ITA Motor Rules

[obxr]

Hi - I recently purchased an ITA Miata. Lower end was old and I spun a bearing and need to build a new one. Just to verify my understanding of ITA rules:

I can go up to .040 over on the bore and remain in ITA.

I can't use pistons that change the CR.

I can't use aftermarket rods.

I can't use an aftermarket crank, change the stroke, etc.

The CR can only be raised a total of .5 - by any means.

Cams and valve timing must be stock.

Correct?

[mossaidis]

The CR can only be raised a total of .5 - by any means.

The only legit way to increase static CR is by overbore, usually .1 increase for every .020 overbore, and head shave.

[timo944]

The only legit way to increase static CR is by overbore, usually .1 increase for every .020 overbore, and head shave.

What about shaving the head???

[Chip42]

shaving the head within OEM allowances (as specified in the service manual for your year/make/model) is OK. head gasket must be stock compressed thickness, and CR can only go up 0.5 total as you already noted.

welcome to IT!

[Greg Amy]

Properly designed - and IT-compliant - overbore pistons will accommodate compression ratio changes via piston pin relocation. In other words, there is no allowance in the IT regs to install pistons that increase compression ratio.

The only way to increase compression and remain compliant to the IT regs is via head and block shaving, both within the factory service limits.

What most fail to understand is that the 1/2-point compression ratio increase allowance is not a performance improvement allowance, such as adding springs and shocks is. Instead, it's a recognition that when an engine is serviced within factory limits of servicing a warped head or block that there will be a resultant compression ratio change.

The fact that we racers take advantage of that service allowance to increase performance is of no concern to the letter and intent of the regs.

- Greg

[quadzjr]

The only way to increase compression and remain compliant to the IT regs is via head and block shaving, both within the factory service limits.

I did not know block was allowed.. I asked about that a while ago.. and was told no.

[mossaidis]

The only legit way to increase static CR is by overbore, usually .1 increase for every .020 overbore, and head shave.

What about shaving the head???

see my bold words above. head shave, shaving the head... same thing.

[obxr]

I think I got it. Thanks for the replies!

[obxr]

I'm still not completely clear on connecting rods. If they are the same weight, and pivot center length, but made of better material and shaped differently - are the legal? So I'm talking about something like eagle or manley - that weighs as much as a stock rod and same length - but stronger.