Overlap, Cam Advance and Idle Quality

[efxr6wagon]

I couldn't find the answer to my question by searching, so I apologise if I am covering old ground.

It is accepted fact that increasing camshaft overlap will reduce idle quality and vacuum, all other things being equal. Please challenge this if I have that wrong.

But overlap is really just the difference between two valve events - intake opening and exhaust closing. So, my question is: which of the two valve events is (mainly) causing the effect on idle quality and vacuum?

Many sources say that advancing the cam - earlier intake valve opening, and earlier exhaust valve closing - improves idle. If closing the exhaust valve earlier improves idle, then it suggests that this is the valve event mainly determining idle quality. Or, at least, earlier exhaust valve closing improves idle quality more than earlier intake valve opening hurts it.

The reason for the question is to determine if going for a tighter LSA (thus, more overlap) while keeping exhaust closing the same (earlier intake valve opening, thus, more advance) will give a similar idle.

Thanks in advance.

[Geoff2]

Advancing the cam closes the exh earlier, so less exh gas to contaminate the fresh incoming charge. Combustion is improved, idles is smoother with more vacuum.

[Walter R. Malik]

Advancing the cam closes the exh earlier, so less exh gas to contaminate the fresh incoming charge. Combustion is improved, idles is smoother with more vacuum.

Exhaust valve closes sooner but, intake valve also OPENS sooner; overlap is the same. The advanced events will also close the INTAKE valve sooner creating a bit more low RPM cylinder pressure.

[RDY4WAR]

Idle quality is a matter of intake vacuum. Overlap is the biggest factor while moving the overlap events (IVO and EVC) around can affect it some as well. When the intake valve opens while the piston is still moving up on the compression stroke, the residual combustion gases in the chamber (at ambient or just above ambient pressure) is introduced the low pressure in the intake and those gases quickly surge up the intake port toward that low pressure. As a result, the pressure in the intake manifold increases (vacuum decreases). The greater the overlap, or open valve curtain area, the more reversion and more vacuum dilution.

When the piston goes down on the intake stroke, the intake pressure is still lower than the chamber and exhaust port pressure and thus exhaust is continuing to reverse up into the intake. It's often not until the exhaust valve closes does the piston start to draw those gases back out of the intake. Only after drawing those gases back in can it start to draw fresh air and fuel.

The larger the combustion chamber, the more residual gases are available to reverse up the intake port. This is why the same cam/head/intake combo will lope/chop more at lower compression than with higher compression. The smaller chamber of a higher compression cylinder means less gas volume to reverse up the intake so less to dilute the next intake charge.

[PackardV8]

All the above is generally true, but there are so many variables. A single plane intake is more affected by overlap dilution than is a dual plane intake and an individual runner intake least of all.

[jeff swisher]

I have had 3 cams ground using the same masters and one on a 106 LSA one on 112 LSA and one on 114 LSA.
218@ .050 sbc 350.
2" difference in idle vacuum from 106 to 112 and you could not tell by ear that you changed cams.
Both cams installed at 102 intake centerline. same cranking pressure 225 PSI si i know the intake valve events were exactly the same.
4 MPG more and then some on the 112 LSA cam.


The 114 LSA cam I thought would idle very smooth with No chop at all but it was choppy and i installed it at 107 Intake centerline 195 PSI cranking pressure and not near as much pulling power as the other 2 cams and it gives up way early in the RPM range.

Ported 601 iron heads and that extra overlap really helps the weak head pull in the mid range and on the big end.

106 and 112 cams pulled the same to 4000-4500 rpm then the 106 trumped it.

I hate the 114 LSA and It gets 3 MPG less than the 112.

I did recently build another 350" with a 230@ .050 cam that I have used many times 110 LSA but this time I advanced it more and it has way more chop when idling.
Actually surprised me as to how choppy it is.

[skinny z]

I have had 3 cams ground using the same masters and one on a 106 LSA one on 112 LSA and one on 114 LSA.
218@ .050 sbc 350.

What was the seat duration of those cams?
Assuming hydraulic measured at .006".

[jeff swisher]

I have had 3 cams ground using the same masters and one on a 106 LSA one on 112 LSA and one on 114 LSA.
218@ .050 sbc 350.

What was the seat duration of those cams?
Assuming hydraulic measured at .006".

That was the 268H Competition cams high energy hydraulic flat tappet masters.

[skinny z]

I have had 3 cams ground using the same masters and one on a 106 LSA one on 112 LSA and one on 114 LSA.
218@ .050 sbc 350.

What was the seat duration of those cams?
Assuming hydraulic measured at .006".

That was the 268H Competition cams high energy hydraulic flat tappet masters.

When working out metrics like overlap and valve open and close events, it was explained to me (by a reputable cam designer/grinder/et al) that the seat timing is what's needed. The cylinder doesn't trap any air when the valve is 50 thou off the seat so that makes sense.
However, he also went on to explain that most people are caught up in the .050" numbers, so when he publishes specs or sends off a cam recommendation, for a hydraulic tappet, it those .050" values that are supplied.
Trying to compare catalog cams is also difficult seeing as different brands measure that advertised duration in different ways. Elgin vs Comp as an example.
Solids are a another matter altogether.

It would be worthwhile to graph the three cams you spec'd and compare them and their associated event values. There's a lot to these experiments and I'm always interested in these kinds of observations and results. (It's no secret that through Covid, all I had to keep the hobby alive was simulations and calculators.)

Thanks for posting Jeff.

[efxr6wagon]

I did recently build another 350" with a 230@ .050 cam that I have used many times 110 LSA but this time I advanced it more and it has way more chop when idling.
Actually surprised me as to how choppy it is.

Are you saying that you advanced the cam and the idle became choppier? That would suggest that of IVO and EVC, IVO has the greater impact on idle quality.

