No life (a.k.a. DattoMaster)  
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2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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There's so many ifs and buts and so forth - almost everything on an engine has an effect on something else, so there's no single best option or anything. Not ever. What it is truer to say is that you have to generally find the combination of 'compromises' that will achieve as much of what you need it to, with as little of the things you can't put up with (in a very abbreviated example - if you wanted max power for drag racing, you'd have a screaming high revver that might only last one competitive season of races, maybe less than an hour of operation at full output before you have to tear it down, and various things like con rods and valve springs may be considered 'consumables')
As a result, this post will go all over the place on many tangents, feel free to skip it, and also remember it's opinion/experience not etched in stone absolute truth.
The thing that has to be kept in mind with mega overbores and longer strokes, and the resultant increase in capacity - even IF you can do it with absolutely ZERO structural or reliability issues, you are STILL having to feed it through an A series head. Even with the worlds best head guru, porting it to perfection, you aren't going to get unlimited flow, and flow correlates pretty closely with power potential. And that power potential isn't massively different (once you start getting capacities beyond the reasonable range the head could support) with a bigger capacity engine vs say, an a15.
What the bigger engine WILL do for any given headflow, is make that 'same' (or similar) power at a lower rpm point than the a15. Within some limits, it can tend to have a slightly wider power band, but once you start going way big on the stroke vs the bore size (and these big capacity engines D is referencing are certainly in that boat) you eventually get to a point where the powerband will close up a little. Still it is a possibility. This lower powerband initially seems like a good thing, but there can be drawbacks. Such as - the lower you make the same hp the higher the torque output (since hp is a factor of torque and rpm combined) . To take a drastic example, if you made 100bhp at 8000rpm, and another motor made 100bhp at 4000rpm, then the one making it at 4000rpm would actually have double the torque of the higher revving engine at that rpm. Realistically that's not a likely real world event, you might instead make the power 1000-2000rpm lower with the extra large capacity engine. And about (estimate, not calculated) 30% more torque. And we must remember that it is actually torque that stresses and breaks driveline components (well if you disregard shock loading, like jumping off the clutch from a standstill at high rpms!) . Even if you didn't over stress the driveline, with a much longer stroke, the actual piston speed at lower rpms might be just as much as that of an a14 at higher rpms. The next thing is about 'useable' powerband. It'd be a bit of a letdown to have a big lower revving torquey engine with a diff ratio of 4.11 (I think that is the ratio in the bakkies but happy to be corrected if I'm wrong) where the engine is screaming (relatively speaking) just out on highway cruises. Of course you could change the diff ratio, I have no idea the availability of other ratios from cars commonly found in SA for parts donors. But if you couldn't find alternate gearing, then the a14 would be relatively a lot happier at highway cruise speeds.
As you increase the stroke (esp if one compares the stroke of an a14 to a 1700cc (or thereabouts) a series based combo, you have less space for a long (long is probably not the right word, more like 'optimal length') con rod, and you can only go so high with piston pin placement in a piston before it is into the oil scraper ring (and whilst such a setup is possible for a shorter lifespan perhaps competition engine, long term it isn't good for oil consumption or general engine life) . You can in fact get pistons made with a higher pin placement and only one 'top ring' - no second ring, just the top ring and the oil scraper rings. This is of course more compact and allows for the pin placement to be higher, which then leads to being able to run a longer rod. The problem is there just isn't enough room to get it ideal. And that means that during period just either side of 90 degrees crank angle, where the rod is at maximum angle, well the shorter the rod, the more severe this angle is and that puts a lot more side thrust on the bore wall. Now the cylinder/sleeve might hold up to it structurally, I'm not suggesting it'll smash out the side, what it does do is lead to far more friction (which can cost power) and accelerated wear, so ring seal will start to deteriorate sooner (all relatively speaking of course, I'm not suggesting it'll happen in a week or two)
i've mentioned it elsewhere, but to give an 'outside' example - the mini a series engine (not the far superior datto one) is 1275cc in its biggest factory guise. It is actually possible with careful block selection and a custom crank to stretch it out to 1600cc. But the bore walls end up so thin that they flex and compromise ring seal, and teh rod angularity/side thrust costs a bit in terms of friction - and the result is that they actually make less power than a 1440 or 1480cc combo (which is mostly achieved with bore increase but also a more modest stroke increase. It's certainly not that bad for the datto a series with bigger capacity, but it at least points out that things only improve for a given amount of change, and eventually you can go too far and backslide with regard to output or reliability.
