Stroking vs Overbore?

[marcc]

Hi guys, looking on the various threads on here it seems that most people stroke the block as opposed to going overbore. Just wondering why this is? I get on a b16b/b18 that stroking is more cost effective as parts are interchangeable. But for b16a the only option I've come across are massively overpriced B.C./ toda kits.

 

[JohnTurbo]

Cunningly swapping b16 for b18 would be the best method of stroking.

 

[Kozy]

Theoretically you can get more power from overboring than you can from stroking, but it involves a lot more work, and involves a lot more risk of problems from errors in the machining.

 

[Murray]

With stroking it will give gains in torque.

 

[elchunk]

Stroking is generally easier, also the cylinder wall isn't that think so there is only so much you can increase the bore. Before you need to do a lot if machine work and set in liners.

 

[Kozy]

With stroking it will give gains in torque.

Despite the common knowledge, increasing the bore is equally effective at increasing the torque. You can make the same torque with a B16 or a B18 crank.

 

[Murray]

Despite the common knowledge, increasing the bore is equally effective at increasing the torque. You can make the same torque with a B16 or a B18 crank.

yeah, as the famous saying goes " no replacement for displacement"
I just see it as having longer leverage on the crank. Could be that it has a slightly different torque characteristic from an overbore.

 

[Kozy]

You're right that it's the displacement that matters. It's easy to see the longer crank throw and think of the bigger leverage, but it's easy to overlook the fact that for a fixed displacement a longer stroke means a smaller bore. A smaller bore means less force generated from the cylinder pressure, pretty much exactly offsetting the increase in leverage.

The big difference is that with a longer stroke, you have to reduce the maximum RPM, which reduces your maximum power. With increasing bore, not only can you maintain the same RPM with increased torque (higher maximum power potential) but the maximum RPM can often increase to as you unshroud the valves.

The power limit of any NASP engine is given by the equation 9.694 x 10^-3 x Bore (mm) ^2 x Number of cylinders.

Bust out the calculator and go check it against any highly tuned NASP piston engine. F1, Nascar, BTCC anything.

 

[KieranEG6]

Essentially destroking a B20vt with a Balanced B18C crank would seem like the most cost effective/ least risky ways of in essence increasing bore but keeping a friendlier rod stroke ratio than the average b20vt. Don't seem to see it done that often though.

Overboring seems pointless on B series when you consider how many larger bore blocks you can use.

 

[Kozy]

Yes that would work, though really the decrease from a B20 stroke to a B18 stroke is not that significant so there seems little point in reality.

When it comes down to it, the only reason you'd destroke an engine is to fit in a specific racing class. If you're capacity is limited like that, it's almost always better to go short stroke and large bore.

On the road, as much as possible of both is best!

 

[Murray]

You're right that it's the displacement that matters. It's easy to see the longer crank throw and think of the bigger leverage, but it's easy to overlook the fact that for a fixed displacement a longer stroke means a smaller bore. A smaller bore means less force generated from the cylinder pressure, pretty much exactly offsetting the increase in leverage.

The big difference is that with a longer stroke, you have to reduce the maximum RPM, which reduces your maximum power. With increasing bore, not only can you maintain the same RPM with increased torque (higher maximum power potential) but the maximum RPM can often increase to as you unshroud the valves.

The power limit of any NASP engine is given by the equation 9.694 x 10^-3 x Bore (mm) ^2 x Number of cylinders.

Bust out the calculator and go check it against any highly tuned NASP piston engine. F1, Nascar, BTCC anything.

My calculator will **** itself if I put that in there
As I said, and so have you, a longer stroke gives a different torque characteristic with the reduced rev limit moving the torque band down the rev range.
Swings and roundabouts really, its all down to what you want from the car.

 

[Kozy]

Haha, it's not that bad, but I built it into an article I wrote here if that's easier.

An interesting one to think about, if you have a look to the bottom of the page there's a 'PI Index' calculator, that will effectively rate your level of tune with a figure you can compare against any other NASP four stroke piston engine.

The higher that score is for any given engine, the less effective increasing the stroke will be at increasing power. Much over about the 900 mark, I would say it's not worth it. When this is the case, you really are better off going with bigger bore to increase power further.

 

[oifovo]

I love these kinds of threads If my calculations are correct a B16B crank and rods in a B20 would bump up the displacement to 1715 cc (compared to B16Bs 1595 cc), combined with light weight high comp pistons and accordingly aggressive cams it should make for a good screamer.

 

[Kozy]

An 84 x 77.4mm B Series would be interesting but maximising the RPM potential of the short stroke to make it worthwhile is the tricky bit. If you can make the most of the 10,000rpm potential then it would be a potent engine for it's capacity, but it's an expensive route to making a 260bhp B series when you can just as easily (relatively of course) make the same with the B20 crank and have a much more useable engine as a result of the increased torque.

 

[Murray]

Haha, it's not that bad, but I built it into an article I wrote here if that's easier.

An interesting one to think about, if you have a look to the bottom of the page there's a 'PI Index' calculator, that will effectively rate your level of tune with a figure you can compare against any other NASP four stroke piston engine.

The higher that score is for any given engine, the less effective increasing the stroke will be at increasing power. Much over about the 900 mark, I would say it's not worth it. When this is the case, you really are better off going with bigger bore to increase power further.

Il get a better look at your site tonight. Try and get my head around some of that