| Author | Message | ||
| rich stephens (Dino2400)
Member Username: Dino2400 Post Number: 624 Registered: 10-2001 |
Mitch and Ulf, you may want to clarify your statements that it makes no use to rev past the listed peak power quoted rpm. We've been through this all before, but there are still people out there who don't understand the concept of gear ratios. | ||
| Mitch Alsup (Mitch_alsup)
Intermediate Member Username: Mitch_alsup Post Number: 1167 Registered: 4-2002 |
At redline there are two times where the piston reaches TDC, a) ignition, b) overlap. During the ignition event, interior cylinder pressure is several hundred PSI. During the overlap event, the intake manifold vacuum is presented to the piston through the openinig intake valve. When manifold vacuum is high there is less pressure in the cylinder and the subsequent conrod loads are higher. This added stress on the conrod is why drag racing engines "let go" after a run (rather than durring a run). | ||
| 911 Fan (911fan)
Junior Member Username: 911fan Post Number: 70 Registered: 5-2003 |
Hugh, If I'm understanding this correctly, after matching revs and downshifting, if you then close the throttle to decelerate, you're stressing the rods, no? | ||
| PSk (Psk)
Intermediate Member Username: Psk Post Number: 1011 Registered: 11-2002 |
Abosolutely. The brakes are for braking. The sole purpose of changing down should be to put yourself in the right gear for exiting the corner ... not for extreme engine braking. I learnt this the hard way after racing an Alfa Sud that had useless brakes ... a few engine rebuilds later my engine builder got real grumpy about excessive engine braking ... Upgraded the brakes (er, buy putting the motor in a different car altogether  ) and never had another engine failure ... er, except when I missed a gear  Pete | ||
| Hubert Otlik (Hugh)
Intermediate Member Username: Hugh Post Number: 1518 Registered: 1-2002 |
911 Fan: In practice, my previous statment could be used as a justification for why rev/matching/heeltoe are important; to keep the motor "spinning" at all times | ||
| 911 Fan (911fan)
Junior Member Username: 911fan Post Number: 69 Registered: 5-2003 |
Mitch/Ben/Hugh, "Intrestingly, rods are under their most stress under tension at redline, when you CLOSE the throttle plates." Does this mean that one should avoid downshifting for the purpose of reducing speed? Or, put another way, can engine braking lead to engine breaking? | ||
| Ben Cannon (Artherd)
Intermediate Member Username: Artherd Post Number: 1018 Registered: 6-2002 |
Rich- makes me wonder if Ferrari had to lower the idle for emmisions reasons (possibly at the cost of some percentage points of longevity?) Perhaps the rest of the oiling system was just re-designed too. Hughbert's got it; Piston speed is the killer. Intrestingly, rods are under their most stress under tension at redline, when you CLOSE the throttle plates. Turns out the rods are quite strong in compression, but you can PULL one apart rather easialy, and the pulling force is radically increased when there's no intake charge to fill the cyl. You'll find race cars always blow motors entering corners, just as they come off full to closed-throttle at redline. Best! Ben. | ||
| Rosso (Redhead)
Member Username: Redhead Post Number: 472 Registered: 12-2001 |
Mitch quote "Ali: does the 575 have more pump capacity than the 550? Allowing it to idle lower?? Mitch, lloking up the part numbers for the supply and the recovery pump, they are the same part number. 153197 and 153198. This would lead me to belive that the pump preasure would be the same on both cars. HTH | ||
| Hubert Otlik (Hugh)
Intermediate Member Username: Hugh Post Number: 1507 Registered: 1-2002 |
Piston speed is probably the most telling quantitative elements from which you can infer that a given engine sees. What's more important is the rod/stroke ratio as it defines the geometry of the motor, and it's ability to "rev." That said, a high revving motor has a short stroke and a short piston/rod assembly; this allows for higher engine speeds, and piston speeds, while reducing the strain seen by the connecting rod; incidentally, all con rods strech -- it's calcualted into their design-- and by vocation the rod sees the greatest degree of stress on decel, not accel. | ||
| Mitch Alsup (Mitch_alsup)
Intermediate Member Username: Mitch_alsup Post Number: 1162 Registered: 4-2002 |
Jim Avery (Boxer12):"Isn't it true that length of piston travel is probably the biggest factor in stress on engine at high revs?" The tensional (pulling) stress in the conrod is maximized at TDC at high RPMs. This stress goes up with the square of the RPMs. I don't remember what the relationship of stroke is linear or quadradic. In American V8s, the valve train is more problematic at high RPMs than the bottom end. Ali: does the 575 have more pump capacity than the 550? Allowing it to idle lower? | ||
