HP/Torque Peak different with engine in car than when engine is out of car?

[XH29N0G]

I assume there are simple rules to this, and would like to hear what people know. 

I have just had an engine rebuilt and tested on a dyno and I started to wonder how much the horsepower/torque curves would change after it is in the car with the full exhaust, power steering, water pump, transmission & diff drag, etc.... (all those parasitic loss things).  I realize some of these increase with RPM considerably and will reduce the peak HP/Torque and shift the RPM where the peak occurs.  When I tried to calculate the change, I ended up with a shift down in the peak for HP and RPM by a few hundred RPM which I think makes sense.  Some questions popped to mind.

From those who have looked at engine vs chassis dyno's, How much is the typical change in the RPM of the peak HP and Torque?  Does it matter whether the HP and Torque peaks are significantly different?


The engine dyno numbers I got were ~ 590 ftlb at 3500 and 530 Hp at 5400 and my sheets have HP dropping to by about 20% torque by less, but with a HP peak dropping by about 500 RPM). I am assuming the reason my calculation has the torque dropping by less is because it has a bigger drop at higher RPM.  Does this fit with other people's real life experience?  I realize this is just math/play, but the car is not together yet so I cannot test it. 

More nerd stuff , I have also been calculating where the rear wheel torque overlaps for different gears to figure out when I should shift....

 

[cdr]

THE rpm that peak t,or hp will not change from a chassis dyno or engine dyno,check out this link,it will help.
http://www.wallaceracing.com/Calculators.htm

[XH29N0G]

Thanks. 

I find this puzzling, but I have been wrong many times.  My thinking is that it seems to me that if the loss is greater at higher RPM it would seem inevitable that the peak would shift to lower RPM. 

I'll keep thinking.

[cdr]

EVERYTHING on the car is just  parasitic  in %, the engine dont care where the load comes from ! its still gonna make power at the same rpm.

[XH29N0G]

OK

[cdr]

we need pics of your project   

[tan top]

THE rpm that peak t,or hp will not change from a chassis dyno or engine dyno,check out this link,it will help.
http://www.wallaceracing.com/Calculators.htm

good link     thanks for posting 

[XH29N0G]

we need pics of your project  

None of engine in car (yet).  I will be posting more when I have some good ones and closer to when done.

EVERYTHING on the car is just  parasitic  in %, the engine dont care where the load comes from ! its still gonna make power at the same rpm.

Some more thought.  This suggests the % loss is similar at most RPM (i.e., a constant % as you write).  Interesting.  

[375instroke]

Couldn't exhaust shift the peaks?

[JB400]

It would shift them some.

[firefighter3931]

Couldn't exhaust shift the peaks?

Absolutely !  

Typical tq peak with a 440ci build :

(1) 1 3/4 in header = 3500 rpm
(2) 1 7/8 in header = 4000 rpm
(3) 2 in header = 4500 rpm

So, if the engine in question was dyno'ed with a larger tube header than what is currently installed in the car it would definately impact the power numbers and where they come in at.

Just to add to the above discussion ; the cam profile also comes into play. A tighter lobed cam works better with headers than manifolds and if the LSA is tighter than 108* it really needs an open exhaust to run descently. If using restrictive manifolds, the best cam choice is something with wide lobe centers in the 112* lsa range to reduce overlap and help scavenging.


Ron