Compression to HP conversion....
06-23-2008, 02:25 PM
#1
Rennlist Member
Thread Starter
Join Date: Aug 2004
Location: Land of the Old People
Posts: 2,097
Likes: 0
Received 5 Likes
on
2 Posts
Compression to HP conversion....
We were discussing this at the track this weekend and I will bring the question here:
If you have a 6 cylinder motor and you have a 30% loss (leak down) in one cylinder how much resulting HP (in %) would you lose? Let's say it's a 360hp motor....
If you have a 6 cylinder motor and you have a 30% loss (leak down) in one cylinder how much resulting HP (in %) would you lose? Let's say it's a 360hp motor....
06-23-2008, 04:14 PM
#2
Drifting
Wouldn't that one cylinder then be acting with an effective CR of something like 7:1 (if it was at 10:1 before)?
This then raises the question, how is HP affected with CR? How much more HP would an 11:1 motor have over a 10:1? What about 12:1? 13:1?, etc.
This then raises the question, how is HP affected with CR? How much more HP would an 11:1 motor have over a 10:1? What about 12:1? 13:1?, etc.
06-24-2008, 02:01 AM
#3
Rennlist Member
Thread Starter
Join Date: Aug 2004
Location: Land of the Old People
Posts: 2,097
Likes: 0
Received 5 Likes
on
2 Posts
You would think so, but I don't think it's linear. A leakdown is done without putting pressure back in the cylinder that an engine would normally do so I don't think there is truly a 30% loss....
06-24-2008, 08:54 AM
#4
Nordschleife Master
When I had that on an old engine of mine, it was pretty close to linear. I would guess you are talking in the 15hp range (formula below says 18).
360hp (current hp) / 5.7 (5 good cyls + 1 at 70%) * 6.0 (6 good cyls) = 378hp
360hp (current hp) / 5.7 (5 good cyls + 1 at 70%) * 6.0 (6 good cyls) = 378hp
06-24-2008, 10:17 AM
#5
Lifetime Rennlist Member
I would say it can not be accurately answered. 30% leakdown means 30% pressure drop at some starting pressure (usually 100 psi) with the engine warm, but who knows how warm. The engine is not running and leakdown does not tell us where it is leaking from. The power loss could be almost anything.
06-24-2008, 10:18 AM
#6
Lifetime Rennlist Member
I understand what you meant but your equation is backwards showing more power from a leaky motor. I think you mean HP * 5.7 / 6.0
Trending Topics
06-24-2008, 01:27 PM
#9
Rennlist Member
Thread Starter
Join Date: Aug 2004
Location: Land of the Old People
Posts: 2,097
Likes: 0
Received 5 Likes
on
2 Posts
Colin-
Here's your formula taken to one more level of detail:
Divide the 6 cylinders into 100 to get each cylinder's percentage contribution (16.67) to the motor.
X (Hp after loss) = [(16.67*5) + (16.67*.7)] * HP before loss.
342 = 95.02 * 360
Therefore 342hp with one cylinder at 30% leakdown or 18hp drop
Now, I still think there's an unknown factor here that we've mentioned. While running, the motor continually builds up pressure thus, there may not be as large a true % loss that a leakdown shows on a non-running motor.
Here's your formula taken to one more level of detail:
Divide the 6 cylinders into 100 to get each cylinder's percentage contribution (16.67) to the motor.
X (Hp after loss) = [(16.67*5) + (16.67*.7)] * HP before loss.
342 = 95.02 * 360
Therefore 342hp with one cylinder at 30% leakdown or 18hp drop
Now, I still think there's an unknown factor here that we've mentioned. While running, the motor continually builds up pressure thus, there may not be as large a true % loss that a leakdown shows on a non-running motor.
06-24-2008, 01:36 PM
#10
RL Technical Advisor
Good man! You've figured out that there are several variables at work here including camshaft profile and static compression ratio that both affect power loss when leakdown figures are not ideal.
Its very difficult to make accurate predictions in these situations as I've observed almost negligible losses on the dyno when leakdowns are below 20%.
