39 replies to this topic

[The Matt]

Well, I was sitting at home last night and was wondering whether anyone has built a short stroke turbine motor? Like a 1340cc bad boy or similar.

I know the common routes are really 1275, 1293, 1330 and sometimes 1380 turbos, but would having a short stroke motor like a 1340 (73.5mm bores with a de-stroked crank) lose out on the torque gains usually associated with a turbo engine?

Also, what sort of revving capabilities would it have? I know true 1340s are rare, as most people believe they have a 1340 when it is infact a 1330, so I guess 1340cc turbos are even rarer!

Edited by mini mad me, 27 January 2006 - 10:15 AM.

[Turbo Nick]

no point in my opinion. as far as i understand the short strokers are for high rev's, with a turbo lump you dont need to rev it past 6.5k ish...

[Dog]

Second what he says

[MAGA7INE]

There are quite definite advantages to a short stroke turbo engine and I am building one for one of the MAGA7INE project cars.

It's getting a 1071 south african crank and a large bore with the K1100 16V cylinder head and efi. These engines in nat asp form will turn 8000 to 9000 rpm with the right cams.

This will get a bit technical unfortunately, I am an engineer and don't like just stating these things without justifying it with hard fact rather than opinion.

The limits of power production in turbo A series engines are a function of the poor breathing of the head and the silly long stroke design which has it's roots in the old RAC horsepower tax of the 1930's where big bore engines were rated higher for tax, so we were lumped with a generation of long stroke sloggers. As engines evolved in the UK, designers stuck with what they knew while the rest of the world used short stroke big bore engines. This has some nasty side effects.

Small capacity long stroke engines are more thermally efficient as low RPM sloggers. (hence 60mpg city E 998 A + motors), good for low down torque, but a disaster for big power production. Our engines' outputs are a triumph of development over the design limitations.

The small bore long stroke nature of the A series engine means the volume of the cylinder increases and decreases dramatically as the engine cycles and produces massive pressure spikes from low to high BMEP, (brake mean effective pressure), as the motor goes through each otto cycle.

These pressure spikes and lower average cylinder volume over the induction period made available by the cam timing made it harder to pile the volumes of air and fuel into the engine that you can with more modern designs and ignite it without causing detonation.

It also gives lower average cylinder pressures and lower power for its capacity and this has a further effect in that the cam timing, boost levels and fueling have to be conservative to keep the engine out of detonation as peak cylinder pressures are held for longer compared to a short stroke big bore engine.

A big bore short stroke engine has much more average swept volume throughout the otto cycle, so can take in more charge per revolution for a given cam duration, and will always produce more HP per litre, have more tollerance to detonation, (which, incidently, Honda proved in the 60's in their motorbike engine development program where they found the higher the RPM the less sensitivity to octane values), entirely due to the reduction in pressure spikes and the greater cylinder volume as you approach TDC and the ignition point, in short, the mixture is squezed harder for a shorter period of time. This produces more power.

The problem with long stroke engines trying to produce extreme power is piston speeds. They are higher with long stroke engines. This is a problem for a number of reasons.

First, it limits the RPM potential of the engine. There is a mechanical limit to the piston speeds that can be safely sustained.

Second, and the reason these limits exist is the loading cycles of a long stroke engine. Despite what you may have heard, upping the boost in engines does not do much to increase mechanical stresses in an engine. They DO increase thermal loads, but that is not the same thing at all.

The loads high boost, (more correctly, high cylinder pressures), induce are compressive in nature. The engines components are much stronger in compression than they are in tension. In fact, compressive forces increase the mechanical strength of the rods and rod bolts.

When designing an engine, an engineer will look only at tensile loads when specifying con rod, rod bolt, pistons and piston pin materials. The reasons for this are simple.

The most violent loads a reciprocating engine sees are those at TDC and BDC where the components are violently cycled from compressive loads to tensile loads.

These tensile loads, (which and engine is weakest under), are a minimum of 50% greater than the compressive loads, (which an engine is stronger under), so you can more or less boost an engine as much as a given fuels octane will allow without that being the cause of the engines demise.

Now it's obvious when you think about it that if a long stroke engine has much higher piston speeds due to the longer length of travel down a bore, then it will experience much higher loads at BDC and TDC and this puts greater demands on the materials.

It tends to break things at high rpm and/ OR under rapid engine acceleration, (like, say in a turbo engine....).

These tensile loads increase with the square of the speed. For example, going from 6000 to 7000 rpm increases the loads that cause engine damage by 144%! Given that in order to produce extreme power, you have to turn higher rpm, this gives the short stroke engine another advantage. Reducing the stroke decreases these stresses.

Third, long stroke engines suffer from greater frictional power losses. These friction loadings also increase cylinder thermal loading and increase bore wear, leading to poor ring seal, charge dilution/octane reduction by loss of oil ring control leading to contamination of the inlet charge with oil. The greater angle of the long stroke also increases friction losses further. (Remember, I am talking about extreme conditions, BMEP's near the practical limits of 400 psi and 8000 rpm, not your average turbo 1380!).

