Swapping long stroke for short stroke and bigger bore

[Grp5L]

Lets take two identical engines; 4 cyl, 16v head, 2.5 litre capacity, mildly modified with a slight increase in CR and some 'bigger' cams, both timed the same.
Both the same apart from one thing: the bore and stoke have been changed:
2.5 LS (long stroke) = 100mm stroke / 89mm bore
2.5 SS (short stroke) = 94mm stroke / 92mm bore

I would say that the power outputs should be (give or take maybe 3bhp/3ftlbs) the same, but i'm being told:

LS made maximums of 198 whp and 215 ft/lbs both @ 7k
SS did a max of 220 whp at 7k and 175 ft/lbs @ 4600. (it made 196 ft/lbs @ 7k)

So the short stroke made 22whp more at 7k.

Now a chassis dyno isn't a good comparator, but what do you think of that outcome?

[jed]

Depends on what u want to do with the engine and how much u trust the data.
John

[RevTheory]

If you have time, there's some good stuff in this thread. It sounds like bore and valve is the way to go over stroke.

viewtopic.php?f=1&t=47765

[Grp5L]

If you have time, there's some good stuff in this thread. It sounds like bore and valve is the way to go over stroke.

viewtopic.php?f=1&t=47765

I will, but it seems to be dealing with engines which have had their potential explored and maximised, the engines i'm referring to haven't. Bore and stroke (and rod ratio of course) are the only things which have changed.

[iainb123]

Hi, long time lurker, first time poster.

In my humble opinion, Before you plan any changes, you need to double check the data you have been given, as it does not add up.

By my reckoning, 215 lbs/ft @ 7k makes 286.6 hp, 196 lbs/ft @ 7k makes 261.2hp. something is very wrong with your numbers. As you expected, usually, small changes in bore and stroke ( giving the same capacity ) will have very similar outputs, unless something has been heavily compromised to achive the stroke or bore increase.

[427dart]

Bigger bore/short stroke will make a bit more top end power and the Smaller bore/long stroke more low end torque.

[Tom Walker]

Large bore and short stroke/ smaller bore and longer stroke comparisons have been debated as long as the internal combustion engine has been around. Each has it’s application where it works, depending on what the engine is designed to do best. In my opinion, as a very general rule of thumb, larger bore version for higher R.P.M. , with it’s ability to fit larger valves for same displacement enables the potential for better breathing. Longer bore for slower turning industrial type engines. Just generalities, not hard and fast boundaries, as there are many variables that come into play and characteristics that accompany which ever design you choose. What is the engine supposed to do best is a crucial question.

[David Redszus]

Lets take two identical engines; 4 cyl, 16v head, 2.5 litre capacity, mildly modified with a slight increase in CR and some 'bigger' cams, both timed the same.
Both the same apart from one thing: the bore and stoke have been changed:
2.5 LS (long stroke) = 100mm stroke / 89mm bore
2.5 SS (short stroke) = 94mm stroke / 92mm bore

I would say that the power outputs should be (give or take maybe 3bhp/3ftlbs) the same, but i'm being told:

LS made maximums of 198 whp and 215 ft/lbs both @ 7k
SS did a max of 220 whp at 7k and 175 ft/lbs @ 4600. (it made 196 ft/lbs @ 7k)

So the short stroke made 22whp more at 7k.

Now a chassis dyno isn't a good comparator, but what do you think of that outcome?

Using your engine data and assuming a SCR of 10:1, a TCR of 7.97, both will produce a BMEP of 10.34 bar.
Using the same rod length (not ratio) and running to a piston speed of 25m/s we find:

LS engine
215 hp @ 7500 rpm

SS engine
231 hp @ 8000 rpm

Since both produce the same BMEP. the performance difference is due to the higher rpm potential of the short stroke engine.

[PackardV8]

Since both produce the same BMEP. the performance difference is due to the higher rpm potential of the short stroke engine.

