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Crankshaft Design
bikecarbfred - 9/9/17 at 07:38 AM

Hi guys.

Please see two pictures of crankshaft

crankshaft A weights 9.8kg for a 1.4 16v 100HP engine
crankshaft B weights 13.5kg for a 1.6 16v 105HP engine

The crankshafts are interchangeable within the blocks.
The advantage is in an average car weight of 1500 kilos approx 3hp.
The advantage is in an car weight of 900 kilos approx 5 hp.

I know it is not alot but with respect do not want to talk about the figures but would like expert help on WHY did the same manufacturer put on the extra fins (hence adding more weight)

I can think of only two things:

1) extra fin to guide/deflect the oil EQUALLY up the cylinder walls
2) crankshaft A was found to be weak

The reason I ask this is I have had 130-135hp through crankshaft A but to get to 160hp-165bhp I require crankshaft B with longer rods. (edit: mistake : shorter rods with crank B)

In your opinion why did the same manufacturer make it heavier in this way?

[Edited on 9/9/17 by bikecarbfred]

Theshed - 9/9/17 at 08:14 AM

Could this not be an issue of balance? You mention longer con rods. The additional weight of those rods would need to be counterbalanced on the crankshaft. However the weight difference is quite a lot more than might be expected from that alone. You do not mention the stroke. Is it the same?

bikecarbfred - 9/9/17 at 08:21 AM

I have made a mistake. Damn!!!

Crankshaft B is be used with shorter rods! So in fact the heavier crank will be using lighter rods.

Also to answer your question:

Crankshaft A stroke: 75.6MM
Crankshaft B stroke: 86.9mm

Dick - 9/9/17 at 11:44 AM

The design of crankshaft A is normally excepted as to being able to rev higher and to be stronger than cranks shaft type B

bikecarbfred - 9/9/17 at 11:57 AM

A stronger than B. Interesting. I thought it would have been the other way around.

Dick - 9/9/17 at 01:19 PM

Sorry wrong way round B is the better I wrote with out looking at order you had posted b b4 a

bikecarbfred - 9/9/17 at 02:19 PM

So in that case getting a company to cut the fins out off B, make like crank A and re-balance it?

Dick - 9/9/17 at 03:28 PM

You can get the crank knife edged to reduce windage and get the faces cleaned up . That always helps. Its rearly down to what do you want to do and what budget

jollygreengiant - 9/9/17 at 06:01 PM

Crankshafts are designed (pretty well) by the manufacturer, to the job they need to do for THAT engine.
Now, you want to change the crankshaft for one that has a different stroke. THIS will change the CC's of your engine. Smaller throw crank on the same pistons = smaller CC's. (remember your school maths), area of a circle = Pi (3.142approx) X the Radius of the circle (half the diameter) Squared. The volume of A cylinder is The area of the circle X the height (the stroke of a crankshaft). The Capacity of an engine is the capacity of A cylinder X the number of cylinders.

With this in mind, I would think your best bet would be to take your existing crankshaft to a PROPER engineering workshop (with the experience of doing it) where they could lighten and re-balance your existing crankshaft after having it crack tested for any flaws in structure, and then have it properly heat treated and nitrided (a means of strengthening the crankshaft).

Also have the flywheel lightened and balance (individually and as a unit attached to the crankshaft.

All this will not necessarily give you more power, but, It will reduce the inertial mass and the amount of power it takes to accelerate that mass before that same power gets distributed to your gearbox.

The crankshaft webs are generally well set on a standard crank to control the reciprocating energy that is produce by the piston of A cylinder and balance that motion across the plane of the entire crankshaft.

If you want more power then there is no substitute for more CC's. Ergo fit a bigger throw crank OR bigger Pistons to the same crankshaft. Changing crankshafts and making them work PROPERLY, tends to be a VERY expensive hobby.

Just my 2p's worth.

snapper - 9/9/17 at 08:02 PM

I would worry more about balancing the piston and rods and then dynamically balancing the crank/pulley/clutch cove combination that worrying about the wright

bikecarbfred - 9/9/17 at 08:44 PM

I'm going to use Crankshaft B obviously as it's going to give me more CC. That's what the project is about however would it be wise to have that lightened? Is 160 bhp alot of a crank weighing 10 kilos?

The crank must bear all the strain in the combustion process? All the force pounding downwards.

jollygreengiant - 9/9/17 at 09:44 PM

I worked once at an engineers, the interesting thing was that, he had built a 1300 Xflow Twin cam (lotus head) using an 1100 Xflow Crankshaft. On fuel injection (Piper) it produced quite prodigious BHP at about 1200rpm. It was a work of art. (this was about 1980), but, it cost a fortune then. Keep your tuning simple and closer to the normal. It will be cheaper, last longer, be just as satisfying and just as much fun. I also went down the off the norm road with a 1662cc Xflow full race, 11:1CR, Flat twin cam pistons, flat head, big valves, Piper ladder rocker box, Twin 40's, etc. Top end 0f 140mph (genuine). It was FUN. but, back in 1980, it cost in today's money the equivalent of 120K (yep, I could have bought a house for that then 3K) 15K to a gearbox rebuild and 20K to and engine rebuild.

As I said, just my 2p's worth.

edit bit.
Oh and longer rods will mean adjusting the block head face, unless you get specially made rods (I would think) and then you might still have issues with rod to block/cylinder wall clearance.

