Don Terrill’s Speed-Talk

What would be optimal bearing clearance on an 1.88 rod journal with GRP aluminum rod on an all out race motor turning max RPM of 8500

I think the word "optimal" may be limiting responses.
Maybe you should post your question in regular engine tech for more views?

WOW?

af2 wrote:WOW?

Please explain.

>0025- .0035.

AF2 : I have GRP rods as well in at .002" or a shade less.I was trying to help the thread.
I have not seen my bearings yet.

Michael Giannone who owns MGP rods and used to own GRP rods recommends .003 to .0035 for aluminum rods...period. Personally since I run his rods I always shoot for that clearance and the bearings always seem happy.

Blessings......Ron.

We just dis assembled our motor with MGP rods, .0035" clearance....bearings were coated, 160 passes and a ton of dyno pulls several weeks ago to work on carbs....... The bearings look perfect !! best we have ever had....... Wished I hadn't taken it apart....... Wanted to change rods........Motor was making 1220hp........ Reher Morrison likes the wider specs. also

Thanks for the info just mocking motor up and had .0037 i thought it was a .0005 on the fat side but talked to some eperts and they said .003-.004 with my combo .

I always like that "little extra" that seems to be too much on any Alum. rod because they always settle a little between the cap and rod in the serrations. That chaffing action between the cap and rod will close up the clearance just a little after they are run. If you are setting it up to run with REAL thin oil you might want to tighten up the clearances some. I've seen engines with loose bearing clearances loose hp on the dyno when real thin oil is tested. It has to do with the film strength of the oil and the crank contacting the rod bearing causing a little drag.

How do you know that the HP loss was not from increased leakage and windage? Was there visible evidence on the bearings?

I still say WOW.

What all out race motor are we talking about?
Blown Alky likes a different rod clearance than a n/a gas deal.
How can you know? Not knowing what motor is being talked about.

No loss of Vac... so not extra blow by... Take the thin oil out and the power came back. I made a few more pulls checking a few other things and quit the session. I've done that test several times with customer engines and seen the same thing a time or two. Esp with big inchers... one of My own little SS/Comp V-6 engines did it too. I took it apart after I took it off my Dyno and the bearings did show signs of bumping the crank. I also called one of my buddies that works in an engine shop for a cup team and he said all their engines that use thin oil are set up with tighter bearing clearances than the engines using heavier oil. He said they had seen exactly what I had noticed. I have also had a non coated cam bearing stick with thin oil. I think Too much load with the heavy roller springs pushed the thin oil film out. I think a tight Jesel belt contributed to te load. Thats the only explanation. I polished the cam and put coated bearings in, ran the 00 oil and had no more problems. I'm pretty sure just puttimg thin oil in an engine doesn't mandate an automatic HP increase. I'm sure you should build the engine esp. for thin oil to get the most benefits fom it.

Thanks for the reply.
BTW I think Super Stock rules. I love those cars.

I've seen engines with loose bearing clearances loose hp on the dyno when real thin oil is tested. It has to do with the film strength of the oil and the crank contacting the rod bearing causing a little drag.

I have not seen to date any engine lose hp from wider clearances as opposed to tighter ones. I do see in some occasions slight hp loses from lighter viscosity oils when using dry sump pumps due to the inability to create the vacuum that a heavy oil creates when going through the scavange sections.