understanding cam overlap for selection bbc

[als427]

Hi all. I understand on basic level how cams work , intake exhaust open close and lift rates / durations.
I get how overlap is calculated. I also get that the less overlap the smoother idle quality and less loss in compression.
Im looking for advice in layman's terms on the below:
If I was to choose a cam based on overlap , ie lower overlap for street use is this a sane idea?
Given that a larger duration cam with a larger lsa can sometimes create similar overlap numbers
Would you choose overlap at 0.006 or @50?
Heres an example , on a 10.1 motor for a street cam:
A 285/295 adv. 230/235 @50 114 lsa cam gives overlap of 62 @0.006 or 4 @50
A 272/276 adv. 216/220@50 111 cam gives overlap of 52 @ 0.006 or 4 @50
I would think the 216/220 would be a better street cam ( lower duration) but if the overlap @50 is what really counts then would they not actually be very similar as they are both 4 degrees? or is the overlap @ 0.006 a more realistic indicator?
I know Im simplifying things but Im curious if this approach is usable.

[CamKing]

I also get that the less overlap the smoother idle quality and less loss in compression.

False. Overlap has no connection to the Compression stroke.

Im looking for advice in layman's terms on the below:
If I was to choose a cam based on overlap , ie lower overlap for street use is this a sane idea?

No, not really. Overlap isn't even in the top 5 things to consider, when deciding on a cam.
Besides that, You can have multiple cams with the same total overlap, but with totally different Intake opening and Exhaust closing points.

Would you choose overlap at 0.006 or @50?

You would use the seat duration(.006" on a hydraulic) to calculate overlap. .050" is meaningless.

I know Im simplifying things but Im curious if this approach is usable.

It's just as usable as a dart board.

[als427]

well that told me!!!
thanks Mike , starting to learn a lot of things , steep curve.

[CamKing]

I hope I gave you some things to think about.
Try looking at valve opening and closing points, and forget about total overlap.

[CamKing]

Here's something to think about.
These 3 cams all have 30 degrees of overlap.
Cam#1: Intake opens 15-btdc, Exhaust closes 15-atdc
Cam#2: Intake opens 05-btdc, Exhaust closes 25-atdc
Cam#3: Intake opens 25-btdc, Exhaust closes 05-atdc

Think about only the exhaust stroke(BTDC), and how each cam would react.
Now think about only the intake stroke(ATDC), and how each cam would react.

[als427]

good point.
On intake opening , the earlier BTDC allows more air fuel in , so more combustion /power
On exhaust closing - the later it close the more time it allows spent gasses out?
You want to plan a cam where the exhaust closes allows all the gasses out for the given amount of air/fuel you have let in?
Too much of one or the other and you got past an effective point? not enough fuel air in or not enough gasses out?
Maybe ive completely missed the point
Bear with me im a woodworker first and foremost -I work to the nearest 1/16 inch , or house !

[pastry_chef]

Overlap isn't even in the top 5 things to consider, when deciding on a cam.

I'd think bore, stroke, RPM are high in consideration. Maybe induction efficiency next in line?

[Stan Weiss]

Hi all. I understand on basic level how cams work , intake exhaust open close and lift rates / durations.
I get how overlap is calculated. I also get that the less overlap the smoother idle quality and less loss in compression.
Im looking for advice in layman's terms on the below:
If I was to choose a cam based on overlap , ie lower overlap for street use is this a sane idea?
Given that a larger duration cam with a larger lsa can sometimes create similar overlap numbers
Would you choose overlap at 0.006 or @50?
Heres an example , on a 10.1 motor for a street cam:
A 285/295 adv. 230/235 @50 114 lsa cam gives overlap of 62 @0.006 or 4 @50
A 272/276 adv. 216/220@50 111 cam gives overlap of 52 @ 0.006 or 4 @50
I would think the 216/220 would be a better street cam ( lower duration) but if the overlap @50 is what really counts then would they not actually be very similar as they are both 4 degrees? or is the overlap @ 0.006 a more realistic indicator?
I know Im simplifying things but Im curious if this approach is usable.

If I use your specs I come up with different results.
A 230/235 @50 114 lsa cam gives overlap of 4.5 @50
A 216/220 @50 111 las cam gives overlap of -4 @50

Stan

[als427]

Hi Stan , I got my numbers from a camshaft website program. You input all the data and it tells you whats what.
Not saying its right , just saying.

[Adger Smith]

Mike Jones
"You would use the seat duration(.006" on a hydraulic) to calculate overlap. .050" is meaningless."
I like that!
Some old slow lazy profile can give you decent @ .050 numbers but at .006 it has numbers that make the engine a slug with way too much seat timing.
To clarify "slug' it doesn't drive well

[als427]

Adger Smith
Thats really useful and thanks Mike as well. I have gone from choosing a basic camshaft to trying to learn about them. Can of worms. Just read a 57 page thread on speedtalk. Understand now overlap on the seat is more what counts not @ 0.50. This also tallies with the david vizards 128 cam thread I just read - Mike , I see you are highly mentioned here in great regard. Whilst im sure its not the only factor - on a like for like cam going from an overlap of say 65 to 55 should noticeably improve street manners? throttle response etc.?
Just curious - what overlap is recommended for a street only / hot street car. Around 4-420 ftlb and HP? With best street manners up to 6k?
On the vizard info looks like a 55-65 overlap , depending on compression.Mine is 9.6-10.2. still waiting tbc.
Thanks trying to learn about this all.
Obviously the whole lsa thing needs factoring in as well. If an overlap is chosen and the lsa is 110 rather than 114 I understand this will give e a better peak tq and throttle response. But might affect engine idle and vacuum.
The one thing thats clear is theres a view that a lot of people choose durations that are bigger than they should be and the wider lsas helps deal with that.
Presumably it would be better for performance to choose a narrower lsa and lower duration , than a bigger duration and wider lsa?
Is this just for out and out performance or is it the same for street cars?
On a bbc 427 with c 400ftlb from 2800rpm up does it really matter either? so long as there is "enough" torque from idle up. Dont need more than can be used. So perhaps I need to choose a cam that doesnt drastically alter my numbers but improves street manners , idle , throttle response.
If i lose or gain 20ftlb/hp here or there less relevant to actual drivability.

[PackardV8]

You're trying to solve a multi-variable regression analysis with grade school math. Change one thing, change everything. Pick one goal and it's easy; try to weigh all the tradeoffs to find the sweet spot and it gets crazy difficult.

Horsepower peak, torque peak, torque curve, idle/vacuum, fuel economy, durability, valve train noise, cost, maintenance; I even had one cam change affect the way the car smelled up the garage when parked.

Continue learning, but the more you know, the more difficult cam selection becomes; understanding stops action.

[als427]

I know , the more I learn the less I realise I know!!
Might just buy some roller skates , no cam required