Front 2 cylinder bearing issue

We run the M soft rod bearings. We made two passes on new bearings. Tune up was 26 degrees on a BAE Fat Head. Good runs, plugs look fine all across, egt's were fine. However, the front two cyclinders mashed the rod bearings. Number 1 was mashed .009 and number 2 mashed .006. All the rest were fine.

I can not explain why only the front two look bad. The plugs look the same all across the cyclinders, plenty of cad left. And the egts never got above 1100.

Any ideas on what could cause the front two cyclinder to do this? I know the blower forces more air in the front two and therefore have bigger nozzels. Could this all add up to just more load on the front two?

 

Are you using a screw blower?

 

no, Kobelco 14-71 Hi Helix Superman at 34% overdrive. It is making 40lbs of boost. Blower is not set back.

 

Maybe you hydrauliced the two front cyl? and i would use the v bearings

 

thats what i was thinking, did any crew members notice any wetness to the headers on the run, mostly leaving the starting line.

 

the pipes are not that wet at launch. The egts leaving the line show all the cyclinders firing, non dead.

 

OK, if it was an oiling problem the bearing would be burnt, If it's not then it has to be hydraulic or piston hitting the head or valve hitting the piston, or piston to tight in the bore,ring pinched, Thats every thing I can think of.P/S if you have a lot of cad then it's to rich,

 

Could detonation cause it Gene? I mean, I don't know why only the front two would do it, but is it possible?

 

He will have to pull a head and look at the pistons, but i'm not to sure about that the motor is rich so it shoulden't detonat, pluss he don't have any timing in it.

 

Do any of the plugs look oily? Possible front seal leak in the blower feeding oil into the front could cause detonation?

 

All good questions and comments. However, no oil on the plugs. And when I say some cad left, I mean just a little is left. I could lean them out more, but I do not want to until I get this solved.

I guess I will just fatten the front two up a lot and do a couple passes and then look to see if it gets better or worse.

 

I think I would lean the front two.If it is to much fuel and its squashing the bearings,I would think it will get worse if you add more fuel.Just my $.02.

 

This was a very common case in the begining of the front opening hi-helix. Plenty of cranks were blackened in the front journal. Cure was to go up at least .005 on the front holes. Stuff like this was one of the reasons for the advent of the set back blower (to dump into the center of manifold).

 

What is the chamber size of that style head?

I agree with Bob Meyer. I think you are lean on those front two. JMO

Rapid

 

What do you mean go up .005?

Set back is not a solution unless I get out the saw and start cutting out the windshield ;-).

 

What is your compression ratio,we ran BAE 5 heads and could not get away with running any less than 11.2 to 1.We actually found that 10.8 was were we ran the best.We also ran .020 shorter connecting rods in 1 and 2 to help band aid our problem.Since then we have learned some guys are running dished valves and 1.6 ratio rocker arms on thier problem cylinders.

 

do you have any crush on any other cylinders at all if not i think i would fatten the front 2 cylinders up .005 sound like a good start like bob said but i believe i would pull the head just to look at the pistons to make sure there is no detionaion problem are hitting the head are back up on the main till it just starts to burn the cat around on each electrode and check the bearings if know crush then it was to lean you sure dont want to loose any parts

good luck

 

take a half a point of compression out of the front 2

 

Richen # 1 & 2 cylinders by .005
Also what was the last tuning change you made before this problem started happening?

 

Hmmm...

I'd try to figure out how to make the rest of them push. M bearings and 5.50's equals 3-6 wide after each run on the safe holes. I use V bearings.

Don't take the cylinder temps as gospel, either. I've widened M bearings 30 (yes 30) on a run that the temps never topped 1,000 degrees. Look at the flow meter, check your CC's per turn to verify that your system is linear and don't forget to make sure your fuel pressure doesn't hit the floor at the hit. If your fuel pressure hits the floor at the hit your nozzles will dry up for an instant at the swap of the feet and knock the rod bearings out of it before the headers cross the starting line.

RG