The main problem for you is high elevation and that gives lower air density and air pressure, lower combustion chambre pressure and due to that a big power loss.

At 7000ft the air pressure is 25% lower and the density is also 25% lower. That results it that your very nice 916cc engine with 11:5 compression will work like 690cc engine with much lower comperssion. If it is 11,5:1 original it will be approx the same as 8,6:1 in compression if I did not calculate it wrong. On top of this it might run very rich if the ECU does not have a correct pressure compensator.

That can only be solved by more cc and higher compression and a maybe a proper ECU.

If you want more power but do not want to compensate for the air density/pressure the first step i would suggest on a S4 is to fit different headers. All 916:s exept for S4 has 45mm headers. The S4 has 40mm.The cheapest way is to find some used ST4 headers and slipons and modify. If you keep your eyes on ebay the ST4 systems comes up now and then. If you do this you will need a power commander and a custom map. I bought my ST4 headers and ST4 silmoto aluminium slipons for 300 Euros at Ebay. I guess that is 400 dollars. Add 300 dollars for a Power Commander and you will need a custm map on top.

This works very nice at low altitude. It might have less effect at high altitude. I dont know.

The next step is to look at ebay for 996 SPS cams. They will make a big difference but it will cost a bit more. I payed 500 Euros for my cams.

I just started the road mapping on my S4 and it has following mods.
ST4 headers, sil moto ST4 slipons, SPS cams timed to 107/107, lightened flywheel and a PC3 USB and it is a big improvment. The plan (dream) for the future is a ST4S or a MS4R engine with 68mm stroke and HC pistons with SPS cams but I guess it will never happen.

Jocke..........

Modified by vij at 12:20 PM 5/14/2005


Modified by vij at 12:22 PM 5/14/2005


Modified by vij at 12:23 PM 5/14/2005