Pondering crankshafts....

[Joey120373]

I know this has been discussed before, but I can’t recall the details, or the thread that it was discussed.

With the 4.375 offshore cranks drying up, I’ve started thinking about billet cranks.

So, questions to the experts:

1, what is the longest stroke that can easily be stuffed into a BBM block?

I think I remember Jay saying that he did 4.5 inch?

2, billet cranks cost ? Iirc they start around $2500?

2a, if they start at ~ $2500, what features can they add on that drive the cost up from there. More time on the machine machining the weight down comes to mind but that’s all I can think of.

 What would you build with the following criteria :
1, custom billet crank
2, max cubic inches easily attainable ( not a ton of grinding )
3, no exotic high dollar rods ( or is it a fools game to spend 2500 on a crank and not spend 2500-3000 on rods ?)
4 street strip application, pump gas motor with a 6500 rpm  limit

[blykins]

I think you'll pay more than that for a billet.

Scat will make you a custom forged crank for a hair less than that. 

At 4.375-4.400, you start having to look at how much rod/cam clearance you have and may have to start grinding on the rods.  You don't have to worry about the block itself, but it's the rotating parts that will give you fits. 

If it were me, I'd do a custom Scat 4.375" stroke.  You can get at least 527 cubic inches that way.  No need in paying ultra bucks for connecting rods.  A set of $625 K1 rods will keep up with 1000 hp. 

[jayb]

I started out with a 4.500" stroke crank in my high riser engine, but didn't like how much I had to shave the Crower rods to clear the cam lobes.  Figured the cam was more important than the extra stroke, so I backed of to 4.375".  However, this was also with big block Chev rod journal and connecting rod.  If I had gone with a 2.100" SBC journal and rod, I think the 4.500" stroke would have been fine with the cam I picked. 

At the PRI show Molnar said they would do a custom forged FE crank for $1800.

[Joey120373]

Thanks for the info, re-read my post and realized I was making a bad assumption.

If I recall correctly someone asked a Scat rep about custom forged cranks, and they said 4.375 was the max.
I had not considered that other companies might do a custom forged unit with a longer stroke, and for my purposes that is a much more reasonable option.

So, if Molnar ( or Scat or brandX ) can do a custom stroke, a 4.5 inch crank with 2.1 rods might be something I would consider. Provided the rods are in the ballpark, say $1k give or take.

After seeing Brent's comments on the quality and weight of the Molnar rods, I have pretty much decided those will be on my parts list.

 

[blykins]

I don’t think it will be cheap to find a SBC rod in a 6.700” length.  I know Oliver makes them.

[Dumpling]

What is this engine going to be used for?

Why the emphasis on max-stroke?

[Joey120373]

. What is this engine going to be used for?

Why the emphasis on max-stroke?   

That is a good question, and one I have been wrestling with for some time now.
Using big standard easy stuff I’m looking at a 505 inch motor.

I can get an extra 15 to 30 cubes going to longer strokes, not exactly a huge jump in displacement.

But it keeps nagging at me to A, do something a bit different, and B, I don’t want to get it all built and constantly think how I could have made it better if I had just spent a little more . ( if that isn’t the eternal rabbit hole I don’t know what is...)

How much more is the question though. This engine will be going in a 4500-5000 pound truck, so the more cubes the better. And I don’t plan on spinning the thing to the moon either, I want low to midrange grunt. All this points to more cubes. When the offshore 4.375 crank was still available, it was kind of a no brainer. A forged 4.25 crank is going to cost $1000-1200 easy, if I can get a custom one for ~800 more that seems like a reasonable deal to me. And if I’m going custom, go for as much as I can get. Brent brings up a good point though, custom rods get real expensive real quick. I sent molnar an email, hoping to hear back from them soon.

[WConley]

There are some downsides to maximizing stroke.  The amount of piston skirt below the oil ring goes down, which reduces piston stability/ increases blow-by.  The bottom of each cylinder may need notching as well, to clear the connecting rod.  This makes the piston stability problem even worse!  I wont even get into piston speed...

The shorter rod really amps up the cylinder wall thrust loads, so beyond a certain point you'll need a good aftermarket block.  There is less overlap on the crankshaft as well.  This reduces strength and increases flex.

