What sort of porting does your engine require?

Some things you should know about porting. Especially street engines. The intent of the article is to educate guys who have questions about this "mysterious-black art". So, lets get down to it and strip away as much mystery as possible.

HISTORY - I've had ported heads from all over the country pass through my hands. Some good, some fair and others were nothing more than polished stock ports flowing maybe 78CFM @ 10". Here's the thing with the ported heads. This is precisely where I see things that cause me to shake my head.

Most times, guys are installing cams at the same time. The cams alone will improve the performance of the bike by a very big factor. There's literally no way for the guy to know if the head was worth what he paid for or not. Often they don't even get a flow sheet back that shows the stock vs ported airflow results.

Most guys don't know anything more than polished is pretty and hope for the best. Once upon a time, I was one of those guys. That was a long time ago. Things I've learned, took years, time & effort. There are not a lot of motorcycle guys around who will even discuss their port work. Everything is a big secret. Well, get ready to have those barriers knocked completely down.

Alright, you've decided you're going to build your engine. Just how much of a fire breather are you looking for? How big is it? Will you drag race it? Will you just ride it on the street?

Those are key questions that rarely get asked and we can't help but wonder way. Just like the big valve mind set, all components of the engine need to work together, in harmony to produce the desired result.

The cylinder head setup that is correct for a 1200cc motor with 34mm carbs, designed to make peak HP at 9500 is not the same cylinder head that is correct for a 1015cc motor with 29mm carbs, making peak HP at 8000. Can you bolt the head ported for the 1200cc motor on the 1015 motor and notice an improvement in power output? Yes, you sure can. You may also discover that it isn't right for the best all around head on that smaller engine though. The small motor can fall all over itself struggling to get enough RPM to use large ports. The ports really do need to be sized with all the engine criteria taken into account.

Effectively, in all engines, the piston demands a given volume of air.  For 100% and greater efficiencies, many factors come into play. The head, cams and carburetors work together to supply the engine with the required air.

There's a lot more to properly setting up a cylinder head for a given engine that just getting in there and grinding away in hope of more peak flow. The ports need to  be of proper cross section for the size of the engine the head will be used on. The flow needs to be a certain minimum amount for the engine to have a shot at exceeding 100% volumetric efficiency. The speed of the air through the ports needs to be correct. You can calculate the mean port velocity but you cannot find local velocity issues through math calculations. That has to be done with a velocity probe and you have to know what  you're looking for.

Are you willing to accept an engine that has plenty of power all packed into a very narrow RPM range?

On the street or at the track, the right cam with the right porting will allow you to maintain a smoother running engine than an oversize, too large cam with a cylinder head that was not ported with the full details of the rest of the engine in mind. Don't get misled. Let's analyze some porting numbers so you'll have a better understanding.

When heads are flow tested, it's generally in .050" lift increments and always at the same test pressure on the flow bench. Super Flow suggests using L/D [lift diameter ratio] instead of .050" increments but the truth is, most don't, even the big name Pro Stock Car guys. Common pressure for these old KZ heads is 10" of water. Why 10" ? Good question, here's the answer.

For years, shops were equipped with Super Flow 110 flow benches. They're good machines but not very powerful as compared to a larger, more powerful bench. They simply can't pull 28" of test pressure which by the way is the preferred pressure everyone has moved on to over the last 15 - 20 years or so. In fact, test pressures are on the rise, not the decline. Why is more test pressure important?

Airflow through the ports may be calm and well behaved at low test pressures like 10". When the test pressure is increased to 28" [about 1 P.S.I.] the previously calm port can suddenly become turbulent. Turbulence is not good in any port. It shows the air is separating and choking off the flow of air through the port.

If there's an unknown condition in the port that will cause turbulence in a live engine, you're not very likely to find it at 10" of test pressure. We spot test all of our ports for turbulence issues at 36" - 40" of water. That gives us a good indication of how the port will behave on a live engine. We've found problems at high test pressures, corrected them, then spot checked back at the common 10" and found the flow had picked up.

As previously stated, flow is charted in .050" valve lift increments up beyond the lift your camshaft will lift the valve. Generally .500" is the stop point for most bikes. You're not going to run a cam with .500" lift on the street so who cares what it flows up that high ?

The engine can care. Heads that back up [begin to flow less air] are not going to be good producers of Horsepower. They have a problem and it needs to be resolved. Good practice is to flow the head at least .100" past the point of max valve lift.

When all the porting has been evaluated, the flow numbers, velocity probe readings and all the numerical data can be reviewed and charted to compare the stock and ported results. This is where you quickly see that the old school mind set of "throw a bunch of cam at it" can be a bad choice in a street motor.

The piston's demand for air for given RPM requirements can be met through good port work & less camshaft lift. This means more performance with less wear on your valve train, better carburetion & behavior on the street while still yielding excellent midrange and top end performance. After all, who wants a cranky street bike that runs poorly on hot days or can't be ridden around town with out irritating carburetion issues?

Other Pages You Should Read

Engine Rebuild Considerations  - Things you need to consider before you budget for your build.
KZ Street Engine Components  - Some combinations that are easy on the budget and perform great.

KZ Data  - Flow Information on the Stock Kawasaki KZ & J Heads plus other information.

Porting Method - Our approach to producing a top performing cylinder head.

Pricing - Prices for porting, valve jobs, etc.

Unsurpassed Customer Service!