Exhaust Lobe Shape

[upsidedown]

Just curious about how someone would select the duration vs lift of an exhaust lobe. It seems that there are various combinations of duration / lift but not sure how that is worked out

Example for a solid roller @ 0.050 over different engine combos that have been specked on speedtalk

276/288 duration so 12 degrees more exhaust
0.763/0.720 lift so less exhaust lift

264/273 duration so 9 degrees more exhaust
0.662/0.639 lift so less exhaust lift

270/276 duration so 6 degrees more exhaust
0.692/0.680 lift so less exhaust lift but not as much difference as the others

278/282 duration so 4 degrees more exhaust
0.736/0.736 lift so the same

So a range of 12 to 4 degrees duration change and 0.043 to 0 lift change

Is it just a matter of the bigger the difference in duration then the bigger difference in inlet / exhaust lift to keep the overall exhaust area more consistent with the inlet

Any thoughts?

[Newold1]

Here is a good simple camshaft design read for you. The more you get into camshafts the more complicated some of the data will get because it starts involving things like acceleration, velocity, jerk, etc. and that starts involving a lot more complex math.

www.elgincams.com/campaper.html

[Walter R. Malik]

Just curious about how someone would select the duration vs lift of an exhaust lobe. It seems that there are various combinations of duration / lift but not sure how that is worked out

Example for a solid roller @ 0.050 over different engine combos that have been specked on speedtalk

276/288 duration so 12 degrees more exhaust
0.763/0.720 lift so less exhaust lift

264/273 duration so 9 degrees more exhaust
0.662/0.639 lift so less exhaust lift

270/276 duration so 6 degrees more exhaust
0.692/0.680 lift so less exhaust lift but not as much difference as the others

278/282 duration so 4 degrees more exhaust
0.736/0.736 lift so the same

So a range of 12 to 4 degrees duration change and 0.043 to 0 lift change

Is it just a matter of the bigger the difference in duration then the bigger difference in inlet / exhaust lift to keep the overall exhaust area more consistent with the inlet

Any thoughts?

It is all determined by what will be most beneficial with the different combinations of parts in that engine, vehicle and how that vehicle is to be used.

There is no "rule of thumb".

[CamKing]

Look at it as two completely different things.
You want the intake lift, duration and centerline, that is most efficient at filling the cylinder thru-out your RPM range.
You want the exhuast lift, duration and centerline, that is most efficient at evacuating the cylinder thru-out your RPM range.
The only commonality between the two, is that they're stuck on the same camshaft.

[andyf]

First step is to figure out what you are trying to accomplish. Are you the crew chief for a Pro Stock team? If so, proceed to cam testing. Are you dead broke but want to put a cam in your street rod so it sounds rumpy? If so, proceed to the Summit website and order something.

These questions are useless since we don't know what your are trying to accomplish. Single pattern cams work just fine for a lot of applications. Cam mfgs have been using lobes from the same lobe family for both I and E forever. Most of the cams in the Comp catalog use the next size bigger lobe from the same family as the exhaust lobe.

If you have a bunch of money for parts, materials and testing then sure, go ahead and try to find the best lobe for the exhaust and pair it up with the best lobe for the intake. I've done a lot of cam testing over the years and I usually give up after 4 or 5 cams in one engine. It just takes so much work to change cams and get the engine back on the dyno that eventually I get wore out. Everything has to be held exactly the same to compare the cams. Often the power difference is very small. Most of the time the cam "experts" disagree significantly on what lobes to use. Even if you stick with the same cam grinder they can send you multiple cams to test before they start to home in on a solution. Nobody really knows the correct lobe to use with a given engine until they try a bunch of lobes since each engine has its own characteristics. Small changes in the valve job for example can really change what sort of lobe profile is required.

I just finished up a test where the cam was supplied by one of the rock stars in the industry. It lost power. The follow up email exchange was basically "well here are 6 other lobes to try".

So good luck with that question...........

[digger]

maybe the discussion should separate from inlet completely and be about duration vs lift and why you might want to achieve the "lobe area" with lower lift but more duration or lower duration but higher lift? its been mentioned you dont want the exhaust to blow down to early so you can use the energy for scavenging but what effect does rpm band, open vs full exhaust have on this...

[af2]

Why are you stuck on the exhaust lobe side?

[GARY C]

Why are you stuck on the exhaust lobe side?

Several head designers and header designers feel there is a lot of untapped potential on the exhaust side. Another thing that would be part of this discussion would be ex valve angle.

[af2]

Why are you stuck on the exhaust lobe side?

Several head designers and header designers feel there is a lot of untapped potential on the exhaust side. Another thing that would be part of this discussion would be ex valve angle.

After how many years and how many designs? I think we ought to think in another box....

[GARY C]

Why are you stuck on the exhaust lobe side?

Several head designers and header designers feel there is a lot of untapped potential on the exhaust side. Another thing that would be part of this discussion would be ex valve angle.

After how many years and how many designs? I think we ought to think in another box....

