NA crank question.

[1 OFF GN]

I'm currently getting my turbo V6 rebuilt. The car had been rebuilt at one point and the engine builder found a NA crank in the block.. The engine has .030 hypereutectic pistons. Stock rods stock heads with stock cam. The NA crank needs to be cut 20/20. My question is will this crank be okay in this engine if I'm running my stock turbo and stock intake etc. I'm building this car to be a cruiser not a rocket ship. Will this crank hold up? Or should I find a turbo crank? Thanks Dave

 

[mikestertwo]

A NA crank will hold up just fine. I have built several turbo motors with NA cranks making some pretty good horsepower with no issues. I don't like to cut more than 10/10 but if the car is just a cruiser you should be ok.

 

[NY Twin Turbo]

A NA crank will hold up just fine. I have built several turbo motors with NA cranks making some pretty good horsepower with no issues. I don't like to cut more than 10/10 but if the car is just a cruiser you should be ok.

I respectfully disagree. Just because.

 

[GNTIM]

GM put a different crank in the turbo motors for many reasons. I unknowingly ran one for a while in a used engine I bought. When I sent the crank out, the machinist said "it looked like a city road map" when he tested it.

 

[dank GN]

Why would you knowingly run a NA crank ? They are weaker then the already weak turbo crank and the one thing to go wrong on these motors is crank flexing . I've gone through 3 and and the crank was flexing so bad it was fretting my mains . So I personally wouldn't run a NA crank . Here is acouple pics of what a flexing crank could do . Also I would get 2 billet center caps minimum if you are rebuilding it . Of course this is just my opinion and you should do what u feel is the right way to go with your build .

 

[boostjunkie]

I built a stock motor with a NA crank 14 years ago . The car is used as summer driver and still runs great to this day .for what it will cost to cut the crank you have I would try to find a crank already cut at .010/.010 and go

 

[Anthony P]

....for what it will cost to cut the crank you have I would try to find a crank already cut at .010/.010 and go

have to remember that the NA V6 engine had a 2bbl carb and was rated at 110 HP.

an intercooled turbo Buick engine in stock trim produces about 290bhp.

both NA and turbo stock cranks were the same "229" castings. NA cranks had rolled fillets on the mains only. Turbo cranks had rolled fillets for BOTH the main and the rod journals.

Remanufactured cranks may have had many or most of the rolled fillets in the mains and rods repaired one or more times and it still qualifies as a "good service part."

I would be very picky here. You have to confirm with the shop what the cost is to prep that NA crank. At 20/20, it is done for any future work.

My suggestion is to pause for a moment. Search for a turbo crank that has been turned no more than 0.010" and has not had any rebuilt journals. What is that crank going to cost you compared to the machining cost of that NA crank? If you're within a hundred bucks, I would go for the turbo crank.

Will the NA crank work on a street car at stock or near stock power/boost levels. Probably not an issue. stock stuff is maxed out at 360bhp.

Again, cost is the key driver here. Why spend potentially more $$$ to grind and polish a NA crank at 20/20 when you might find a ready to go turbo crank for similar cost?

Billet main caps on #2 and #3 are not needed at stock or near stock levels. once the combination starts hitting mid-400 to 500bhp then billet caps are most necessary plus a girdle as the main webbing is the weak spot on any of the stock blocks.

"EDIT ADDED INFO: besides a turbo Buick stock crank, the stock crank in the '89 turbo trans am will work also, if you can find one."

Nick Micale noted the same concepts in this old thread. old thread yes, but the information does not change. there is a ton of info to be learned by just searching the archives here.

http://www.turbobuick.com/threads/rolled-fillet-crank.45294/

 

[bison]

NA crank that has been magnetic particle checked and has good thrust and journals is fine. A rolled radius crank has no benefits to reducing flex of the nodular iron crank. They are all going to flex at higher output levels. I've seen cracked caps on stock engines. The most common failure point I've experienced on stock engines is the pistons. Next is the block and caps due to flex. I've never had a crank failure. These examples all 700+hp. Had a car I tuned and ran multiple mid 10 sec passes and when we were updating the shortblock it had a non rolled radius crank in it. I've known of a few .020/.020" stock cranks that went 9's. The most important part of the longevity is the tune.