Another point to the short stroke big bore engine as they rev much faster than small bore long stroke motors, (the reason becomes clear when you realise that the short stroke big bore motor having a more consistant dynamic BMEP as the average swept volume is more consistant through otto cycles also producing more average torque under the curve = more hp for a given capacity).

So there are sound engineering reasons to build a short stroke turbo engine.




Superior cylinder filling, building more low rpm torque providing the turbo is baised towards low rpm volume flow.

This superior flow allows the use of a higher flowing turbo without running into surge conditions than would be the case for a long stroke engine.

This also translates into more high rpm power as the turbo can be sized to provide higher volume flow at lower boost pressures for higher adiabetic efficiency, leading to lower inlet temps, higher charge denisty and lower thermal loading. Remember that the ideal turbo match will produce as much flow as possible at as low a pressure as possible.

Lower piston speeds reducing the tensile loads on engine components.

Faster engine acceleration.

Less sensitivity to fuel octane allowing more ignition advance plus more fuel and air to be crammed into the cylinder before the onset of detonation.

More power per cc.

Allows more radical cams with less loss of average cylinder pressure.

It's not just theory, 80's F1 engines made 1500 bhp out of 1500cc on the basis of the research they did following the principles they discovered above, (and a witches brew of fuel helpled!).

[Brett]

It's not just theory, 80's F1 engines made 1500 bhp out of 1500cc on the basis of the research they did following the principles they discovered above, (and a witches brew of fuel helpled!).

some nice info there

also can i say that in the 80's the bmw 1500cc f1 engine block was not made specifically for racing but was taken out of used road car's :saywhat:

[Bungle]

can i ask a quick question about all this

as i understand it a short stroke engine has

a short stroke (up and down bit)
b large bore (big hole)

but everything i have ever seen says you dont bore a turbo engine out more than a 1293

so if you have the bore of a 1293 and a short stroke dont you end up with a small cc engine ?

[The Matt]

can i ask a quick question about all this

as i understand it a short stroke engine has

a short stroke (up and down bit)
b large bore (big hole)

but everything i have ever seen says you dont bore a turbo engine out more than a 1293

so if you have the bore of a 1293 and a short stroke dont you end up with a small cc engine ?

the short stroke is achieved by offest grinding the crankshaft so effectively the pistons have less travel, so yes, it would make a small engine even have an even smaller CC, but the advantages of shorter stroke engines is their abiltiy to rev (in effect I guess it's because of less centrefugal force).

I was just asking the question, because I have seen people do 1380 turbos and quite a few 1330s too, but you never see short stroke turbo builds in the mini scene (that I knew of).

So, in the world of normally aspirated engines, there are some massive advantages of having shorter stroke engines (like the trusty, but quite rare 1340). The main one being that you will have an engine capable of high revs, larger power band and more driving in each of the lower gears.

Edited by mini mad me, 28 January 2006 - 09:12 AM.

[Jammy]

Must just add, I have no engineering experience and very little engineering knowledge, but I managed to read all of that and understand it, I'm really looking forward to the magazine when I comes out!

[The Matt]

Yeah thanks Maga7ine, that was some really good info! :wink:

[MAGA7INE]

can i ask a quick question about all this

as i understand it a short stroke engine has

a short stroke (up and down bit)
b large bore (big hole)

but everything i have ever seen says you dont bore a turbo engine out more than a 1293

so if you have the bore of a 1293 and a short stroke dont you end up with a small cc engine ?

Hi, well there are a few 1380 turbo engines around now. I think that was another case of someone, (Vizard I think), assuming that it could be the case that thicker cylinder walls were desirable. In fairness to him, he never did state this as a fact, it was just a thought which was quoted over and over. In practice it has not proven to be a problem. Modern materials and tighter production tollerances in piston and ring sets have also helped.

Think about the magnitude of the forces involved. The most BMEP you will ever see on the far side of competition level turbocharging is 380 to 400 psi. 380 psi is not going to deform a steel cylinder wall.

To be on the safe side, I am boring my block oversize, sleeving it and the finished bore size will be 72 mm. That is only because the engine will be buring methanol with a small, (10 bhp), shot of nitrous at low rpm to kick the massive turbo over the surge line at the drags

The capacity isssue is a very good point. Unfortunately due to the design limitations of the A series, we can't do as motor bike engine manufacturers do and have 90mm bores

Yes, you are sacrificing some capacity, just over 100cc with a 1071 crank and 73.5 mm bore, more with a 1293 bore as you correctly stated, but you are making far more use of that capacity due to the reasons given above.