[Edited on 9/9/17 by jollygreengiant]

bikecarbfred - 9/9/17 at 10:49 PM

I made a mistake in my post. It is shorter rods! that will give more CC combined with crankshaft B

jollygreengiant - 10/9/17 at 04:26 PM

Originally posted by bikecarbfred
I made a mistake in my post. It is shorter rods! that will give more CC combined with crankshaft B

The length of the conrods will make absolutely NO difference to the CC'age of your engine, that is affected by A, the bore (diameter) of the pistons. B, The Stroke of the crankshaft, and. C, The number of pistons.

The length of a conrod ONLY allows for the clearance of the bottom of a piston between the crankshaft web journal at the bottom of its stroke and opposing crank journals/webs at the top of their stroke.

Again this is just my 2p's worth, but IF you don't understand what/how things work in an engine, should you be playing with the dynamics of it. I feel that this approach could end up being VERY costly for you.

bikecarbfred - 10/9/17 at 05:09 PM

The picture does not show it well. Crankshaft B rod journals are arranged different.
This crankshaft has been tried n tested to convert 1.4 to 1.6. With shorter rods combined with Crank B gets you the extra CC.

I just thought if the engineer shop cut just completely cut of the extra fins so it looks like Crankshaft A.

minibull - 11/9/17 at 01:10 AM

As previously said length of rods make no difference to cc they need to be long enough to allow bottom of piston to avoid hitting crank and short enough to prevent top of piston hitting head. If you know that the larger throw crank with the shorter rods and your pistons will fit use them as is. Heavier crank will have been designed the way it is for a reason and although if you know what you are doing it would no doubt be possible to improve on it for your own engine, it would be unlikely simply lightening would be effective. On the bright side larger webs will be easier to balance and heavier reciprocating mass will increase torque, on the downside heavier crank and shorter rods will reduce ability to rev. Personally I'd be tempted to use longer rods and shorter pistons but custom made pistons aren't cheap!

bikecarbfred - 11/9/17 at 07:07 AM

Yeah @ minibull/

A set of new pistons is costign me 200. These are std and good enough because no boost.

Custom pistons are about 1000

I wonder how much to lighten and balance a crank is.

You talk about the design made for a purpose but I don't see it as they go into the same block design.

The only reason I can think of is the way is deflects the oil equally on both side of the cylinder from the oil that squeezes out of the big end bearings of the rod.

mcerd1 - 11/9/17 at 08:01 AM

Originally posted by bikecarbfred
I wonder how much to lighten and balance a crank is.

Not that much - probably less than 150 for a full bottom end balance, inc pistons, conrods, crank, flywheel and clutch cover

If your going this far you really need to do all of them together - conrods and pistons will need the dimensions checked and weights matched.
and as above a full dynamic balance of the bottom end (crank, flywheel and clutch cover) will help and will be needed if you try to remove any mass or knife edge the crank.

Removing mass from the crank will not give you any more power - it will help it spin up faster though (making the thing more rev happy) but it will also reduce the rotating momentum so lumpy idles are more likely....

Finding a machine shop that knows your engine well will help alot - they should know exactly how much they can remove without it going off like a grenade

Originally posted by bikecarbfred
You talk about the design made for a purpose but I don't see it as they go into the same block design.

The only reason I can think of is the way is deflects the oil equally on both side of the cylinder from the oil that squeezes out of the big end bearings of the rod.

The "fins" have nothing to do with deflecting oil from the bearings - the difference I can see between the 2 cranks is that the longer stroke cranks has had the additional mass required to balance it added (and its been positioned such that it doesn't increase drag too much when its spinning in the air/oil)

your also making a lot of assumptions about the materials - who says they are even made from the same kind of metal ?

for all you know the heavier one is make from cast steel and the lighter one is forged iron/steel - both could be the same strength, but the heavy cast one would be cheaper to make and the extra mass could help with a nice smooth idle too, thats enough reason for any mass market manufacturer to make the change (and they don't have to be consistent across all the version either)
- of course I don't know that they changed any material on your engines...

bikecarbfred - 11/9/17 at 08:59 AM

The conrods weigh 471g, 471g, 472g, 472g

I am going to leave the crank B as it is. The journal placement is different to allow the longer stroke so like you said they maybe changed the design/added extra fins to the crank to help the balancing for this.

I'll just go for a lighter flywheel to compensate.

Daf - 11/9/17 at 12:12 PM

Out of interest what engine are the cranks out of?

bikecarbfred - 11/9/17 at 09:07 PM

1.4 & 1.6 polo

thanks for help lads

02GF74 - 12/9/17 at 08:16 PM

Interesting stuff. The counterweights balance the rods and pistons. One reason the one crankshaft has less counterweights is the weight may be even external I.e.
on the flywheel, else heavier rod/piston.

Adhoc mixing new matching of crank/rods/pistons isn't going to end well.

bikecarbfred - 12/9/17 at 09:59 PM

"Adhoc mixing new matching of crank/rods/pistons isn't going to end well. "

It's using the bottom end of another engine. It's not mixing and matching.

The only thing that's changing is the head.

Also exchanging the rods for h-beams.

So the the crank is the right crankshaft for the length of rods and piston type if you get what i mean.