The issues of spinning rod bearings is complex. The basic problem is that the number 2 rod needs a constant supply of fresh cool oil to float the bearing. If that oil supply is diminished by any means then you can have enough loading to have the rod and crank make contact. This can lead to failure in seconds.

Now there are probably 10 factors that lead to insufficent flow to the rod bearings. Some is directly flow of oil into the bearing and others related to increasing flow requirement as the oil thins and clearances and rpm increase.

In my experince there is no single magic cure. It is my estimation that the oil flow margin to the #2 bearing is low. Such that if the system is working to 100% perfection you may only have 20% margin to spinnign a bearing on #2. Evidence suggest that you may have 25% margin on #3 and maybe 50% on number 1 and 4. This why we always spin number 2 first. Less margin. For a street motor that is fresh this more than enough. For a motor that is older and raced that margin decreases. When you go negative you spin a bearing.

So all things that improve margin are encouraged. Baffling the pan helps, correct oil level helps. changing be balnce of oil flow in crank helps as reduceing margin on number 1 and 4 to help number #2 and 3 is going to help overall life. Oil coolers help by reducing oil temps and changing bearings (rod and mains) by reducing gaps and thus creating less require oil flow to float bearings.

One key point to remember is oil flow to the rod bearings comes from block to first the main bearings. Excess oil at the main bearings is forced into the crank which goes to the rods. So large gaps at the main bearings mean less excess oil there and therefore less oil to the rods. Larger and/or more oil feed holes to rod journals will tend to improve oil flow to these areas. This will help up until you start starving main bearings. The good news is that while this can happen I have never seen nor heard of bearing failures at 1 and 4 rods or main bearings. I have seen only cases were #2 or #3 rod bearing fail and some cases with balance shaft bearing failires, but these are rare.

Anyway given all these basic issues the rules have been geared to allow lots of mods to improve the margins at the number #2 rod bearings. We then let the racers decide which mod they want to spend the money on.

I run a cross drilled crank, turbo oil cooler, baffled pan, highest quality oil of a thick weight for temperature performance, I check the oil each session to ensure full but not overfull and change rod bearings periodically.

loftygoals wrote:


BJ- here's the proposed wording on that rule:

ISV / Auxillary air valve can be deleted or disabled. Associated lines must be plugged if deleted. It is recommended to maintain factory idle control.

loftygoals wrote:


BJ, this is interesting. Anyone ever had a flow bench on a 6R vs 8R head? My motor at Nationals had a max shaved 6R head, and it made great torque, but really fell off at high RPM vs. the cars with 8R heads. I'm not sure my results are typical, but they seem to fit with your theory.

I ended up having to use a 6R head on my engine. Seems to work fine.

My guess is that the combustion chamber shape of the 8R head is slightly better, but I don't have any data to support this.

joepaluch wrote:


Great summary of the issue, Joe. I should have mentioned that I agree that #2 gets the least oil and it is the most commonly effected when there is a problem.

Sterling Doc wrote:


Sounds good, but why is it recommended to retain? Is the deletion of the venturi valve covered under updating and back dating? I don't believe the 88's had the venturi.

Sterling Doc wrote:


OK, just rummaged through the heads in the shop. 6R heads are off of 83's (all the 6R's I have have an 82 casting date). On the 8R heads, not only are in intake ports larger, but the exhaust ports are larger. I never took the time to measure the difference in CC's though.

There's a lot of data to support my theory about power:

1. Decking to minimum will retard the cam timing.
2. Retarding cam timing moves the torque curve higher in the RPM range, and thus peak power occurs later.
3. To make power at higher RPMs, you have to have the air flow to support it.
4. The 6R head has lower volume ports which should lead to less total CFM.
5. Choking the intake increases torque.

This would explain why the motor makes great tq, but not top end hp. To confirm all this, your dyno would be helpful. The peak tq should be higher than stock. Also, your should see the A/F getting richer as RPMs increase. If both these things are true, we have a pretty good explanation of what is going on.

Assuming everything above is true, your motor would really benefit from advancing the timing one tooth. This would move the torque curve and peak hp lower. This should be optimal for the minimum thickness 6R head. I would guess that peak tq would stay about the same, but you might pickup as much as 5 hp. With that kind of curve, you would have fewer usable revs, though. I would expect to shift at 6k or before, not redline.

-bj

Upon further reflection, I misspoke above. "Optimal" was not the right word. Advance would be better. For optimal we need adjustable cam gears. Honestly, adjustable cam gears may be the only way to truly equalize the performance between the minimum thickness head with low compression and the 88 high compression motors.

The real cost associated with the adjustable cam gear is not the gear itself, but the tuning associated with it. Of course the real problem with performance differentials is created by the rules in several places:

1. Raising compression via milling the head has been allowed for a number of years. There are quite a few cars built this way already.

2. Dyno tuning is already allowed using AFM and FQS adjustments.

3. 88 motors and/or high compression pistons are allowed and there are a number of racers using them.

4. Updating and backdating of parts from 83-88.

Thus, big expenses are already a part of the rules.

I'm not arguing for adjustable cam gears, just presenting it as an option for solving the problem of keeping the power equal across all the motors. I'm having a lot of fun figuring out the best combination of parts and the optimal tuning under the rules as they are.

Another option for adjustable gear would be to allow them, but require all cars to have the cams indexed at a predetermined setting relative to TDC. The problem with this is that it is hard to index a 944 cam because of the cam tower configuration.

-bj