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Cam Question (RB26)

RB26zcar

By RB26zcar,
in Engines & Forced Induction

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RB26zcar Enthusiast

RB26zcar

Posted
Posted

Hi again, I'm getting ready to have the cams I bought installed into the head (motors being built up as we speak) and I'm wanting some suggestions...

I bought a pair used from a member that were suppose to be 264 9.7 cams, but he sent me a 264 and a 256, 9.7, saying the other 264 was rust pitted.

So now I have a 264 and a 256. QUESTION: considering I'll be using my Electramotive TEC2 standalone, the CAS drive is not important to me, so which would you install where? (I forget which one has the CAS drive gear in it)

Like I said, I will not be installing the CAS mechanism, so either cam can go into either side... so which would be better where??

Thanks for the help

joel

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Sydneykid Grand Master

Sydneykid

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Hi Joel, I would go with 256 inlet and 264 exhaust, I have seen this (longer duration exhaust) work on a number of GTR's. What's the lift on each cam? My RB31DET has 268 inlet and 272 exhaust, both at 10.5mm lift.

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RB26zcar Enthusiast

RB26zcar

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Hi Joel, I would go with 256 inlet and 264 exhaust, I have seen this (longer duration exhaust) work on a number of GTR's.  What's the lift on each cam?  My RB31DET has 268 inlet and 272 exhaust, both at 10.5mm lift.

Thank You Sydneykid for the responce. The cams are both only 9.7 on the lift. I had the head set up on a end mill (for the lobe clearnce needed), and the notchs came out sweet ....

I also have the Tomie adj cam wheels to go with the cam upgrade, so if you would, please give me some advice on where to start with the placement of the cams.

thanks again

joel

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Emre Proficient

Emre

Posted
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Duration is the amount of the time the valves are open for.

Lift is how far the valves open. Consider it as the washer in your tap; as you turn the tap on further, the washer lifts higher to let more water through. With the higher lift cams, the valves travel further into the the combustion chamber, thus the modification to the head is required for clearences.

As with longer duration cams, higher lift cams have their advantage in the higher rpm and hurt low rpm response. However, how much this rule effect the forced induction engines always puzzled me. Ben is running a huge turbo with crazy cams and has amazing response, does 400cc make that much difference...?

Back to you SK.

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Sydneykid Grand Master

Sydneykid

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Thank You Sydneykid for the responce. The cams are both only 9.7 on the lift. I had the head set up on a end mill (for the lobe clearnce needed), and the notchs came out sweet ....

I also have the Tomie adj cam wheels to go with the cam upgrade, so if you would, please give me some advice on where to start with the placement of the cams.

thanks again

joel

Hi Joel, 256 @ 9.7mm and 264 @ 9.7mm is a good combination, if tuned properly it will make power and run very nice with that combo.

As for cam timing all engines are different, I start at 1-2 degrees advanced on the inlet timing and 3-4 degrees retarded on the exhaust. The exhaust timing makes the most obvious difference to the power, but don't ignore the inlet. It helps the response as well.

Hope that helps :thumbsup:

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Merli Grand Master

Merli

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SK,

Interesting that you've found that shorter duration inlet cam works better than the reverse!

Tomei sell two PONCAM setups for the RB26:

Type A - 260 degree, 9.15mm lift intake / 252 degree, 9.15mm lift exhaust

Type B - 260 degree, 9.15mm lift intake / 260 degree, 9.15mm lift exhaust

I went for the Type B set for my car, but was wondering what the theory behind running a larger duration exhaust cam was?

Wouldn't it be best to have a longer duration intake cam to fill the cylinder and push out the gasses out of the exhaust valves as opposed to having a longer duration exhaust cam and relying on scavenging vacuum to suck out the gasses?

Hmmmm...

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Sydneykid Grand Master

Sydneykid

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SK, Interesting that you've found that shorter duration inlet cam works better than the reverse!

Tomei sell two PONCAM setups for the RB26:

Type A - 260 degree, 9.15mm lift intake / 252 degree, 9.15mm lift exhaust

Type B - 260 degree, 9.15mm lift intake / 260 degree, 9.15mm lift exhaust

I went for the Type B set for my car, but was wondering what the theory behind running a larger duration exhaust cam was?

Wouldn't it be best to have a longer duration intake cam to fill the cylinder and push out the gasses out of the exhaust valves as opposed to having a longer duration exhaust cam and relying on scavenging vacuum to suck out the gasses?

Hmmmm...

Hi Merli, I didn't really specifically choose the cams, they were available at the time at right price. I was really more interested in the higher lift (and lower price), than the small diff in duration. :cooldance

I have heard and read that longer duration inlet is better for the reasons you put. And they certainly sound valid. I have also heard and read that longer duration exhaust is better, based on the fact that the inlet is under direct boost and the exhaust isn't. So you have to run longer duration on the exhaust to match. I have also heard and read that a longer duration inlet cam makes the engine lumpy, so its not a matter of running a LONGER exhaust duration, but in fact running a SHORTER inlet duration. :confused:

There are plenty of arguments either way, I really have no idea which is correct as I haven't done or seen a proper comparison. I suspect that there may be a boost threshold where the longer inlet duration argument has substance. But I have no proof, it's just a suspicion. :headspin:

Most of the engines I am involved with, I suggest the same duration for both inlet and exhaust cams. Maybe that's the right answer. ;)

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