Do we accept this as a given with today's technology? Honda has been building undersquare designs which have very high redlines; way beyond 7,500, so where does that limitation come from?

[woodaca3680]

What about the friction horsepower lost due to the longer stroke.

[David Redszus]

Since both produce the same BMEP. the performance difference is due to the higher rpm potential of the short stroke engine.

Do we accept this as a given with today's technology? Honda has been building undersquare designs which have very high redlines; way beyond 7,500, so where does that limitation come from?

The piston speed limit comes from recommendations by Mahle. I think it is based on material strength and survivalbility rather than pure performance. Some drag racers have exceeded that piston speed limit but usually with frequent piston replacement.
A Formula One engine running at 18,000 rpm does not exceed the 25 m/s piston speed limit.

What about the friction horsepower lost due to the longer stroke.

The friction increase of a longer stroke could be somewhat negated by increased ring friction due to greater bore area.

We have not considered the potential power increase that would be obtained by an increase in valve size with the short stroke engine.

[pdq67]

As for Honda's little S2000 engines.

I figure that regardless of bore, as long as the rod to stroke ratio is OK, it really doesn't matter??

But I may be off here..

But if a small bore chokes off air flow, then it get's important here also, as do valve sizes.

pdq67

[Grp5L]

This is the graph in question:

2.5ss vs 2.5 ls.png

[Bazman]

Lets take two identical engines; 4 cyl, 16v head, 2.5 litre capacity, mildly modified with a slight increase in CR and some 'bigger' cams, both timed the same.
Both the same apart from one thing: the bore and stoke have been changed:
2.5 LS (long stroke) = 100mm stroke / 89mm bore
2.5 SS (short stroke) = 94mm stroke / 92mm bore

I would say that the power outputs should be (give or take maybe 3bhp/3ftlbs) the same, but i'm being told:

LS made maximums of 198 whp and 215 ft/lbs both @ 7k
SS did a max of 220 whp at 7k and 175 ft/lbs @ 4600. (it made 196 ft/lbs @ 7k)

So the short stroke made 22whp more at 7k.

Now a chassis dyno isn't a good comparator, but what do you think of that outcome?

Looking at your graph - I have corrected your figures.

LS made 198whp and 149ft/lb at 7k
SS made 220whp and 165ft/lb at 7k
LS peaked around 160ft/lb at 4600
SS peaked around 175ft/lb at 4600

The LS made significantly more TQ under 2500rpm, the SS significantly more from 5000 up. The SS also made more peak TQ and has more area under the curve.... the LS motor only shines under 2500rpm. Results agree with logic, but the difference is way bigger than I'd expect. There's a lot of very capable long stroke motors out there so other factors come into play I suspect

[Grp5L]

Lets take two identical engines; 4 cyl, 16v head, 2.5 litre capacity, mildly modified with a slight increase in CR and some 'bigger' cams, both timed the same.
Both the same apart from one thing: the bore and stoke have been changed:
2.5 LS (long stroke) = 100mm stroke / 89mm bore
2.5 SS (short stroke) = 94mm stroke / 92mm bore

I would say that the power outputs should be (give or take maybe 3bhp/3ftlbs) the same, but i'm being told:

LS made maximums of 198 whp and 215 ft/lbs both @ 7k
SS did a max of 220 whp at 7k and 175 ft/lbs @ 4600. (it made 196 ft/lbs @ 7k)

So the short stroke made 22whp more at 7k.

Now a chassis dyno isn't a good comparator, but what do you think of that outcome?

Using your engine data and assuming a SCR of 10:1, a TCR of 7.97, both will produce a BMEP of 10.34 bar.
Using the same rod length (not ratio) and running to a piston speed of 25m/s we find:

LS engine
215 hp @ 7500 rpm

SS engine
231 hp @ 8000 rpm

Since both produce the same BMEP. the performance difference is due to the higher rpm potential of the short stroke engine.

But what would both of them be delivering at 7000 rpm?