How much vacuum does an intake port really see?

[cboggs]

what if some of that high lift flow gets "choked off" in favor of smaller
throats etc, for increased air speed into the chamber?
Curtis

Would you also change the IVO and IVC points to take addvantage of the increased speed?
Later IVC?

Randy

Randy,

Yes & no, .. that's likely a question for Lloyd the cam guy in my shop, ..
we do cams by committee in my shop, ..

if you have the airflow CD's from Don, go back and listen
to what Mike Chapman refers to as "down force", ..
the energy of the charge entering the chamber, .. ..

I think there's a 100 ways we could "play" this, .. but air speed
has always proven to make power, .. and air speed has energy
in inertia , . .

Well if you reduce the throat area to increase air speed through the throat,
a by product of the smaller throat will be better mid lift flow numbers
and in some cases reduced high lift numbers?

I'm not saying this is right or good, .. just throwing it into the conversation.

Curtis

[shawn]

I've always tried to think of filling the cylinder with as much energy as i can, up to the last possible moment. Much like a pitcher accelerates his arm trying to create his highest velocity at the point he releases the ball, or the way a golfer try's to create the highest club speed at the moment of impact. My vision of filling the cylinder has me trying to create as much velocity, without it slowing or backing up, as high as i can into the lift range. Once that mixture is put into motion into the cylinder, it's much easier to keep it going, then it is to speed it up, slow it down, and try to speed it up again. I believe swirl has some effect on how well this can happen. Again this is just my opinion, but the trend is not to see high swirl characteristics in good race heads. I believe it limits the "downforce" that Curtis describes. I also think that the slow down, or back up of air flow occurs probably sooner into the lift range than most of us realize. My data from flowing heads at the highest depression I can, and the results gathered so far, indicate that this is true. When I make a small change to a head that allows it to flow better at higher depression, and it gains more power than the cfm gain would lead you believe, I have to think that it's because there's a large flow restriction based on the higher velocities going on in the port in an actual running engine.
I always smile when someone won't put "x" cam in an engine because "the heads quit flow at .500 lift, so I won't put anymore lift in it than that.". I ask them if i flow it at 40"'s and it quits at .400 lift, would they not put one in there any larger than .400? It usually draws a blanks stare.
shawn

[randy331]

Would/could, there be a power gain with an increase in throat speed, even without a VE increase? I can see a power gain if the increased speed improved mixture quality, but is there other reasons for a power gain with faster throat speed,but no VE improvement?

Randy

[cboggs]

Would/could, there be a power gain with an increase in throat speed, even without a VE increase? I can see a power gain if the increased speed improved mixture quality, but is there other reasons for a power gain with faster throat speed,but no VE improvement?

Randy

Hummm, .. I gotta "chew" on that for a while but in general yes there is
a performance gain even if there's no VE gain in a few cases I've seen.

99% of the time however I'm seeing VE gains, .. and I think that's the primary function.
Also think about how a smaller throat with a sharp edged VJ
flows backwards, .. rick seems to be into this lately, .. combine that
with a steep top angle, . .

It seems like the current trend is huge throat sizes, steep angles and big peak flow numbers, ..
all of this sacrifices the airspeed into the chamber, .. not a good thing I think.

Curtis

[randy331]

Also think about how a smaller throat with a sharp edged VJ
flows backwards, .. rick seems to be into this lately, .. combine that
with a steep top angle, . .

I'd think any reduction in reverse flow, without a reduction in foward flow, would gain power at some RPM. If the gain is in the RPM range you race at??

It seems like the current trend is huge throat sizes, steep angles and big peak flow numbers, ..
all of this sacrifices the airspeed into the chamber, .. not a good thing I think.
Curtis

Is the csa of the throat increasing for a given application, or just the % of the valve size.


Randy

[cboggs]

I'd think any reduction in reverse flow, without a reduction in foward flow, would gain power at some RPM. If the gain is in the RPM range you race at??

