It utilized 10.25:1 Sealed Power pistons, together with mildly ported
straight plug 186 head castings. On the induction side, an Edelbrock Victor
Jr. intake was used, along with a 650 cfm double pumper electronic Quarter
Mile-O-Dial Holley for rapid mixture optimization. Exhaust dumped through
1 3/4-inch headers, and from there through the dyno cell's 'zero restriction'
muffler system. The dyno used was at Advanced Performance Technology's
(APT) Riverside facility. Being a SuperFlow computerized dyno, it was set
to take measurements while the engine was accelerating at the commonly
used 300 rpm per second rate. Chevron Unleaded Premium gas was used throughout
our tests, and by optimizing the engine's quench clearances, mean best
torque timing could be achieved without detonation.
The cams tested were hydraulic and measured out at 292 degrees at the SAE standard of 0.006 inches valve lift. The graph, Fig. 6, gives pertinent cam details and dyno results. Curve #1 is for a cam on 105-degree LCA, Curve #2 on 108, and Curve #3 on 111. On the face of it, Curve #1 looks marginally superior as it produced the best peak torque, horsepower and volumetric efficiency.
Curve #2 is slightly down at the low end. In a couple of places further
up the rpm range it marginally exceeded the output of the 105-degree LCA
cam. However, these curves alone don't tell all. The 105 cam had only just
come on the cam at the first reading of 2250 rpm. You'll note that it produced
less torque here than the 108 cam. The 108-degree LCA cam would pull right
down to about 2000 rpm and run satisfactorily. At 2250 rpm it was positively
on the cam, and at this point it produced noticeably more torque than the
105 cam. Curve #3 represents a cam on 111-degree LCA. As can be seen, it
is consistently 10-20 hp down at the top end over cams #1 and #2.
So far tests don't show too well for 111-degree LCA cam, but this angle cam did idle almost like a stocker, and produced a higher vacuum than the other two cams. It would be easy to assume that cams on such wide centerlines don't work, but this is not so. They just have different characteristics and you, as the end user must decide where your priorities lie. Remember that good vacuum is an important factor for a vehicle that has vacuum accessories such as power brakes, vacuum operated air conditioning controls, etc. The tighter the LCA you choose, the shorter the cam must be to preserve vacuum and idle. This is so because the overlap comes back to roughly the same as that given by a longer duration, wider LCA cam. Obviously a shorter cam on a tighter LCA won't make as much top end horsepower, so again there is a balance of tradeoffs to consider.
RACE ENGINE LCA’s
Choosing the LCA for a race engine becomes simplified because compromises are virtually nonexistent. We are no longer concerned with anything other than maximizing engine output over the RPM range used. That's good, but to be successful it's necessary to make a better job of maximizing output than the next guy. To do that you need to understand those factors affecting the optimum LCA for the job.
The easiest way to explain how optimum LCA’s can change is to use a base spec engine which has been dyno-optimized as a starting point. By making hypothetical changes to this engine it becomes easier to see how the optimum LCA is affected. Let us assume the following: 355 CID from 4.03 inches x 3.48 inch bore/stroke combination, a set of reasonably well ported heads, 12.5:1 compression ratio, a nonrestricted exhaust, a single 4-barrel carb on a race manifold, a single pattern flat tappet cam at 310 degrees seat duration and about 265 at 0.50-inch lift, and 1.5:1 rockers. Such a combination usually produces the best all around results at about 107-degree LCA.
To better understand how the required LCA changes, always consider that it is strongly tied in with the cylinder heads' flow capability and the displacement the head must supply. In its simplest form, this equates to a ratio of cfm per cubic inch. With that in mind, let's start with the affect changes in bore and stroke have on the optimum LCA.
Okay, here we go-pin your ears back and pay attention! Assuming no change in head flow efficiency, we find that any increase in the displacement requires an decrease in the LCA. For a typical 350, every additional 15 CID increase requires a reduction of one degree LCA, and vice versa.
Now let's fix the displacement and see how head flow affects the optimum
LCA. The same air flow to displacement trend also holds true here. If flow
capability over a large part of the valve lift curve increases, the optimum
LCA will spread, and if it decreases the reverse is true. If a dramatic
increase in intake low lift flow is achieved, the tendency is to require
less overlap. This means the LCA spreads, and this may have to be used
with shorter intake duration. However, the reduced overlap is the most
critical aspect. An increase in low lift flow without a compensating reduction
in the overlap area can reduce output right up until very high rpm is reached.
The intent here is to restore the overlap triangle, in terms of cfm /degrees,
back to its original optimum value. Sure, it's tempting to analyze thousandths
of valve lift and degrees around TDC, but the engine does not recognize
valve lift as measured by a dial indicator-only flow capability. This means
all overlap characteristics should be related in terms of cfm/degrees not
inch/degrees. Achieving an exceptionally high flow at low lift on the intake
can cause the engine to react as if it has 20 or so degrees additional
overlap. This often proves way over the top for an engine with previously
optimum valve events. An increase in low lift flow is potentially good
for added power but, if substantial, usually requires a revision of the
valve opening and closure points.
