| Author | Message | ||
| Mitch Alsup (Mitch_alsup)
Junior Member Username: Mitch_alsup Post Number: 141 Registered: 4-2002 |
Ah, yes, the mistery of balancing cranks and power delivery.......... By numbering the banks as Left and Right and ignoring the piston numbers, you can get a sense of the intake and exhaust breathing patterns. A 90 degree crank (typical of american V8s) has a breathing pattern of LRLLRLRR (Chevy, Dodge) or LRLLRRLR (Ford). These patterns allow each bank to have one piston rising, one decending, one at Top and one at bottom at all times. This has inherently good secondary balance. But consequently, to get the headers to work correctly they use cross over pipes (X-pipe or H-pipe) to get balanced exhaust and minimize the burble of the engine note. In order to balance the breating pulses at the intake, there has to be a common plenum after the throttle body which decreases throttle response. In addition, due to the size of these engines and the engine bays they sit in, the intakes are contorted to fit between the banks adding numerous curves and flow restrictions. All real racing V8s plus Ferraris and Lotus use 180 degree cranks and have an breathing pattern of LRLRLRLR. These engines are more like two 4-cylinder engines mounted to a common crank with a 90 degree bank angle. two pistons are rising/stopped and two pistons are falling/stopped at all times. They breath very well, and have inherent primary balance while sacrificing a little smootheness (secondary balance) for breathing. No common plenum is required for balanced intake breathing, and this allows the throttle plates to be placed right above the valves which improves throttle response. The excellent breathing allows small engine blocks and allows for straight airflow into the airbox(es) and straight flow from the airbox to the intake valve. The lack of curves in the inlet is one reason the Ferrari engines have 'tone'. The balanced breathing at the exhaust enhances the 'tone'. | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2004 Registered: 12-2000 |
Sorry Erich, I should've went into further detail that the bank of cylinders (#5-8) fires in the same sequence as the other bank (#1-4), except, its 90° later. That's what I meant about the pattern... | ||
| Erich Walz (Deleteall)
Junior Member Username: Deleteall Post Number: 124 Registered: 12-2001 |
"but 2 of those pistons are actually in the other bank so they're at about the mid-position their bores." Steve: Do'h!, that explains that. Thanks for the clarification. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1039 Registered: 1-2001 |
Erich -- 1 and 5 don't fire at the same time -- 5 fires between 1 and 3. The figures on the site that you show are a little misleading -- for example, in the V8 flat-plane figure it shows 1, 2, 5, and 6 on a crank pin in a direction that seems like TDC, but 2 of those pistons are actually in the other bank so they're at about the mid-position their bores. Just look at any single V8 distubutor cap -- the spark plug wires are equi-spaced (i.e., only 1 spark plug wire is fired at a time). | ||
| Erich Walz (Deleteall)
Junior Member Username: Deleteall Post Number: 123 Registered: 12-2001 |
If I'm reading Peter's firing order pattern correctly it makes sense (1 and 5 fire at the same time, 3&7, etc). But that seems contrdictory to Steve's comments. So I'm really confused now. Here's one of the sites I found on this regarding the position of the crank pins. http://autozine.kyul.net/technical_school/engine/smooth4.htm | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 448 Registered: 12-2001 |
One other thing to note with the ferrari flat crank on the V8, is it has alternate bank firing unlike a 90 degree domestic. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1034 Registered: 1-2001 |
Erich -- I think there's something wrong with your assumption -- with a flat (or even non-flat) 4-throw crankshaft in a 90 deg V8 only 2 pistons are at TDC simultaneously (and only 1 at the end of its compression stroke) so you get 1 firing every 90 deg of crankshaft rotation in all modern V8s. | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2000 Registered: 12-2000 |
The order: 1-5-3-7-4-8-2-6 If you split that in half you get: 1-3-4-2 5-7-8-6 Do you see the pattern? (1=5, 3=7, 4=8, 2=6)  | ||
| Erich Walz (Deleteall)
Junior Member Username: Deleteall Post Number: 122 Registered: 12-2001 |
I had been told long ago that on v8s two cylinders fired at the same time. That made sense because it seemed like with a 180 degree crank there would be four cylinders at TDC. I saw a post the other day that said this wasn't true. So what's the story? Thanks. |