Bore to sweep ratio

[youngoldy]

does anyone have any links or info on the topic?

thanks

[Dave Brand]

Do you mean 'bore to stroke ratio'?

[youngoldy]

no, 'bore to sweep ratio' in the cylinders

[zefarelly]

sooty and sweep ?

mines 82mm x 73mm in round figures, oversquare I believe

as far as I'm aware shorter strokes rev well, longer is good for torque ?

[Dave Brand]

Originally Posted by youngoldy

no, 'bore to sweep ratio' in the cylinders

It's the same thing really, but 'bore to stroke ratio' is the more common term - I did a Yahoo search on "bore to sweep ratio" & got no results; "bore to stroke ratio" came up with 373! Taking the "" out of the first search came up with 103,000, with some relevant ones.......

Originally Posted by zefarelly

as far as I'm aware shorter strokes rev well, longer is good for torque ?

Shorter strokes give lower piston speeds. Bore/stroke ratio has, surprisingly, a negligible effect on torque - for a given BMEP torque is pretty well constant, irrespective of whether the engine is over- or under-square. I'll try to find some calculations I did to illustrate this a few years ago. Note the key words: for a given BMEP!

[phoenix]

This is a quote:

"The size of the bore does not affect the volumetric efficiency, assuming the valves are properly sized. The big factor with regards to the stroke is not the length of the stroke, but the piston speed (because the inlet Mach index is proportional to piston speed). An engine with a short stroke running at high rpm can have the same piston speed, and thus similar performance, to a long stroke running at a low rpm."

so there!

[graham bahr]

Originally Posted by zefarelly

sooty and sweep ?

mines 82mm x 73mm in round figures, oversquare I believe

as far as I'm aware shorter strokes rev well, longer is good for torque ?

longer strokes do tend to produce more torque, if for no other reason than the piston/rod assembly has more leverage on the crank, although to get significant amounts of torque you need a very long stroke, but them possibly start to loose out again due increased friction, and possible cylinder head breathing restrictions caused by the smaller bore size that goes with a long stroke engine

[icemachine]

I always figured a big bore, long stroke gave you the best torque.

Somethin about no replacement for displacement

[Al Weyman]

Sorry to talk Small block chevys again.

The 302 CI engine as fitted to the original 1st gen Z/28 and that size to be under 5 litre to compete in Trans Am racing had a short stroke crank and good sized bore (4 inch) and a 3 inch stroke and was a vey 'over square' engine. These were very revvy and tuneable and came out from the factory with 350bhp I believe.

However the later 305 ci (5 litre as well) stock engines that found themselves in virtually every later Chevy model although almost exactly the same displacement had a 3.75 inch bore and a 3.5 inch throw crank. This configuration is not reckoned to be so tunable as the 350 which has the 4 inch bore and the same crank. For a start if you fit the large valve heads to the 305 they perform worse than the smaller valves because of shrounding. Chevrolets thinking behind this configuration was to produce a higher torque hauler that would produce good power at low revs but not one for the racers!

I built a custom 400 cu motor once and have the dyno figures somewhere. It had a 4.25 inch bore block with a 3.75 inch throw crank and I fitted the longer 350 rods and shaved the crowns of the pistons. This combo produced a tree stump pulling 475 ft lbs if torque at well under 3000 rpm!

[Dave Brand]

Originally Posted by graham bahr

longer strokes do tend to produce more torque, if for no other reason than the piston/rod assembly has more leverage on the crank, although to get significant amounts of torque you need a very long stroke,

For a given cylinder volume, torque will be more or less the same, assuming the same BMEP, for a long- or short-stroke engine. Why? Because if you reduce the stroke you increase piston area, therefore the pressure resulting from combustion acts on a larger area, producing a bigger force. As torque is the product of force & distance, a bigger force acting at a shorter distance will give the same torque as a smaller force acting at a biggger distance.

[Al Weyman]

What about Grahame's good point about leverage, I have read that a few times in articles by very knowledgable engineers so it must be correct.

[ian_w]

I concur with Dave Brand, for a given cylinder pressure the torque output will be independant of bore-stroke ratio.

Torque is Force * Distance. The 'leverage' from a long stroke engine increases the 'distance' factor but the smaller bore size reduces the 'Force' by the same amount.

The above argument is based on the 'same cylinder pressure' assumption. What you will tend to find is that a long stroke engine of a given displacement will have a higher peak torque then a short stroke engine and it will occur at a lower engine speed. However its peak power will also occur at a lower engine speed as well and ultimately this peak power will likely be less than the short stroke version.

So, is a high torque low speed engine of a given power better than a low torque high speed engine of the same power? Below is an example of two completely different engines to illustrate the point:

2 litre car engine: 150 bhp @ 6000 rpm, 190 Nm @ 4000 rpm

1 litre bike engine: 150 bhp@ 12000 rpm, 110 Nm @ 8000 rpm

At first sight most people would pick the car engine; its greater torque must give better acceleration - wrong!

As the engines make the same power, the car will have the same top speed if it is geared to reach max speed at max power rpm. Say, the car engine runs a 3.0:1 final drive ratio, the bike engine will need to run a 6.0:1 final drive as its peak power speed is twice that of the car engine ( 12000 vs 6000 rpm )

What makes a car accelerate is torque at the wheels, i.e. engine torque multiplied by gearbox ratio and final drive ratio. If we assume both engines are in top gear which has a ratio of 1:1 we can calculate the peak wheel torques:

Car engine: 190 * 3.0 = 570 Nm, 4000 / 3.0 = 1333 wheel rpm

Bike engine: 110 * 6.0 = 660 Nm, 8000 / 6.0 = 1333 wheel rpm

So from these calcs, you can see that the bike engine gives a significantly higher peak wheel torque ( 660 vs 570 Nm ) at the same wheel speed, i.e the bike engine will accelerate the car faster.

