No, don’t talk about EVACS!

I’m not sure how many years it has been since the infamous evacs thread on we-crash. I could no longer find it, just a re-hash poll that was in 2013, so the original discussion must have been a few years older than that even. Full discretion, I’ve never bothered with installing evacs on any of the motors I’ve ran, nor insisted that we should install them on my brothers or dad’s motors, that’s why I generated a category called musings. However I do find them fascinating, partially because people seem to latch on to them as part of their identity, with fierce opposition to anyone questioning their efficacy, and also because they are a kind of mystery. “Do they work? Under what conditions do they work? Are the benefits substantial?” The list goes on.

What recently piqued my interest was an episode of Engine Power on YouTube. If you are not familiar with this show, two guys build engines and then dyno them, that’s pretty much it. On this episode though they decided to try out some “tricks” to improve power on a worn out 305. (10) How To Achieve Power Gains Without Taking Apart Your Engine – Engine Power S8, E2 – YouTube

So adding header evacs was the last test they did, after maxing out the timing, fueling, air filter, and thin oil. This worn out 305 with a good amount of blow-by visible out the valve cover was up to 208.5 HP and and 280lb-ft torque. According to their test the evacs gained 1.6HP and 3.5lb-ft of torques at peaks. That is less than 1% horsepower increase and a 1.25% torque increase. They did not overlay the curves to see if the miniscule gain was throughout the curve, or just at the peak. So I can look at this two ways;

  1. On this engine it did nothing exceptional to increase horsepower and torque, so evacs would be of little use on a stock block/cam/heads bonestock style engine.
  2. If it helped this wheezy 305 even just the teensiest bit, then evacs could potentially be of use on an engine even with just a cam upgrade.

As others have stated in posts long ago, I would like to recount some of my assumed pro’s/cons.


  • Prevent oily breathers on valve covers and their potential to drip on headers.
  • Reduce air pressure in crankcase
    • Theoretically better ring seal on some engines (doubtful on 5/64 cast rings)
    • Lower pressure should reduce work required by pistons to displace crankcase air volume
      • Theoretical potential power increase
      • Theoretical lower crankcrase air temp by lower pressure area
  • Theoretical increase in oil performance/life due to blow-by being evacuated instead of coalescing with the oil.
  • +10 Cool guy points, intimidation, identity confirmation
  • Windage effects?


  • Expense in dollars and time to add to engine and headers.
  • Dubious power effects on stock style engines and what is your power goal anyway…
  • If installed improperly or if engine has leaky gaskets elsewhere, potential to generate air flow and EVACuate oil out of the engine.
    • Same potential issue if an engine has excessive blow-by, the blow-by will carry oil with it as it is EVACuated out the headers.
  • Windage effects?


If I were to ever run them I would be paranoid about checking my oil for a while to make sure it wasn’t disappearing. Besides that, the main reason I would consider running evacs would be for to get rid of the crappy push in breathers that like to fall out or get sludged up and drip oil.

Unpopular opinion:

On a few derby motors I have left the PCV valve in place and ran to factory intake port. This factory solution works well to prevent oily dripping breathers, reduce crankcase pressure (when intake has vacuum), and may also theoretically function as a crude type of egr, (depending on amount of blow-by). EGR probably deserves its whole own post, as the actual purpose of EGR is to lower combustion temps, which once again theoretically could have potential derby engine benefits.

Down the rabbit whole with me:

One thought on “No, don’t talk about EVACS!

  1. i dont run evacs. I dont usually build a high dollar motor because its a big waste when the thing pukes. for my lets put an intake, 2g, zoomies, and a cam motors, I dont need em.


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