So the argument seems to be that a cap is insufficient because of the potential to affect the power curve. I think there could be a way to structure a rule that mandates some sort of curve. Spec E30 has a rule like this.

Everett

dancing944 wrote:


Exactly this. Of all the legal dyno charts I have examined, which is around 20 now that we have a compliance dyno travelling with us around the Southeast, there is always a consistant trend of peaks and valleys in the torque curve. The horsepower curve is generally a constant slope with a few variations but also has a couple small bumps right at the same time the torque curve does. Changes to cam, head, or tune or anything else are going to be noticeable on the dyno chart and reason for further inspection. I have a back to back dyno chart of an aftermarket chip versus OEM on a 944Spec car and they are not alike.



Torque Curve: Peak at ~3000, valley at ~3500, peak 3700, peak at 4400, falls off after 5000.
Horsepower Curve: linear up to a peak around ~5600-5900 then falls off with small noticeable bumps around the same RPM as the peaks on the torque curve.

tcomeau wrote:


What you are recommending is for 9.5:1 CR engine heads to be shaved to minimum thickness as allowed by the rules. That leaves no lattitude for a further plane cut after the head is used because it will measure below the minimum thickness. The other problem with shaving a head that much is that it starts to impact timing. This seems to impact various engines differently but research and math we have done is that it retards timing somewhere in the 2-2.5 degree range by moving the cam wheel closer to the crank negating the benefit of a bump in compression from cutting the head.

Im also not so sure that the combustion chamber isnt impacted by a head shaved that much. So sure the overall peak HP/TQ is right where it needs to be once the engine starts revving high enough for everything to work as designed again, but it leaves a lot to be desired in the bottom end of the power band. I think this is why an engine with 10.2:1 CR pistons and a head that measures at OEM thickness has better mid range and low end torque. It also allows for the head to be planed more than a few times over the course of its life on a race engine.

We messed with the 2 degree offset key on a low compression engine that had the head cut to your figure and it did nothing to change the power output.

tcomeau wrote:


This was in response to RD's comment about how the cars with fancy aftermarket pistons are going to now fetch a premium on the resale market. Already happening with cars advertising Dirks built engines but the max typical resale value of a 944Spec race car is still only $10K and there are a lot more factors that go into that besides what pistons are in the engine. Show me pedigree, dyno sheets, a quality build and that will fetch the higher price.

rd7839 wrote:


Cheaters gonna cheat no matter what the rules say. Ive already explained how its not expensive to police: a $35 blue tooth bore scope off Amazon that connects to your phone stuck down a vacant spark plug hole will tell you what CR pistons are inside an engine. If you see no dish, you know the piston is illegal. Beyond the design of the top of the piston, nobody will ever know if its an aftermarket piston, not even now, because we dont tear down engines anywhere in the country. This is why we have the dyno rule, so we dont have to tear down engines. Post a reference HP/TQ curve to the rules along with the max allowable and let that be the catch all as it is now.




rd7839 wrote:


This mentality will kill our class.

Jason,
Some good points to consider, however:

Once a head is shaved to max for LC pistons, it doesn't need to be shaved ever again unless it gets warped. (A well-operating cooling system will prevent that.) Just a valve job freshening periodically. If it gets slightly warped, you can use the "repair" head gasket which is 1.4 mm vs. 1.1 mm. Also, heads are readily available and cheap. I have 15 in my shop.

Yes, shaving impacts valve timing, but that can usually be corrected with the cam key of 2 or 4 degrees. When you install the timing belt, you can drop the teeth into the cam gear slightly advanced OR retarded. The last engine I built needed more than the 2 degree key to put the valve timing back to optimum, which is why I suggested the rule change to include different keys like the rule used to be. If you use too radical a key, you'll hurt HP, so this is self limiting. Back to zero is what we want.

Good point about the combustion chamber shape change with max shaving. I've wondered about how the air flow thru the chamber and flame front of the spark are affected, however, we've been getting between 138 and 140 HP from our engines here, so I haven't dug into that further. I do index the spark plugs so the gap is not facing the nearest wall of the chamber.

As Ron Dale said, anybody should be able to assemble one of these engines in their own garage and make power AND be reliable. No need for designer engines. From my point of view, it's much more important to deliver reliability than get that last 2 HP. That keeps cars on the grid and the class growing.

Ron Dale,
If you find cookies or phonies wheels for $25, SNATCH THEM UP!!!! You can sell those to guys in the class and pay for your next race fee. I'd spin them up to ensure straightness before buying, especially the ABS offset phonies.

The "944 used parts dealers around the country" list has been started on face book.