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
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