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.

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!



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.

Mark C.


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