Don Terrill’s Speed-Talk

How much duration do you add to the exhaust with the addition of a supercharger?

10 to 12 years ago pro stock exhaust ports could be made to flow more that they did. If exhaust flow was imported enough to port to max flow, why didn't they; and reduce the split ( 18 to 22 degree) that they were running?

Lots of assumptions being made that you can actually gather any useful data "flowing" an exhaust port.....

shawn

Now that sounds just plain silly. The heads you see on the market today are directly the result of flow bench and dyno data. Do you know how long it takes to develop a head strictly by the "try this and then dyno it" approach? Holy Jesus.. we'd be another 20 years behind in cylinder head development right now and only rich people could afford to have fast engines. Thank's to flowbenches dyno and CNC we can open a box, bolt the parts on, and race with the bigger dogs.

It's just data. Knowing what to make of it and how to design the rest of the engine around the data is the tougher part. Has absolutely nothing to do with the data being useless.

steve316 wrote:10 to 12 years ago pro stock exhaust ports could be made to flow more that they did. If exhaust flow was imported enough to port to max flow, why didn't they; and reduce the split ( 18 to 22 degree) that they were running?

Because absolute power potential relating to the exhaust port has far more to do with balancing out pumping losses, scavenging effects, and more importantly.. VELOCITY.. than just total flow capability. If you expect to size the primary pipe or adjust merge sizing to fix some shortcoming of the port then it's likely you didn't design the exhaust port as effectively as possible. The camshaft is then developed around the displacement/head/induction/exhaust design and engines usage model to make the most of what you have available from the exhaust port.

Now, can you have far too much exhaust port PEAK flow for any given inductions/intake port/valve/cam capability? Sure you can. In that case you have most likely reached an oversized state and ultimately killed that ports velocity profile and now need to compensate with cam timing and lift figures to crutch it. No need to try and get out of the engine what can't get in.

OTOH, can an exhaust port really flow FAR too much at low and midlift flow(say anything under .400" valve lift) for any street engine? For a particular cam design that was originally designed for a much more restrictive intake tract(intake port included) and lower flowing exhaust port, no doubt. But plenty of people have found that stock cams and street motors LOVE more low/mid-lift flow capability. Upsize the exhaust valve and run more rocker ratio and you will see the results in the first half of the throttle pedal travel. And the motor won't lay over as badly up top of of even a lo-po cams powerband either. Mileage improvements can also be seen on stock restrictive manifolds and catalytic converter combo's as well.

My advice to the guys who are diminishing the exhaust port as just being "whatever's left over for space in the combustion chamber" and cam timing being irrelevant to the exhaust flow capability equation.. is to talk with any good cam designer about what cam spec's he would grind for a regular combo of parts with a really good intake port and an average exhaust port.. vs same head with a really good exhaust port that flows 40 cfm more air on a static bench. Flow window adjustment and port/pipe velocity has substantial impact on the final cam design.

Just how much cylinder pressure do you still want after blow down / ABDC? Just how far from peak valve lift / max exhaust port flow are we ABDC?

Stan

CGT wrote:

CamKing wrote:
That's why you calculate the potential mass/pressure in the cylinder, then you use the exhaust flow to calculate the duration needed to evacuate the cylinder.

Thank you. Goes right back to the "no need to try and get out what couldn't get in".

To further extrapolate the point. Now try to imagine if Mike would possibly develop that cam differently if the PEAK flowz were identical between 2 cylinder heads(same exact engine combo).. but one of them had a considerably smaller exhaust port with deeper bowl and high port exit design?

CGT wrote:

CamKing wrote:
That's why you calculate the potential mass/pressure in the cylinder, then you use the exhaust flow to calculate the duration needed to evacuate the cylinder.

shawn wrote:Lots of assumptions being made that you can actually gather any useful data "flowing" an exhaust port.....

shawn

I agree with Shawn.

And how do you "calculate" the mass in the cylinder, or the pressure decay from TDC?
Let me guess FLOWZ ??

Randy

groberts101 wrote:
Thank you. Goes right back to the "no need to try and get out what couldn't get in".

