There are two types of V8 crankshafts, cross-plane (crank pins at a 90° angle) and flat-plane (crank pins at 180°). Cross-plane being the type of crank in the LT1 and virtually every other production V8; but what are all of the advantages and disadvantages of these two types?

I know that the primary advantage of the cross-plane is that it runs much smoother. But the disadvantage is that it requires counter weights which make it heavier than the flat-plane which has none and is thus much lighter and rev happy.

Also are there any aftermarket flat-plane V8 cranks available for small blocks?

 

Well, the 180° cranks really made about the same power & torque as a 90° crank, so no real advantage. Don't forget all the hassels with getting a cam ground, injectors timing, etc...

 

Why does a flat-plane or 180 V8 crank not need counterweights?
If it were a boxer (flat) engine, that's ok, but with a 90 degree V, I believe it still needs some.

With a 180 crank you can treat the engine as two 4-cylinder engines. Inlet and exhaust tuning is easier because you don't have two cylinders on the same bank firing at a 90 degree interval. I believe the production Ferrari 360 (V8) has a flat plane crank. I'm not sure what they do to minimize the vibration, but maybe Ferrari owners don't mind. The original IRL Aurora 4L was also a flat plane, I believe. Some organizations (NASCAR) prohibit them now.

 

I'm not 100% sure but I believe that I read somewhere that flat-plane cranks don't need counter weights.

Why did NASCAR ban them?

 

Not sure why. Maybe it was a start of the "level playing field", or "everything pretty much alike" movement. Maybe it was an attempt to save teams a little money. If you had to develop both single and two-plane crank engines costs would go up.

For a while, but not recently, as far as I know, some teams used 180 degree headers which pair the cylinders like a flat crank. It sounds different, like the 2 four cylinders it mimics. Many/most(?) Cup engines seem to be 4-2-1 headers, vs 4-1 of a few years ago. Perhaps this exhaust tuning, which is basically 4 cyl tuning, is better than 180 degree headers or approaches flat-crank tuning.

I suspect 4-7 firing order swaps are also used, which again may help.

Speaking of Cup engines, here's a quote from

www.fordracing.com about Newman's Dodge engine:

"SECRET IS OUT
With all the talk about Ryan Newman’s race strangling fuel runs of late it’s no wonder that many of his peers are frustrated to the point where they’re hurling accusations. But the story behind Newman & Company’s fuel secret crawled out of the bag and into the lap of TFR.

The secret to the formula — bigger is not always better in Winston Cup. The NASCAR specifications for a carburetor allow a carburetor will flow up to 830 cubic feet per minute maximum. Newman’s men have found if they run a carb that flows 750 cfm that they give up only two horsepower to what the bigger carb delivers. That smaller carb, without 2 hp, brings the team about 0.5 miles per gallon extra in fuel consumption.

Taking that 0.5 mpg and working it with the 22-gallon NASCAR-specified fuel cell, you see that Newman has an 11-mile cushion compared to his competitors. Meaning that Newman could run an extra seven laps at a 1.5-mile track, such as is found at Lowe’s Motor Speedway.

Unfortunately for the Alltel team the secret is starting to filter out to some of the other teams, however, the power lost to the smaller carb is not quite at the level that the Penske motor men are capable of recapturing, but darn close. Give those who have started to experiment with the smaller carb a few weeks and any deficit should soon disappear. "

 

A flat crank V-8 has advantages WRT exhaust tuning. It can be treated as two four cylinder engines operating on a common crankshaft, so the exhaust design is made like a traditional 4-2-1 set up to take maximum advantage of pulse tuning effects. To do this on a conventional V-8 requires a very large, heavy exhaust system. With a conventional crank, wouldn't cylinders from opposite sides have to be "matched up" to gain maximum advantage from exhaust tuning? I wish I could explain this better, I can visualize it and diagram it better than I can write it.

I am unsure about the balance issue. But I understand that flat crank V-8's are "rough". AFAIK, most purpose built racing V-8's utilize flat cranks. They sound "weird". Very flat, not nearly as nice as a conventional V-8.

Rich Krause

 

Yep, a four fires every 180 degrees of crank rotation. An eight every 90 degrees of course. To mimic 4-cyl tuning, you need to pair cylinders firing every other point (180 deg) in the firing order.

with a 1-8-4-3-6-5-7-2 firing order (even numbers on one bank, odd on the opposite), the pairing for these "180 degree" headers is:

1-4-6-7
8-3-5-2

So each side of the exhaust has 2 right bank and two left bank cylinders into it's collector. Two pipes have to cross over from each side. I think that's what Rich is visualizing. The plumbing is complex, especially on an engine set very low in the chassis and not alowing over the top headers.

A typical 2-plane V8 has the distinctive sound (with true duals) because of the 270*-180*-90* firing spacing on each side. Most of us have come to identify (and love) that sound with a V8.

Inline fours have an inherent imbalance (second order, I think). That's why we see balance shafts in larger displacement fours. With a flat crank, a V8 has this on each bank.

My $.02

 

The Ferrari 360 does have a 180° crank and it shakes worse than a [*********]. To absorb these vibrations, the ecm actuates electro-hydraulic dampers in the motor mounts. This shock-absorber system is said to add about $10 000 to the cost of the car (small change when you've paid 200 large for the car).

 

Stranger, you do have a way with words. Thanks for the info.

 

Ferrari is the only manufacturer, who uses flat-plane design in production cars. That's why they make such specific sound compared to other V8 engines.

I have driven 360 and didn't notice any strange vibrations, my camaro is much worse.

 

The Subaru boxer also has a flat crank.

 

In a boxer design that's about all that will work well.
I think Est96Z28 meant flat crank in a V8 engine.

 

A Honda 6 cylinder Gold Wing boxer motor has a 120 degree crank, and a Porsche 911 has 60 degree crank.

 

Good thang we got that straight, cuz I was thinkin' jus bout evry inline injun has a flat crank. An some o them r jus nat'rally balanced so as ya don hafta mess round wit them pesky counterweight thingies. Them there beemer sixes r bout as smooth as cow snot on a warm day, an they jus about as fast as snot goin backwards.

 

I stand corrected. Thanks. That makes sense on a 6 cylinder engine. No wonder 911s are so expensive to rebuild.

In an earlier post I said IRL engines were originally flat cranks, but that was changed to 90 degree. Oops! Bassackwards. They were 90 degree, are now 180 degree. At least one has counterweights. They do sound cool at 10,700 max rpm!