Well I have been staring at my rod intensely the con rod that is, anyway there is another issue that should be known, at least it is an issue when you use Honda Nascar rods, maybe normal chevy rods too. Because the bore spacing is different on the Chevy versus Porsche. The porsche is 122 mm V the Chevy at 111.8 mm a difference of basically 5.1 mm per rod per journal.

So to get the rod centralized in the bore I decided to turn the rod around and have the opposite side chamfered so that it doesn't catch the radius of the crank and this is all fine except for the bearing will be in the way of the chamfer. So to solve this problem I am going to have the rod grooved and the bearings moved over just enough so that they don't catch the radius. So this means that the bearings are as close to the outer side of the big (near the radius) end as they can be, this takes them slightly off centre from the beam of the rod.

It is off by about 0.75 mm certainly less than 1 mm, the bearing moves more towards the central part of the crank which is good as this means it is centralized in regards to the oil flow. Now my rod bearings are 20 mm wide exactly. I think the Porsche standard bearings are 23 mm from memory. My bearings could be theoretically wider, maybe 21 mm but what all this means is by having a crank throw that is narrower (in terms of width not diameter) than the standard crank, it seems to me that it is impossible to get the bearing totally centralized under main beam of the rods.

This may not be a big problem, it is certainly something I didn't know, you could use a narrower bearing as Nascar do, some of their rods are only 0.840" whereas ours are 0.940" but this means even smaller bearings and I think more bearing width is better than less. Maybe this factor plays into the side loading issue, I suppose it depends on which way the rods are run or if you are using the "special" Carrillo rods. Anyway just thought I would advise of the issue.

Greg

 

after all this, now I am curious as to how many strokers are using Chevy rods in them without checking these rod issues!?!?!?!


Wonder if the engine owners are aware of this......



(if their cranks were cut for the [unmodified] Chevy rods, then their rods are offset, but I still wonder if there any complaints/incidents of this affecting the NON-extreme HP strokers that have already been built---?)







--Russ

 

Hmmm...this is the stuff that makes building engines so very tough...and it is just left over stuff from the first guys that built these stroker engines. Heh, I think they did a good job..for the first time through. However, it's alwasys the same...one guy does one thing and the entire pack behind him does the same thing..over and over again.

Greg, you are getting close to making that rod fit...just a few more modifications and it will work. The old "H" series Clevite bearings came with the entire edge of the bearing cut with a champher, so the bearing would not hit the radius. How about that approach and leave the bearing in the same location? Pretty easy to make a little fixture to modify the bearing, in the lathe. The newer "HN" series, they just left that entire portion of the bearing out...backing and the radius...why have the backing there, if there is no bearing material above it? I doubt that making your bearing slightly narrower will hurt anything.

Here's a great story:

I was with a team, in 1993, at Daytona practice. We were running a brand new964 RSR. I was in charge of car set-up. I decided that we only needed enough wing to hold the car on the track safely...no more. We kept backing the wing down, lower and lower...and kept going faster and faster. The drivers resisted, because everyone else had their wings cranked up as high as they could get them. However, we worked on the handling, to make the car stick with suspension changes and kept lowering the wing. The car went faster and faster. Someone on one of the other teams had a radar gun and they went out and recorded all the speeds at the end of the straightaway. Turns out we were 5mph faster than anyone else. The officials were advised and pretty soon there was a giant meeting and an inspection of our car. Porsche Motorsports checked the engine. The car was re-weighed. They checked everything they could. Finally, in a big meeting in the garage, one of the other team owners finally asked, "Well, how do you explain being 5mph faster than anyone else here....if your car is the same as everyone else's."

This is one of those moments that sticks around forever....

I looked down this endless line of RSR's backed into the garages, with their wings stuck at crazy angles, smiled, and said "I have no clue...maybe we just have faster tires."

Here's the really crazy thing about this whole story. I went back the next two years, and we did the exact same thing....no one ever stood back and looked. We were faster every year we went! It was stupid.

Well, here's what I've done with the 928 stroker engines: I threw the entire mess away and started with a clean sheet of paper. New cranks, new rods, new pistons. New headwork, new cams, new manifolds. They are some better, now. Certainly not perfect...because an engine is never really done...just some better.

 

By GregBRRDThat is a good story, that is the crazy part, when Colin Chapman invented the ground effect car, at least he tried to throw them a red herring, he said it was secret diff btw.

