What's the difference between cams that...

[horse123]

cams that just idle roughly sounding like this: https://www.youtube.com/watch?v=P7M27YmIT70&t=60s

and cams that cause actual lope like this: https://youtu.be/oaJ3w_fAdoI?t=37s

What's the physical difference between them, and what do they cause the engine to do that makes the difference between an arrhythmic popping idle and a rhythmic lope?

I'm aware both are somehow related to valve opening overlap, but clearly there's a difference between the 2.

 

[gregk24]

I don't know, they both sound awful to me.

 

[Dyusik]

Sounds like the firing order is different, but I'm no expert. Hennessy I imagine will tune their cars to be sexy sounding to all, including tone deaf teenagers of rich parents.

 

[CT8]

The lobe separation is why there is the difference in the sound.

 

[RedOakRanch]

Before variable everything, the worse it idled the better the HP! I love the rough idle sound in the first clip.

 

[DemoFly]

They both sound like similar grinds. The ZL1 just has a higher idle.

The prominent popcorn sound in the second video is from valve overlap.

Quote:
"Valve overlap is the period during the valve timing where both the intake and exhaust valves are open. Occurring towards the end of the exhaust stroke, the intake valves are opened just before all the exhaust gases are released, providing more time for the intake air to enter the engine."

 

[spasm3]

Lobe separation angle. Overlap.

 

[Silk]

I've generally found a performance cam to give a lumpy idle, but had a 1600 Kent engine that had a splashy idle. I have no idea what cam it had, but it I spent a lot of time getting a better idle...top end was always good, but that splashy idle was annoying.

They will both have low manifold vacuum.

 

[Shannow]

Lopiest car I've ever seen is this one...the first few seconds are genuinely the lope on the thing



You should see Batto try to get BlownLux onto the trailer on grass...no throttle, and it's throwing rooster tails of sob every second or so.

 

[Silk]

As young fellas, you could pull the choke out a bit to get a lumpy idle...but no one was fooled if it was a sidevalve Hillman Minx.

 

[FlyNavyP3]

The first cam is SIGNIFICANTLY larger than the second in the camaro. The stable lope in the camaro occurs once duration at 0.050" lift gets to around the 224-234 area. The cam in the first car that just chops instead of lopes is likely more in the vicinity of 260+ @0.050.

 

[horse123]

Originally Posted By: Shannow
Lopiest car I've ever seen is this one...the first few seconds are genuinely the lope on the thing



You should see Batto try to get BlownLux onto the trailer on grass...no throttle, and it's throwing rooster tails of sob every second or so.



Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.

Originally Posted By: FlyNavyP3
The first cam is SIGNIFICANTLY larger than the second in the camaro. The stable lope in the camaro occurs once duration at 0.050" lift gets to around the 224-234 area. The cam in the first car that just chops instead of lopes is likely more in the vicinity of 260+ @0.050.


So a lope will happen when you have performance cams that aren't really "huge"?

 

[funflyer]

Originally Posted By: RedOakRanch
Before variable everything, the worse it idled the better the HP! I love the rough idle sound in the first clip.


Maybe, maybe not. If there is one thing I've learned about cam choice over the years, it's that most "rickie racer" type people choose the wrong(too large)cam. They actually kill the torque and move the HP so far up in the RPM range that the rest of the engine build can't spin that high.

 

[OVERKILL]

Originally Posted By: FlyNavyP3
The first cam is SIGNIFICANTLY larger than the second in the camaro. The stable lope in the camaro occurs once duration at 0.050" lift gets to around the 224-234 area. The cam in the first car that just chops instead of lopes is likely more in the vicinity of 260+ @0.050.


Not necessarily true, as LSA plays in there as well. Two cams with the same duration but different separation angles can sound significantly different. The wider one will sound far more civil because it won't have any, or will have significantly less overlap.

I had a split pattern cam with 221/225 (@.050) on a 112 in my Mustang and it almost sounded stock. In comparison, the single pattern FMS E cam I had in my Lincoln loped along pretty good despite having less duration (220/220) because it was on a narrower 110 degree LSA.

I've often found that "older style" grinds, which don't take advantage of modern lobe profiles, sound significantly more aggressive than modern grinds. I've met a few old hot rodders that bought a specific camshaft, usually one that was quite dated, just because of how it sounded. They were not concerned with the fact that a newer style camshaft would idle better and make more power.

 

[Shannow]

Originally Posted By: horse123
Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.


Could you explain blower surge ?

It's all taking place at closed throttle.

Genuinely interested, as I agree with the surging power part, just don't get the physics.

 

[ls1mike]

Originally Posted By: Shannow
Originally Posted By: horse123
Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.


Could you explain blower surge ?

It's all taking place at closed throttle.

Genuinely interested, as I agree with the surging power part, just don't get the physics.

Maybe no bypass?

 

[ls1mike]

Originally Posted By: gregk24
I don't know, they both sound awful to me.


You made my day... LOL

To each their own...Don't worry the Jetta and Honda have NO chance of ever sounding like that.

 

[mjoekingz28]

Originally Posted By: Shannow
Originally Posted By: horse123
Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.


Could you explain blower surge ?

It's all taking place at closed throttle.

Genuinely interested, as I agree with the surging power part, just don't get the physics.




At idle, the motor goes lean which causes the rpms to rise. Which then cause a rich condition and rpms drop.


That is the best I can do to paraphrase




Please help clean up the link

 

[clinebarger]

Originally Posted By: Shannow
Originally Posted By: horse123
Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.


Could you explain blower surge ?

It's all taking place at closed throttle.

Genuinely interested, as I agree with the surging power part, just don't get the physics.


The ZL1 Camaro in the video was cold & running rich coming off the trailer, I guarantee the idle got better once it warmed up & went into "Closed Loop".

My Camaro does that when cold & it doesn't have a Blower.

Actual blower surge is usually a fuel tuning issue that I have only seen on old (GMC Style) roots blowers, ALL modern blowers/superchargers worth a pi$$ have vacuum or electronic controlled Bypass Valve that completely bypasses the blower at idle & cruise conditions.

 

[ls1mike]

Originally Posted By: clinebarger
Originally Posted By: Shannow
Originally Posted By: horse123
Actually I think that's blower surge. Cam lope shouldn't be powerful enough to jerk the car like that.

Blower surge is more common on cars that are tuned for absolutely nothing except drag racing.


Could you explain blower surge ?

It's all taking place at closed throttle.

Genuinely interested, as I agree with the surging power part, just don't get the physics.


The ZL1 Camaro in the video was cold & running rich coming off the trailer, I guarantee the idle got better once it warmed up & went into "Closed Loop".

My Camaro does that when cold & it doesn't have a Blower.

Actual blower surge is usually a fuel tuning issue that I have only seen on old (GMC Style) roots blowers, ALL modern blowers/superchargers worth a pi$$ have vacuum or electronic controlled Bypass Valve that completely bypasses the blower at idle & cruise conditions.


My S/C 3800 had by pass valves. If you overrode it cold (not open) you could get some surge.