Don Terrill’s Speed-Talk

Has anyone done any testing with the lower viscosity oils and tighter bearing clearances?
I was trying to decide whci way to go this winter if its worth it power wise to use the lighter oils vs the heavier wt.
I never really seen much gains from the 0/30 to a 20/50 once the oil was up to temp.....But for a drag motor it maybe worth it because its a little tough getting the oil up to temp in the lanes....

You'll want to use the tightest bearing clearance you CAN use without any bearing distress.
Finding that point with your combination is usually a tedius process with a lot of disassembly and checking bearings.

Another variable is "has the block been line-honed?" I'd always be willing to accept a bit tighter main bearing clearance on a line-honed block than an unknown.

FWIW, some of the old OEM Shop Manuals are an interesting read: Main bearing clearance - .0005" to .0025".

thnx, jack vines

I do not agree with the tighter bearing clearance at all. Maybe for strret engines, but NOT FOR RACE engines

JOE SHERMAN RACING

I've had experiences that make me a believer in looser bearing clearances. The downside might be a low oil pressure reading at idle, that's mainly a problem if the car winds up in the hands on an average non car guy Problems don't occur at idle but when the engine is stressed and bearing bores don't stay round and pinch bearings!

rmcomprandy wrote:You'll want to use the tightest bearing clearance you CAN use without any bearing distress.
Finding that point with your combination is usually a tedius process with a lot of disassembly and checking bearings.

I agree with above post.

On Cup engines we work real hard on less clearance to control leakage better. 1.5 to 2.0 is normal on our 850+ hp 9500rpm engines. Our oil temps do run higher then a most for a longer time.

whtrthanu wrote:a drag motor it maybe worth it because its a little tough getting the oil up to temp in the lanes....

He said it's a drag racing motor, I agree with Joe Sherman on this issue.

For the life of me I can not figure out on a drag racing motor why anyone would want those tight clearances. Stay in the .003 for mains and .003 to .004 for rods and the bearings will always be happy.

That's my experience anyway........blessings........Ron.

Ron C. -
EVERYONE, including Joe, is entitled to their own OPPINION but, when actual test results differ from your theory; I believe the results.

Barbapapa -
IF the components were stressing to the point of distortion you had better believe that a "pinched" bearing WOULD show some sign of distress upon inspection.

Using tight bearing clearances is not to be ventured into with a blind eye; it takes a lot of experamentation to get it just right FOR THAT PARTICULAR COMBINATION.
If all you are looking for is a rule of thumb - I wouldn't go down that avenue.

I use .0025"-.003" on about everything I do. IF the block has been line honed I might go as tight as .002" on the mains. But I don't like to.

Don

On small blocks i go .003" on the mains and .0025"-.003" on the rods, i prefer the rods to be in the middle of that range

With clearances looser you want a higher volume pumpe but with them tight you dont want a high volume pump

Randy, If I am wrong tell me.
Drag racing has a whole lot of cold oil hence the bigger clearance.? I know mine doesn't see hotter oil until I reach the finish line. I feel the point's from the other poster's are well taken.

I also agree if you have the proper oil temp you could close the clearance as said.

So, what did I just say?

The better the parts are, the tighter you can run them. And there is advantages in running tighter clearances/thinner oil both from a HP and durability standpoint.

This thread will either get interesting or die real quick!

Hmm.. lest see....

$13,000+ engine vs. MAYBE a few hp through better oil ctrl...

Without a NASCAR engine building budget....I'll take the larger clearance.

stealth wrote:Hmm.. lest see....

$13,000+ engine vs. MAYBE a few hp through better oil ctrl...

Without a NASCAR engine building budget....I'll take the larger clearance.

X 2 !!!