Only if the reverse flow is a problem to begin with, ..

Is the csa of the throat increasing for a given application, or just the % of the valve size.
Randy

I'm talking about bigger throats with steep seats, .. it's the "bigger is better"
mentality that always gets head porters in trouble.

Ok, it's late, .. I'm going to bed.

C

[jmarkaudio]

In a sense, doesn't using a smaller throat also give the incoming charge a more gentle turn around the head of the valve? Just like you would want a larger short turn radius in the port?

Mark Whitener
____________

Good work isn't cheap and cheap work can't be good.

[randy331]

I'd think any reduction in reverse flow, without a reduction in foward flow, would gain power at some RPM. If the gain is in the RPM range you race at??

Only if the reverse flow is a problem to begin with, ..

Wouldn't reversion be a problem, at some rpm on almost any engine?
I don't mean at peak ve or peak hp, but maybe 3000 rpm, or so below peak hp. I don't see a gain if the engine is ran near peak hp but say on a street engine, or circle track that see's a wide rpm range on the track. A reduction in reverse flow, without hurting foward flow, would have the POTENTIAL, to increase power below peak tq. IMO.

Randy

[MadBill]

I think it's safe to say that any race engine with optimum (or close) valve timing has significant (IVC-region) reversion up almost to peak power RPM. I know that when using the Dynomation program, lack of reversion (as shown by the pressure graphs) much below peak power is a sure indicator of too-early IVC.
In fact some builders claim that a fully optimized system produces max power with some reversion. Again in Dynomation, I can attest that getting the velocity up via optimum minimum port area shows (besides significant direct power gains) indirect gains due to now-later optimum IVC .
Don't know about any others, but DM has no provision for entering reverse flow numbers, which would be a factor in determining acceptable reversion.

[randy331]

I think it's safe to say that any race engine with optimum (or close) valve timing has significant (IVC-region) reversion up almost to peak power RPM..

Do you belive reducing reverse flow, at low lifts, when measured on a flow bench, would result in a reduction in reversion, in the running engine, and result in a power gain at some rpm?

I know that when using the Dynomation program, lack of reversion (as shown by the pressure graphs) much below peak power is a sure indicator of too-early IVC. .

If the program doesn't let you input reverse flow, what does it use to calculate reversion? It just assumes a certain amount of reversion based on valve diameter,seat angles,lift/vs piston motion etc?

Again in Dynomation, I can attest that getting the velocity up via optimum minimum port area shows (besides significant direct power gains) indirect gains due to now-later optimum IVC .

Engine pro shows a power gain with a smaller valve flowing the same, but not a different cam requirement.

Randy

[MadBill]

I think it's safe to say that any race engine with optimum (or close) valve timing has significant (IVC-region) reversion up almost to peak power RPM..

Do you belive reducing reverse flow, at low lifts, when measured on a flow bench, would result in a reduction in reversion, in the running engine, and result in a power gain at some rpm?

I know that when using the Dynomation program, lack of reversion (as shown by the pressure graphs) much below peak power is a sure indicator of too-early IVC. .

If the program doesn't let you input reverse flow, what does it use to calculate reversion? It just assumes a certain amount of reversion based on valve diameter,seat angles,lift/vs piston motion etc?

Again in Dynomation, I can attest that getting the velocity up via optimum minimum port area shows (besides significant direct power gains) indirect gains due to now-later optimum IVC .

Engine pro shows a power gain with a smaller valve flowing the same, but not a different cam requirement.

Randy

(getting too much to separate all the quotes, so...)
1. Yes, lower reverse flow should help, but short of a reed valve, I'm not sure a lot can be achieved.
2. I think DM just assumes the same flow in reverse. I know if you don't input low lift flow numbers (for sure below 0.200", best down to 0.050"), the predicted power results are low and unresponsive.
3. I'll have to try small valve, equal flow with DM. Don't know how it reacts.