What if someone designed some clever gadget that forced more air in, what would that do to the in/ex valve ratios?

randy331 wrote:
I agree with Shawn.

And how do you "calculate" the mass in the cylinder, or the pressure decay from TDC?
Let me guess FLOWZ ??

Randy

JMO, on how that might work with no more combustion after max cylinder pressure and no EVO.

Stan

4vpc wrote:

groberts101 wrote:
Thank you. Goes right back to the "no need to try and get out what couldn't get in".

What if someone designed some clever gadget that forced more air in, what would that do to the in/ex valve ratios?

Easy one. It would be akin to making the old OEM Cleveland 4v intake port smaller via stuffed floors or port plates. Run that with a factory exhaust port and even the old cast manifolds and what happens to the power curve?

Would you actually think.. "exhaust port don't need any improvements in "flowz"" to help maximize the now larger amount of mass that gets squeezed in"? Or just rob peter to pay paul back some added duration and/or lift over on the exhaust lobe and call it the same end result anyways?

An engine is just a huge series of compromises to get towards the end result. All I'm getting at with all this is that if you trivialize or underestimate one piece of the puzzles worth or ability to affect some other piece?.. then you're downplaying the physics involved with the whole deal. I come here for the first hand experiences and physics breakdown of it all.. not diluted comments like.. "doesn't matter.. you won't be able to measure it or feel 5 horsepower on the street anyways". I can get that crap on some other sites.

From the standpoint of properly timing and sizing the flow window.. which is what all this needs to be dwindled down to anyways.. there are various ways to go about moving the same mass. You can vent larger amounts of mass out the cylinder throughout the low/mid range and limit lift and slow the valve down all the way to max lift. Or you can lower the low lift flow capability of same port(smaller valve size with 55° seats?) and increase the valve open speeds(rocker ratio/roller lobe?). Purely rhetorical but which combo is best? Best for what?.. valvetrain life.. higher rpm? Do both combo's make same power under 6k? Optimized cam requirements wouldn't change either?

Stan Weiss wrote:

randy331 wrote:
I agree with Shawn.

And how do you "calculate" the mass in the cylinder, or the pressure decay from TDC?
Let me guess FLOWZ ??

Randy

JMO, on how that might work with no more combustion after max cylinder pressure and no EVO.

Stan

ab-cp-st.gif

If you add EVO into it and relate it to various rpm.. what happens with the cam/rocker requirements of each engine?

I would sum up my opinion of what absolute exhaust flow potential means by saying that the cylinder and IN/EX tracts only know what the flow window size and timing is at any given moment in time. Adjust even one parameter and you'll need to readjust another. Changing that affects another.. and another. The combinations are almost infinite.

WTF is "Flowz"?!?

groberts101 wrote: Would you actually think.. "exhaust port don't need any improvements in "flowz"" to help maximize the now larger amount of mass that gets squeezed in"? Or just rob peter to pay paul back some added duration and/or lift over on the exhaust lobe and call it the same end result anyways?

One big problem with this Idea is, it makes the assumption that you had "ideal" EX flowZ before the change.

groberts101 wrote: All I'm getting at with all this is that if you trivialize or underestimate one piece of the puzzles worth or ability to affect some other piece?.. then you're downplaying the physics involved with the whole deal. I come here for the first hand experiences and physics breakdown of it all.. not diluted comments like.. "doesn't matter.. you won't be able to measure it or feel 5 horsepower on the street anyways". I can get that crap on some other sites.

I'm not trivializing the physics involved at all, I'm saying the physics going on in a static test on a flowz bench and the physics of an actual ex port in a running engine aren't the same. And deciding we do something to the cam based on ex port flowz is .

Randy

Warp Speed wrote:WTF is "Flowz"?!?

I think it may be Latin or an Olde English spelling of FLOW.

CGT wrote:

Warp Speed wrote:WTF is "Flowz"?!?

I think it may be Latin or an Olde English spelling of FLOW.

Nope - - an unfortunate lapse into the Social Media shtick which increasingly infests science based publications