BORE & STROKE CHANGES
If head flow is reduced, the LCA needs to tighten up. Now why would anyone want to use a head with less flow? Well, no one wants to, but a long stroke/small bore combination may force the situation. A long stroke engine has less room for valves than a short stroke, so may have less breathing capability on that score. This causes a long stroke engine to need tighter LCA's than a short stroke.
High and low lift flow capability can also affect the picture. We have
already discussed what can happen when low lift flow is increased, now
let's look at high lift flow. An increase in high lift flow only, during
the last 60-70% of the valve lift envelope used, requires a slightly tighter
LCA. This only comes about because it allows the intake valve to be closed
a few degrees earlier for the same peak power rpm. However, for most practical
purposes we can ignore its effect without incurring a performance loss.
By leaving the cam timing unchanged, a slightly higher rpm capability is
produced along with some extra power.
The effect of changes in compression ratio used on the optimum LCA is rarely dealt with, but it can be significant. The first step towards understanding why the CR affects the LCA is to appreciate the difference between the cylinder pressure plot of a high and low compression engine.
In a low compression engine, peak combustion pressures are lower than
in a high compression unit. But percentage-wise, the pressure doesn't drop
off as fast as it does in a high compression unit as the power stroke progresses.
At the higher rpm a high compression motor is likely to run at, it needs
a little more time to blow down the cylinder. This we can do by opening
the exhaust valve earlier than with a low compression engine. This proves
possible with little or no penalty because a high compression means more
work on the piston at the beginning of the stroke and less towards the
end. So the higher the CR, the wider the LCA can be made by virtue of extended
duration by opening the exhaust valve earlier. A rough rule of thumb is
to open the exhaust valve 1-2 degrees earlier for every point of compression
increase from a previously optimally timed cam. Opening the exhaust valve
2 degrees earlier means the LCA has spread by half a degree.
Engine geometry other than the bore and stroke also influences the most favorable LCA. The connecting rod length to stroke ratio has a measurable effect on the position of the piston in the bore at any point of crankshaft rotation.
It is important to understand that the induction system does not know how far around the crank has turned. It only recognizes piston position and velocity, and it's subsequent effect on gas speed throughout the valve lift cycle. If the LCA and valve events were optimal then changing the rod/stroke ratio a significant amount will require a new cam profile to restore the original event timing.
Take, for example, the rod length tests done for a well-known tech magazine
a couple of years ago on a 330-inch engine. For the experiment, the connecting
rod length was changed by a whole inch, from 5.5 inches to 6.5. What effect
would this have had on the required cam event timing? If the original cam
were a 280-degree piece on a 110 LCA, then to restore the original parameters
the new cam would have to be 279 degrees with a LCA of 109. These changes
in the required cam spec, especially the LCA, would have measurably affected
the results this test produced; though the trends would still have been
The rocker ratio used can have a strong influence on the LCA. We've seen, like the rod length test, back-to-back dyno tests of various rocker ratios that have indicated a far more complex picture than is actually the case. Such tests showed that on occasion, high lift rockers don't work yet offered no reason why. From the point of view of the gas dynamics in an under-valved 2-valve engine, high lift rockers up to ratios of 1.8-1.9:1 always works if used correctly! The most likely reason for negative results when switching to higher ratio rocker is because the overlap triangle on an optimized engine was already as big as the combination would tolerate. If the LCA is already optimal on a big camed race engine, changing to high lift rockers will usually reduce the output, especially if used on the exhaust.
For a 2-valve engine, possible power reduction from high lift rockers becomes less likely and of lesser proportions when cylinder head flow per cubic inch drops. That's the situation for bigger inch small-blocks or really big-inch big-blocks. To make the most of high lift rockers, the reoptimization of the LCA is necessary. This means spreading the LCA’s. By how much depends on the head flow to cubic inch ratio. Generally, large engines require little or no change, whereas small engines may need as much as 2-3 degrees greater spread.
In the same way, a change from a flat tappet to a roller cam can affect the LCA required. To avoid a very lengthy valvetrain dynamics discussion to explain why, it is suggested you read the book "How To Build & Modify Small-Block Chevy Valvetrains," published by and available through MotorBooks International, and Competition Cams or any good bookstore.
For cams under about 270 degrees, changing from a flat tappet to a roller
will need a slight tightening of the LCA, about 1-2 degrees. From 270 to
about 285 it holds constant, but over 285 the LCA will need spreading a
degree or two.
All you have read so far might indicate there is a lot to this area of cam design. However if you absorb this, then as an aid to specking out and building a high performance engine, it will prove a valuable tool. In a sport that puts so much emphasis on technical capability, knowledge of camshaft lobe center angles can make the difference between winning and losing.
Most people when deciding to upgrade their engines look to change the camshaft. The cam is probably the least understood component in the engine. Hydraulic, mechanical, roller and hydraulic roller are the 4 basic types of cams in use today. There are IR designs as well, which only add to the confusion.
For the majority of people will just pick a cam out of the parts book, or worse still let the local counter boy make the choice for them, there is very little hope. For the people who don't want to settle, we present the following article.
(continue) Cam Basics p. 3
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