Sent from my iPhone using Tapatalk

 

[750H.P.V6]

Brian,

I agree with you that the tune is the most important factor in regard to engine longevity. I've only pulled 1 broken crank out of a Buick V6 motor over the years and it was a N/A crank that had been installed at some point. Personally I'd get a rolled fillet crank out of a 4.1 liter motor rather than install a N/A crank or spend the $$ to turn one that will need to go 20/20. As far the overall strength of the cranks both the N/A crank and turbo crank have the same metallurgy, the rolled fillets on the rod journals were added to the turbo crank to increase the longevity. A good used rolled fillet crank shouldn't break the bank cost wise and is the better option.

Neal

 

[Nick Micale]

I had a BMS forged crank break on the dyno at 3500 RPM, seen TR leave his crank on the track after a couple 13 sec. passes, so a 30+ year old, .020 stock crank in a new build is certainly a "crap shoot" at best!

 

[NY Twin Turbo]

Just because.............

If you love the car the way it was made, and you plan to keep it as your baby, and you don't want to upgrade because it was fine the way it was, That's OK. But then why would you want to downgrade anything the Buick engineers thought was adequate? Especially a part that literally acts as the backbone of everything that's great about these cars. There's no 2 ways about it. The NA crank is not as good as the turbo crank. Even if it's one trillionth of a percent weeker.........it's still weeker. We are not talking thousands of dollars here. Were talking small money and maybe some time. And most of us have the time because we don't use our Buicks to drive to work everyday.

If you plan to sell it, then you will have to tell the new owner it has a NA crank in it. Right?.... Or you won't. And then this makes you the kind of person you would probably say bad things about. I know I wouldn't pay the same for a TR with an NA crank in it.

If you plan to keep it forever, some day you will die (we all do) and you will leave it to someone you love.

If you don't care about any of these things and you want to use the car to haul Sheetrock and concrete blocks like I do to all my disposable beaters, just throw a used carbed Chevy 305 in it and turn it into a Monte Carlo. Those motors only cost 38 cents.

Sometimes it's not about "will it be OK". Instead, it should be about "is it as good or better than it was before."

And sometimes the reason is....... Just because.

 

[earlbrown]

Years ago I almost put a ''turbo'' crank and a new N/A crank in a large press and break them.

I have a feeling I could put 100 of them in the press and not be able to tell which was which.


That rolled fillet stuff just doesn't impress me. IF the rolled fillet does do anything it's probably just a stress riser. Now a radiused fillet can add strength (by eliminating stress risers). But removing meat in a corner might allow flex while minimizing cracking, I can't know for sure without testing.


These cranks aren't steel, they're cast iron. Cast iron in crystalline in nature, there's no grain like steel. Pressing on cast like you do with a forge won't work with cast.


If I had a 'N/A' crank that passed magnaflux and was dimensionally true, I'd run it for a stock build without thinking twice.

 

[captflash]

I respectfully disagree. Just because.

I broke a NA crank and it was cut 20/20. The motor wasn't too radical and was running mid to high 11s

 

[Anthony P]

Surprised no one else has brought up the cost to get this NA crank ready to go. I brought it up in post #7. Is this a case of penny wise and pound foolish?

What is the cost to magnaflux the crank, grind it and then polish it? I'm thinking $300 - $400 to inspect and grind it and another $100 to polish it.

Why spend that $$$ on a 30 years old cast iron crank? Maybe my concept of cost is not correct?

Eagle forged 4140 crank is $440 shipped to your door from Summit. Eagle part no. 523134005967

EDIT: posted link doesn't go anywhere. just use the part # to search on summitracing dot com.

 

[750H.P.V6]

It's not quite that expensive to mag and turn a crank but your point is a good one. Truth be told a cast iron crank really has no place in a performance motor. I'm not saying every engine application needs a forged crank but the reason our engines came with cast iron cranks is that OEM's are cheap and deemed the crank good enough for the application. Along that same line they also felt that having rolled fillets on the rod journals was beneficial and necessary or they wouldn't have done it because if they can save a buck they will. I spoke to one of our mechanical engineers to make sure I wasn't off base and he confirmed that the rolled fillets compress the metal structure and add fatigue resistance to the rod radius which is the most common place for crankshafts to crack.

Neal

 

[Jerryl]

I'm currently getting my turbo V6 rebuilt. . . .The NA crank needs to be cut 20/20. . . . I'm building this car to be a cruiser not a rocket ship. Will this crank hold up? . .

There will always be examples on both sides of the spectrum. Some have broken cranks whole others continue running.

Cost and future goals aside: You should be good for *your purpose* IF magnafluxed and not cracked.

Many have determined that realistic performance and longevity are in the tune.
Good luck with your decision.