Catch Can Math doesn’t make a lot of sense to me.


I understand that under hard driving there will be more blow by.

I understand that introducing those fumes back into the intake could be detrimental.

-Reasons given by Catch Can Sellers include, increased risk of detonation, due to decreased octane.


What I don’t understand is the math of the whole thing.

Seems like most properly operating engines are generating a few ounces of liquid in the catch can every 3-5,000 miles

That volume does not seem like enough to affect the performance of the engine. Here is why:

- If you are getting 20 mpg, you will burn 150 gallons of fuel over 3,000 miles. That’s 19,200 ounces

- If you are getting 3oz of fluid in the catch can during those 3,000 miles...that bad stuff accounts for .000156% of the total fluid going thru the system

- A large portion of the fluid in the catch can is condensation and introduced into the intake would have no effect

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How would 1 1/2 oz of oil, introduced as vapor, over 3,000 miles have any measurable effect on octane?




Really, just trying to justify NOT adding a Catch Can. I hate all of the extra hoses under the hood.

 

Or do it the practical way. Go out to your truck, take the throttle body off and wipe around the inside the intake with a rag then consider this is the stuff thats not burned away.

 

I understand that there will be a small amount of oily residue in the TB

I am questioning the claimed impact of the portion of that residue that makes it to the cylinder...is burned...and how that could cause the claimed change in octane; resulting in detonation.


How much would you actually have to lower octane to produce detonation in a properly tuned, EFi, computer controlled engine?

 

Very interesting analysis!

One thing that needs to be considered is the PCV is not operational 100% of the time. How much that is introduced depends on engine temperature, speed, and loads (and probably some other factors). So you'd have to at least subtract a % of time of when the valve is not operational.

Also need to consider that not all oil vapor will be caught by the can itself. There will still be some that bypasses and gets burned. So there certainly can be more that collectively could be enough to reduce octane; but the catch can reduces enough that the effect becomes insignificant (theorizing here).

And finally, I'm sure some octane sensitivity comes from the fact that it's hot crankcase gases being introduced. With a catchcan in place, there is time to cause some amount of cooling of the vapor before it's introduced back into the intake system. Therefore the intake charge becomes cooler as well. As a matter of fact, for best catchcan efficiency I believe they recommend putting it in a cooler area of the engine compartment so the oil more easily drops out of suspension.

 

In 99% of cases it's a waste of money and a feel good item. I'd imagine there are some instances (forced induction comes to mind) where a catch can would be helpful because of the added pressures and blow by but for a NA engine in good working order it's just not worth it in my opinion. I checked my heads when I pulled the intake off at around a 120k miles and there was a minor amount of crud on the valve stem and the runners were stained but nothing that would possibly affect performance in any way. The intake itself was oily but no build up in it. I keep threatening to make some out of a beer can and steel wool as that's really all you need.

 

If you're boosted, and if you don't like oil leaks, replacing rear mains and/or valve cover gaskets: you need to vent the motor

If you're NA, and your rings aren't atrociously worn/sized wrong: you can get by with the factory PCV system, without much issue. Some oil ingestion will occur, no matter what vehicle/engine/age or mileage. If you have an internally lubricated engine, its going to burn a smidge of oil, no matter huwat

Take note that the factory breather setup with boost will pressurize the crankcase via the passenger side valve cover. With that in mind, the largest surface area gaskets at risk are the valve covers (easy) and the rear main (yeah thats gonna take more time to replace than building a catch can setup)

Even if you don't use a can, you still need a way to let that pressure out of the crankcase. Oil vapor does reduce octane, but generally we are talking about significant amounts of oil: like leaking valve seals, bad rings, bad intake gaskets (engine design dependent)

All in, I run catch cans and I feel good about it, except the smell gets obnoxious. Next setup for my truck will be sealed

 

Like with most everything in performance to each his own, myself I run a catch can for peace of mind. My engine is a cam and full top end swap. I had a POS eBay can and never got anything so almost didn’t believe the hype, once I got a decent can and plumbed better, I got about 2 to 3 ounces after 500 miles of hard driving since this is not my daily. I was surprised and glad at the same time because I don’t want that **** in my intake.

 

I've been wanting a catch can but never got around installing one. Run the truck pretty hard (not abused) since I built the engine two years ago. Haven't noticed any abnormal oil consumption. I was almost certain when pulling the heads the piston tops would be relatively clean with my driving style. Nope... had a nice layer of carbon on the pistons which is probably normal using the factory PCV system. Think I'm going with the Moroso catch can as it has the bracket that's made to fit on the GEN 3/4 alternator bracket.

 

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It is my understanding that the PCV and a 3-Port Catch can function differently, because the 3-Port pulls vacuum, so that it functions during idle and other low stress operating scenarios.

 

I have seen it suggested that the Catch Can should actually be mounted in an area of higher temperature, so the water component evaporates and the vapor goes back to the intake.

There is speculation that the Catch Can manufacturers suggest that it be placed in a cool area, because they want as much total liquid to be in the can when you empty it. More of a WOW Factor. Internet Conspiracy Theory?