How much advance for a less than optimal cam ?

[skinny z]

Assuming that the events below .050” is the same is a big assumption.

The engine sees seat-to-seat duration, not .050”. You can’t compare a Mike Jones designed lobes to Billy Godbold has designed lobes to Harold Brookshire designed lobes look at .050. Mike’s cams look smaller at .050” than they run to, Harold has “unsymetrical ness” and Billy has symmetrical cams to non symmetrical cams, same with many of the designers, and they all use different lash ramps. It’s all “it depends.

This is my understanding as well.
Or even between a Jones lobe of 286 adv with 234@.050' (or 288 with 242@.050"!) vs one his 286 adv with 226@.050".

Looking at .050 and peak lift is like deciding what a house looks like based on a measurement 5’ off the ground and the roof peak, and ignores everything in between.

Exactly.

[F-BIRD'88]

Assuming that the events below .050” is the same is a big assumption.

The engine sees seat-to-seat duration, not .050”. You can’t compare a Mike Jones designed lobes to Billy Godbold has designed lobes to Harold Brookshire designed lobes look at .050. Mike’s cams look smaller at .050” than they run to, Harold has “unsymetrical ness” and Billy has symmetrical cams to non symmetrical cams, same with many of the designers, and they all use different lash ramps. It’s all “it depends.

This is my understanding as well.
Or even between a Jones lobe of 286 adv with 234@.050' (or 288 with 242@.050"!) vs one his 286 adv with 226@.050".

The cams of same .,,050" spec and same events open close @ .050" on all the various high perf hyd flat tappet sbc cams will run MORE ALIKE THAN DIFFERENT.
the real different in relative seat to seat real durations will be vey small +/-.. The practical design mechanical limitations are the same.. unlesz you are willing to accept a cam that is NOISEY. or needs extreme spring force. or does not LAST LONG.
you cannot win any race you do not finish.

Who do you think designed to Comp magnum hyd cams...
That are practical modern aysemetric cam designs..
Same guy who designed the Lunati. street strip and Voodoo cams. They are more alike than different.
Even the so called lazy cams. Elgin, etc etc run very similar on the dyno and down the track in a sbc .
One 234° cam runs very close to another 234° hyd cam.
One 244° cam runs very same as another..
The LSA thus open close events and installed position. of same .,050" lifter rise sbc hyd cams makes THE MOST DIFFERENCE. The basic .050" "shoe size" MAKES THE MOST DIFFERENCE.

[skinny z]

The cams of same .,,050" spec and same events open close @ .050" on all the various high perf hyd flat tappet sbc cams will run MORE ALIKE THAN DIFFERENT.
the real different in relative seat to seat real durations will be vey small +/-.. The practical design mechanical limitations are the same.. unlesz you are willing to accept a cam that is NOISEY. or needs extreme spring force. or does not LAST LONG.
you cannot win any race you do not finish.

A little off the topic but it's the longevity dilemma that'll will have to be worked through eventually.

[F-BIRD'88]

The cams of same .,,050" spec and same events open close @ .050" on all the various high perf hyd flat tappet sbc cams will run MORE ALIKE THAN DIFFERENT.
the real different in relative seat to seat real durations will be vey small +/-.. The practical design mechanical limitations are the same.. unlesz you are willing to accept a cam that is NOISEY. or needs extreme spring force. or does not LAST LONG.
you cannot win any race you do not finish.

A little off the topic but it's the longevity dilemma that'll will have to be worked through eventually.

Yup. a bad cam lobe design is a bad cam even if it looks
"intense" on paper.
It won't run for long.

If it beats up the valve seat in 4000 miles on a street car. where are ya?

[CamKing]

Taking all the variables out of this conversation.
Let's assume the optimum can for your application is 226/234@.050" on a 110 LSA, with a 106 ICL.
Now, assume you have a cam from the same exact lobe family, so the acceleration rates, ramp design, and lift are the same, but it's a 230/230@.050" on a 110 LSA.

How do you make that cam work like the optimum cam? You don't
If 226@.050" intake profile is optimum, going to 230 will move peak torque up in the RPM band by 200-300rpm, and hurt power below peak torque.
You can get some of that back, by advancing the 230 to a 104 ICL, but that won't get it all back, because the longer duration 230, will reduce intake velocity over the valve.

If the 234@.050" exhaust profile is also optimum, going down to a 230 will make the power curve fall off 200-300rpm sooner, because you don't have enough duration to evacuate the cylinder. Putting the intake on a 104 ICL, puts the exhaust on a 116 ECL. This means you're now closing the exhaust valve 4 degrees before the optimum point.

[skinny z]

Taking all the variables out of this conversation.
Let's assume the optimum can for your application is 226/234@.050" on a 110 LSA, with a 106 ICL.
Now, assume you have a cam from the same exact lobe family, so the acceleration rates, ramp design, and lift are the same, but it's a 230/230@.050" on a 110 LSA.

How do you make that cam work like the optimum cam? You don't
If 226@.050" intake profile is optimum, going to 230 will move peak torque up in the RPM band by 200-300rpm, and hurt power below peak torque.
You can get some of that back, by advancing the 230 to a 104 ICL, but that won't get it all back, because the longer duration 230, will reduce intake velocity over the valve.

If the 234@.050" exhaust profile is also optimum, going down to a 230 will make the power curve fall off 200-300rpm sooner, because you don't have enough duration to evacuate the cylinder. Putting the intake on a 104 ICL, puts the exhaust on a 116 ECL. This means you're now closing the exhaust valve 4 degrees before the optimum point.

This is the kind of info I was after in a previous thread.
Excellent. Thanks Mike.

Quick question as we're comparing a single pattern to a dual pattern cam.
Perhaps not easily answered but what is the defining factor for establishing a duration split? I/E ratio? Exhaust system efficiency (as in open headers vs a full system)?

[CamKing]

Quick question as we're comparing a single pattern to a dual pattern cam.
Perhaps not easily answered but what is the defining factor for establishing a duration split? I/E ratio? Exhaust system efficiency (as in open headers vs a full system)?

Back in the 80's, we used Exhaust to intake ratio, but as the ports got bigger/better, that was no longer accurate. When the port gets really big, a 10% gain in port flow, does not mean you're increasing the cylinder fill by 10%, so you wouldn't need a 10% bigger exhaust port to evacuate the cylinder.

Using the specs on the engine, including the intake port flow data, I design the intake profile for the desired RPM range.
With that information, I can now calculate lbs/hr at max RPM.
Using the exhaust port CFM, I can then calculate the duration needed to evacuate that mass(lbs/hr).

In short. I design the intake profile for optimum cylinder fill at desired peak HP RPM, then design the exhaust profile to fully evacuate that "fill".

[skinny z]

Quick question as we're comparing a single pattern to a dual pattern cam.
Perhaps not easily answered but what is the defining factor for establishing a duration split? I/E ratio? Exhaust system efficiency (as in open headers vs a full system)?

Back in the 80's, we used Exhaust to intake ratio, but as the ports got bigger/better, that was no longer accurate. When the port gets really big, a 10% gain in port flow, does not mean you're increasing the cylinder fill by 10%, so you wouldn't need a 10% bigger exhaust port to evacuate the cylinder.

Using the specs on the engine, including the intake port flow data, I design the intake profile for the desired RPM range.
With that information, I can now calculate lbs/hr at max RPM.
Using the exhaust port CFM, I can then calculate the duration needed to evacuate that mass(lbs/hr).

In short. I design the intake profile for optimum cylinder fill at desired peak HP RPM, then design the exhaust profile to fully evacuated that "fill".

Excellent.
Thanks as always.