Mismatching Cams

[sweetr33]

hey guys, whats your oppinion on mis matched cams like the hks step ones?? its on rb25 engine, reckon its better to stay with same profile cam for intake and exhaust or more on exhaust side is good??

[LTHLRB]

hks 256IN 264OUT is a killer setup... but you gotta be running a bigger turbo to get the full effect.. its a straight drop in too.. thats what im going to be running with a 35/40

[WARLORD]

ive got a set and i love them .

[sweetr33]

hey yeah ive got a gt35/40 with .82 ex housing... build motor with head work also...cheers

[LTHLRB]

crazy, sounds like a pretty hardcore setup, im running 35/40 .63 but standard plenum and head with rebuilt std bottem end.

[Hungryhippo]

what are the advantages / disadvantages of running more duration on the exhaust cam?

[Sydneykid]

what are the advantages / disadvantages of running more duration on the exhaust cam?

Two schools of thought;

1. The inlet cam duration is what makes an engine lumpy, by keeping the inlet duration down you avoid unnecesary lumpiness. By lumpy we usually mean rough idle, not smooth cruise, cranky when cold, requires higher rpm to make power etc. The exhaust cam duration has LESS effect on lumpiness, so you can run slightly more duration and not suffer from it.

2. The exhaust valves on RB's are slightly too small when you increase the boost, they are OK at standard boost, plus a bit. But when you start feeding them 1.5 bar or so, the exhaust valve size becomes an issue. Using slightly more duration on the exhaust cam is a band aid for that problem. The real fix is to use oversize exhaust valves, even as little as 0.5 mm larger diameter makes a huge difference.

cheers

[MBS206]

I like lumpy idle...

Put them in the other way around...

SK: What is stock duration on an R33 GTR and stock on GTS-t

[sweetr33]

Two schools of thought;

1. The inlet cam duration is what makes an engine lumpy, by keeping the inlet duration down you avoid unnecesary lumpiness. By lumpy we usually mean rough idle, not smooth cruise, cranky when cold, requires higher rpm to make power etc. The exhaust cam duration has LESS effect on lumpiness, so you can run slightly more duration and not suffer from it.

2. The exhaust valves on RB's are slightly too small when you increase the boost, they are OK at standard boost, plus a bit. But when you start feeding them 1.5 bar or so, the exhaust valve size becomes an issue. Using slightly more duration on the exhaust cam is a band aid for that problem. The real fix is to use oversize exhaust valves, even as little as 0.5 mm larger diameter makes a huge difference.

cheers

thanks SK, your a legend, thats exactly the info i wanted to know, i didnt understand the reasoning behind it but all clear now... thanks alot