[MAGA7INE]

can i ask a quick question about all this

as i understand it a short stroke engine has

a short stroke (up and down bit)
b large bore (big hole)

but everything i have ever seen says you dont bore a turbo engine out more than a 1293

so if you have the bore of a 1293 and a short stroke dont you end up with a small cc engine ?

the short stroke is achieved by offest grinding the crankshaft so effectively the pistons have less travel, so yes, it would make a small engine even have an even smaller CC, but the advantages of shorter stroke engines is their abiltiy to rev (in effect I guess it's because of less centrefugal force).

I was just asking the question, because I have seen people do 1380 turbos and quite a few 1330s too, but you never see short stroke turbo builds in the mini scene (that I knew of).

So, in the world of normally aspirated engines, there are some massive advantages of having shorter stroke engines (like the trusty, but quite rare 1340). The main one being that you will have an engine capable of high revs, larger power band and more driving in each of the lower gears.

This is almost right It's not centrifugal force that is the issue with revving, rather the piston speeds, tensile loads and the fact that long stroke engines suffer from having LESS average swept volume which limits the rate of acceleration. (Though the shorter stroke and the lesser inertial energy and lower induced tensile loads from lower piston speeds does load the three main bearing bottom end less, so you are quite correct in that sense, I think that's what you meant).

If you think about it, imagine the volume of the SSBB engine as it starts going down the bore. Because of it's larger bore and shorter stroke, for each degree of crank rotation the engine gains more volume more quickly than a long stroke smaller bore engine. So for a given pressure ratio, (boost level), the engine is taking in MORE air fuel mixure than a SBLS engine at a faster rate. Also consider what the cam is doing. This fast cylinder filling also happens to coincide with peak cam lift more in the big bore engine than in the small bore engine.

At peak lift in a typical A series street cam, the big bore engine will have cycled through a much larger percentage of it's total capacity in relation to cam duration. This in turn leads to greater efficiency due to the much better cylinder filling. The short duration cam is closing before the long stroke engine is done trying to fill the cylinder.

But this gets worse! A long stroke engine then needs more cam duration to make more power, but because of its long stroke, a long duration cam reduces effective cylinder pressures and turns the motor into a slug!

A short stroke engine can stand more cam lift and duration, (ignoring for now the effect of the 5 port head...I am not using it), and will make better use of an increase in duration without killing low rpm power as much.

There have been alternative cranks in the history of mini production, the 970 crank was used in the big bore blocks, and the one I am using, the 1071, which has the same stroke as an 850 mini.

While on that subject, small bore 850 factory cranks have a great deal of potential when fitted into 998 blocks bored out to the new minispares oversize, (over the previous maximum of +60 thou), when fitted with the custom thrusts. I have built an 850/998 combo and it was a jewel of an engine, but that's another subject, one that will be in the magazine later this year.



Thanks for the kind words Jammy! Unlike some other magazines, all the staff on the mag are engineers and scientists and even the designers drive and work on their minis, so you can expect a lot of indepth tech like this in the mag backed up with project engines that are a bit different from the other mags. You will love the first issue! (Though we do have lots of feature cars as well as the tech....)

[The Matt]

can I subscribe to this magazine? I like the sounds of it!

plus, can you build me a short stroke turbo then! lol :wink:

[MAGA7INE]

can I subscribe to this magazine? I like the sounds of it!

plus, can you build me a short stroke turbo then! lol :wink:

Thanks! The magazine will be available from all good newsagents and worldwide direct off our secure web site either as a single issue or a years subscription and you can pay by credit or debit card on line. Any bought via the web will receive a £1 per issue discount, even if you just want to try it out for one issue. This will reward all our forum members who have been following our progress! Subscriptions also help us plan for the future more effectively and will allow us to expand the magazine content much more quickly.

We have waited until the first issue is done and ready to purchase before offering subscriptions as we don't want our readers to have to wait before they get their magazines. There will be prizes awarded randomly to our subscribers who take out a full year subscription, (4 issues), in the first issue period including iPods and other lovely gifts!

The magazine is way ahead of schedule as originally mapped out and we will announce on this and other forums and in the motoring press when we are ready to accept subscriptions. We also have a very very special companion web site under construction which will launch at the same time that will be a mine of this sort of information for all mini enthusiasts!

We will keep you all posted!

Build you an engine, well, there MAY be the odd competition you may like to enter next year.....(hint hint...)

[chairchild]

sounds like your mag is the kinda mag I like then! :wink:

all techy, with proper tech advice! (not bloody telling you to sink your valves into the head - like a certain someone did, in a certain mag)


as a sort of related Q, what sort of power would you expect a LONG stroke engine to make, before it starts ripping the conrods apart?

for example, a 1098, with a 12g940 head. Would you expect it to cope with 100+Bhp's worth of boost?





more curious as to anything else really

[Jammy]

....including iPods and other lovely gifts!

Noooooooo! Look into other MP3 players, the iPod maybe mor popular by aesthetic design, but there are other players that are much better. Firstly the battery has to be replaced after a year on the iPod which costs £50, and you HAVE to use iTunes. Its like giving away a car but saying you can only use certain roads and the engine will have to be replaced after a year.