Just answering a post on the exhaust length thread and remembered an interesting engine setup that a few autograssers have run in the past (and now)

So called "Big Bang" engines. Basically usually run on 4 cyl or 6 cyl engines, pairs of cylinders are fired together.

I knew of a chap (actually Paul Exon of Quorn Engine Developments) running a Vauxhall XE big bang engine a few years ago.

The camshafts were made specially to give identical valve events on cyls 1-4 and 2-3, and the engine management set up that way also. The engine then runs as a big twin cyl.

The principle was to give huge lowdown torque to get off the startline and out of corners.

Sounded VERY interesting when revving at 8000 revs!!!!!!

Problem was the shock torque loading continually destroyed gearboxes, and this was why he abandoned the idea.

Was still producing over 250hp from 2.0 XE engine though!!

There was another chap running a Honda 2.7 V6 as 3 twins in a similar fashion, but I have not seen that car for ages.

No use at all to us locosters but an interesting point all the same!

scat V4 were the same incidentally .iirc

half a nascar v8 .

the bigger the bang , the more torque - hence why lots of cylinders will make hp if you rev it enough , not necessarily torque .

the volvo 4 cyl is preferred at 2.8L in rallycross purely because of this , even tho 5/6 cyl engines at that displacement are common .

When they forst introduced these on 500grand prix bikes all 4 cyliders were being fired at once. the riders initially found them to be unridable but after a couple of years they were reintroduced I suspect with a slightly softer power profile as everone switched over to them.

"Big bang" has been used in Moto GP for some time originally on the V4 500 two strokes to make the power delivery less vicious and to improve tyre life.
Odd ball crank phasing is still used today on Rossi's 4-stroke Yamaha to make his inline 4 cyl motor sound and behave like a 'V4'.
Good eh?

Hows do the physics work here - the flywheel acts as a large damper, so my intuition says whether it sees 2 power strokes per rev or 1, it wouldn't really make much difference.

Gary

Even fire engines have power pulses evenly spaced apart. By having cylinders fire close together ther is a longer pause before they fire again. This allows the tyres to regain any lost traction. This is the reason v-twins did well in superbike. They could get more power to the ground as poser pulses were further apart allowing the tyre to regain grip more easily.

quote:

---

Originally posted by garyo
Hows do the physics work here - the flywheel acts as a large damper, so my intuition says whether it sees 2 power strokes per rev or 1, it wouldn't really make much difference.

Gary

---

quote:

---

Originally posted by bernie955
Even fire engines have power pulses evenly spaced apart. By having cylinders fire close together ther is a longer pause before they fire again. This allows the tyres to regain any lost traction. This is the reason v-twins did well in superbike. They could get more power to the ground as poser pulses were further apart allowing the tyre to regain grip more easily.

---

it must work, or race teams wouldn't use it, but it does strike me as an agricultural version of traction control, and the best way of transmissing the most torque to the ground would be *just* maintaining grip at a contant level, rather than delivering spikes of torque which continually break and regain traction. Maybe this is a way of tuning the power stroke frequency and drivetrain mass so that they're in the sweet spot with simulates a very good traction control system?

tyres don't work in terms of on/off maintaining grip, it's all about slip angles and friction curves.. fairly complicated physics. with normal tyres if memory serves you get most force with some percentage of slip, in the teens of percent or something. depends on the tyre and surface.

I must say I agree with the theory Blueshift but I have read of the pulsing v-twins of the ducatis etc allegedly being better at delivering their power to the tarmac particularly out of corners at GP power levels.

I guess the way I was thinking about it was ABS in reverse with a level of slip in between pulses of drive....

Be interesting to read something definitive on the subject though

Geoff

quote:

---

Originally posted by NS Dev
Yes, the power will be little different because there is a big torque impulse and then a ruddy great gap, but the torque will be greatly increased.

---

Big bang Moto GP engines solved the traction problems for the riders without the necessary throttle control. Mick Doohan didn't need a big bang motor to win his championships; neither did Rossi when he rode the 500s.

The theory behind the big bang was that it might not give you the ultimate torque all the time at the back wheel, but that it gave you as much as you needed without ruining through overheating the tyre with wheel spin.

As above Doohan, and to a similar extent, Rossi, have shown that careful use of the right hand (foot!) with a (what are they called now? The two strokes were called screamers, not sure about the four bangers.) normal power distribution motor can beat a big bang motor. At our level it is much easier to just fit traction control, and it's much cheaper than custom cams.

This is like some v8 and v12 engines. They are called flat plane crank ie if its a v8 they have two cylinders firing at the same time giving four pairs of firing cylinders. Look up falt plane crank in google and you'll find loads. Some tvr's i believe had them. Most american V8's dont though.