Big Bang engine

oldeskewltoy wrote: hmmmmmmmmmm, don't I remember an old Norton motorcycle having both cylinders fire simultaneously???

Actually I think you are referring to the 360* crank on the Norton and most other British vertical twins. The pistons came up together and the spark fired on both cylinders each time. But one cylinder was at the end of the compression stroke while the other was ending the exhaust stroke. So although the plugs fired every rotation, the cylinders fired one rotation out from each other.

I remember that Sears had a two stroke motorcycle engine called the "Twingle" where two cylinders shared a common head and 360* crank so both cylinders fired making you think you had a 250cc single. We took a few in on trade but they weren't anything to brag about.

The current Yamaha MotoGP and sport bike arrangement is no longer big bang, but instead has one piston every 90 degrees. The benefit is that while two pistons are stopping at the bottom and top, the other two have peak momentum, and the result is a smoothing of overall angular momentum, reducing driveline (and contact patch) noise. That is, accelerations and decelerations which may cause work which isn't aligned with the desired result.

It does result in a somewhat odd firing pattern...

Yamaha crossplane crank

ransom wrote: The current Yamaha MotoGP and sport bike arrangement is no longer big bang, but instead has one piston every 90 degrees. The benefit is that while two pistons are stopping at the bottom and top, the other two have peak momentum, and the result is a smoothing of overall angular momentum, reducing driveline (and contact patch) noise. That is, accelerations and decelerations which may cause work which isn't aligned with the desired result. It does result in a somewhat odd firing pattern...

Ooh, good point. Whenever I think "big bang" it translates in my head to "uneven firing order" and not multiple cylinders firing at the same time.

One crank throw every 90 degrees sounds like a half-a-V8. Which did work in midgets and such, but everyone eventually went to even firing crankshafts as it got to be cheaper to buy those instead of screwing with OEM cranks. And then they went to actual four cylinder engines.

Still, the idea of taking an LS1/LS2 that blew a cylinder wall out (should be trivial as this is a common failure) and blanking off the dead bank's deck is a continual favorite of mine. They even made it extra easy by sealing the heads to the valley and using a separate valley pan! Just blank off the water pump to that bank and throw a couple Weber-like throttle bodies on the remaining bank.

What you wind up with is a 3-liter-ish four that can make naturally aspirated power on par with a $20,000 Esslinger engine for a fraction of the price.

Still, the idea of taking an LS1/LS2 that blew a cylinder wall out (should be trivial as this is a common failure) and blanking off the dead bank's deck is a continual favorite of mine. They even made it extra easy by sealing the heads to the valley and using a separate valley pan! Just blank off the water pump to that bank and throw a couple Weber-like throttle bodies on the remaining bank. What you wind up with is a 3-liter-ish four that can make naturally aspirated power on par with a $20,000 Esslinger engine for a fraction of the price.

Reminds me of the pontiac trophy 4, half a 389. It's funny I have thought the same thing about running a half v8 with a high flowing head and big ci.

Dont large 4s have nasty harmonic issues? Last I checked, 4s larger than 2.0 litres had secondary vibrations in the crank that would cause things to fragment, which is why the 2.5L Pontiac made was a slow rev engine(Redline at 5k rpms). This why in 95, the Quad4 got balance shafts, in 88 the Iron Duke got them, and the EcoTec got them from the start. And yet, the LN2 2.2L in my S10 doesnt have them. Hmm

In reply to AquaHusky:

Indeed. Inline 4s have unresolved second-order harmonics (vibration at twice the RPM). That's because the pistons accelerate at a different rate at the top of their stroke than their bottom, due to the angle of the con-rod in relation to the angle of the crank. Basically, they get yanked down harder than they get pushed up.

You can get away with bigger four-bangers by adding balance bars, but at that point you're just adding stresses and losses within your engine. Keep in mind you would need to spin those balance shafts at twice the engine's rotational speed, and friction from bearings is not linear with speed.

And that's another reason I like the idea of a twisted-crank four. You DON'T get coincident varying piston acceleration/deceleration, and you don't get the dead time at TDC/BDC when all reciprocating mass slows/stops. So it shouldn't have the horrible harmonic issues that a large four normally has.

What you do get is an uneven firing order, which is a problem in itself, but in this case it is a problem that we're (I'm) actively looking to get.

The Pontiac Tempest engine was even-fire. And yes, being a 194ci four with no balance shafts, it shook a lot. Note that 194ci is .2l larger than a 968's engine.

AquaHusky wrote: And yet, the LN2 2.2L in my S10 doesnt have them. Hmm

The LN2 holds up because it is a tank of a boat anchor engine, I ran one more or less on the 6000rpm redline for a couple years, pretty sure I burned the rings out of that car I beat it so hard, was kinda down on power by the end.

Whenever I hear big bang motors mentioned I think of Wayne Rainey, Kevin Schwantz, Wayne Gardner and Eddie Lawson. Mick Doohan opted out and rode a crazy wheelspin generator but the rest were bangers and the eary 90's WSB and MotoGP were some of the best racing ever.