Just a personal take - but the main advantages of such big capacity a-series are really only if you need a lower powerband and aren't allowed to run a turbo (perhaps for a certain racing class) - it'd probably do very well in a rally car. But it'd be a VERY expensive build indeed, and even if it was good on fuel (which isn't actually necessarily the case) you'd have to be running it for a few hundred thousand miles for the fuel savings to account for the cost of the engine build itself. It would also do pretty well in a performance oriented but regularly street driven vehicle, where for whatever reason the owner doesn't want to turbo or supercharge it.
I've talked about the limitation of head flow. Well with turbocharging, the head 'technically' flows about the same, but it is (under boost) ingesting much denser air so it essentially gets you right around the flow restrictions. Of course you'll get more power still with a ported head and turbocharging, but done on a massively tight budget, boost tends to be the only real player.
When you start running boost, you are limited in the sort of compression ratios you can safely run for any given fuel/boost level. And the bad news there is that the lower the static compression the worse the part throttle fuel economy. You can get around it a little bit by running std compression ratios and a fair rate of water injection, but that too is not without issues (it has been seen to cause some corrosion when used in very high volumes on some engines, so whilst it can be great while it works, at high flow rates, the head isn't going to be fantastic in a few years time, potentially. WIth the sort of water flow rates that most people use this is unlikely to be an issue, you have to get up above something like 1:2 ratio (for every 2 parts of fuel, one part of water is metered in) for the effect to really set it. It can also find its way past the rings so you have to keep an eye on oil condition and investigate slightly more frequent changes if the oil shows sign of water contamination.
The other thing that is done of course is to run a richer mixture under boost. the extra fuel isn't burned, it is used for its cooling effect to help prevent detonation/pre-ignition (same reason the water is used). The more water you run the less rich you need it under boost (still well rich of stoich mind you). But the fact remains that when you are driving it hard, it'll be running rich and using a lot more fuel. Whilst technically you can get ok economy running off boost for economy cruising at part throttle, if my own experience is anything to go by, once you have a car with a turbo, you'll find a thousand reasons to be driving it on the limit :)
What else - as mentioned a turbo (or supercharged) engine basically breathes in as good as the head can flow, but the volume of air/fuel it is ingesting is much denser. Theoretically at about 15psi you'd be taking in twice as much as a non boosted engine that was otherwise the same. In practice, you'd probably have to go to 18-20 to get there. double the air/fuel ingested (if burned effectively, which isn't too hard) will double the power. It'll also (of course) double the torque. This has two effects - for one thing, you can run much higher gears (lower numerical ratio - like say 3.9 or 3.7:1) and still out accelerate any non turbo engine even one with 1700cc. But you are of course back with the problem of the extra torque being a drivetrain killer. On this front, however there is a little bit of good news. It's about how the engine burns the fuel that matters. When you ingest twice as much air/fuel, then it will burn and produce higher cylinder pressures, BUT it also means that as the piston moves down the bore, the pressure will still be high enough to exert a significant 'push' for a lot longer than a non boosted engine. So it's sort of a longer burn. Which basically means that for X amount of torque, the boosted engine would be more gentle on the drivetrain than if it made the same amount of torque in an NA engine. In a similar way a 2 litre 4cyl engine with X amount of torque would only have 2 firing events per revolution, a small v8 engine that made teh same amount of torque would have 4 firing events per revolution, each producing half teh amount of force as the 2 events on the 4cylinder - meaning the small v8 would be easier on the drivetrain considerably (of course there aren't too many 2 litre v8s out there so this example is theoretical)..
the big advantage of turbos is just how easy they make power, and anyone with access to welders and enough practice/skill to use it well, can make the necessary 'plumbing', and even a real 'ghetto' low buck combination can produce remarkable results. Of course ported head, new cam with more optimal specs for a boosted engine etc will yield even better results but it's pretty decent even without all of that.
Posted on: 2013/6/28 11:12
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