Big Valves or Not ?
Some things you should know about big valves.
 

Do I need big valves? That's a question that comes up quite often and it's a good one. Here's the straight story on big valves for the Kawasaki KZ.

First, let's look at what's really involved with installing the 37.5 & 31mm Valves in a KZ head. We'll look at it from the cost perspectives as well as  the performance side. The first thing you really have to know is what is the customer expecting performance wise. Why?

Because just poking a big valve into that head doesn't mean you've improved it any. Actually we've seen heads that were not improved by the big valve installation. It comes down the efficiency of the valve. Valve efficiency is measured by it's "coefficient of discharge" or [Cd].

The air flow through the port and ultimately past the valve and into the combustion chamber is measured in CFM [cubic feet per minute] at a standard test pressure [commonly 10" on these old motorcycle heads]. From there we break it down further into CFM per Square Inch of Valve.

If the correct amount airflow isn't gained through the installation of the larger valve, then the larger valve can actually flow less cfm/sq.in than the stock valve does. This lowers the valves "Cd".

Result = wasted money & effort.

Hang on.....let's investigate the mechanical perspective.

When installing large valves, one of the things that needs close attention is the camshaft the customer is expecting to run in the engine. On part of the engine's cycle, at TDC, beginning of Intake Stroke / end of exhaust stroke, both valves are open. This is referred to as TDC Overlap. Why is that important?

Different camshafts have different profiles. Some have a lot of TDC overlap lift while others have less. The more the TDC overlap lift, the closer parts are to becoming intimate with one another. Not a good thing where vales and pistons are concerned.

When it comes to avoiding valve to valve clearance issues, you have two approaches. Sink the valves in the head and/or time the cams for high lobe centers. When you sink the valves, you have limitations on how deep you can sink them before you run out of valve clearance. You can do what we call "tipping the valve" where we remove material from the stem but that too has limitations......WAIT...it gets even better!

You probably never knew this but all Shim On Bottom Valve Buckets are not the same thickness. Here are some bucket thickness specs for you. With some cams you really have no choice in the matter. For reliability with certain cam profiles the thicker buckets are a must.

Stock KZ S.O.B Buckets - .100"

APE KZ S.O.B. Buckets - .118" [9 shim sizes thicker]

PM   KZ S.O.B. Buckets - .125" [12.5 shim sizes thicker]

As you can see, switching from Stock KZ S.O.B. buckets to either of the aftermarket high performance buckets starts to work against you as well......There's a solution. Small Base Circle Cams.

Stock KZ 900 /1000 cams have 1.110" base circles. Aftermarket cams are available with smaller base circles. Web Camshafts will custom grind the cams you need as will other companies. Small base circles buys you a couple of things. One of them is the clearance you need to sink big valves deep enough for cams with lots of TDC Overlap lift. Why is this important to know?

Because if you buy camshafts prior to anything else and expect to run big valves & aftermarket buckets, you may get told that your cams will not work with the other components! You could be stuck trying to resell the cams you purchased prior to knowing all the related information in this article.

It's important that whoever does the work on your cylinder head has all of the related information / parts so they can proceed without glitches or surprises. In fact, it's best to discuss your objectivs with them prior to buying any parts.

All of this information is not meant to turn you off about big valves in your Kawasaki KZ. It's meant to educate you about the potential pitfalls that might crop up. Used properly it can save you money and headaches. Big valve installations are quite common in these heads. The details about them is not so common. After all, its your motor and we think you'd like to know before you start buying parts.

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!