Performance Improvement V12 camshaft


(Paul) #1

My 1974 XJ12 (Daim VDP 2X6) carbie engine is being rebuilt, including the head.
My mechanic has asked if there are any performance enhancements that can be made to the camshafts or camshaft timing (emissions is no longer an issue)
Cheers
Paul


(Frank Andersen) #2

**
Jaguar configured cams and valve timing for ‘street use’, Paul - making compromises…

But tuning these parameters is a highly specialized area - maximizing one aspect will influence others. Altering cam profiles will require expertise, which may or may not be readily available - but minor adjustments to cam timing may be attempted on a trial and error, using driving and/or dyno testing. However, care must be taken to avoid valve damage…

Likewise, some experimentation with ignition timing is possible - but the performance changes might not be suitable for ordinary use. And may be counterproductive…

Frank
xj6 85 Sov Europe (UK/NZ)
**


(Doug Dwyer) #3

There are some ‘performance’ camshaft offerings out there intended (I think) for the E-type V12. My guess is that they offer more power at a higher RPM range…which probably isn’t well suited to the much heavier XJ sedans.

https://www.terrysjag.com/category/PERFORMANCE-CAMSHAFTS.html

http://www.v12sales.com/Jag-Engine.html

Cheers
DD


(MarekH) #4

The problem with performance camshafts for the v12 is that only the wrong ones are commonly available. The v12 has so much low down torque (and many cars are geared with a 2.88 differential) that the average road user will spend the majority of time below 3500rpm. If the performance were to be optimised or enhanced for this lower rev range, then it would indeed be an upgrade, but most “sports” enhancements are for the higher rpm range. Sadly, road users don’t ever really need to go there, so attempting to boost performance above 3500rpm will yield little positive benefit for the average user.

kind regards
Marek


(Carl Hutchins, Jr. ) #5

The add’s language is a bit misleading, or I am not reading what it says.

Road, road racing or street?

As Marek mentions, torque is a king. Match the "sweet spot’ to the need, and the car is alive.
One might look in to increasing the lift, if the valves clear, and leaving the duration as is.

Carl


(Thomas Cummings) #6

For a street only car, the best way to increase performance during a complete engine overhaul is by enlarging displacement and going with a higher compression ratio. If you are considering camshaft changes as well, a larger displacement will help to swallow up the normally lower rpm torque loss of a camshaft with a performance profile in a smaller engine. An otherwise equally built 6 liter should make at least 10% or better power vs. a 5.3 liter. And so on depending how far up the displacement increase ladder that you go.

I have no experience building a performance Jaguar V12, but much experience building common Detroit V8’s. I once had a '67 El Camino that I built a torquey low revving 406 V8. At the drags with a nothing special setup of 3.08 gears and leaving the transmission in drive for full throttle upshifts at a leisurely 4,200 rpm, it ran a consistent 13.10 e.t. at 106 mph. The camshaft I used would have been somewhat lumpy in a 350 V8, but the extra 56 cubes of the 406 swallowed it up and helped deliver gobs of torque.


(Paul Wigton) #7

Pretty good advice: there is also the complicating factor of manual vs. auto ‘box, that makes what kind of performance mods to do, even more touchy.


(Paul) #8

Thanks for the advice folks :slight_smile:


(Paul Wigton) #9

Ed Iskenderian, who, like the Farmer’s Insurance guy, knew a thing or two about camshafts, was asked about which cam was the best for all-round street performance: his answer?

“The one it came with, from the factory.”


(knojag) #10

I’ve devoted some time to the modification of the Jag V12 and from my experience, there are some cams available out there but as has been said earlier, you have to consider the whole package. And then it gets really expensive for the performance increase. The best performance upgrade is a manual transmission conversion. Check Harry’s Garage XJ12C video.


(David Judd) #11

The best performance upgrade is a manual transmission conversion. Check Harry’s Garage XJ12C video.

Or of course you could always install a GM T700 R4.


(Doug Dwyer) #12

I went from 2.88 final drive to 3.31 and am very pleased with the performance increase. Might not be the best choice for those who spend a lot of time on the open road at highway speeds, though. It’s a little busy. But the response for passing is wonderful :slight_smile:

Cheers
DD


(Paul Wigton) #13

Flow chart for these kind of mods…

  1. Is it your car…yes?

  2. Have you researched, and spoken to others who have done the same mod…yes?

  3. GO for it!


(Jochen Glöckner) #14

Doug,

my car has the 3.31 as well and it maxes out at 4700 RPM and around 110 mph. With a 12 cylinder car and the same ratio you have around 1300 RPM and +100 hp left to take you to 140 mph. That should do for most places … Yet, I admit that 3000 RPM at highway speeds feels a bit more than necessary and I’d prefer the 3.07 axle that was installed in continental cars at the time.

Still, I guess the SI 2.8 MOD would end up pretty much at the same final drive ratio as the 3.31 (4.09 x 0.8 = 3.27).

Best

Jochen

75 XJ6L 4.2 auto (UK spec)


(ronbros) #15

my approach ,5.3,is 700R4 , rear gear 3.73 -1 LSD, in overdrive final ratio is 2.60.

but 1st gear is great for accelleration with a 12-1 ratio, other words engine turns 12 times for 1 rev of the wheels .

700 trans has instantainios shift to second gear!wild%20cat%2Cburnout%20006 .
ron


(ronbros) #16

i had CRANE CAMS custom grind some cams that gave some more lift , without to much increase in duration, slight overlap!

i have no problem revin 6500rpm , altho 7000rpm can be done , BUT its past the HP peak!

my torque peak is around 4200rpm, its close to the best of both worlds!

if i were to do another engine , i would deffenatly go for more cu.in. Displacement , torque IS KING for a street car!
plus some odd&end engine mods!

now i must say the 5.3 Jag does not have much torque,stock, CONTRARY to what most people think!!

and very true its the whole package of mods that count , and overall weight is the ENEMY.
ron