Head Games

[Crazcnuk]

I beleive the reason for the advance keys was to overcome the cam being set retarded from the factory.

The key actually put them back where they should have been in the first place.

I am not sure what the rationale was behind retarding the valve timing.

But there was a big difference before and after installing the cam keys.

You have to remember that at the time the average 350 was putting out around 165hp stock. They were so badly detuned it was a wonder they ran at all.

[TADracer]

My thought is that engine mods of any type that alter the factory tolerances ie. head or deck milling, piston change, valve seat grinding, camshafts, should come with great attention to the resulting clearances. As Dan says, degreeing your cams is nothing more than a method of tuning. Degreeing your cams will not result in a power increase that will blow you off of the seat but rather will result in subtle changes to adjust the power curve and when it occurs. So if one is going through the time and exense of modifying a motor then having the ability to degree your camshafts is a nice feature. Slotted cam sprockets, if installed correctly, will not move. I check mine routinely every Spring and they never move.

But a word of caution about checking your valve/piston clearances that has not been mentioned in this thread. Do not just check your V/P clearances at TDC. When checked at TDC it appears you have miles of clearance that you might not actually have. The actual closest V/P clearance will probably occur off of TDC by as much as 15 degrees. So check at 5 degree intervals from 15 degrees before to 15 degrees after TDC. Mine happen to occur at 10 degrees ATDC for the intake cam and 5 Degrees BTDC for the exhaust.

[Dan Hodges]

VP Clearance @ ATDC & BTDC

When measuring the valve to piston clearance, the following must be taken into account. The intake valve is closest to the piston at 10 degrees ATDC and the exhaust valve is closest to the piston 10 degrees BTDC. If you install larger valves, you must also take into account the clipping clearance between the valves (interference) at their closest point as they open and close and you need .020 minumum. The camshaft duration, lobe lift and event angles will effect the clipping clearance between the valves. In the Eleven head, you can use 39 mm intake valves in conjunction with 34 mm exhaust valves without a problem but larger than that and camshaft selection and phasing becomes critical. All of this is due to the fact that the Elevens head has a large amount of included angle between the valves vs some of the newer stuff that have shallow valve angles. In the XS motor as in every motor, there is at least 1100 things that need to be checked if you want it right. When you mount your degree wheel and dial indicator on your motor to check your cam phasing it should look something like this.



With the degree wheel and dial indicator properly mounted on #1 cylinder, you can with a light checking spring installed on each valve of #1 cylinder ( weak enough to allow you to compress the valve with your finger) check your VP clearance. Mention is made, I use a weak inner valve spring for this but any spring will do as long as it's stiff enough to properly hold up the valve with the keeper and retainer installed yet weak enough to enable you to depress it with your finger. Relative to this, the assumption is made that the valve relief's have all been cut the same which would have been checked before you began this procedure. Once the VP is checked, you can degree your cams. This VP thing is kind of a pain to check because you have to use a light spring on the valves which requires putting on the head and then taking it off to reinstall the regular spring. Some folks have made tools that compressed the valve with the regular spring however I tried this and could not get a dead nut measurement and subsequently prefer the light spring method. If you are using stock cams and pistons your VP clearance is suffcient and you can skip this step if you prefer however if you are using a cam that has over 250 degrees of duration @.050 lift and something more than .360 of valve lift you need to check your VP clearance, ditto when you change the event angles of the stock cams. What is the duration, event angles and lobe lift (valve lift) of the cams in a 80 XS, at the opening ramp, @ .040 lift, @.050 lift @.100 lift as compared to a pair of cams from the 78-79? Unless you degree them, you will never know why and how one set differs from the other and offers different performance parameters i.e. torque, horsepower and unburned hydrocarbon emissions. Lastly I offer up a picture of the perfect 2 valve performance motorcycle head.



These heads are made from a raw casting and yes you can get one made for an Eleven if you want to spend the money. They are primarily used on 1500-1600 cc Susuki and Kawasaki Pro/Stock drag bikes but because the Yamaha is similiar in design the head with proper machining is applicable to all three. These 1600cc inline 4 cylinder Susuki's are running 6.90's at over 190 mph in the quarter on gasoline and carburetors @ 625 pounds including rider with this 2 valve per cylinder head configeration. They could run a water cooled 4 valve per cylinder motor if they wanted but for many reasons they don't. Happy tuning. Dan