Valve Clearance

Jacer Rocker Valves Formula VeeI’ve been asked to write something about valve lift. It?s a pretty topical subject amongst a few people I?ve spoken to. I?ll try to cover the rule as it is now and how we got there rather than the how to’s.

From my memory it was the early nineties that the practice of removing metal from the area around the underside of the rocker where it fouls on the valve spring retainer first was noticed. Occasionally before this you might find people lightly clearancing one or two rockers out of a set when the rocker would foul on the retainer or just shim the rocker posts out (which was the legal way). The advent of grinding the rockers for advantage was a step further away from legal.

It was determined that this process was “balancing”. Once the precedent was set open slather became the norm for the next 20 years. We move on to the later 2000?s and lengthening of pushrods up to around 8mm longer than std, grinding the 1600 rockers through to the oil feed holes to be able to stand the rocker screw somewhere around parallel to the rocker post stud was becoming the norm.

Machining of the rocker posts to stop the screw from falling off the edge of the valve end during its lift cycle all combined with excess wear in the valve train. Rockers would break from being so weak around the ground section. Pushrods would break from binding in the pushrod end of the rocker from over centering.

Valve guides would wear from the harsh angles and lack of valve rotation. People were machining up their own offset rocker studs to try to lower the rocker gear to help with some of these dramas.Towards the end of 2007, the best I think I saw on our car was around 10.8mm lift on the inlets, and 10.2 on the exhausts.

Some I believe would be getting quite more from 1.25:1 rockers and the use of lash caps both of which were generally considered not to be legal in 2007. To get 10.8mm though meant you needed to make up a jig to measure rockers looking for the machining variations through production. I can remember in late 2006 checking a 100 ltr container of rockers for 3 sets of „good ones?.

It took roughly 5 hrs to do but gains were there to be had. We move on to now with fixed valve lift which was introduced to help reliability and „parity?. I?m not going to go into the how?s etc with the new system other than to say that the lift isn?t a consistent figure and will change with wear and heat. If you replace a tappet screw it will have an effect on lift. If you break a pushrod and the one you replace it with isn?t the same length the lift will change.

If you pull a head off and change a valve unless the end of the valve is the same length as the one you replace lift will change. If you do a top end and re-cut the valve seats lift will change. I believe that moves are heading towards a 1.25:1 rocker being introduced which will make it easier to get lift required but may not be the perfect ideal.

As with most things progress over the next couple of years will determine the next set of problems for the tech guys to argue over.


Jacer DynoHi again for one last time in 2010. I’ve been asked to write a little about dynos. Hopefully it may be of some interest. I feel the need to make a couple of points first before getting into this.

First, in my opinion, a dyno is a handy and helpful tool, but don’t consider it to as the be all and end all to your on track performance.

Secondly, not only do all dynos read slightly differently, but you will get different reading depending on the method the operator uses.

Thirdly, having your car tuned and making a million horsepower at 9 o’clock at night doesn’t mean you will have a million horsepower at midday the next day.

Finally, a dyno should be used for back to back testing of components and tuning, and that before you get there the basics should have been checked. The intention of the final point in the previous paragraph is meant to indicate that unless you are trying to find an oil leak or source a dodgy wiring problem, then it is wise to make sure the car has all the basics covered. At anywhere from $150 to $200 per hour having a flat battery, running out of fuel, or having the engine dump the contents of it’s sump on the floor, isn’t money well spent.

On the subject of fuel, it is wise to make sure your car is dyno’d with the fuel you normally run. On an initial run for a carb, you may find you will have to change float levels at the race track after your trip to the dyno to deal with cornering flat spots. This will affect you mixtures to an extent.

Points one and two of the previous statement relates to the weather, or more specifically, air density. Most of the front guys, when they dyno their car, tend to have at least an air density gauge with them. Some guys will run their own lambda sensors in the exhaust. Peak hp readings will be taken after final jetting/tuning. Lambda (or CO) readings along with the air density readings will also be taken down.

At the racetrack, as the air density changes, calculations will be made to the jetted carb depending on what you have weather-wise on the given day. Sometimes these changes may be in the region of .025mm to .05mm. That sort of sums up the first three statements in that the h.p readings given are only a snapshot of the conditions of that time, but can be of great value at the racetrack if used correctly. I could spend time talking about jetting the pilot circuit, the main circuit, the air correction circuit, or why cars foul plugs in the pit or at Wakefield.

I could go on about what a difficult carb we run, and how the restrictor plate leans off mixtures on throttle opening leading to the reason of fouling plugs.

But I may leave that for another time. See you at the track.