Roller Rockers and head flow
I need some input regarding the use of roller rockers on a race
prepared TR6. If you are running a cam with .500 lift and 300
degrees
duration, does the use of a higher ration roller rocker (1.65)
really
add that much more power? This is assuming that the head is well
ported
and you are running the largest intake/exhaust valves that fit
in the
head. I was just made aware of some engine testing recently that
indicated that there was only a 2 h.p. gain by using roller rockers
with the above modifications. It also indicated that the increased
ratio caused more wear on the cam, tappets, etc. The logic as
presented
seems to be that you can only draw so much flow through the head
before you get diminishing returns. Any thoughts on this?
Allen Washatko
The primary reasons for
using roller rockers are to decrease side thrust on the valve
stems, reduce
friction in the valve train (pin bearing in the rocker and a
roller) and
reduce reciprocating weight. The only way that translates into
higher
horsepower is to take advantage of the opportunities these factors
enable. A
lot of speed equipment is sold to people with no plan to take
advantage of the
factors the equipment was invented to improve. If your cam won't
benefit from
a higher rocker ratio then you won't gain anything from it. If
you're not
prepared to experiment with shorter intake valve guides to improve
flow then
the reduced side thrust won't give more horsepower, just better
guide
wear--which none of us care about.
You can get a lot of horsepower from roller rockers, but not
by bolting them
on.
Bill Babcock
Bill,
That being said, the dyno tests involved a fully developed race
engine
run with stock rockers and again with roller rockers. The difference
was 2 h.p. - maybe a 1-2% difference overall. Hardly close to
a 10%
gain as some promise as a bolt on improvement (I would assume
that this
figure is for a stock situation). It is clear that the pin bearing
and
roller will reduce friction but the increased leverage will also
add
more work at the tappet end. As in everything, there is a limit
to
improvement. As machines become more refined, the next increment
of
improvement usually becomes smaller and more difficult to obtain.
My
point in asking the original question was whether or not roller
rockers
really added more horsepower due to a longer valve duration assuming
that all the other bases were covered - friction aside. I would
assume
that there is a limit to just how much air/fuel mixture you can
suck
through a Triumph head as fully developed as it may be. The dyno
test
would seem to suggest that.
Allen
Allen,
Bill B.is correct. The big advantage to roller rockers
is
reduction of valve stem/valve guide wear. High ratio rockers
for your
car - 1.65 - greatly increase valve train strain and wear.
Even the
fastest TR6 in current SCCA Production form is only runing
1.55. We
sell a goodly number of roller rockers and do not recomend
or sell the
1.65.for your application. Jack Drew's comment about filming
roller
rocker tips certainly has merit. But, at what enginespeed.
The engines
in our cars, othe than the small Spit engines, don't see 8500
- 9000 rpm.
Ted schumacher
I have yet to trial any roller rockers but it is planned for
the in-progress
rebuild of our race GT6 engine. I can however offer some real-world
comments
on head flow and related matters that might clear up some mis-conceptions
in
this thread.
Owing an engine shop means I have a bit of a chance to experiment.
We have
made various mods on a single 2500 head (casting # 219021) and
flow tested
each one of them. At the simplest end, it was dead stock, followed
by
various versions of most folk call a "port & polish".
The final end of the
head was fully downdrafted, with the head chopped away and a
35mm pipe
pressed in from the top corner of the head downwards at about
60 degrees. A
bit of port putty and some die grinder work, made quite a nice
job of the
merge.
In all cases we had the stock valves and factory width seats.
The guides
were progressively shortened as the porting got more radical.
The stock head kept flowing to 0.500" and rapidly flattened
The port work increased the flow by about 20% up to 0.300" lift
where the
results started to come together. At 0.450" lift, the fully
downdrafted
port flowed EXACTLY the same as the stock port.
This tells me the stock valves/seats/chamber etc are the limiting
factor in
max flow, not the ports or lift.
I now have some Titanium race valves with 7mm stems to trial.
I'll reduce
the seat width and further modify the chamber and see if I can
get any
better results.
There have been a few comments in this thread about roller
rockers and cam
lift. Because these engines are so restricted by the valve & seat,
then the
only way to improve them is by having very fast lifting cams
and therebye
having the vale open for the longest possible duration. The rocker
arrangement has little to do with this. The biggest influences
are the
diameter of the cam follower and the mass being moved. The stock
lifter can
be enlarged (considerable cost as machining is time consuming)
and very
small changes make a lot of difference. Because of this, just
reading
published cam data is often a waste of time. Any reputable cam
grinder will
advise how fast the cam acceleration ramp can be for any given
lifter
diameter. Anything that can be done to lighten the valve train
is a good
thing.
In relation to the prior comments about dyno results being
used to
prove/disprove the worth of roller rockers, I have to cast
some doubts over
making definitive statement on measurements of 1-2%. Only the
very best
temperature and humidity controlled dyno cells can achieve
such
repeatability. We operate both engine and chassis dynos, and
generally
ignore results unless they show more than 3% change, once corrections
are
made for ambient temperature and humidity. For example, when
testing a 2500
race engine, a change in engine water or oil temperature of
about 10degress,
can make a 3-5% difference in max power. Some other engines
. like the well
known 4AG Toyota, will show much larger swings from temperature
changes.
