Should I do a KLG4 Crank Swap?

[SuperK]

I was talking to my machine shop today (picking up some stuff) and I asked about his opinion on which crankshaft I should end up using. The KLG4 crank I have has been polished and he said that the condition of the crank is excellent. He said that it's a tiny bit loose but once I get him the rest of the components he'll know what that'll mean.

He suggested that just a tiny bit of clearance could be good, less risk of seizing on the bearings. If it's too lose I'll get the crank reground.

I explained how I had two cranks, the steel nitrided crank and this cast iron one. The machine shop does a lot of muscle car work and they're the best in the area. I asked him which one should I use, because I could switch before the assembly is balanced.

Here's his opinion on the matter.

"We use cast iron crankshafts on our v8 muscles. Cast Iron crankshafts are more forgiving than a steel crankshaft. It is a more flexible metal than steel and will allow more bend before it breaks. Especially a lower quality steel is more rigid and has a higher chance of cracking or breaks. In your application, a small v6, I would recommend staying with the cast iron crankshaft. The weight reduction will be worth it. The only time I would recommend switching to a steel crankshaft would be if you're running nitrous or large amounts of boost, which you're not.

Just make sure that your rotating assembly is balanced, which you are, and the engine harmonics are good and youll not have any issue with your crankshaft failing. "

I asked him about running a high RPM application and he said, "That doesn't matter. Most of your strain will happen if you're shooting nitrous or boost."

[stevesei]

Gentlemen,

The last post hit a nerve, so I looked up some crankshaft test literature from the Univ. of Toledo, done for the American Iron and Steel institute. It's a long report, but very definitive. The entire report is at this site: http://www.autosteel.org/~/media/Files/ ... ummary.pdf .

Summary:
Steel is stronger
Steel fatigues at a slower rate
Steel resists impact better
Steel resists bending stresses better
Steel doesn't allow crack growth as quickly
Steel is (or can be) lighter for the same strength

Personal note: Nitriding is a high end treatment not available on most cars. It is a GOOD THING.

Conclusion: Stay with the steel if it's a good crank. HOWEVER, Mazda started using cast iron, probably as a cost measure, but they wouldn't use it if it wasn't satisfactory (keeping in mind that most 626s are ATX family cars, not high performance in the least).

I'm sure someone will dispute this, but the facts are the facts. The "speedshop" had it exactly backwards, with their good results depending on the fact that most cranks are outrageously overbuilt, as are the stock K8/KL cranks. Oiling, bearings, and condition are much bigger issues.

[Pritchett]

Your right, the 626 isn't a high performance car, but the kl-g4's made 170hp, where all the other non jap KL's made 160-165. Also the kl-g4's were made in 97-98+ I believe, so one could say a successor to the kl engine family. I had pipe dreams of building a kl and had planned on using the kl-g4 crank, I say go for it and I eagerly wait your positive results

[MrMazda92]

The only reason the G4 was only used in the 626 is simple... The other KLDE cars were discontinued before it was produced... It would have been thrown in the MX6 and Probe GT otherwise, I imagine. While neither are "sportscars", they're both sportier than a 626.

I WILL be using a G4 crank in my build, although I'm staying NA with it(at least for now)

For what it's worth, Goldmember had a junkyard G4, and AFAIK the G4 crank was kept even after the Millenia S rods were installed, meaning it survived 500 HP just fine. Does anybody know off the top of their head if he used the G4 crank after he bought the Millenia S rods? I know he ran upwards of 340 WHP with the G4 crank. Hopefully that will ease some worried minds...

[fowljesse]

V8s make their power in the low end, having each power stroke hit the crank relatively far apart, causing flex in the crank. Since our cars make power higher up, there isn't as much twist. I think the G4 crank will be fine. I had this exact discussion years ago, with the same ideas. If I tear the engine down that far, I'd go with the G4, too. My rotating assembly is balanced far beyond specs, and I don't push the low end, so I wouldn't be worried. It's a good point about hpddwn, too. There was another guy, maybe Gold Member that had a boosted G4, or at least G4 crank. Do it, and report back!

[xxxmonoxidechild]

KLG4 crank FTW!

im the one that posted the comparison thread with pics showing the weight differences on PT a few years back. and i have a KLG4 crank in the KL hybrid im building.

