[xj-s] Dyno curves

XJ-S
1

After reading the dyno charts on the V12 it appears that the engine is
throttled badly. I understand that the HE head has an relatively
restricted exhaust valve pocket. Is this the major problem with its
breathing? Has anybody done any experimenting with serious head work to
improve this area, perhaps with the older head design? With the large
bore relative to the cylinder volume, and the amount ot real estate open
for valves, it would seem that improving volumetric efficiency would be
basic. I also understand that the HE head was primarily developed to
improve fuel mileage, but maybe retrofitting a more modern fuel
injection system with a MAF sensor could get the mileage back without
killing the top end. I used to lament my XK-E having such a low redline
due to its stroke and resultant high piston speed, now the more modern
engine has the same problem for a different reason.
The small block, pushrod GM engines of about the same displacement are
making about the same power at the same rpm so their breathing , or at
least BMEP must be about the same.
Tom Wilson 87 coupe with no (currently) leaks

2

Question is, is this power drop off caused by

  • Intake runners being long/narrow enough for low end torque
  • Cam profile more suited to low end torque
  • No exhaust headers
  • The fuel injection being mapped conversatively/incorrectly at high RPMs
  • The stone age ignition of the pre-marelli not being able to get the
    optimal
    high RPM advance.
  • The valve size being suited to low RPM operation.

or is it actually the HE head design?

Perhaps someone in the know could tell us…-----Original Message-----
From: Tom & Teresa Wilson twx3@wcsonline.net
To: XJ-S Digest xj-s@jag-lovers.org
Date: Sunday, March 25, 2001 12:57 PM
Subject: [xj-s] Dyno curves

After reading the dyno charts on the V12 it appears that the engine is
throttled badly. I understand that the HE head has an relatively
restricted exhaust valve pocket. Is this the major problem with its
breathing? Has anybody done any experimenting with serious head work to
improve this area, perhaps with the older head design? With the large
bore relative to the cylinder volume, and the amount ot real estate open
for valves, it would seem that improving volumetric efficiency would be
basic. I also understand that the HE head was primarily developed to
improve fuel mileage, but maybe retrofitting a more modern fuel
injection system with a MAF sensor could get the mileage back without
killing the top end. I used to lament my XK-E having such a low redline
due to its stroke and resultant high piston speed, now the more modern
engine has the same problem for a different reason.
The small block, pushrod GM engines of about the same displacement are
making about the same power at the same rpm so their breathing , or at
least BMEP must be about the same.
Tom Wilson 87 coupe with no (currently) leaks

3

Tony Bryant wrote:

Question is, is this power drop off caused by

  • Intake runners being long/narrow enough for low end torque
  • Cam profile more suited to low end torque
  • No exhaust headers
  • The fuel injection being mapped conversatively/incorrectly at high RPMs
  • The stone age ignition of the pre-marelli not being able to get the
    optimal
    high RPM advance.
  • The valve size being suited to low RPM operation.

or is it actually the HE head design?

Perhaps someone in the know could tell us…

If someone could tell me the following I can run it through a good dyno
program. My hunch is that the motor is severely restricted on the intake
side. I need to know:

inlet valve dia
exhaust valve dia
average runner length and dia, measured from head to first major opening
head - average port dia
head - port vloume
head - bench flow data in cfm at Lift/Dia =1/4 and how many inches of water
camspecs at 1mm or seat
throttle bores
exhaust dia

The more info and the more accurate the data the more accurate the results

Rohan Christmas

4

If someone could tell me the following I can run it through a good dyno
program. My hunch is that the motor is severely restricted on the intake
side. I need to know:

inlet valve dia
exhaust valve dia
average runner length and dia, measured from head to first major opening
head - average port dia
head - port vloume
head - bench flow data in cfm at Lift/Dia =1/4 and how many inches of water
camspecs at 1mm or seat
throttle bores
exhaust dia

The more info and the more accurate the data the more accurate the results

Still doesn’t cover what the ECU or Ignition does at high RPM.

Although the ignition can fairly closely be guessed at. Does any have an
accurately measured curve handy?

Hopefully sometime in the next 6 months I’ll extract out the map entries in
the ECU, since I’ll building another, and I’ll need a baseline.