Help OverSquare or UnderSquare Engines?

[ShakeyC]

Actually is a point with my question too help decide which block to go for my future e30 (sort of planning ahead) Havin a thicko day again understanding what ment by Over Square / Under Square Engines something doing with engine blocks bore crank ?

[Turbo-Brown]

Over square is where the diameter of the bore is larger than the stroke of the crank, and vice versa.

Why you askee out of interest?

[Jhonno]

oversquare engines tend to be rev happy and make more power

[ShakeyC]

I been offered choice of 3 engine blocks (bottom half) rest is identical for arguement sake Block A B C. I like to turbo charge one block and I can choose any stroke bore rod crank and piston for same price. Surely i want the best for my money its a case for me working out which would be best to turbo within reason.

A is standard height
B is 1" taller
C is 1" taller and larger bore

As price of each botom half is the same and I can choose from 2.0 L capacity (Block A) upto 2.5 litre with (Block C) sure 2.5 L will give me more power then the guy went on about over/under square revs and lost me after that!

[Jhonno]

taller block is no good to you unless you are looking at putting a longer throw crank.. but 1" is a massively longer throw

so A imo

tho.. what has block height gotta do with over/undersquare?

[Turbo-Brown]

oversquare engines tend to be rev happy and make more power

And your bonus for 10, why is that?

[ShakeyC]

That where im stuck understanding? I guess taller block will allow for longer rods or crank throw but what affects engines ability to rev higher? Is gaining extra 0.5 L capacity better than say being able to rev extra 2000RPM?

[Jhonno]

shorter throw in the crank, less reciprocating mass, more bore area so more room round the valves

[Jhonno]

That where im stuck understanding? I guess taller block will allow for longer rods or crank throw but what affects engines ability to rev higher? Is gaining extra 0.5 L capacity better than say being able to rev extra 2000RPM?

1" taller is alot tho, it will end up needing decking

high revving turbo's are uncommon, so the capacity is prob more important, depends on what you want

[ShakeyC]

Ideally want a very high revving turbo engine that make lots of bhp and torque for longer, with vast choice in spec im lost

[ShakeyC]

Actually are ther some sort of software to simulate engines? those spreadsheets from google seem to give different answer to each other not knowing which is correct even when if typing in figures for an engine that is known.

[Turbo-Brown]

The length of the crank throw has nothing to do with it, it is all to do with the area available to fit valves in.

The reciprocating mass will need to increase to fill the larger bore too.

The basic point is that you won't make an engine more 'rev happy' by just taking an undersquare engine and giving it an overbore.

[Jhonno]

the longer the throw the more energy in the rotation surely? therefore slower to respond to applied force?

The basic point is that you won't make an engine more 'rev happy' by just taking an undersquare engine and giving it an overbore.

Definately

It is a complicated relationship..

[Turbo-Brown]

Indeed, but the bigger the pistons the more energy there is too.

The biggest factor in the rev-happiness debate is the ability of the head to breath.

[Jhonno]

yeah but is it not to a lesser extent as the 'circumference' followed is smaller..

That stupid power Norris Evo 6 is undersquare, yet revs to 8k odd iirc

[tomstickland]

Oversquare, bore is larger than stroke, good for power because they allow large valve area and also short stroke allows higher rpm for the same stress levels in the con rods.

[Turbo-Brown]

yeah but is it not to a lesser extent as the 'circumference' followed is smaller..

That stupid power Norris Evo 6 is undersquare, yet revs to 8k odd iirc

The S5x engines are undersquare and like to rev too

Now are we talking about engines which will eat up revs (in which case the breathing ability makes all the difference) or ones which will rev up quickly as being rev-happy?

[Jhonno]

i thought the S50B30 was 86x86? its the b32 thats undersquare (86x88?), yet revs higher from the factory than the b30 iirc..

revving up quickly is a product of the weight of the engine internals surely? breathing being the engines ability to make the revs..

ahh i can see where you saw the confusion coming in

[tomstickland]

I've just been doing a basic analysis

Bore B
Stroke S

Volume is proportional to B^2 S
For a given cylinder pressure, torque is proportional to B^2 S

Hence torque is proportional to cubic capacity regardless of what bore/stroke values are chosen.

Reciprocating acceleration is proportional to RPM^2 x S
Reciprocating force is proportional to piston mass x acceleration
Piston mass is roughly proportional to B^2
Hence stress is proportional to
B^2 S x RPM^2
which is
volume x RPM^2

ie: reciprocating stresses are roughly the same regardless of the bore/stroke relationship.

So that pretty much just leaves valve area as a big advantage of large bores. There might be something to do with frictional losses and wear rates being better due to the lower piston speeds in oversqaure engines etc.

[HairyScreech]

your forgetting a longer crank throw will generate higher rotation speed (not rpms) at the big end and a higher piston speed for the same engine RPM as the crank and piston will have to travel further.

[tomstickland]

I'm not forgetting anything;

RPM^2 x S

ie: tip speed is proportional to stroke.

[Jhonno]

your forgetting a longer crank throw will generate higher rotation speed (not rpms) at the big end and a higher piston speed for the same engine RPM as the crank and piston will have to travel further.

big end velocity will stay constant.. Piston velocity will change tho yes

[ShakeyC]

This getting interestingg now for argument sake again head will breath plenty as OE, the big end will rev to 8000RPM all day with either block A B or C. Say aiming for 10000RPM it will make good power but want the torque all over the shop too. From reading piston speed + rod length seem to be thee factor in making torque for longer, bit im confused on is if longer rods will increase piston speed? if longer rods will create less rod angle with more turning force on to crank? will this limit its ability to rev cleanly using same crank in any block? or would using larger crank help?

[Jhonno]

longer rods wont increase piston speed, but will move the pistons further up the bore