What’s the relationship of lobe separation to vacuum at idle?
Lobe separation and vacuum
- Thread starter Moparfiend
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When the lobe separation is tight, the valves are open at the same time at TDC for longer. That's called overlap. And reduces vacuum.
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Yep. A 118° lobe sep will have a bunch more idle vacuum than the same cam on a 106°. And a lot smoother idle, less cam lope too.
But the 106 will make more low-end torque and hp. The 118 might catch up at 6000. Maybe.
But the 106 will make more low-end torque and hp. The 118 might catch up at 6000. Maybe.
Got it so the overlap doesn’t have enough time to build vacuum. Makes sense thanks! Been looking at LSA vs HP and TQ and see the trade offs there just couldn’t find one comparing vacuum at idle.
Is there a way to calculate LSA based on performance? Ot is that not possible? I don’t have any cam specs.
Is there a way to calculate LSA based on performance? Ot is that not possible? I don’t have any cam specs.
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Calculate? Not to my knowledge.
But my limited experience says that most fuel injected engines like 114-118 (nitrous likes wide seps too, that is often what makes a cam a nitrous cam)
Race engines seem to like 104-106. (I have an old cam dynamics roller ground on a 104)
110 seems like the normal compromise. Possibly why about 95% of comp cams street grinds are on 110.
But my limited experience says that most fuel injected engines like 114-118 (nitrous likes wide seps too, that is often what makes a cam a nitrous cam)
Race engines seem to like 104-106. (I have an old cam dynamics roller ground on a 104)
110 seems like the normal compromise. Possibly why about 95% of comp cams street grinds are on 110.
INTMD8
Well-Known Member
LSA is a result of choosing valve events rather than choosing valve events based on LSA.
Performance trends engine analyzer is nice.
Performance trends engine analyzer is nice.
Does that software estimate vacuum?
Rebelrouser
Well-Known Member
Buy a good engine program, I use performance trends, it will calculate really close your idle vacuum. When a customer asks me to build an engine many say I want a lope at idle but I want it to clear up as soon as I touch the throttle. I know idle vacuum of about 16 inches will do just that, input the specs for the engine and order the parts, has worked without fail for 20 years now.
Geoff 2
Well-Known Member
Look up David Vizard's 128 rule on the net for determining LSA. Designed for SBC, but works well for parallel valve heads in general.
There is a you tube Video by Cattle Dog Garage, 34 min [ not the 1hr plus video! ], with dyno tests & validates DVs 128 rule. Sorry, do not know how to link it.
There is a you tube Video by Cattle Dog Garage, 34 min [ not the 1hr plus video! ], with dyno tests & validates DVs 128 rule. Sorry, do not know how to link it.
451Mopar
Well-Known Member
Idle quality depends on the amount of overlap duration near TDC when both the exhaust and intake valve are open at the same time.
LSA by itself is just the angle between center of the exhaust lobe and the center of the intake valve in camshaft degrees (x2 for crankshaft degrees.)
All the rest of the cam specs are in crankshaft degrees.
Because LSA is just the angle between lobe centerlines, the degrees of overlap will increase with increases in cam duration, and be reduces with less duration.
So a small duration cam with a tight (106 or less) LSA might have the same amount of overlap as a larger duration cam with a wider LSA (like 112+)
Example:
1. 260/260 degree (@0.006"), 106 LSA has 48 degrees of overlap (using the 0.006" duration numbers.)
2. 272/272 degree (@0.006"), 112 LSA has the same 48 degrees of overlap (using the 0.006" duration numbers.)
LSA by itself is just the angle between center of the exhaust lobe and the center of the intake valve in camshaft degrees (x2 for crankshaft degrees.)
All the rest of the cam specs are in crankshaft degrees.
Because LSA is just the angle between lobe centerlines, the degrees of overlap will increase with increases in cam duration, and be reduces with less duration.
So a small duration cam with a tight (106 or less) LSA might have the same amount of overlap as a larger duration cam with a wider LSA (like 112+)
Example:
1. 260/260 degree (@0.006"), 106 LSA has 48 degrees of overlap (using the 0.006" duration numbers.)
2. 272/272 degree (@0.006"), 112 LSA has the same 48 degrees of overlap (using the 0.006" duration numbers.)
lewtot184
Well-Known Member
From my experience there's no free lunches. Everything is a trade off to some degree. I've even found rocker ratio to have a positive or negative effect to low rpm vacuum.
Yes, part of the reason almost all new cars are built with variable cam timing.
BSB67
Well-Known Member
If you tell us your engine size, and what idle vacuum you want at a given rpm, some folks here will be able to get you close on overlap. Understand that solid verses hydraulic and lobe rates also impact vacuum for a given overlap.
INTMD8
Well-Known Member
Yes you guys could do that ^
There are a lot more variables however.
Sure valve events and therefore resulting overlap/lsa influence vacuum at idle but so does idle speed, timing, tune up, compression ratio/etc/etc/etc
Just my opinion but better to start with a goal. What are you trying to achieve overall (power? et/mph, what are you building?) rather than starting with a focus on LSA and idle vacuum.
There are a lot more variables however.
Sure valve events and therefore resulting overlap/lsa influence vacuum at idle but so does idle speed, timing, tune up, compression ratio/etc/etc/etc
Just my opinion but better to start with a goal. What are you trying to achieve overall (power? et/mph, what are you building?) rather than starting with a focus on LSA and idle vacuum.
451Mopar
Well-Known Member
In my post above I should have posted the valve open close events in the comparison because the intake closing point can also effect idle and cylinder pressure.
Example:
1. 260/260 degree (@0.006"), 106 LSA (installed split overlap 106) IO: 24 IC: 56 EO: 56 EC: 24 Overlap: 48
2. 272/272 degree (@0.006"), 112 LSA (installed split overlap 112) IO: 24 IC: 68 EO: 68 EC: 24 Overlap: 48
For fun, the 272 installed at 106 ICL:
3. 272/272 degree (@0.006"), 112 LSA ( 6 advance overlap 106 ) IO: 30 IC: 62 EO: 74 EC: 18 Overlap: 48
Now the overlap center is shifted to before TDC, but overlap is still the same, and all the valve events happen 6 degrees sooner compared to the pistons position.
Example:
1. 260/260 degree (@0.006"), 106 LSA (installed split overlap 106) IO: 24 IC: 56 EO: 56 EC: 24 Overlap: 48
2. 272/272 degree (@0.006"), 112 LSA (installed split overlap 112) IO: 24 IC: 68 EO: 68 EC: 24 Overlap: 48
For fun, the 272 installed at 106 ICL:
3. 272/272 degree (@0.006"), 112 LSA ( 6 advance overlap 106 ) IO: 30 IC: 62 EO: 74 EC: 18 Overlap: 48
Now the overlap center is shifted to before TDC, but overlap is still the same, and all the valve events happen 6 degrees sooner compared to the pistons position.
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