All of that stuff can be managed with good parts and sensible preparation.  Just remember that more stroke is not free from drawbacks.  I would go with a fully engineered, proven stroker kit.  You'll save yourself way more in the long run.

Here's a decent article on the subject, with advice from some big guns (Jon Kaase and Sonny Leonard to name a couple...)

https://www.enginebuildermag.com/2008/03/stroker-tips-from-the-pros/

[Joey120373]

The shorter rod really amps up the cylinder wall thrust loads, so beyond a certain point you'll need a good aftermarket block.  There is less overlap on the crankshaft as well.  This reduces strength and increases flex.

Got the block covered, i hope, poor BBM block has been sitting in its crate for about 3 years now.....

Thanks for the advice.

[Joey120373]

Got a reply back from Tom Molnar, a reply same day, so far big +1 for Molnar.

He said that they don't offer ( or rather haven't offered ) FE cranks but do offer Rods.

He warned against using standard BBC rods as the offset was wrong and would not center the rod in the bore, good info.

He also cautioned that a smaller crank pin would make the crank more flexible and prone to cracks/breaks.

Feel free to correct me here, but my thought is on a sub 6500-rpm motor, that will not be raced ( much.... ) a .100 smaller rod journal would not really matter all that much.

So I am still pondering it, but I now leaning towards a scat ( light weight? ) custom forged 4.375 unit and Molnar or K1 rods, very much similar to Brent's 449 build here :

http://fepower.net/simplemachinesforum/index.php?topic=8911.0

of course i am assuming that the Scat custom forged crank used in this build falls in the price range Brent eluded to earlier. More cubes and a lighter rotating assembly might well be worth it.

[blykins]

Tom tends to err on the side of caution.  Sometimes too much I think.

We all have done FE cranks with SBC rod journal diameters.  As a matter of fact, there are some custom FE cranks out there running SJ SBC and Honda journals.   I wouldn’t say it’s common but it happens from time to time.  The only issue is finding a rod.  To my knowledge though, Oliver is the only one making long SBC rods. 

As for the standard BBC rod warning, practically every aftermarket FE rotating assembly kit uses a standard BBC rod and there are zero rod offset issues.  I don’t think he’s aware of how. many. thousands. of those combinations are out there running flawlessly.  This topic comes up once so often on some of the other forums and it seems to be one of those things that guys run to fix but there’s no problem to fix.  It’s a non-issue. 

In terms of strength and weight though, you can’t beat a Molnar or K1 rod.

[Joe-JDC]

One thing I don't hear discussed with all these stroker combinations, is the fuel mileage.  I know the gasoline prices are down somewhat at the moment, but if the prices were to go back up much, then mileage would be a real deterrent in my opinion to these maximum stroker kits.  I would think spending money on a smaller, efficient rebuild, and then adding a transmission/overdrive would be the way to go with towing, or hauling a load occasionally.  Don't get me wrong, I am building a 530 FE myself, but not for street duty, and I certainly would not like to feed it on a regular diet of 93/race fuel for the street on a regular basis.  Trucks worked fine for decades with 360/390 cubic inches and pulled their weight easily when built for the job.  I towed a 6400# travel trailer all over the USA for several years with my F-150 with 351M/4 speed OD, then 408C/ 4 speed OD.  Just trying to see the reasoning behind maximum when 100 cubic inches less would do the job with lots better mileage.  JMO, Joe-JDC

[jayb]

Got a reply back from Tom Molnar, a reply same day, so far big +1 for Molnar.

He said that they don't offer ( or rather haven't offered ) FE cranks but do offer Rods.

He warned against using standard BBC rods as the offset was wrong and would not center the rod in the bore, good info.

Hmmm, maybe I got them mixed up with somebody else.  Callies, maybe? Some fairly well known reciprocating assembly manufacturer came by my booth last year at PRI and told me they were able to do the FE cranks, at what sounded like pretty reasonable prices.  Wasn't Crower or Scat or RPM or Eagle, somebody else...

[Royce]

Probably Sonny Bryant...

Molnar? Moldex?   Moldex does our Y Block cranks  (allows die grinder rpms)

[475fetoploader]

The last time I called Winberg I believe they would make any FE crank I wanted for $2800 + balancing.  Don’t quote me on that though please.  In my opinion, it would be difficult to find a nicer looking piece.