| Ali Haas (Aehaas)
Junior Member Username: Aehaas Post Number: 143 Registered: 5-2003 |
Both the 550 and 575 Maranello have the piston cooling sprayers. The 550 idles at 1050 while the 575 idles at 650. The idle oil pressure in a hot engine with 30 wt oil was about the same, around 30 - 35 PSI. It is the same despite one having almost 2x the RPM at idle. aehaas | ||
| Jim Avery (Boxer12)
Member Username: Boxer12 Post Number: 322 Registered: 6-2003 |
Isn't it true that length of piston travel is probably the biggest factor in stress on engine at high revs? | ||
| Mitch Alsup (Mitch_alsup)
Intermediate Member Username: Mitch_alsup Post Number: 1157 Registered: 4-2002 |
Ferrari V8s tend to idle around 1000 RPMs because this provides enough oil pressure after a hot run to keep the bearings lubricated as the engine slowly cools. In the more modern V8s, there is an oil jet under each piston that sends a stream of oil to the back side ofeach piston for cooling and lubrication. After a hot run, you want the oil system to still have enough oil pressure to send that cooling sream of oil to the piston. Idle is choosen as a balance point of minimum gas consumption and minimum wear. Lowering the idle speed in a high strung Ferrari might require cams that have more TQ in the low end, and thus affect the HP at the top end. If the oil system drops pressure at lower idle speeds, more wear can occur. So, while American V8s idle at 650-800 to save gas, Ferrari V8s idle at 1050 to save wear. Maximum power occurs when the rising RPMs are countered by falling TQ. Beyond peak HP, TQ is falling so fast that additional RPMs does not produce any more power (indeed, less power is produced). RedLine is set at the point where the mechanical parts in the engine can take the stress of running. This stress is quadradic (x**2), so a small increase in RPMs creates rather dramatic increase in stress levels. If you took you mondial T engine and put titanium con rods, and beefed up the valve train, nikisil cylinder liners in your Mondial T engine would be ready for the same 8500 redline that the F355 engine has. However, your cam would still be limiting peak HP to 7500 RPMs. | ||
| Ulf Modig (Ulf308qv)
Junior Member Username: Ulf308qv Post Number: 85 Registered: 3-2001 |
There are several factors that affects an engines character (or power and torque). Displacement, cam profile (duration, valve timing, lobe separation & and lift) intake & exhaust runners (length & diameter), compression and cylinder head design to name a few. There has been several discussions on effects on various engine modifications. If I recall correctly the drop in HP after 7,200RPM is quite dramatic hence no need to rev the engine higher than 7,400RPM. Another reason is to keep the engine wear at reasonable levels. | ||
| Ben Cannon (Artherd)
Intermediate Member Username: Artherd Post Number: 1017 Registered: 6-2002 |
Dave- idles are generally chosen to reduce or eliminate even order harmonic vibrations of the engine and mounting structure, and chassis. (read, they're chosen to reduce vibration :D) 1,000rpm is generally a good number to insure the vibration coming from the engine is always too high in frequency to excite the car's chassis into resonance (much shaking and unplesantness!) I do know the F355 is still pulling HARD at it's 8750rpm fuel cutout! Best! Ben. | ||
| Robert McNair (Rrm)
Member Username: Rrm Post Number: 687 Registered: 5-2002 |
Good question Dave. I have driven cars that have rev limiters were the power is elec. cut off, I have driven cars which just stop making power at a certain rpm and I have driven cars that still make power over redline on the tach. | ||
| Dave Wapinski (Davewapinski)
Member Username: Davewapinski Post Number: 615 Registered: 8-2001 |
Haven't been on for about a week. Earlier I had asked if the Mondial t (and most manual shift Ferraris) had rev limiters. Then the conversation turned to torque. According to the Mondial t Owners Manual, max torque is 4,200 RPMs. Most vehicles seem to idle at 1,000 RPM. Why is this? Anyway my t idles at 1,000. When I first got the t, I was conservative in starting from a standing stop. I tried starting at 1,000 to 2,000 RPMs but this was almost lugging the engine. The t starts much better at 2,000 to 3,000 RPMs or higher. This makes sense if max torque is 4,200. Max. power is listed as 7,200 RPMs with redline at 7,400. I thought that power would increase as long as RPMs were increased. What else is affecting it so that power is not increasing after 7,200 RPMs? Air? Fuel? Something else? There may be other factors, but it would make sense to set redline at 200 RPMs over max. power. |