As to the rod mod, I will do what ever is the easiest I suppose, I do think that moving the bearing has its benefits as the bearing moves to a more central part of the crank, this will be good for the hydro dynamic wedge, i.e better oil pressure, I mean I don't want to fix my rod centralization issue and cause another one in the name called oil pressure or lack of it.Remember I will try to run a thinner oil, I am thinking 15w 40 at this point.

How wide are the chevy bearings that are used in the 2.1" rods? You are right about the chamfer, the bearings I have are CB 1663 H.

Cheers Greg

 

I'll measure the width of a "new" style "HN" bearing, for you, when I get by work.

 

Here's a pic of what Greg is referring to with regard to using stock offset SBC rods in the 928.

Sorry for the crappy resolution of my camera phone.

This is the 928 Int'l Scat Stroker crank using Oliver Rods sourced by Motorsport SLC.

This first pic has the digital caliper jaws spread at .940". It doesn't appear on the caliper screen because my battery was low and the reading was flashing. I tried a few times but couldn't get the timing down to catch the reading in the pic. The centerline of the two rods line up at this spec, which according to Greg is the SBC spec.



Here is a pic of the where the rod orients itself on the wrist pin on cylinder #3.
As can be clearly seen, the rod is closer to the rearward side of the piston leaving more wrist pin exposed on the forward side of the piston.




#7, which is the matched rod pair is just the opposite, with the rod closer to the forward side of the piston and leaving more wrist pin exposed on the rearward side.

So on the power stroke, combustion will have more leverage on the forward half of piston #3, and on the rearward half of piston #7. If the small end of the con rod acts as a fulcrum, then the #3 piston would rock forward in its bore and #7 would rock rearward. By the same token, con rod #3 would be leveraged forward and #7 would be leveraged rearward.

The insane rod ratios on these strokers already seem to defy physics. Now to have this extra stress in the equation, it is really a wonder that the strokers based on this model run with any consistency at all.

Is this ideal? No. But there enough of these motors out there running this configuration that I'm not concerned about it for my personal motor. If I were building a motor for someone else I would definitely make the revised rods a part of the program.

 

Adam have you tried turning the rods around? If you did that other than for mock up you will need to machine the rods to clear the radius. Also if you decide to do mock up be careful not to damage the bearings.

Greg

 

I haven't tried turning them around, but I don't think you could get an accurate measurement without radiusing them for the fillet first (like you mentioned). Can you even have the two radiused sides facing each other (if you were to turn them around)? I kind of assumed that the non-radiused sides of the big ends were machined flat to act as a thrust surface.

BTW, anyone have the tolerance spec off hand for the gap between the rods?

 

Adam as far as I am aware, the thrust faces on both sides of the rods do the same things, the chamfered side runs on the big end of crankshaft thrust surface, so this is no different to the rods running against each other, it may even lower friction, there is no real side loading anyway, it is more of a guide anyway, remember that Nascar use guided rods, so the rods are kept in line by wider small ended rods, as such the side loading can't be too great because if it was the rods would flex as these rods are over six inches long.

If you like you can just work it out with a set of calipers. Is your block Nicasil or Alusil? Adam I know what your on about being a hassle building these things, have to wonder about the friction if all these forces are as great as they are supposed to be. Friction may have been the down fall of a few strokers, I think this may have been Devek's secret to longevity, when you think about it, the close tolerance that Marc specified for the piston wall clearance which was as far as I was aware was at the small end of the Porsche clearance, this tight clearance kept "potentially" everything pointing the right way so to speak.

Just an opinion.


Greg

 

Adam:

That's the problem, exactly.

Greg:

The "new" HN bearing is actually 20mm wide, also. Apparently, they must have moved the tang over on the bearing. If I measure from the edge of the bearing, I measure the center of the tang at 3.3mm.

 

Although I am not building a stroker, I would be interested in seeing these data.

 

So I just read in an automotive mag that the original Boxster (M96?) motor had the wrist pins offset 0.8mm on the rod? Anyone familiar with this?

 

common to have the wrist pin offset in the piston it reduces piston slap NOT common to have the rod not centered on the wrist pin.

 

Louie,

Do you know why the HP/TQ cross at 3800RPM VS 5252 RPM on this chart..? Just curious..

 

Probably because the scale is different for Torque and Horse Power....
This is pretty common when there's a big difference between the two.