77 replies to this topic

[Jim Drago]

Please let us know what you think...

WHAT DO YOU THINK?
SPEC MIATA
The CRB requests member input on whether or not the use of .25 mm(.010 in) over size pistons in all SM engines should be
permitted. The Spec Miata advisory committee asks that members take into account the following in addressing this question:
• The cost of new crate motors is approaching $3500. Many used blocks are too worn to be suitable candidates for
proper SM engines. The SMAC has decided to explore the possibility of allowing 0.25 mm (0.010 inch) over bores.
The intent is solely to contain costs. While sleeving blocks is allowed, few machine shops do it well and the costs to
do it properly are substantial.
• Over bore pistons and rings are readily available from Mazda and the cost is the same or less than standard. The
basic engine rules would not change in regard to OEM parts since only Mazda parts would be permitted.
• The performance gain is negligible. For a 1999 Miata making 128 HP, the gain from the over bore pistons would be
no more than 0.77 HP based on the added displacement.
• The purpose of permitting this over bore is not to create a performance enhancement nor as a parity adjustment
measure. However, the advisory committee is mindful that this may become the perceived "standard" and that many
competitors will feel that they "must" build to the new over bore allowance. To prevent this, cars using the over bore
allowance would be assessed a 15-20 pound weight penalty. That will assure that there will be no performance
advantage to using the over bore. But, a competitor with a running core motor will be able to build it without spending
$3500 for a crate motor and will not sacrifice any performance or build quality due to worn out bores.

[MPR22]

No brainer. Yes with no weight added. Too hard to regulate.

[trimless]

No brainer. Yes with no weight added. Too hard to regulate.

Agree, give the 1.8 .010 and the 1.6 should get .030. That should work for equality.

[FTodaro]

For those of us who run a steady program between SCCA and NASA, we already are working harder each weekend to come into compliance between the two groups. My question would be, would I be DQ ed in NASA for doing so. As far as the logic of the rule, if its truly .77 hp I can live with that.

My only other comment is my last crate motor cost me 3,000 deliverd September 2010. I hope it has not jumped up 500.00

[DionJ]

Yes bore, no weight penalty....

[dstevens]

Johnny D beat you to it with this thread from yesterday...

http://mazdaracers.c...686

Most of the budget guys aren't going to have programs where they can exploit that extra bit of displacement. As far as NASA and the changes, I'd bet they adopt them as well. If you really want to talk about helping budget racers other things like allowing OEM spec non Mazda parts and Snug Tops could help some folks. Good to see the CRB take this under consideration.

[Rob Burgoon]

No brainer. Yes with no weight added. Too hard to regulate.

No brainer. Yes but with weight penalty.

Does it make any sense to require a note in the logbook about their claimed bore?

[Rob Burgoon]

For everyone that is suggesting allowing overbore without a weight penalty, are you out of your f*ing minds?

[Tom Hampton]

Oh Boy. Here we go.

[Johnny D]

For everyone that is suggesting allowing overbore without a weight penalty, are you out of your f*ing minds?

yes.

Do you think it will end up like the RP?
1.6 big bore.
1.8 mid bore.
99 little bore.
J~

[MPR22]

For everyone that is suggesting allowing overbore without a weight penalty, are you out of your f*ing minds?

So you think .01 overbore will make that much of a differnce? Or Do you think you could actually tech the overbore. If someone did the work how would you suggest we determine they needed weight? Pull the head? Are you posting the bond on that work for everyone you think is not carrying the additional weight?

I don't really care, but its just a matter of being practical.

[GregA]

No brainer. Yes with no weight added. Too hard to regulate.

First thing I thought of. How would it be regulated...the honor system?

[Rob Burgoon]

Regulated the same way we handle anyone cheating with too much displacement now. If you get torn down, you'd better hope your measurements are within tolerance.

[dstevens]

A 10 thou bore is nothing. Let's say .77 hp. If you have it done just for performance sake, it's about US$800 if you do it send the block out and do the wrenching yourself with parts from the Zoom Zoom discount, about a grand if you have them do it all. For the sake of argument let's say $800. That's about a grand per hp for the mod. If guys want to spend that kind of dough for that small of a performance gain alone, let them. That 3/4 hp ain't why they are beating you. I'll take that grand and go get a good dyno tune from a pro shop or some seat time and tires or put it toward a pro head. A $1200 head is going to get you much more than 3/4 hp.

[Lee Tilton]

+1 for overbore, no weight penalty. I would be in favor of .030 for 1.6s, obviously.

[KentCarter]

10 over is a very good idea, but can tech really detect it without a rip down. Will allow a little more compression, by Whistler, but not by measurement.

I have a sleeved motor with scored bores... can I go 10 over?

I think the weight penalty will be hard to enforce.

[Zauskycop]

I think a lot of 1.6 owners will automatically rebore, since they are carrying that extra weight anyway

[ToddW]

Great idea on the bore. Do not add the weight.

[Sam Barnett]

Great idea for cost savings to allow the slight overbore. Many junk blocks will suddenly become useful which is good for SM and good for the environment.

[Astro]

How can the rules continue to change after 10+ years of experience? I vote for freezing the rules. If there is something that makes one gen more competitive than the others then people will move to that model over time. Changing the rules every year gives the perception that some group is getting screwed and is driving people out of the SM community. Again, I vote "NO MORE CHANGES!"

P.S. 0.77 HP is a BIG DEAL.