Looking at something other than the intake runner would be outside of the box but most keep going back there looking for more power and then cast the same exhaust runner on the head and bolt up a header that is probably based off of 20 year old technology and call it a day.

[upsidedown]

Why are you stuck on the exhaust lobe side?

Not stuck on it. I feel like I have a good handle on the inlet and how to work that out. It's just the exhaust that I could do with some more feedback on in regards to duration / lift. I know its based off exhaust flow or mass flow but was hoping there was at least some sort of relation to the inlet profile. So if you picked a 6 degree bigger exhaust duration than inlet do you need to make the lift less to keep the inlet / exhaust area the same?

[groberts101]

Why are you stuck on the exhaust lobe side?

Not stuck on it. I feel like I have a good handle on the inlet and how to work that out. It's just the exhaust that I could do with some more feedback on in regards to duration / lift. I know its based off exhaust flow or mass flow but was hoping there was at least some sort of relation to the inlet profile. So if you picked a 6 degree bigger exhaust duration than inlet do you need to make the lift less to keep the inlet / exhaust area the same?

I think you're in the ballpark but also think about not only the volume but also the available time in relation to that volume.

IOW.. bigger cylinder moving air at slower piston speeds takes a different exhaust lobe compared to a smaller cylinder moving air at much faster piston speeds. The mass flow could be somewhat similar between them but one cylinder has much less available time to evacuate the mass than the other. So, mass flow, area AND time.

To add to that.. take an older and weaker OEM port design and compare it to another more current efficient design. Then go even further by porting that already decent port to make it larger and higher flowing. Each port design and size would potentially require a different lobe design and even affects the intake lobes requirements due to varying degrees of overlap flow associated with weaker or stronger exhaust flow potential. Make the available window bigger and you can get by with less aggressive exhaust lobe. Same goes for the intake lobe as well.

[RDY4WAR]

Why are you stuck on the exhaust lobe side?

There's a lot of unknowns with the exhaust that we can't replicate on a flowbench such as 100+ psi cylinder pressure when the valve cracks open and 1500+*F temperatures. That pressure and temperature will vary depending on the compression ratio (expansion ratio) and rpm. Unlike on the intake side, the pressure in the cylinder on the power stroke is still pushing the piston and turning the crank (making torque) when the exhaust valve opens. Finding that balance between sufficient force to the crank and allowing enough time for the cylinder to be evacuated of leftover exhaust gases isn't something you can simulate so easily. It takes trial and error.

The intake side, for the most part, is much simpler and easier to replicate.

[RDY4WAR]

Why are you stuck on the exhaust lobe side?

Not stuck on it. I feel like I have a good handle on the inlet and how to work that out. It's just the exhaust that I could do with some more feedback on in regards to duration / lift. I know its based off exhaust flow or mass flow but was hoping there was at least some sort of relation to the inlet profile. So if you picked a 6 degree bigger exhaust duration than inlet do you need to make the lift less to keep the inlet / exhaust area the same?

The opening and closing events are the most important parts of the cam puzzle. If we want to push the power range to a higher rpm, we have to open the exhaust valve sooner in order to keep the timing of the scavenging wave on point and allow sufficient time for the cylinder to blow down and evacuate the remaining exhaust gases. If we have an exhaust port and valve that flows pretty decently, opening the exhaust valve sooner may satisfy the timing for the scavenging wave but may weaken the wave if the exhaust velocity falls from too much flow area. We control this with the valve, keeping the lift a little lower to keep up the velocity of the exhaust gases passing around the valve and through the throat, so the scavenging wave is timed correctly during overlap, still evacuates the cylinder accordingly, but still enough flow area that we aren't making a notable impact on pumping loss.

In general, a wider split of intake and exhaust duration will tend to put more separation between peak torque and peak horsepower as the shorter duration, earlier closing intake valve will build more dynamic pressure at a lower rpm for torque, and the exhaust valve opens sooner to move peak horsepower to a higher rpm. This isn't always the case, but it's usually the intended goal.

If you open the exhaust valve later, you want to run more lift as you're less concerned with moving the rpm range higher, and more concerned with just getting the exhaust out of the cylinder in a shorter amount of time.

A lot of shelf cams that have more lift on the exhaust side than the intake are just taking the next size bigger intake lobe and slapping it on the exhaust side. From a business perspective, it works because there's no way of knowing what the customer may have in regards to exhaust flow such as stock exhaust port vs ported or aftermarket, shorty or longtube headers vs tuned length headers, compression ratio, spark timing, etc... which all have an effect on the exhaust flow. The power lost from ill timing or weakening the exhaust wave is less than the power lost from not evacuating the cylinder so throwing on a bigger exhaust lobe with more lift is just a CYA move to make the cam viable, but not ideal, in a wide range of potential engine combos.

[pdq67]

Please do a search for the long cam discussion with several of the guys involved, CK and UDH are two, and read it a couple of times!!

Here it is!!

https://www.speed-talk.com/forum/viewtopic.php?f=15&t=50056

Great info and I really do miss Harold!!

pdq67