[Al Weyman]

I can see that argument, a low revving torgue producing engine is obviously what the GM engineers had in mind with the longer stroke configuration to meett the CAFE fuel consumption reqiuirements and it works if my 1999 V6 Vortex engined Blazer is anything to go by as that thing will pull from 500rpm in 5th gear!

[graham bahr]

Originally Posted by ian_w

Car engine: 190 * 3.0 = 570 Nm, 4000 / 3.0 = 1333 wheel rpm

Bike engine: 110 * 6.0 = 660 Nm, 8000 / 6.0 = 1333 wheel rpm

So from these calcs, you can see that the bike engine gives a significantly higher peak wheel torque ( 660 vs 570 Nm ) at the same wheel speed, i.e the bike engine will accelerate the car faster.

i can see you numbers, but in practise its not that simple, for a start transmission losses go up with rpm so a lot of the extra torque the bike engine is supposed to deliver to the wheels wont actually arrive at them.


having raced against bike engined kit cars that weigh half the weight of my car, i would only say they accelerate faster due to the fact they have much less weight to lug around, not to mention an extra cog in the sequential gearbox, they tend to get the jump on me off the line and out the slower corners, but DONT actually accelerate faster because i then reel them in again.

sorry but were not comparing like for like here, otherwise everyone would be building tiny engines that reved to hell and back and just geared them down,

just look at F1 they are slowing the cars down by making them have SMALLER engines which WILL rev higher, or has bernie got it wrong and they should be getting bigger engines, i think not.

[Al Weyman]

Does'nt conrod length also have an effect on torque, the longer being prefered for more power, the shorter as in the standard 400ci chevy small block, for more torque or have I got that wrong.

[graham bahr]

Originally Posted by Al Weyman

Does'nt conrod length also have an effect on torque, the longer being prefered for more power, the shorter as in the standard 400ci chevy small block, for more torque or have I got that wrong.

it certainly can shift the torque around, as the rod length varies how long the piston hangs around at TDC, altough i think it has most effect on 2 valve engines which are always going to be struggleing in terms of head airflow

[graham bahr]

Originally Posted by phoenix

This is a quote:

The big factor with regards to the stroke is not the length of the stroke, but the piston speed

so there!

how high you can go with piston speed is merely dictated by the weight and constrution of the piston and rings etc, not to mention the strength of the conrod

[phoenix]

Originally Posted by graham bahr

how high you can go with piston speed is merely dictated by the weight and constrution of the piston and rings etc, not to mention the strength of the conrod

Not at all, I'm afraid.

It is easy to make things strong enough to withstand high piston speeds, but without managing port speed to maximise the charge in the cylinder you won't maximise Ve.

Piston speed effects the speed of the inlet charge through the port.
It is essential that the gas speed in the port is kept subsonic, or the flow will suffer and in some cases even halt and reverse at Mach 1, thus reducing power. So on high revving engines, although materials can be found for rods, pistons, cranks etc. to take the forces involved, a short stroke because of the lower piston speeds associated with a short stroke become essential.

[graham bahr]

i suppose a lot depends on if were talking about modifying production engines or bespoke race engines

[zefarelly]

Originally Posted by graham bahr

i suppose a lot depends on if were talking about modifying production engines or bespoke race engines

but for most of us aren't we trying to turn the former into the latter ( and on a budget!) or in English, a case of optimising what we have within the rules of our series

[phoenix]

Originally Posted by zefarelly

but for most of us aren't we trying to turn the former into the latter ( and on a budget!) or in English, a case of optimising what we have within the rules of our series

If that is the case, then we are probably talking about a capacity increase and the argument is whether to over bore or increase stroke to get a larger capacity, then I would go with what has been done successfully previously with the same type of engine.

There are many examples where engine output has been improved by both methods. In some cases to get an 25% increase in capacity stroking is the only option.

If the decision has been made to get more power by increasing engine rpm by 25% or more then maybe the flow limitations of the ports need to be considered before deciding to increase the stroke as well.

[ian_w]

The point I was trying to make with my Bike vs Car engine example is that you need to be careful when looking at raw engine torque numbers - you also need to consider engine speed. At the end of the day, if you do all the maths then what matters is Power at any given engine speed.

In reply to Graham - sure friction losses increase slightly with rpm but is a very very small effect so not significant in the calculations. I guess what is happening with his bike engine competitors is that they have a higher power:weight ratio and thus accelerate faster. They probably have less power than him and also possibly more drag so their top speed will be lower than his - i.e. he starts catching them at the end of the straight.

To answer the original question, if you are building the ulimate race engine ( i.e. F1 ) then you want the biggest possible bore combined with a short stroke. The big bore results in big valves, i.e good breathing whilst the short stroke gives lower pistons speeds for a given rpm and thus lower stresses on the bottom end of the engine ( or more rpm for a given stress ). You may be interested that for 2006 the FIA specifies a maximum bore size for an F1 engine of 98mm, this gives a stroke of 39.8mm for a 2.4 litre V8 - now thats a pretty 'over square' engine!!

For more 'normal' race engines then a big bore is still a good thing but its probably not worth going as extreme as an F1 engine.