mhwarner

This may be a dumb question, but I am honestly perplexed. Searching/reading various posts here, I have come across statements that say p-ported engines are louder than other types of ported rotaries. Is this true? More importantly, why is this? Why would modifying the intake portion of a rotary's combustion cycle affect how much noise it puts out?
-Mark

Kenku

They're not necessecarily loud really. However, when people talk about pports, they're almost always talking race motors... and, like any other motor with big exhaust ports, they're more sensitive to backpressure, which means either straightpipes / minimal mufflers, or very carefully designed exhaust. Since, again, everyone is thinking in terms of race motors when pports come up, it's hard to point to many examples where the exhaust is designed to be any quieter than noise restrictions at race tracks.

I'm pretty sure it's possible to have an exhaust setup for a race style pport that would be acceptably quiet (not as quiet as stock, but quiet enough for street duty) but I doubt most of the kind of people who would put a pport in a street car care about sound levels that much.

Jaime Enriquez

Not to mention what open throttles sound like with such huge intake ports sucking in all that air!

Maxthe7man

The overlap makes the engine have a sharper exhaust note, along with the fact it has to idle at much higher idle speed than it did previous...

diabolical1

iTrader: (3)

higher idling and minimal muffling is what i would have offered.

peejay

The intake ports, to be literal, do not make the engines louder.

However. There are many other factors at work.

Engines with a lot of overlap do not like restrictive exhausts. For a rotary, the ONLY quiet exhaust is a restrictive one. So you either kill a lot of power or you have a loud exhaust.

When people port engines, many like to open the exhaust ports earlier. (Mazda did not on their race housings, infact they open *later* than stock...) This makes the exhaust louder. Very much so, actually.

A decently tuned PP will idle around 1000-1200, mine could idle at 600rpm if I wanted to any my carb would meter correctly at that point (EFI is the answer!) BUT, at lower RPMs there is very little manifold vacuum. (Mine will idle at about 8 in-Hg average at 1900ish, only 5 in-Hg average at 1500) Manifold pressure is an idicator of load, so basically the engine is under more load at idle than a stocker, so it will be a bit louder that way. When cruising there is no difference relative to a sideport engine with the same exhaust port.

Lots of people run mega mongo oil pumps and supersludge oil and brag about their engines pegging their oil pressure gauges at idle. I ask you, why do you need 100psi+ of pressure at idle? You don't, you're just wasting power. And you're putting the engine under more load to do that, which makes it louder. (You can easily detect a 200-300rpm drop in idle speed on most engines after an oil change after the oiling system re-primes, this is due to the additional load imposed by the oil pump! Abd those are stock engines with stock pumps and light weight street oil)

Maxthe7man

Its not under more load at idle at all, the vacuum is lower, because of increased overlap its effectively pulling on both the exhaust and the intake port at the same time, instead of just the intake port with a few degrees of exhaust port closing comparitve to a stock motor of peripheral exhaust and side intake design..
By the way the oil pressure will be whatever the oil reg is set at it, if you have a good system to supply 100 psi peak oil pressure, and it happens to be able to do it at idle, there isnt much you can do about it.. I have a externally adjustable oil regulator, adjusting the idle oil pressure from 25 psi to 75 psi has no effect on idle or vacuum, its such a minimal drag to begin with...

peejay

My oil pressure is 30psi at idle cold, about 10-15psi idle hot. 60psi regulator. Stock weeny 12A oil pump, stock regulator, just ported the runners at the oil pump and modified the oil pickup.

If you have an externally adjustable regulator, it can be assumed that you have a drysump system, which is going to be much more efficient than the factory blunderbuss system.

I maintain that it is under more load at idle. Yes some of the loss of vacuum is from exhaust pull-up. However, I maintain that the engine is under more load (pulling more air through the carb/throttle body) because the exhaust dilution means you need to pull in yet more air and fuel in order to get a burnable mix. And even then, when you're sputtering/brapping there at idle, the engine is only firing about half the time, so it has to be running harder the times that it is actually firing in order to carry it through the times that it is missing.

Maxthe7man

No I dont have dry sump system its stock oil pump with hose/plumbing upgrades and some oil system blueprinting...
Its all exhaust dilution, which by the way also causes the miss, which also lowers vacuum...

peejay

So you're saying that the engine hitting only half the time at 1200-1500 requires the same amount of air as an engine smoothly idling at 600? Ignore the "internal EGR", yes it's lowering vacuum but it's not responsible for *all* of it.

How is your oiling system externally pressure adjustable? I'm curious.

Maxthe7man

I see what angle you are looking at it from that the greater throttle input for higher idle speed is the cause of the lower vaccum which is true when you add load and give more throttle, but in the case of the overlap idle, there really is no added load, its just that we increase the engine speed to maintain a certain port velocity to overcome the reversion of the exhaust port, the diluted charge necesitates the added engine speed.
I externally plumbed part of the oil system... The thought of dropping the pan everytime to dial in my oil pressure for my turbos ability to stay sealed kinda of turned me off doing the typical RB adjustable pressure regualor or shim gig...I needed good oil pressure and flow for the rpm band of my setup, its scary how good it works when you get rid of the systems bottlenecks....

peejay

What we *need* is a nice comparison on a airflow meter car to see how much intake air it required stock vs. with beefy ports.

Interesting.

mhwarner

Looks like I'm hijacking my own thread ;-)

Max, can you give more details on how you externally plumbed part of your oil system? Sounds pretty interesting. I'd like to know more, if you don't mind...
-Mark

peejay

I know about the "loop line" (running oil directly to the passage for the front main, instead of having to go through the tension bolts and three or four right angle turns) and running the oil outlet from the front housing instead of the front cover (elminates three or four right angle bends, and an O-ring) but I'm unaware of any oiling system modifications that have an external pressure regulator.

Really curious.