Swapping long stroke for short stroke and bigger bore

[digger]

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.

Both have the same swept volume so with each degree the piston sweeps the same amount. If with the same head i don't think equating the rpm where piston speeds are the same is correct.

[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.

Both have the same swept volume so with each degree the piston sweeps the same amount. If with the same head i don't think equating the rpm where piston speeds are the same is correct.

The rpms were based on the practical maximum piston speeds of 25 m/s. It does not address whether the induction system can handle the air mass flow at those piston speeds. If we hold rpms identical, we get differing piston speeds; and vice versa.

[Grp5L]

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.

Both have the same swept volume so with each degree the piston sweeps the same amount. If with the same head i don't think equating the rpm where piston speeds are the same is correct.

The rpms were based on the practical maximum piston speeds of 25 m/s. It does not address whether the induction system can handle the air mass flow at those piston speeds. If we hold rpms identical, we get differing piston speeds; and vice versa.

But it hasn't answered the question which was asked.
We know the SS has potential to make more power as it can survive a higher rpm limit, but at 7k what would both of them be putting out?

[Roundybout]

These theoretical engines always make my head spin. If we take the same exact engine and only differ the stroke something mechanical has to be different to accommodate the change. Rod length? Piston pin height? Taking either engine and changing the stroke makes it a different engine. The one that was optimized for its given package now has a difference. Now if we can optimize and gear each engine, give me the short stroke one that reaches a higher RPM every time.

If we can't change anything but the stroke, I'll still take the higher rpm potential engine. Even if we took the short stroke engine and added whatever was needed to the deck (same pistons and rods, just the stroke ) to give the same combustion space for the long stroke, it's still a different engine

[Grp5L]

These theoretical engines always make my head spin. If we take the same exact engine and only differ the stroke something mechanical has to be different to accommodate the change. Rod length? Piston pin height? Taking either engine and changing the stroke makes it a different engine. The one that was optimized for its given package now has a difference. Now if we can optimize and gear each engine, give me the short stroke one that reaches a higher RPM every time.

If we can't change anything but the stroke, I'll still take the higher rpm potential engine. Even if we took the short stroke engine and added whatever was needed to the deck (same pistons and rods, just the stroke ) to give the same combustion space for the long stroke, it's still a different engine

It isn't a theoretical engine.

[Ted Gerstenslager]

Some years ago, with an A class offshore race boat to which the rules were 1 1050 carb, roller cam, Iron fully ported heads, 510" max inches.
Engine A was short stroke combo 3.750 x 4.625 with a rpm area of 76-7800. The boat was not a lightweight and was a consistent back of the pack finisher. It would haul ass down the straights , but die a miserable death in the corners.

Engine B was built with the same 510" rules, but long stroke, 4.5 and small bore, 4.250, roller cam, ported iron heads that started life as oval and were turned into small rectangular port heads, 1050 carb, and rpm range was 68-7000

Boat would now handle the corners, they did not have to trim the drive to keep rpm up, it would just walk around the corners and accelerate down the straight. It required an entirely different drive, prop set up (bigger), and was flat lining at the top of the straight, but it became a mid/upper pack finisher.
They both made about the same power, but the rpm range was different and the torque of the long stroke engine gave it some ass to get around the corners.

[Belgian1979]

Some years ago, with an A class offshore race boat to which the rules were 1 1050 carb, roller cam, Iron fully ported heads, 510" max inches.
Engine A was short stroke combo 3.750 x 4.625 with a rpm area of 76-7800. The boat was not a lightweight and was a consistent back of the pack finisher. It would haul ass down the straights , but die a miserable death in the corners.

Engine B was built with the same 510" rules, but long stroke, 4.5 and small bore, 4.250, roller cam, ported iron heads that started life as oval and were turned into small rectangular port heads, 1050 carb, and rpm range was 68-7000

Boat would now handle the corners, they did not have to trim the drive to keep rpm up, it would just walk around the corners and accelerate down the straight. It required an entirely different drive, prop set up (bigger), and was flat lining at the top of the straight, but it became a mid/upper pack finisher.
They both made about the same power, but the rpm range was different and the torque of the long stroke engine gave it some ass to get around the corners.

In a car you just install a bigger rear gear to compensate.

[groberts101]

Not an apples to apples but gives an idea of power curve differences between engine designs. Skip to the 8:00 minute mark.

https://www.youtube.com/watch?v=vtdtGgf1BF0

I think it depends on what that power curve shape is going to be matched up with. A lighter car with more gear and the short stroke big bore motor will will be plenty of fun to drive if you like to rev em'. A slightly heavier car with a little less gear might be just as quick in a straight line but likely not as fun in the twisties.

[digger]

Not an apples to apples but gives an idea of power curve differences between engine designs. Skip to the 8:00 minute mark.

https://www.youtube.com/watch?v=vtdtGgf1BF0

I think it depends on what that power curve shape is going to be matched up with. A lighter car with more gear and the short stroke big bore motor will will be plenty of fun to drive if you like to rev em'. A slightly heavier car with a little less gear might be just as quick in a straight line but likely not as fun in the twisties.

that isn't a very good example at all

[AMXstocker1]

Not sure if it was mentioned but the bigger bore combo could have some gain due to increased airflow if valve shrouding was an issue with the smaller bore.

[midnightbluS10]

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

Can you get correct points from a graph that isn't in the correct scale? Since the lines don't cross at 5252, the scale of each axis isn't exactly drawn to any sort of standard is it? How could the numbers he gave be so far off from what the graph shows?

[kimosabi]

Trend today modern engines is undersquare. Why?

[pdq67]

If anybody wants to look, the old HPBook, "How to Hotrod Big Block Chevy's", in the way back of it has a section on this as well as a graph of the two engines used to produce it. Page 128...

One was a short-stroke 4.25" b x 3.46" s = 392" and the other just a stock 4.09" b x 3.76" s = 396". Both were hopped about the same except the 396 was at 12 to 1 CR whereas the 392 was at 11.3 CR. The 392 out powered the 396.

Same deal with the big bore/short stroke 4.44" b x 3.47' s = 430" vs the stock 4.25" b x 3.76" s = 427"... But the 430" was dirtier smog-wise so GM went with the 454"...

pdq67

[kimosabi]

Heads were not as good back then and the inline valve placement definitely favors a bigger bore.

[Grp5L]

Trend today modern engines is undersquare. Why?

Because it suits the way the majority of us drive in everyday life.