Because of this, comparisons have to be made under as near
to identical
circumstances as possible. Peak power readings can rarely achieve
what you
need to know, so you have to run acceleration tests and load
cycles to make
valid comparisons.
I'd be happy to swap real results with anyone else who might
have the same.
I'd really like to hear from folk with cam data and anyone
else who has
experimented with larger valves and lifters.
Terry O'Beirne
Terry, thanks for sharing this with us. I found the same thing
with
my flow bench work on both Spitfires and TR6's.
Thanks for telling us about the straight port experiment - you
saved
me all the work of trying this next.
One thing that I found interesting is the flow I measure on
both 4
and 6 bangers with the valve totally removed did not increase
as much
as I expected, and like you, could only conclude that the diameter
of
the seat was pretty limiting. I have found that the 3-angle valve
seats act as advertised and are the biggest single improvement
that
one can make.
Jack Drews
Years ago a sharp machinist in Long Beach, CA did what he called
a spherical
seat on the intake of my Matchless G80CS desert sled (single
cylinder 500cc
motorcycle) Gave it a noticeable power boost with no apparent
change in the
power band. I always figure an improvement you can feel with
the seat of your
pants is at least 10%. The "spherical seat" was just
a curved seat--no
angles--that was lapped by hand to seat the valve.
Bill
Rocker ratio has everything to do with valve velocity. Lifter
velocity
times rocker ratio equals valve velocity. Boring the block and
installing .874" Ford lifters instead of .800" stock
lifters allows the
cam to be ground for 13.6% more lifter velocity. Changing from
stock
1.45:1 ratio to 1.65:1 ratio rockers increases the valve velocity
13.8%. Do both and you get a 29% increase in valve velocity.
Richard Good
Richard is correct. The caution is boring out the lifter bore.These
are
on a 4.5 degree slant so you can't run straight into the block
and
overbore the lifter bores.
Ted Schumacher
On our current race motor for the TR6 we bored the lifter holes
for ford
lifters...Ted is correct...the lifter bores are not perpendicular
to the
block face....What we did is used a CNC machine and wrote a program
to do
the job...the blocks are between 1 and 2 thousands from one to
another
using the locating tabs on the pan flange surface...aside from
calculating
the degree of offset, each hole center was found, and lastly
the lifter
directly in front of the cam drive needs to be moved forward
about 25 thou
when using the 875 lifters...it is also important to do aprox.
a .900 or so
cut initially in the very top of the bore to remove the webbing
curve to
give a flat face to start the boring process and get a clean
cut and not
destroy your bit..
Chip
I stand correcting......... for any given camshaft, the average
valve velocity will change with a rocker ratio change. For example,
if changing the rocker ratio increases valve lift to say 16mm
instead of 13mm, then the average velocity increase is 16/13
or 12%. this is because the lift occurs in the same time (cam/crank
moves the same amount).
What I really should have said originally, is, the biggest useful
influence over the rate of valve opening, is the camshaft, because
flow testing suggests you need a big increase in rate of lift
early in the cycle, not just averaged.
I will take some measurements of cam rates on various Triumph
cams and report back.
To the various folk who emailed me about lifters, I understand
various sized lifters can be fitted---its just a case of boring
out the 12 lifter bores (tedious job). Cam grinders will specify
the minimum cam follower diameter their cam will successfuly
work on. The faster the acceleration rate of the cam, the larger
the lifter diameter required. If you dont get the lifter diamter
correct, the cam will edge ride. This means the lower edge
of the lifter will try to dig into the cam, rather than ride
up and over it.
I would be deligthed to ehar form anyone who has such cam data,
or even what is the most agreeive cam people have found to
work on stock lifters.
Terry O'Beirne
just as reducing the rocker weight isn't an overall weight reduction
issue.
Skip that second donut and you'd be way ahead for a lot less
money. It's all
about making life easier for the tappet, camshaft, and the valve
so that you
can either make the head flow better or rev the engine higher.
So to answer
the original question as succinctly as possible, you can do much,
much better
than 2% with roller rockers if the overall engine has been designed
to have
roller rockers. If not, then you won't.
A high performance racing engine designed for standard rockers
probably won't
benefit from roller rockers to a large degree. But a good cam
designer knowing
exactly what's going to be in the engine can shape a more radical
profile for
a roller rocker than a stock one, accelerating the valve more
harshly, and
holding it at maximum lift longer. Just as they can if they know
you're going
to have roller followers (only moreso). The head porting guy
that knows you're
using a roller rocker with properly and precisely set valve geometry
can
eliminate the valve guide from the intake port, knowing that
the remaining
stub is going to be enough.
As with any other flowing system, as the valve gets open further,
it has less
influence on flow. Its why you don't use a gate valve to throttle
and why
carbs flow about the same from 3/4 to full opening. It's pretty
well
understood that the value of the higher rocker ratio isn't really
ultimate
lift, it's acceleration during the early part of the opening
cycle. Given that
rocker ratio doesn't influence duration of the cam, or the point
that it
begins to open, it does have an effect on early flow since the
valve is
accelerating faster.
The problem with conversing with me about this is that I'm re-reading
the
Engine Builder's Handbook, and trying to translate all the great
V8 knowledge
in that book to puny tractor motors. A frustrating practice that
makes it
impossible for me to look at any individual part without considering
the
entire system.
Bill
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