cbuzzetti wrote:


Charlie there are no 9.7:1 pistons. The only US spec pistons made were 9.5:1 or 10.2:1. Euro pistons are flat topped at 10.6:1 and not legal both by rule and by the actual compression they produce. 944 Turbo pistons are 8.0:1 and useless in our motors. They are also not legal by rule.

Now the head shaving rules are not intend to show how much can be shaved before problems result, but are intend to be an alternate method to determine compression ratio when other tools are not available. Measurement with calipers is simple and easy and this a clarification as it merely publishes clear the limit the cars are expected meet. If you happen to current have a car that does not meet the these limits you had an illegal motor. Charlie, yours was close and I don't do feel it is smart to back off the limit. Just like weight you can run legal at 2600lbs even, but all it takes a slight issue with measurement and you earned a DQ.

That is what I try to run a little over weight in all cases. Same here. Best to run a little over the limit just to be sure.

Now what works best on the car may not be shaving heads to the max. I have seen it over and over that guy who try to shave to the very limit tend to have more relabiliy issue that those that keep it mild. However that is your choice as a driver. I have had many opprotunity to shave my heads, but always opted to have the machiend shop remove the minimum while ensuring flatness. Always seemed the best way to have my motor work well and I have never regretted it.

I am opposed to the head thickness rule, at this point, because I have questions that I believe need to be addressed PRIOR to this rule being adopted. The "unintended consequences" issue and because I believe there is no need for it at this time because the current rules adequately deal with the issue.

This rule is being proposed as an easier way to ensure that the maximum compression is not being exceeded but it actually adds another way to DQ someone regardless of whether their head compression is too high or not.

Joe, from what I read here, your number, for an 88 piston motor, may be thicker than a head can be and still be at 10.5/1. If there is any desire to make the head thickness a DQ item, it MUST be at the exact minimum to meet the compression rule or even slightly LESS than your calculated minimum thickness in order to not DQ a motor that is still at or under the 10.5/1 compression.

The reason that I am suggesting that it may need to be slightly less than the minimum thickness is that Chuck is saying that how the valves and seats are installed can affect compression with any given head thickness. I take that to mean that the valve seat could be lower, creating more compression in a thicker head or higher, creating less compression, in a thinner head.

I suggest that the head measurement be used as a "guide" to determine if more testing is required. While equipment may not be available at the track to check the compression directly (which is the justification for this proposed rule), the rules provide that the head can be impounded and checked later if there is a concern.

Once again, NO NEW RULES unless there is compelling evidence that they are needed and, until there is a greater understanding of all of the nuances of this proposed rule change, it is not ready to implement.

Our rule has always been 10.5 to 1 and I believe this rule should NOT be implemented. We have the compression rule and it should be left as it is.

Jim

Additional question for Joe on the head thickness issue.

Joe, if a head ends up too thin, because of machining over time with rebuilds, would a thicker head gasket work to increase the volume to keep the compression below 10.5/1?

If that is possible, is there any reason, from a performance standpoint, not to allow an after market gasket on a thin head so that the head does not have to be replaced at considerably higher cost than the head gasket - read "lower cost".

Jim

Big Dog wrote:


Don't need to go after market. Porsche oem, 1.4mm, Part number 951.104.374.50

yep there are a few different gaskets . the non standard ones are pretty high cost. or they were .

Jim,
10.5:1 is the limit on compression.

The numbers I listed get you there as well. Remember that head thickness is only one fact in compression. Measuring compression perfectly is a hard job. It requires head removal and piston surface checks. A whistler is the perferred method for tech in most cases. The head thickness check is another method. We do not intent to remove the 10.5:1 as that covers all aspects. The head thickness measurement was first used in 2009 at Nationals and really should be clearly stated in the rules to avoid any confusion and to be used effectivly.

If you want to built a 10.49999:1 motor you may, but realize that any carbon build up can bump the compression over the 10.5:1 putting you in DQ territory. This distance measurment is easy check to make sure we are building good solid motors and not planning that it would take too long to check compresion at most regional races. Push the limits comes with risks.

I consider this just like weight. We all know the class min is 2600lbs. However how close do you want to run there. Run too close to min and you risk that the scales of the day are unkind and DQ you.

Same here. Build for not 10.49999:1, but build for 10.3 or 10.4. Remember the intent of allowing anythign over 10.2 on 88 piston cars was to give room to shave a head for flatness and not be over stock. It was not intent to build to the max. I know people have done that, but it was never intended to be like that.

Now I will not change the 10.5:1 limit as it has been long established. The head shave we have shown will get us right there and combined with 10.5:1 limit is just another tool we have to ensure compliance. It also a good way to check your head builder to make sure the don't try to build something that would get you in trouble.