flat plane cranks

toughy V8 · Post by **toughy V8** » Wed Feb 14, 2007 7:58 pm

can some one explain to me the reasons behind flat plane cranks like the cerberas, i mean like what benefits and drawbacks do they have compared to a cross plane crank

sidecar · Post by **sidecar** » Wed Feb 14, 2007 8:42 pm

One of the reasons (there maybe others) is that due to the way a normal cranks are made they are usually out of balance. They are made by taking a flat plane crank out of the forge and twisting it whilst its still red hot.
The throws do not end up at exactly 90 degrees but there is enough meat on the big end journals to machine them to 90 degrees, the counter weights are not machined so remain a few degrees out.

The other thing with this process is that it puts stress into the crank where its twisted.

The other reasons may be to do with obtaining a better firing order and more effective header pipe connections but I'm not sure.

Draw backs would include the need for a special camshaft to be ground to suite the firing order and also having to mess about with the HT leads for the same reason.

Motors fitted with such cranks are supposed to sound better!

Regards,

Pete.

toughy V8 · Post by **toughy V8** » Wed Feb 14, 2007 9:04 pm

well ive heard them cerbera's and i think they sound awful like a saxo with a big bore exhaust. didnt the rover powered tuscan race cars use a flat plane crank

bill shurvinton · Post by **bill shurvinton** » Wed Feb 14, 2007 9:28 pm

It's all about power. With a twin plane crank it is very difficult to tune the exhuast system to get beneficial scavenging as you need to link pipes from opposite banks. As a flat plane crank makes the engine behave as 2 seperate I4 engines from a pulse tuning perspective you can use the standard 4-2-1 designs on each side and get all the benefits that gives.

This is why the 4.5 cerb puts out a lot more power than the 5 litre RV8. The sound is not the nice lubba lubba of a twin plane though. Although Ferrari seem to manage to get a nice soundtrack, Lotus have has less luck with the esprit V8

kiwicar · Post by **kiwicar** » Wed Feb 14, 2007 9:56 pm

mostly as above, however a few of detail points.
1 most dual plane forged cranks are non twist designs these days, cast cranks are cast to be in two plans so don't get twisted, forged cranks are now forged directly into their final shape. true 15 years ago they both were twisted, not now.
2 a twin plane crank has good secondary ballance a single plane crank needs a ballencer shaft or two running along side the crank.
3 ferrari engines use generally use 72 degree angles between the banks so the exhaust note is different. Although more difficullt to get primary ballance the secondary imballance tends to cancel itself out so less need for a ballancer shaft.
4 a single plane crank needs less counter weight to get primary ballance so the crank is accelerating less mass with each firing so is stronger for a given weight (still have to accelerate the ballancer shaft but it also acts as a crank damper so you gain some back by having a smaller damper mass, on a race engine you are less bothered about secondary ballance and pasanger comfort so you only damp out enough to stop it shaking itself to bits)

v8alex · Post by **v8alex** » Wed Feb 14, 2007 10:39 pm

As above, a flat plane crank removes the need for huge counterbalancing that you will see on the dual plane cranks. Less counterbalancing also means a lighter crank with less inertia to overcome, so it spins up quicker, giving higher rpm capability and better acceleration.

Alex

Boosted LS1 · Post by **Boosted LS1** » Wed Feb 14, 2007 11:42 pm

As far as I know a flat plane rv8 crank can't be balanced properly. That said, plenty of people race with them but they tend to be billet items so won't break immediatley

Heavy feckers though, I wouldn't have one as a gift!

Boosted.

sidecar · Post by **sidecar** » Thu Feb 15, 2007 8:21 am

Well you live and learn !

All good stuff.

kiwicar · Post by **kiwicar** » Fri Feb 16, 2007 11:37 am

just been reading up and the cerbera engine is a 75deg bank angle as well, I think for a flat plane crank to work beyond a brief few runs then shaking itself to bits, you really need ballancers.
I have a feeling the cadillac northstar/ Arora (nice engine)

engine and that fancy lexus V8 may be flat plene cranks, but I expect to be corrected on that one

Mike

toughy V8 · Post by **toughy V8** » Fri Feb 16, 2007 5:49 pm

i dont think either of them engines have flat plane cranks definately not the lexus anyway ive heard them and they sound like a normal v8.

HairbearTE · Post by **HairbearTE** » Fri Feb 16, 2007 8:13 pm

The aroura engine was developed to a very high spec for racing in the states and these versions perhaps had a flat plane crank but i very much doubt the oem engine did. The cerbera engines were based on a detuned judd racing engine and that is why they have many features found on racing engines. I believe the current straight six tvr engine is also a further developent of the same basic design.

ChrisJC · Post by **ChrisJC** » Mon Feb 19, 2007 1:29 pm

Further to Bills comments about exhaust scavenging, the Ford GT40 managed it with a dual plane V8 crank.
http://gpma.org/Archive/1021a.jpg Note that the collectors take the out two from one bank and the inner two from the opposite.

But you can see that wouldn't be practical in most cars!

I have animations of both flat plane and dual plane V8s on my website:

http://www.cowdery.org.uk/cranks.php

Comments on exhaust tuning are also there.

Chris.