. I was a transmission test engineer and I spent several years in the '80s running M1 tank powertrains in dyno cells for performance and durability. The AVCO Lycoming turbines were needed to test my transmissions. I learned a lot about those engines because they were fairly new and had a few issues. I ended up being an engine test engineer of sorts just for self preservation in getting my job done.

[@bryanst.martin7134]

@@bitsofwisdom460 I also had the opportunity to work on a test cell with a turbo I had recently rebuilt. It takes a little nerve and great care to stand inches behind a spinning 14' prop to adjust the fine pitch of the propeller control...

[@DUCKSAREEVILLLLLLLL]

10:36 and on. Where the nickel dropped for me. I have a motorhome with tag axle brakes activated by vacuum from an outlet just below the carburetor. Even at high throttle, there's still enough vacuum to make them work OK.

[@bitsofwisdom460]

Most vacuum brake systems have a reservoir to hold vacuum for when the throttle is open. Then when you back off it will charge again.

[@DUCKSAREEVILLLLLLLL]

@@bitsofwisdom460 The 1987 tag axle still has a fluid reservoir that holds about 10 ounces of fluid. Maybe when vacuum is low, the hydraulic braking kicks in? (They were made by Dexter, by the way, which no longer makes hydraulics brakes for vehicles. I think they're focused almost exclusively on trailer brakes now, especially electric brakes. Fortunately, all the hardware, drums, and shoes are identical to those for Ford E350 vans, which are still being made.)

[@bitsofwisdom460]

I don't know exactly what system you have, but from what you describe, it sounds like it could be vacuum over hydraulic. Those systems have a vacuum brake booster driving a hydraulic master cylinder. These have some type of actuator that provides a signal to that vacuum booster. It might be a hydraulic line from the main brakes that triggers the system for the TAG axle.

[@DUCKSAREEVILLLLLLLL]

@@bitsofwisdom460 Yes, you are right, the tag axle has its own booster and the rear hydraulic line is connected to the tag axle hydraulic system, but I had to bleed the system from the Dexter brake fluid reservoir before they would work. Previous owners probably just ignored the tag axle brakes, which is pretty dangerous, since the hydraulic and vacuum systems are all interconnected. I'm just amazed that the valve in the booster and everything else still work after 37 years.

[@Freemanpressx]

Excellent.

[@bitsofwisdom460]

Many thanks!

[@somaxusa]

Hello. How much does a vacuum typically change on SC and turbo engines? Also, how would you assure if any PCV changes necessary when modding a car say with a bigger turbo? Thanks.

[@bitsofwisdom460]

A supercharged gas engine still needs to be throttled to control power, so at idle and lighter loads, it still should have vacuum similar to a normally aspirated engine. That vacuum should decrease and transition to boost as throttle is opened to increase power. I haven't experimented much with SC / Turbo so I don't have a good example. Regarding PCV, I don't know. NA engines go to minimal vacuum at WOT and PCV reduces until throttle is closed. I would look at how boosted engines are plumbed to use as comparison. It may depend on whether the boost is blow through the throttle body / carb or draw through.

[@somaxusa]

@ is my logic accurate? When boosted air rushing in under the load it would cause more blow by gases that need to be vented out to mantain pressure range..? That is what I’m trying to understand with some people modding the cars by say sizing up a turbo or changing pulley sizes, throttle body and etc. I don’t think I have ever heard anyone talking about a need of altering PCV system to avoid premature failures. Or am I missing something and generally speaking OEM PCV should handle these changes just fine…? I look at some people 3x the power and that’s what I started thinking after watching your excellent video! Thank you for sharing all of your wisdom with us!!

[@bitsofwisdom460]

@@somaxusa I think that your logic is reasonable in the thought that higher cylinder pressures from boosted engines would likely cause higher leakage past the rings and more blowby. I don't know the capacity or margin in the systems. The engines with PCV valves tend to regulate flow. Some newer engines have fixed orifice systems that don't regulate. My engine friends used to call the old ones "fresh air" and the newer fixed orifice "foul air" systems because the fixed orifice wasn't as efficient at removing the blow by, (yet those engines seem to work just fine). If you go all the way back to my '60s car, the PCV inlet and filter is the oil fill cap in the valve cover. If PCV became overloaded under power, the pressure relief would be leakage back-flow out the fill cap. Newer cars have inlets in different locations. Now, just thinking, is that the main risk for high boost is probably crankcase sealing. If pressure builds up in the crankcase, the crankshaft main seals could be at risk of damage at some level. If it was me, I would probably look at others who have been down the high boost path to see if they need some form of pressure relief or custom PCV.

[@somaxusa]

@ That pressure relief valve was there for a reason for sure, granted with the emissions control came down the changes. But in reality, one would think a modified oil cap to somewhat match old style or better yet use of some sort of pressure measuring device(manometer) would be ideal during at least test stage to compare baselines. I’ll have to ask modders around to see if I’m overthinking this potential issue, but it’s hard to believe someone would actually have any calculations. A lot of them seem to be just super excited to get some bolts on to get as much as possible from the engine.

[@bitsofwisdom460]

@@somaxusa Yes, I was thinking about suggesting pressure measurement. A manometer check would be good, I just don't know a good threshold for maximum pressure. The lip seals for the crank would seem likely to be most sensitive.

[@aidan9958]

Could this explain why the oil filler hole/dipstick tube pulsates air in and out during idle? I understand there's PCV connected to the crankcase, but I was a bit alarmed by how much air appeared to be coming out of the oil filler hole during idle. If I placed a sheet of paper of the hole, it would rapidly push and pull the paper to the hole as if it was "pulsating" Or is this just air displacement within the crankcase from the pistons moving up and down at a rapid rate..

[@bitsofwisdom460]

There can be some pulsating, but most of the pulsation (in most engines with even number of cylinders) should be neutral because as a piston moves down, another moves up. There will be some net flow outward due to the "blowby" or leakage of combustion gasses past the rings. If you have a cylinder with higher leakage you might get a stronger outward pulse when it fires. The entire intent of PVC is to pull these vapors and burn them.

[@Mike-yq6jg]

on a diesel engine will vacuum give high blow by ?

[@bitsofwisdom460]

I would not think so. Conventional diesel engines don't have much vacuum because they are un-throttled. Blow-by is usually related to ring seal. It is true that vacuum can upset ring seal, but I don't think it will apply to a diesel.

[@Eli-kr5bm]

I always wondered, why engines worked with vaccum, in my head is so inefficient, so many loses

[@bitsofwisdom460]

You're correct that it seems inefficient, yet that is a key part of the way standard gasoline engines work. Many have tried alternatives, but they are difficult and have drawbacks. It's too complex to try to explain here, perhaps I'll need to try a follow-up video.

[@PlanetTwilow]

We are standing on the bottom of an ocean of air that covers the planet Earth. The weight of the 100 miles of air above us, is 14.7 Lbs. Air weighs a lot less than water, don't try this at the bottom of the Pacific.

[@bradsobin9741]

So this is why my engine sucks.

[@PhilipMaguire-iy2th]

Vacum leak