question...possibly stupid...but i'm curious
 

i remembered today, for some reason, the law of conservation of matter (nothing is created or destroyed in any process, just changes state...etc)

why, then, are intake ports and exhaust ports so drastically different in size? if, in theory, everything that goes in, has to come out, wouldn't you want same sized ports for optimum performance? i mean, when you throw air and fuel into the chamber, it combusts making power, then must be expelled...are the resulting gases less dense afterwards or what?

Due to the temperature of the exhaust gases versus the intake, the viscosity (of the gases flowing out of the exhaust) is much less. These less viscous exhaust gases flow with relative ease through a given size port.

Steve

ah....makes sense

Gasses move from one place to another due to differences in pressure (differential pressure or "delta p"). At the point where the exhaust valve opens (blowdown), the pressure differential (delta Pp) between the cylinder and the atmosphere might be a hundred or more times as much as the delta p in an intake. "Hit it with a bigger hammer and it will move through a smaller passage."

There is also a lot more exhaust volume (but not mass) than intake. If you look at what is making good (great?) power, especially in 2 valve engines, the relative size of the intake port/valve is increasing even at the expense of the exhaust port/valve.

We flowbench check intake and exhaust ports at the same delta p, but that's not what the engnie really sees.


Jon

in direct injected engines there is more mass flowing out of the exhaust port than mass flowing into the intake port...

Yep, about 8%, right?

DI engines generally have more static compression than the non-DI version of the same engine as well as other changes the make more power, so both intake and exhaust flow are increased.

The GM 3.6 high-value V6 makes about 20% more peak hp in the DI form. It would be interesting to compare the ports/flow of a DI and non DI version.

Jon

Old SStroker already said this but it bears repeating. Remember when the exhaust valve opens it is at the end of the power stoke where the combustion process has just taken place. There is still a lot of pressure in the cylinder.

So when the exhaust valve does open that residual pressure forces the exhaust gas out the valve. Then that is followed by the upward movement of the piston to force out the remaining gases.

The intake side has no such luxury. It depends solely on atmospheric pressure in a N/A situation to force the intake charge into the cylinder. Old SStroker used deltas to correctly explain it being the engineer he is.

Then the question is with the excellant flowing heads and good exhaust systems being put on our cars today do we really need a big split in the intake/exhaust duration?

Maybe that is why single pattern or reverse split cams are comming into vogue. Then again you look at what GM has done with some of their products lately and they have a huge split on the intake/exhaust durations. Got me folks. I'm sure Old SStroker has the answers if he is willing to give up some secrets.

ok now i understand. this info will help me with cam selection in the future. thanks for the info guys, anyone else feel free to chime in or keep the discussion going :nod:

1989GTA, the big secret is that OEMs and good aftermarket engine designers give the engine the valve duration and lift it likes. Others should try to analyze why it likes certain valve events. I certainly don't have all the answers, nor do I have any "deep throat" inside GM.

Keep on thinkin'. It's good for you, and it's fun to excersize your neurons.

Jon

Thanks for the reply Old SStroker. Regarding the big cam splits GM has come up with and from what others have said is that the intakes systems and the heads flow so good that they need the longer exhaust duration to move out the expended gases.

That scenario makes sense to me. However GM probably has multible reason with their multi-million dollar research budget and facilities for research. I can think of fuel economy and emissions as some other factors.

Whoops, starting to hijack the thread. Sorry. :hijack:

FWIW 2000_SS there is no such thing as a "stupid question" if you don't know the answer. I meant to say that in my first post.

I have seen some questions that are only a RCH away from "stupid", but certainly not yours.


Jon

As Old SStroker suggests, paying attention to "split" is not particularly useful, possibly even actively misleading. It's valve events we're interested in, and OEM valve event timing is unique to the special mix of requirements for an OEM engine.

Basically, GM is balancing performance with driveability, fuel economy, and emissions. One way to get more horsepower for a given displacement is to allow the engine to breathe well at higher RPM, leading to greater horsepower. In a race engine, high RPM breathing is facilitated by late intake valve close (IVC), early exhaust valve open (EVO), and lots of overlap (late exhaust close overlapping with early intake valve open). Lots of overlap is not tolerable in an OEM application, because it hurts emissions, driveability, and fuel economy. However, late IVC and early EVO have minimal impact on part-throttle fuel economy, and basically no impact on emissions and driveability, so the OEM cams tend to have late IVC, early EVO, and thus extended RPM range for good top end HP.

Looking at the overlap issue, to keep overlap down we have to close the exhaust earlier, open the intake later, or some combination of the two. Closing the exhaust too much earlier isn't good, since it leads to incomplete emptying on the exhaust stroke, resulting in contamination of the intake charge. So an OEM cam tends to have a late intake open instead.

Putting that combination of valve events together in a cam grind - late IVO and IVC, early EVO and middling EVC - will give you smaller intake duration relative to exhaust, large LSA, and late ICL. This is exactly what we see in the OEM cams.

But. A performance cam is different, since we are more tolerant of the side effects created by more overlap, and overlap in combination with a tuned (long tube header) exhaust greatly aids breathing and power. Compared to the OEM cam of similar RPM range, the biggest difference will be earlier IVO - meaning greater relative intake duration (less "split"), tighter LSA, and earlier ICL.

The point being, just because an OEM cam has a large difference between intake and exhaust duration does not mean a performance cam should also - in fact, "split" by itself tells you almost nothing about the real characteristics of the cam. IVC, overlap, and EVO are the most critical specs.

all good info here, more than i expected, but all helpful. thanks guys.

so in theory, if there was some way we could create a vaccuum on the exhaust side to evacuate the cylinder faster, it would allow the intake charge to enter, almost like boost? is this even possible...just thinking outside the box here...

like could we plumb the header tubes with nozzles with something like a PCV valve and use the engines own vaccuum to pull exhaust out? there's a picture in my head and i dunno if that accurately describes it. in other words, make it so when the intake valve opened, it wouldn't have a neutral atmosphere in the cylinder, but a vaccuum to pull the air in...make sense?

If you used some kind of pcv setup, it wouldn't work because when you are pulling a high vacuum, the engine is presented with a low load, like during cruising or idling.

Conversely, when you want to go fast, the engine is pulling a low vacuum.

This is all generally speaking, of course.

ok.....hm....so we make/mount a vacuum pump on the motor in place of the A/C :D

????

The point of a long tube header is to use the wave dynamic energy of the exhaust pulse to create a vacuum during the overlap cycle. That does indeed help pull the intake charge through the intake valve. This effect is worth hundreds of horsepower in highly tuned race motors.

See this article by David Vizard:

http://www.superchevy.com/technical/...exh/index.html

According to Vizard, the suction from a tuned exhaust during overlap can accelerate intake flow up to 100 ft/sec - before the piston even starts the intake stroke.

yeah, i was kind of aware of that, scavenging more or less...just wondered if there might be a cheap way to amplify the effect...???

Yes. Get the entire engine combination spot on in the rpm range you want the most output. Easier said than done, however, and rarely cheap.

Jon

HOLY CRAP!!!!!