Compression And Ignition Requirements For A Catcam 4900339 Cam

[Anthony 1,000]

My current engine is a rebuilt 1.9 8v bottom end, QEP fettled XU10 head, Kent PT36 cam, all running on standard Jetronic management and clockwork dizzy. It goes pretty well considering the fairly modest spec, able to stick with an average 205 Mi16 and noticeably quicker than a friends 306 S16 upto 100mph or so.

 

I've recently managed to lay my hands on a Catcam 4900339 8v cam (274 degree @ 0.1mm, 11.9mm lift, 3.0mm lift @ TDC) to replace the mild Kent PT36. The cam is quite a bit hotter than the existing PT36 with more overlap, and hence I'm expecting the idle to be lumpier than school custard and low rev running to be less than ideal with the pulsing the AFM is going to get, but I'm not too fussed as the 205 isn't my daily-driver anymore

 

The two questions I have however are...

 

1) What sort of static compression ratio will I need for this cam? Currently running approximately 10.2:1 with the existing cam, but seeing as I've got to remove the head anyway (one of the valve stem seals is leaking badly) it's easy enough to get a bit more material skimmed off to raise the compression a little. Still running standard cast pistons, so would rather eer on the side of caution...

 

2) How will the ignition advance requirements differ with the hotter cam, and how unsuited will the standard 1.9 dizzy advance curve be? I'm guessing that because of the poorer cylinder filling at low revs I'll need more advance low-down, and because of the better filling on-cam I assume I'll need less advance in total? This is just a cheap fun engine and hence mappable management aren't on the cards (although I appreciate that it would benefit), although a dizzy rebuild by someone like H&H Ignition Solutions I can stretch to if the standard dizzy will prove hopeless, or perhaps consider a Megajolt mapable ignition ECU.

 

Any advice on the above two questions? And yes, I know a GTi-6 engine would be quicker out of the box, but it's staying 8v.

[Mattsav 1]

10.2/10.5:1 is fine with that cam.

You'll always have an ignition problem with the std dizzy - Pinking at 3k if the high rpm advance is anywhere close to optimum.

[pug_ham 242 3 Cars]

Is this the Catcam that is nearly identical on spec to the old PTS Max torque cam? (I@ve got one fitted to an XU10 head unused but can't remember the model number).

 

I'm going to swap this to the head I've just bought from Ash after sending it away for some extra flow work.

 

Can the XU10 inlet valve be fitted into the XU9 head?

 

Sorry for the slightly off topic question.

 

Graham.