Don't tell me there's a bunch of different cams too!!!!
I should have just bought an 88 to start with.

RacerX wrote:


There is cam for 83-85 and 85.5 to 88. Slight difference.

Again most of this stuff is down in the weeds as you have to remember there are tolerances on every part in the engine. A certain stack up of tolerances in any one motor can cause a shift in hp. If you want to chase that you can, but now you starting getting into even more effort.


To be honest even in a stock class there is not limit to how much you can spend on a built motor. You can infact chose to build 4 or 5 motors at 5k each and then choose to run the best one. So that might mean a 25k motor.

There really is no way in the rules to stop that sort of thing. The only hope we have as rules makers that such an effort will bring only small insignificant power gain. The idea being why spend 25k for 1 hp.

This where 10.2:1 pistons have stood for years. On paper there is a power gain, but when all things are considered is that power gain burried in normal variation of engine to engine build and dyno to dyno measurments.

If you are comparing junkyard unrebuilt motors vs typical rebuilt motors then the effect of those pistons is marginalized. Comparing well built motors then maybe less so.

This is central issue. Are we now at the point that due to motor development and driver development that there is noticable on track difference between 10.2 and 9.5 pistons? If so where does that difference come from? And are there things we can do to adjust the rules to minimize the performance impact on the track while also controlling costs and not upsetting the driver community?

As of now we directors feel it is the right time to investigate this in some detail and try to replace facts with conjecture such that we can make the right decisions going forward. The right decision that balance trying to ensure equal peformance and minimize "must have" stuff and also keep a lid on costs for both new builds and existing racers.

RacerX wrote:




There are two spec legal cams:

Early - 155.05
Late - 155.09

The intake profile, lobe centers, and lift are the same on both. The late cam just has slightly more duration on the exhaust profile. I honestly wouldn't expect much in the way of a perceivable difference using one vs. the other.

While I'm talking about motor differences and this thread has gone way off topic, lets look at the other differences:

Here are the picks I have comparing the intake and exhaust ports:

Intake:
6R head - Notice the pocket at the bottom.


8R head



Exhaust:
6R head - Notice the light gray area: that's the exhaust pocket.


8R head


Sorry the pics for the 6R head are so dark, but you can tell that there is much more port material on both sides of the 6R head.

Here's some shots of a stock thickness head vs. a head shaved to minimum thickness for low compression pistons:

Head removed: 24.15mm


New head: 22.68mm


The mating surface for all the heads look pretty much the same. The thing to notice is that when shaved to minimums, the depressed area that mirrors the piston bore is eliminated, changing the shape of the chamber.

So the question becomes is this good or bad? In my opinion it is a positive. These aren't the most advanced heads. They are simple 2 valve heads with a old school wedge combustion chamber design (nope, no hemi's in these cars). Compared to modern heads, there is very little to cause swirl and promote fuel and air mixing. Just for reference, modern heads are usually a modified hemi design. They have more of a kidney bean shape with ridges between the valves to promote turbulence. The valve combustion area also is usually smaller than the diameter of the cylinder bore. The flat parts of the head that covers the bore creates an area called the quench area. This is where the piston gets very close to the cylinder head surface. Reducing the space between the piston and the head causes the air to get squished into the combustion area more rapidly. This promotes turbulence which in turn mixes the air and fuel better. The faster moving air also cools the edges of the head's combustion chamber reducing the likelihood of pre-detonation or knocking.

So increasing the quench area is a good thing on these heads. It promotes better A/F mixture and reduces the chance of detonation, which is good since we are raising compression.

The other thing that can effect the effectiveness of this quench area is the shape of the piston. Here are what each of the pistons look like:

Low Compression - 9.5:1 Piston (83-87)


High Compression - 10.2:1 Piston (88)


As you can see, the tops of both pistons are very flat with the only difference being the dish in the piston itself. The flat edges on the low compression piston will work well with the new tighter quench area to promote the benefits listed above.

Thus, in some ways the minimum thickness low compression piston configuration is superior to the stock 88 high compression configuration.

So what does all this mean? It means that a low compression piston motor with a minimum thickness head should perform at least as well as an 88 high compression piston motor (assuming that the late head and cam are used with the low compression piston motor).

The one gotcha in all of this is the cam timing. Using a minimum thickness heads makes it impossible to set the cam timing to TDC. It must be retarded by a few degrees or advanced by a slightly larger margin. I don't know without dyno testing, if either of these are advantageous or not.

