View Full Version : @ 750 rpms, how many combustion events per second?


BondoSpecial
02-21-2011, 04:28:56 PM
Hi here's a dumb car math question...if my SBC is idling at 750 rpms, how many power strokes per second are occurring? I know there are 12.5 crank rpms per second @ 750 rpms and the crank has to rotate twice in a 4 cycle engine per complete cycle, but I want to know how many power/firing events there are per second @ 750 rpms?

Thanks

Steve

frankz
02-21-2011, 04:32:52 PM
I say 50.

High Country Z
02-21-2011, 04:33:14 PM
I say 50.
Sounds right to me.

Twisted_Metal
02-21-2011, 04:34:30 PM
(750/2 * 8)/60 = 50

tmv36830
02-21-2011, 04:40:06 PM
Hi here's a dumb car math question...if my SBC is idling at 750 rpms, how many power strokes per second are occurring? I know there are 12.5 crank rpms per second @ 750 rpms and the crank has to rotate twice in a 4 cycle engine per complete cycle, but I want to know how many power/firing events there are per second @ 750 rpms?

Thanks

Steve

I am thinking 50 but I will be willing to bet someone will know for certain. This board is full of smart engineering type people.

High Country Z
02-21-2011, 04:48:00 PM
I say 4 x 12.5 = 50

tmv36830
02-21-2011, 04:52:48 PM
Hi here's a dumb car math question...if my SBC is idling at 750 rpms, how many power strokes per second are occurring? I know there are 12.5 crank rpms per second @ 750 rpms and the crank has to rotate twice in a 4 cycle engine per complete cycle, but I want to know how many power/firing events there are per second @ 750 rpms?

Thanks

Steve

Let's try this.

750/60=12.5 750 revolutions per minute divided by 60 seconds
12.5/2=6.25 12.5 revolutions per second divided by 2 equal 6.25
6.25X8=50 6.25 compression rotations multiplied by 8 cyclinders equal 50

BondoSpecial
02-21-2011, 05:13:05 PM
Thanks!!

K5JMP
02-21-2011, 05:22:10 PM
you burning a prom or something?:confused:

BondoSpecial
02-21-2011, 05:38:09 PM
No I was messing w/ the z28 today and whether this engine has been carb or EFI, at idle one bank has a very slight puff-puff-puff and I'm wondering if that is one single cylinder maybe having an exhaust valve sealing issue... I made a video of the idle comparing both banks. I was curious if one cylinder was causing the puffing how many puffs per second I would get.

K5JMP
02-21-2011, 05:43:49 PM
interesting approach though.

BondoSpecial
02-21-2011, 05:46:02 PM
Once I get a car actually driving nice I start looking for any tiny little problem to take it back apart.

K5JMP
02-21-2011, 05:48:49 PM
Once I get a car actually driving nice I start looking for any tiny little problem to take it back apart.

also guilty of that here..:screwup:

BondoSpecial
02-21-2011, 05:54:26 PM
These heads have maybe 100 miles on them since I had a valve job done on them. This was back before I did my own valve jobs though and it was to correct the previous owner crashing a few valves. I was not 100% thrilled at how narrow the shop ran the seat widths, they basically did a competition valve job even though I told them it was for street only, I personally would have run wider valve margins. So I am a little worried if I have an exhaust valve that is not quite sealing. I actually have pictures of the heads before they were repaired so I could figure out which cylinders were the ones where the seats even after grinding were not quite uniform all the way around to my liking, and note that when I leak down test the engine. It might have been my imagination but it seemed like #5 when I was pulling wires one at a time at idle made the puffing (which no one other than me would notice, you'd have to have your ear to the pipe almost) stop...

Vince's79
02-21-2011, 07:34:09 PM
Let's try this.

750/60=12.5 750 revolutions per minute divided by 60 seconds
12.5/2=6.25 12.5 revolutions per second divided by 2 equal 6.25
6.25X8=50 6.25 compression rotations multiplied by 8 cyclinders equal 50

I think this is wrong. You have 6.25 revolutions a second on the crankshaft. If two rotations of a crankshaft has eight power strokes, then 6.25/2x8=25

tmv36830
02-21-2011, 08:50:27 PM
I think this is wrong. You have 6.25 revolutions a second on the crankshaft. If two rotations of a crankshaft has eight power strokes, then 6.25/2x8=25

And you may well be correct. I reference my first post. I am sure there is someone here on this board that is an engineer and will tell us know for certain.

But please note that there are 12.5 revelutions per second. I had already divided by two to compensate for the two turns per power stroke.

Todd80Z28
02-21-2011, 09:20:37 PM
I think this is wrong. You have 6.25 revolutions a second on the crankshaft. If two rotations of a crankshaft has eight power strokes, then 6.25/2x8=25
Nope, 750RPM = 12.5RPS. In a 4 stroke, 90* V8, there will be 8 firing events per 2 crank revolutions, or 4 firing events per crank revolution, so 12.5*4=50 fires per second.

Rich Schmidt
02-21-2011, 09:30:51 PM
Both sides of any V-8 with dual exhaust will have an exhaust pulse. Thats because on each side there are 2 cylinders that fire into the collector at the same time on each side of the engine. In the left side of the engine it is 5 then 7,on the right side of the engine it is 8 then 4. Thats why exhaust systems that put all the cylinders into1 pipe (like a single exhaust or X pipe) will typically be quieter and sound smoother then an engine with full duals. It also causes uneven fuel distribution on engines with dual exhaust(even open headers).

1972L
02-22-2011, 07:05:41 PM
The Distributor spins 1/2 crank speed.
750rpm/2= 375 distributor rpm
375 distributor rpm/60 seconds= 6.25 distributor revs per second

1 revolution of the distributor fires 8 times in a V8

Therefore:
6.25 (distributor) revs per second X 8 sparks per revolution = 50 spark events per second.

OldCamaroNut
02-22-2011, 07:23:44 PM
The Distributor spins 1/2 crank speed.
750rpm/2= 375 distributor rpm
375 distributor rpm/60 seconds= 6.25 distributor revs per second

1 revolution of the distributor fires 8 times in a V8

Therefore:
6.25 (distributor) revs per second X 8 sparks per revolution = 50 spark events per second.

We have a winner. Nicely put.