oil and bearing clearance ?

hey guys just looking for some opinions here on oil and bearing clearance in hemi's,I run a BAE hemi billet block, MGP billet rods and bryant crank, we run 0.0075- 0.008 clearance on the rods and 0.0065- 0.007 on the mains, the engine is in a TAD that see's 10000rpm plus, I have assembled a few other engines for some of my customers and this seems to be the normal clearance for a blown alcohol hemi, we run a 70 weight oil and run around 140-150 psi pressure , from what ive seen and people ive spoke to these all seem to be pretty std numbers give or take a little.
This all just seems a very old school way of thinking, big clearance, heavy oil and tons of pressure. With the advances in oil, bearings and oil delivery systems ie dry sump and i mean proper 4-7 stage dry sumps not those 2 stage deals that most of us run on our hemi stuff, how come people havnt changed there way of thinking in nearly 40 years.
If you look at a pro stock engine 25 years ago they all probably ran 0.004-0.005 on the mains and 0.003-0.004 on the rods and lets say 80psi oil pressure now they are around 0.0015-0.0018 on the rods and not much more on the mains and they run up to 7 stage dry sump oil pumps and 0 weight oils with not much over 45-50 psi oil pressure, why couldnt a hemi be the same. im not saying run clearances that tight or so little pressure or oil that light but what about 0.004-0.005 clearance on both rods and mains a 5 stage dry sump system like Bill Daley makes and say a 15/50 multigrade synthetic like joe gibbs or any other brand and 70-90 psi pressure, has anyone tried anything like this?
dont get me wrong the way most guys (including me) still run these engines is working ok , just seems like we are still building all these engines the same way as they were in the 60's and 70's and all we have done is make everything bigger and stronger instead of fixing the problem in the design, im not meaning to offend anyone at all hell my engine runs the big clearance big pressure and heavy oil, im just thinking out aloud, just looking to see what everyone thinks here, maybe there is a better way? let me know what you think guys
thanks

 

One reason for the large clearances is distortion in the block, crank, etc. at high loads (at least that's what we think). We have seen some trend towards slightly smaller clearances, and 50w oil. and maybe this is due to forged blocks, better rods, etc.

 

I agree about the parts becoming more rigid so the need for such large clearance wouldnt be as necessary, i know years ago when people were using std factory bbc blocks in high horsepower applications you had to run really big main clearance because they moved around so much it would hurt the bearings now with all the aftermarket blocks available they are really strong and rigid so you dont have to run anywhere near the clearance also they have re designed the oil system which im sure helped to.

 

Well keeping in mind that "I'm just a welder", but have an investigative mind, I decided years ago to do a little test with the flathead (Donovan) that we were running at the time. We took dialbore gauge readings of the mainbore at 78* room temp, then closed the door of the Jetclean machine and ran it 15 minutes. Fast as we could we pulled it open and temp checked the block at 224* and the mainbore had shrunk .00035, which sort of suprised us. It would be interesting to know what the true temp of the main saddles were at the end of the track.
The idea for this test was done of course after several beers and someone wanted to bet the holes got bigger because "aluminum grows". I said "yes it does, toward any open area, so I'll take your bet 'cause I think it'll get a couple thou smaller", but I'll admit I was suprised it was almost double my guess.

 

Did you put the bore guage in the jet tank too?

 

I agree that the rod bearing clearance starts out at .0065 to .007 with new rods, but if you've ever measured them again after only one or two runs, the bearing clearance shrinks to about .005, which is about where you want it. At about 10 passes, it gets down to about .0035, which is why some racers re-size them or replace them at this point. I'm sure the aluminum is just stress relieving, which is causing the collapse of the big end.

The mains of the block tend to do that too, but not to the same degree. After a fresh align hone, the main bores, especially the rear, which sees a lot of heat, shrink down to about .004 or .005 clearance after 20 or so runs.

I don't think a blown hemi could ever be run with the light oil and tight clearances. We need the cushion of the heavy oil and larger clearances to withstand the high cylinder pressures and detonation that is prevalent with a screw blower or a high end roots blower.

 

I have been meaning to do a similar test bob but with a conrod and put it in an oven, i have been told that the tunnel size would get larger but i might do the excersize and see what really happens

 

Thermal Growth

Our experience is that the housing bore of a rod grows with heat. As a matter of fact an alcohol rod will grow something like .0025"+/- from 68 to 200 degrees F.

Brian
GRP Connecting Rods

 

Brian, good rods & am glad that the second set weigh in at the same as our first for a simple drop in exercise, even after 2 1/2 years.

 

Now that was :::::::::BOREINNNNNGGGGG.