Gti6 Cams In Xsi Head

[petert 555]

I did some measuring recently and discovered that the standard springs in an XU10J4R head will go spring bind at approx. 10.5mm of lift. Thus if you fit an XU10J4RS cam into a standard XU10J4R head you are seriously on the edge of spring bind. The RS cams have 10.0mm of lift. So leaves just 0.5mm. Most info on the net suggests allowing 1.5mm of clearance. Another generally accepted practice is 0.18mm between each coil. Thus for an R spring with 7 coils, the clearance can be 1.26mm. Doing the maths on a standard R setup:

 

Cam Lift = 9.14mm

Spring Bind Allowance = 1.26mm

Min. theoretical lift for spring bind = 10.4mm

Actual std. spring bind = 10.5mm

 

As you can see, the standard R cam is on the acceptable limit for standard R springs. Fitting a cam with any more lift is like playing Russian roulette.

 

It's thus no surprise that the RS has double springs.

 

Don't be discouraged however, as 9.14mm is still a lot of lift for that head. ie the same as an Mi16. Keep the lift down and increase the duration and you'll still make impressive power.

[Anthony 1,000]

Which XU10J4R head is that though Peter? There's three different variants of springs used in the J4R head depending on age.

[welshpug 1,627]

interesting to note, I think the 7JP4 is similar in most respects (valves and springs)

 

basically the key is more duration and opening the valve faster, rather than more? and it doesn't hurt to have a little less lift at this milder lever of tune presumably.

[petert 555]

I wasn't aware there were three spring variants for the XU10J4R, but I don't believe it would make much difference given the design. The head was from an XSi, circa 2001.

 

So yes, more duration is needed as there's no free lunch available with lift.

 

I could only find two different springs, according to the parts CD, 095282 and 095289.

Edited April 21, 2010 by petert