If we really want to be able to equalize the performance, or at least give all motors the chance to make the same peak horsepower, we need to allow for adjustable timing gears. I know I previously posted that I had no position on this issue, but after pondering it further, I have taken a side.

Now, an alternative to using completely adjustable gears would be allowing for modification to the stock gear to bring the cam back to stock timing. This modification would involve having a new key hole cut in the gear to return them to stock timing when installed on a minimum thickness head. I think this could be done for around $100.

Here's how I would amend the rules:

1. Require stock timing on all cars
2. Cars with head thickness below the factory specified minimum thickness (not rules thickness for this definition) would be required to run the modified cam gear.
3. Have a specified supplier for the modified gears, to keep from allowing "cheater" gears.
4. Allow 1-2 seasons for compliance.

This should equalize the performance potential for all motors while controlling costs and eliminate the need for dyno tuning. It would also allow you to run the stock low compression motor with out modifications (although you may give up a few ponies).

Enforcement would be simple:

The gear could have 3 clearly marked settings. 1. Stock, 2. LC Piston & < Stock Thickness, 3. HC Pistons & < Stock Thickness (Not sure the last setting would be needed. I need to compare the HC minimum thickness to factory minimums.)

A steward would measure the installed head thickness. Based on the measurement and pistons installed, a particular setting would be required. The markings for the setting could be made on the gear so they are visible through the cam gear TDC inspection window.

What do you guys think?

-bj

Joe,

I didn't see your post before I finished mine. Please see my comment below.

joepaluch wrote:


I believe the answer to this question is yes. I have seen a huge shift here in the Texas region over this one season. At the beginning of the season there was a clear leader in the class. He would out drive the rest of the field. By the end of the season, every single racer in the region rebuilt their motor with high compression configuration (either with a 88 motor or LC and min shaved head). The results? The driver that was winning at the beginning of the season can't keep pace with the rest of the cars down the straights.

joepaluch wrote:


That's exactly what I'm trying to do. I don't want racers to feel like they have to have an 88 motor to be competitive, whether it is true or not.

joepaluch wrote:


I'm not a director, but I want to see this series continue to grow and prosper. I never want to see this series become what some other spec classes have become.

As for developing real data, that's hard to do without money. We could build three fresh motors, an 83, and late LC, and an 88 and compare all the numbers on an engine dyno. The problem is financing it. Hell, I'd be willing to donate the time to do this, but we are still talking a lot of money.

In reality, what we have to work with is a little data and lots of conjecture. Here's what I do know:

* Porsche published specs for the 88 motor rate it at higher horsepower.
* There are a number of design differences between 83-88
* Using general engine building knowledge, you can see that these differences should equate to more performance.


The rules address these items in the following general ways:

1. Allowing updating and backdating of parts
2. Allowing compression to be raised when using low compression pistons

The first point is fine. I have no issue with this. The second allows compression to be raised by decking the head. This is acceptable, except for the obvious difference in timing. My proposed timing rule would fix this.

The beauty of the combined timing gear/shaved solution is that it allows everyone to build to 1988 specs without sourcing an 88 motor or pistons. The only costs added to a LC motor rebuild to achieve this is a $100 cam gear. That sounds affordable to me.

-bj

BJ,
Eric has been in discussion with the NASA National office on this topic as well. They are fully in support of dyno testing to gather data. They have asked us to come up with motors to test and they will cover the costs of the dyno time.

So right now we are working on finding the right candidate motors and desinging a good test plan. Some of the discussions here are centered around having some understanding of what factors could influence the testing. As you can imagine we cannot test all combinations of everything so we have make some educated guesses.

As for cam gear. We know there is some effect, but not sure what it is. One idea is do testing with an adjustable gear and an effort to find the right adjustment for shaved heads. Then we may try to go with an offset key between the cam gear and h the cam that locks in adjustment. It might be all we need. Then again maybe not.

So testing is really the only want to convert guessing into useful changes. This is clearly why we could not make and change for 2012. It is going to take time to gather the data to make informed decisions.

BJ... thank you for your input.

joepaluch wrote:


I agree there is no way to make this happen for 2012. This is just the time of year to get discussions going. If we have the means to gather the data needed, then by all means let's do it!

-bj