Thanks everyone for all the feedback.

[David Redszus]

Overlap period
The overlap period is defined as that period during which both the inlet and exhaust valve are
open simultaneously. Its duration is therefore calculated as the sum of the inlet open angle and
the exhaust close angle. Once again, pay attention to the checking clearance that has been used.

Inlet open angle.......24....deg BDC
Exhust close angle.....29...deg ATC
Overlap duration.......53...degrees

Overlap area
The overlap area is defined by the minimum area present, for either the inlet or exhaust
valve during the overlap period. True overlap effects are not governed by overlap angle alone;
area and pressure ratio are vital.

During the exhaust stroke, most of the exhaust gas is expelled through the exhaust valve, but
some exhaust gas will always remain in the combustion chamber. The higher the compression
ratio, the smaller the combustion chamber and the smaller amount of residual burned gases
that will remain in the cylinder. The purpose of valve overlap is to assist the volumetric efficiency
by scavenging the chamber volume above the piston at TDC

The minimum valve curtain area of either valve will determine the overlap area. Since inlet and
exhaust valve lift are changing with crank angle, the overlap valve curtain area will also change
with crank angle. Since the overlap period occurs when both intake and exhaust valves are
at low lifts, and the piston is moving very slowly, the low lift Cd values are very important.

Of even greater importance is the pressure ratio during overlap. The intake pressure Pi, must
be greater than cylinder pressure Pc, which must be greater than exhaust pipe pressure Pe.
The following condition: Pi > Pc > Pe, must exist in order for fresh mixture to flow in while hot
exhaust gases flow out. Sometimes this is abbreviated as Pe / Pi < 1.0, and must exist no
matter what the valve area or duration might be.

When the Pe / Pi < 1.0, the difference in pressure will assist the scavenge process; the
remaining exhaust gases present in the combustion chamber will be sucked out through
the exhaust valve, allowing fresh inlet charge to enter, and will improve volumetric efficiency.

When exhaust pressure is greater than the inlet pressure Pe / Pi > 1.0, the exhaust gas will
always flow backwards, from the cylinder through the overlap area, into the inlet pipe. The
cylinder will then take in a combination of fresh mixture and increased exhaust gas.

Since engines are controlled by throttling, the effect of overlap when Pe / Pi > 1.0, must be
considered. A throttled engine will cause the inlet pressure to be reduced and cause the
Pe / Pi ratio to become greater than unity and cause overlap flow reversion, with an increased
fraction of exhaust gas in the cylinder.

The valve overlap event can be represented by a cylinder with no overlap plus a bypass
passage between the inlet and exhaust pipes, whose diameter reflects the overlap area.
Note the critical importance of valve CD on equivalent overlap flow area.

Overlap equivalent area
Inlet valve diameter ........2.120....in
Inlet valve Cd..................0.820
Inlet valve lift..................0.050....in
Inlet valve curtain area.......0.273....in2
Exhaust valve diameter......1.875....in
Exhaust valve Cd...............0.690
Exhaust valve lift...............0.048.....in
Exhaust curtain area...........0.195.....in2
Max overlap area...............0.195.....in2
Equiv overlap diameter........0.498.....in

The total volumetric efficiency (Vte) can be represented by a chamber volumetric efficiency
Vce, plus a bypass volumetric efficiency Vbe. The overall cylinder volumetric efficiency can be
improved by proper design of the overlap area.

Overlap efficiency
A = bypass flow area..................0.283....in2
C = Bypass area Cd....................0.76.....ratio
Ap = Piston area.......................12.56......in2
Sp = mean piston speed...............79.........ft/s
a = sonic velocity of inlet.............1100......ft/s
phi = compressible flow function....0.84
Vbe = bypass efficiency................1.32......%

[Tom68]

Many sources say that advancing the cam - earlier intake valve opening, and earlier exhaust valve closing - improves idle. If closing the exhaust valve earlier improves idle, then it suggests that this is the valve event mainly determining idle quality. Or, at least, earlier exhaust valve closing improves idle quality more than earlier intake valve opening hurts it.

Haven't heard it put like that but many many cams are 4 degrees advanced from straight up.

More for good overall performance than for idle quality I'd suggest.

Don't forget with a common plenum motor if you have 18" of idle vacuum and 0" in the exhaust, regardless of piston position or travel direction, whilst both valves are open, exhaust will flow into intake.

[Stan Weiss]

268 - 106 LSA - 102 ICL - Overlap 56
32/56 - 64/24

268 - 112 LSA - 102 ICL - Overlap 44
32/56 - 76/12

So you have 12 degrees less over lap while you open and close the exhaust valve 12 earlier.

268 - 114 LSA - 107 ICL - Overlap 40
27-61 - 75-13

Exhaust 1 degree different than 112 LSA 4 degrees less overlap and intake events 5 degrees later.

Would have been interesting to see what the 114 LSA did at 102 ICL

Stan

[digger]

I've advanced more than 1 cam over a 12 degree spread and the idle quality was never notably affected. IMO its normally a second order effect. Perhaps if you have a lot of overlap then it may be become more perceptible but the big driver is still the OL area.

IMO with the same lobes a 104LSA is not going to idle like a 110LSA just by adv/ret cam to maintain either the same ICL or ECL that would be fanciful thinking.

[Protech Racing]

I have a cam that can run either hydro or solid lifters. Ive run it both ways. I had a cheap customer buy my cam but not the expensive shim under bucket package . His car made 1 More HP than my car did . with the same peripherals and my tune .
The solid lifters idle better and the total off the seat time is about 11- 12 degrees shorter vs the hydro package.
The hydro package makes more power on the same cam for me. Very limited lift situation at 4.25 in. Around 299 OTS.