[MrMazda92]

Hell yeah ;D Well thank you for posting it, It's one of many threads I've added to favorites, and poured through many times. Ahh, is that build posted here, or on PT?

[SuperK]

my KLG4 crank just got polished and balanced. Just waiting for the block to get overbored. Taking a while so I should have it in my possession maybe by the end of this month.

It's taking SOOO long.

[MrMazda92]

Believe me man, I can relate... I'm just ITCHING to get some progress on my build, but ever present financial strain is holding me back. I cleared my debts finally, and am slowly working my way towards my goal, but it's taking way longer than I'd like.

It's coming together, but I'm still only about 1/3 of the way there.

[Josh]

Gentlemen,

Conclusion: Stay with the steel if it's a good crank. HOWEVER, Mazda started using cast iron, probably as a cost measure, but they wouldn't use it if it wasn't satisfactory (keeping in mind that most 626s are ATX family cars, not high performance in the least).

I'm sure someone will dispute this, but the facts are the facts. The "speedshop" had it exactly backwards, with their good results depending on the fact that most cranks are outrageously overbuilt, as are the stock K8/KL cranks. Oiling, bearings, and condition are much bigger issues.

If you know any history on the engine and what Mazda did it was not cutting costs. Redesigning a new crank would have cost more after they had probably produced hundreds of thousands of them at that point, they created the cast crank just for this application. And continued using forged cranks through 02' in KL series motors just not the KLG4. It was to increase power bottom line, by lighting the rotating assembly. It is lighter there for reduces rotating mass and increases HP, bla bla bla and takes a way a bit of TQ. so what. a forged crank is more brittle as it is harder. It will withstand a lot more abuse due to this and takes a lot more to get it to the point of braking. The speedshop guys were correct in what they said as they have real world experience with the application, not looking at the properties and tables and evaluating the metals being used based on that alone. the forged crank is overkill and Mazda knew this thus they could lighten up the rotating assembly and create additional power. The cast crank will bend and flex more than the forged steel crank will, if you compared the two side by side. Not talking metal properties but the cranks as a whole as they were produced. that makes a difference. The forged steel crank will not it will simply break but its breaking point is a lot higher.

You are correct when saying the steel crank is stronger, but that is not where this was going, the cast crank is it better for the application and is still more than strong enough. that is what the speed shop guys were getting at.

IMO if you are building an N/A only motor I would go with the G4 crank all day long. Perhaps even knife edge it to lighten it up even more that is what I regret not doing to my BP build...

[SuperK]

well I got all my parts back from the shop. The balanced crankshaft looks a lot different. I weighed it, got some interesting results. The KL31 crank is about 43-45lbs. The KLC7 crank is about 38-ish. This crank, balanced with a clutchmasters flywheel and the k1 rods/wiseco pistons turned out to be ** lbs. I guess I wasn't expecting that.

[davmac]

Good information in this thread. Good luck with your build. You confirm an old thread that said same: klg4 crank (stamped klc7) is about 5 lbs lighter than the de or ze cranks. With lightened flywheel (and maybe for some lightened crank pulley) the differences should add up to a much more responsive engine - maybe even better fuel efficiency.

For posterity I'll post some of the pictures from the old post:
http://mazda626.net/topic/38012-oil-adv ... ge__st__30
All credit and respect to monoxidechild

kl31 crankshaft weight:


kl31 markings:


klc7 weight:


klc7 markings:


klc7 left and kl31 right:

[Josh]

Very very nice.

The only other thing you can do is knife edge it to reduce even more. on the BP you can shave like 2lbs off I would think on this crank you could get 3lbs or so.

Here is a pic off a knife edged crank I pulled off google pretty bad a--.

[SuperK]

I don't think I'd be brave enough to knife edge it. I already feel like I'm asking for trouble :S
it's not hitting the sump oil in these cars, which I thought is the reason for knife edging? I dunno, it didn't really look attractive to me.

[fowljesse]

If I were rich, I'd knife edge that crank, and save weight everywhere possible. Every lb you save off the rotating assembly is worth about a HP, and when added to the rest of the mods, multiplies a little.
That's the theory of my build; a lot of little things added up.