Is there tolerance in the engine displacement categories

[mdcrose]

I have a 3 liter motor and I want to rebuild it an use forged internals.  The rules are quite brief and do not address if the motor can be bored slightly (.030 or .040) and still run in the 2.01 - 3.00 liter class.  Is this allowed?
 
 -Michael

[Gary Perkinson]

All that matters for engine-class purposes is the final displacement number. But it's interesting that you asked, because my 302 is bored .030 over, which makes it a 306--and those four cubic inches take it out of the "D" engine class (261 through 305 cid) and into the "C" class (306 through 372 cid). So if your stock engine size is right on the line between two classes, cylinder boring can make a difference. If you're in the middle of the class, it probably won't....
 
 Gary

[RichFox]

Gary, If you have not progressed to far I don't understand why you don't get a 289 crankshaft and stay at the top of "D" instead of the bottom of "C". Michael, In bulding my 2 and 3 liter motors I went .005 oversize to cleanup the bores and still be within class limits. Perhaps you can do the same?

[dwarner]

Each displacement break is allowed up to the next break. For eaxmple if the top of your class is 305ci you can go as far as 305.99999999...9.
 
 As soon as you measure 306 and larger you are into the next break.
 
 Dan

[Gary Perkinson]

Rich:
 
 If I had any chance of coming close to a record in either class, I might consider doing that, but the record in C/CGALT is 238 mph and the record in D/CGALT is 224 mph, neither of which I'll be threatening, at least not this time around....    
 
 Gary

[panic]

Watch carefully: some sanctioning bodies define Pi/4 as .7854 (slightly oversize).

[dwarner]

The displacement formula we use is bore x bore x .7854 x stroke x # of cylinders.
 
 have not had too many problems in the last 41 years using this formula. A little care should be exercised when building the engine when you are running at record speeds.
 
 Dan

[mdcrose]

Thanks for the info guys.  Looks like I will be getting a new block cause the smallest off the shelf pistons are still .040 bigger and I can only bore .010 and I don't think that is enough to clean up my block.  Not a bad thing at all cause the new block comes with a 4 bolt and a forged crank where as mine is 2 bolt and cast crank.  Hope to be out there in 2007.
 
 Have a Merry Christmas!

[ddahlgren]

The way I learned it a long time ago was like this..
 (4.030 / 2)^2 X 3 X 8 = 306.13375
 
 Pi/4 gives the same answer in this case
 4.030 X 4.030 X (pi/4)X 3 X 8 =  306.13375
 
 0.7854 is Pi/4 rounded to 4 decimal places so it is an approximation as are all things that use the # PI
 4.030 X 4.030 X 0.7854 X 3 X 8 = 306.13447
 
 the difference is 0.00072 cubic inches out of 8 cylinders approximately 0.00009 cubic inches difference per cylinder..
 
 What would you use to measure the bore or stroke to this precision ?
 
 using the textbook formula from the 60's the first one lets just vary the bore by 0.0001 inches
 
 (4.0301/2)^2 X 3 X 8 = 306.14895
 
 That is a difference of 0.0151956 cubic inches total and 0.0018994 per cylinder
 In the highly controlled environment at the tech line as well as the average machine shop how might you measure this. If the guy doing the boring says he is accurate to 0.0001 in absolute numbers you paid too much or someone lied. If they can do this I would suggest that they send a set of instruments to a NIST certified lab and have them calibrated for an 85 degree f day 10% relative humidity and a baro of 25 in Hg and you should be close. Or you could set up a controlled area with std temp and pressure and humidity along with some NIST technicians to make the measurement. Or you could intentionally build it to 305 and have a whole cubic inch to play with in error. With all the prize money at stake I do understand the dilemma though.
 
 For those that might not get it.. this is supposed to be funny... or point out when we are working too hard..