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
duration, does the use of a higher ration roller rocker (1.65)
add that much more power? This is assuming that the head is well
and you are running the largest intake/exhaust valves that fit
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
seems to be that you can only draw so much flow through the head
before you get diminishing returns. Any thoughts on this?
The primary reasons for
using roller rockers are to decrease side thrust on the valve
friction in the valve train (pin bearing in the rocker and a
reduce reciprocating weight. The only way that translates into
horsepower is to take advantage of the opportunities these factors
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
a higher rocker ratio then you won't gain anything from it. If
prepared to experiment with shorter intake valve guides to improve
the reduced side thrust won't give more horsepower, just better
wear--which none of us care about.
You can get a lot of horsepower from roller rockers, but not
by bolting them
That being said, the dyno tests involved a fully developed race
run with stock rockers and again with roller rockers. The difference
was 2 h.p. - maybe a 1-2% difference overall. Hardly close to
gain as some promise as a bolt on improvement (I would assume
figure is for a stock situation). It is clear that the pin bearing
roller will reduce friction but the increased leverage will also
more work at the tappet end. As in everything, there is a limit
improvement. As machines become more refined, the next increment
improvement usually becomes smaller and more difficult to obtain.
point in asking the original question was whether or not roller
really added more horsepower due to a longer valve duration assuming
that all the other bases were covered - friction aside. I would
that there is a limit to just how much air/fuel mixture you can
through a Triumph head as fully developed as it may be. The dyno
would seem to suggest that.
Bill B.is correct. The big advantage to roller rockers
reduction of valve stem/valve guide wear. High ratio rockers
car - 1.65 - greatly increase valve train strain and wear.
fastest TR6 in current SCCA Production form is only runing
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
rocker tips certainly has merit. But, at what enginespeed.
in our cars, othe than the small Spit engines, don't see 8500
- 9000 rpm.
I have yet to trial any roller rockers but it is planned for
rebuild of our race GT6 engine. I can however offer some real-world
on head flow and related matters that might clear up some mis-conceptions
Owing an engine shop means I have a bit of a chance to experiment.
made various mods on a single 2500 head (casting # 219021) and
each one of them. At the simplest end, it was dead stock, followed
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
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
In all cases we had the stock valves and factory width seats.
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
results started to come together. At 0.450" lift, the fully
port flowed EXACTLY the same as the stock port.
This tells me the stock valves/seats/chamber etc are the limiting
max flow, not the ports or lift.
I now have some Titanium race valves with 7mm stems to trial.
the seat width and further modify the chamber and see if I can
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,
only way to improve them is by having very fast lifting cams
having the vale open for the longest possible duration. The rocker
arrangement has little to do with this. The biggest influences
diameter of the cam follower and the mass being moved. The stock
be enlarged (considerable cost as machining is time consuming)
small changes make a lot of difference. Because of this, just
published cam data is often a waste of time. Any reputable cam
advise how fast the cam acceleration ramp can be for any given
diameter. Anything that can be done to lighten the valve train
is a good
In relation to the prior comments about dyno results being
prove/disprove the worth of roller rockers, I have to cast
some doubts over
making definitive statement on measurements of 1-2%. Only the
temperature and humidity controlled dyno cells can achieve
repeatability. We operate both engine and chassis dynos, and
ignore results unless they show more than 3% change, once corrections
made for ambient temperature and humidity. For example, when
testing a 2500
race engine, a change in engine water or oil temperature of
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
Because of this, comparisons have to be made under as near
circumstances as possible. Peak power readings can rarely achieve
need to know, so you have to run acceleration tests and load
cycles to make
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, thanks for sharing this with us. I found the same thing
my flow bench work on both Spitfires and TR6's.
Thanks for telling us about the straight port experiment - you
me all the work of trying this next.
One thing that I found interesting is the flow I measure on
and 6 bangers with the valve totally removed did not increase
as I expected, and like you, could only conclude that the diameter
the seat was pretty limiting. I have found that the 3-angle valve
seats act as advertised and are the biggest single improvement
one can make.
Years ago a sharp machinist in Long Beach, CA did what he called
seat on the intake of my Matchless G80CS desert sled (single
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.
Rocker ratio has everything to do with valve velocity. Lifter
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
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 is correct. The caution is boring out the lifter bore.These
on a 4.5 degree slant so you can't run straight into the block
overbore the lifter bores.
On our current race motor for the TR6 we bored the lifter holes
lifters...Ted is correct...the lifter bores are not perpendicular
block face....What we did is used a CNC machine and wrote a program
the job...the blocks are between 1 and 2 thousands from one to
using the locating tabs on the pan flange surface...aside from
the degree of offset, each hole center was found, and lastly
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
give a flat face to start the boring process and get a clean
cut and not
destroy your bit..
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.
just as reducing the rocker weight isn't an overall weight reduction
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,
than 2% with roller rockers if the overall engine has been designed
roller rockers. If not, then you won't.
A high performance racing engine designed for standard rockers
benefit from roller rockers to a large degree. But a good cam
exactly what's going to be in the engine can shape a more radical
a roller rocker than a stock one, accelerating the valve more
holding it at maximum lift longer. Just as they can if they know
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
eliminate the valve guide from the intake port, knowing that
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
carbs flow about the same from 3/4 to full opening. It's pretty
understood that the value of the higher rocker ratio isn't really
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
begins to open, it does have an effect on early flow since the
The problem with conversing with me about this is that I'm re-reading
Engine Builder's Handbook, and trying to translate all the great
in that book to puny tractor motors. A frustrating practice that
impossible for me to look at any individual part without considering
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