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Port Velocity and Sizing
Hi Randy:
My question is to ask if you've ever done any port modeling using port
velocity, as measured in feet per second, over your many years of engine
building? Let me explain. A highly respected performance engineer (David
Vizard) has written several articles and books which discuss air velocity
in four cycle cylinder heads for drag race and street car engines. He
provides baselines and lots of formulas on how to make an engine breath
well at certain rpms with certain camshafts, etc. It's interesting reading
because he was able to develop a performance baseline which applies to
almost any 4 cycle engine.
Have you ever done any modeling like mentioned above? That is, does your
performance mastermind have numbers that say, for example, a port needs to
move air at 125 feet per second in order to turn 8000 rpm. And in order to
move this much air, the port needs to be a certain size and shape.
John Marles, a West coast racer who you probably know, provided a great
port map (gave port sizes and heights, no air movement numbers though) for
a 2.5 200 on S&F about six months ago which makes something like 330hp
at 7800 rpm. Do you have any port map and horsepower details like this in
your arsenal of knowledge for the various engine groups (2.0's, 2.4's,
2.5's)?
And while I'm at it, do you have any tips to get my Viking to 85 mph? I'm
running 83.7 on gps with a 150 V-6 2.0 Merc spinning a 15x32 Mazco Cleaver
at about 6100rpm. I'm reworking some heads (milling and cc'ing) to bring
compression way up and run race gas (I'll write this up for a feature
article later this summer...it's part 3 of 3). The motor is box stock
except for relieved exhaust. I've lightened the boat up and it's down to
about 510 pounds. It runs and handles great on top end and don't want to
go inside the motor yet. I keep meticulous notes on testing (if you need
details) and highly respect your input.
Thanks!
Mark Booker (Boatnik)
Mark, (Boatnik)
I think we could have some interesting conversations if we ever get to
meet. Here's how I feel on the port velocity subject.
On almost all two stroke motors, the port shape and it's aiming of the
fuel change is much more important. It does no good to have high air
flow volume if it is directed in the wrong place. A two stroke uses the
air flow to scavenge the cylinder. Also the airflow is not anything like
the four stroke so an airflow number comparison is not good for
anything. You can read up on such things in two strokes in a book called
"two stroke tuners handbook" It was written by Gordon Jennings
over 30 years ago. My point is that the two stroke is a very old and
very researched piece. Most everything has been done already.
A neat item that most overlook is the fact that two strokes respond to
exhaust tuning (expansion chambers) and they respond to intake tuning as
well. These include velocity stacks and intake airbox shapes. Done
correctly the intake length can be tuned to increase, you got it, intake
velocity and volume. I see guys take a perfectly tuned airbox on
snowmobiles and remove it thinking they'll go faster because they made
the engine " breathe ". Little do they know they just lost
several horsepower. There is a lot of thought that goes in to some of
those designs. Land & Sea successfully tuned the intakes on
outboards in the 80's. The ram tubes didn't fit under a hood so nobody
wanted them. Porting is a fine art that comes with experimentation and
the will to destroy a few motors to find out what works. I've found that
what makes big numbers on the dyno, 3 out of 4 times don't work good on
the boat. For this reason it is best to be conservative with port specs.
There are some very good engine port specs out there, but each has its
place. Port a 2.5 to make 330 or 350 hp at the required high rpm (9000+)
and you have a motor that wouldn't be worth anything on, let's say an
HST.
The most common term for engine efficiency is Brake Mean Effective
Pressure, or BMEP. This represents an engines ability to pump air. It's
not all about volume either. A BMEP figure in the 150 range is a very
good engine. I've heard of 500cc GP bike engines with 180 BMEP numbers.
These engines make just under 200 hp at 13000+ rpm. Try that with a 2.5!
Sorry to say our 2.5's have some pretty pathetic numbers compared to the
cream of the two stroke powerplants.
Where am I going with all this? I don't want to sound preachy but it's
probably my writing style. Anyway here's my advice to any boater out
there. Stick to maximizing the set up and balance, along with good prop
work. These things will get you more speed than horsepower. Yes
horsepower helps, but if everything is the same (with set up and such)
it takes 10hp to gain 1 mph. You are talking some serious hp increase
needed to gain 5 mph. Granted if the boat runs slightly drier from more
power, the gain will be more, but usually just adding power is
disappointing.
That is just touching on the porting subject, so now to the most
important thing, your boat. The 2 liter 150 is a very good motor to work
with. If I had your boat I'd add the #41941 exhaust pipe in place of the
stock one. This would let the engine build power into the high 6000 rpm
range. It's worth about 10 hp. I'd think about having the prop cut to 14
and one half inches diameter. This would help top speed and
acceleration. For heads run the stock SST 120 head. It has 26cc chambers
and with its shape you can run 92 octane gas. The motor will make close
to 200hp and still run nice. I'd chuck the oil injection and jet up
about 3 or 4 sizes with the timing at 23 to 25 degrees. It's a place to
start. You should be able to jet down from that start point, but better
safe first. Lastly you could try the crossdrilled SST intake manifold.
It's tricky to jet so leave that for last.
I'll bet you'll go 90!
Randy
Please submit
whatever questions you have to IHRTechTalk@yahoo.com.
All questions and answers selected will be displayed on this page as they
come in. Note: though all boaters are welcome, priority may be given to
I.H.R.
members. Randy can not be held responsible for any advice
given. Though his information and expertise is second to none, he
has no control over what you do with your boat. It is up to you to
boat safely and act responsibly, and his advice is only to be used as a
guidance for your high performance boat/motor of which you are the one
responsible for the risks involved.
Thanks,
Mark C.
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