Baldy

Ok, with any motor, piston or rotor, non-turbo: I always read where people say that opening up the intake ports allows "more air in" the compustion chamber. If the engine is sucking in the air, how can more air fit in than the maximum displacement? Unless, without porting, there is a negative pressure in the combustion chamber before compression. I mean, if air isn't being forced in (turbo, etc.), how can more air be fit it?



The way I understand it is, with porting there is less restriction, so less work is needed to suck the air in. This would let the power stroke of the cycle put more power to the drivetrain, and use less power to suck in air (among other functions). Just like opening up exhaust allows only for less restriction, not more air to be shoved out of the engine (you can't shove out more than there was to begin with!), right?



Are people just throwing terms around ("more air in") without realizing the actual meaning, or am I misunderstanding something? I'm just trying to grasp this.

75 Repu

DJ Rotor

A less restrictive intake will improve volumetric efficiency, or what percentage of the chamber is actually filled with intake charge. Because of the interaction of pressure waves, and also inertia tuning (both of which involve the length of the intake tract and exhaust header) it is sometimes possible to get more than 100% VE.



Think of it this way: the engine has a limited time to suck in the intake charge. The more restriction, the less it will be able to take in in that time because there is only a limited amount of intake stroke to work with.



So "more air" is actually correct.



J

Baldy

hmmm...interesting



yeah, I just went and read about intake tuning, (edit: www.howstuffworks.com) with the pressure waves and stuff, so I can see how that works



so, with constrictive intake, and no intake tuning...at the close of the intake cycle, before compression, is there actually a negative pressure in the chamber?

banzaitoyota

There is no such animal as "negative pressure". There is always pressure, may be very little but it does exist

Baldy

sorry, I meant less than atmospheric

DJ Rotor

There is negative gauge pressure, not negative absolute pressure. Gauge pressure is just referenced to atmospheric, and that was what you were thinking of - and you're right about there possibly being a slight negative pressure *gauge* in the chamber at the beginning of the compression stroke - depending on the exhaust flow and port overlap, that is.



J

Baldy

ok, I think I've got it straight now. Thanks!

DJ Rotor

If you want to dig into this stuff get A. Graham Bell's "Four-Stroke Performance Tuning" - very excellent book. No rotary-specific content, but the concepts are mostly the same - especially in areas of intake, exhaust, carburetion, etc



J

Baldy

if I ever feel the urge to read a book, I'll look for that one