Stan.Shaw@Excell.Net

I think Joe has 1 year on his motor and it is being rebuilt. Mark A. had near 10 years on his. Also Joe's new motor came on to the scene after I started accumulating my build parts. I am not saying his motor isn't better, just that I prefer the more proven path.

slate blue

Sterling I completely understand what you are saying, I was coming off the statement below by Stan.

I would have thought external balance is not as good as internal balance as the vibrations at the point of imbalance don't get countered till the rear or front of the crank, I would think this is not good for longevity. I think Stan is going the right way finding lighter componants.

On another note I think there probably is no definative answer to which is better as far as crank design goes, 8 cwt for the street and constant higher revs, versus a lighter 6 cwt for racing quicker response more variable revs. Maybe I was too cautious with my design as I have cut a huge amount of rotating weight out and as such I could have gone down the 6 cwt path with safety. Live and learn! My rotating assembly is still lighter than stock by about 2 kgs and the weight that is spinning at the greatest distance from the centre point of the crank is very light, hopefully there will be a benefit there. I would still like to know what a Moldex 6 cwt crank weighs and what a 8 cwt Scat/ Devek crank weighs.

Greg

SwayBar

I think Mark's previous motor was 4 years old.

GregBBRD

I have new Moldex cranks in stock, so I can weigh one of those. I have an 8 counterweight Scat crank, that I can weigh, too. I'll try and do this tomorrow.

Joseph's pistons and bores look exactly like they did when the motor was assembled. The Nicosil is holding up very well. That is not the weak link in that engine.

Mark's engine was 4 years old when he took it out, redid his car, and put it back in. That was over 4 years ago. He hurt the rod bearings the first weekend in the redone car....and we "rolled" in a new set of bearings and sealed it back up. I'd think his engine is somewhere between 8-10 years old....with a couple of sets of rod bearings thrown in. I know it has many, many hours of use on it. Interestingly enough, that engine used the "original" Moldex crank that had slightly different stroke and used 928 rod bearings....which are pretty soft and fairly easy to "hurt".

gb

slate blue

Looking forward to hearing what they weigh, mine is apparently 62 lbs.

Thanks Greg

928SS

don't want to restart that debate again... but fwiw, 911 rebuilders, motorcycle/snowmobile (2 & 4 stroke) and avaition industries have been using that setup for over 2 decades, and many auto OEM's also use AL forged slugs w/nicom plated bores (nissan, benzo, bimmer, even that monster 3.8L 997 twin turbo). the nicom has better wear and olification properties than alusil, but is more costly from a large scale manufacturing perspective. course I doubt anyone would notice the diff when driving as much as in the wallet

some interesting short reading re: nicom and forged stuff:
http://www.motormeister.com/suz/perf...erf_racing.htm
http://www.lnengineering.com/type1.html

even the porsche CGT:



a 62lb crank? holy lump of 4340!! about like the 454 crank I used for a mooring for my boat in baja

dr bob

With a couple seized rods on the end? Call that a fluke?

Stan.Shaw@Excell.Net

I am just waiting for all the good 928 track experiences to be touted before I go in that direction. In the meantime I will stick with what has been proven to work in the 928 world, and hope to complete my build in this lifetime. Not sure when that means my nicom build will happen...

928SS

PO spun a rod bearing on a suburban, I bought for 1k / built a monster baja 'burban kinda like my 928 story... buy it cheap and dump a ton of $$ into it, hahaha

GregBBRD

Scat 8 counterwight crank....with Mallory.....65.5 lbs.
Moldex 6 counterweight crank...out of box....60.5lbs.
Carrillo "A" beam rod....597 grams.

gb

slate blue

Thanks Greg, appreciate the input and we have probably all learnt something here. So I wonder what that Moldex 6 cwt cranks ends up at when balanced? I would imagine that it could end up at 61 lbs, if that is the case I have done alright at 62 lbs.

Also when I was trying get to sleep pondering my decision about the 8 cwt thingy. I thought that it is probably still a good but conservative decision. The reason for this is that the weight spinning furthest from the crank centreline on my crank is the lightest of all 928 cranks. The reason being the Honda journals are only 1.888" versus 2.10" We also need to add the rotating weight of the rods too. My rods which are 6.2" weigh 542 grams. The stroke is shorter to at around 3.55" . So weight distance and speed are big factors in mass force.

I think the reduction in weight will be a blessing for a long block life. Joe at Moldex said and I must admit I understand the principles of balancing but not the finer details, that my bob weight was around 1500 and he said that most of these 928 stroker are approx 1750. If that makes sense to anybody please explain it to me. The reason I say that is my pistons, rods weigh in total 1083 grams. So where does 1500 grams come from?

The other factor that I had completely factored out and shouldn't have was the gyroscope effect of the crank on road performance, obviously smaller weight the better, this is why I'm sure Porsche is paring grams of that GT3 engine of theirs.

Greg

Stan.Shaw@Excell.Net

Greg G.,

If you look back at my first post, you will see what numbers are used in arriving at the bob weight. The weight would be higher because the rotational weight of the rod is doubled when included.

slate blue

Thanks Stan for lesson 101 in crank balancing, I added mine up roughly and got to what they told me around 1520 ish. Have no idea why the rotating part of the rod is added twice but no worries, it does make it clear why you need those lighter componants and really it is the only way to go, filling it up with lead so to speak would be a real shame. Have you had any more progress?

Best of luck.

Greg

Stan.Shaw@Excell.Net

Lesson 101 must be what I have learned in the past week then

I have been looking for the actual calculation, but haven't found it yet...

I am investigating the Carillo rod option first and am looking for the bob weight of the 597g rod.

Stan.Shaw@Excell.Net

Actually the machine shop questioned whether it could be balanced at all internally due to the short counterweights.