4age TURBO cam selection

**30psi 4agte** · 04-07-2006, 01:06 AM

I know there are a few threads around on cam selection but none are really dedicated to turbo application as the cams can differ greatly from those used in atmo engines.

A rule of thumb for turbo cam selection is to go with high lift short duration.
Short duration to cut down on overlap and hence pushing all your air out the exhaust.
High lift to make the most of cylinder filling

I.E how much duration is too much for a turbo ??
I recently purchased some cams from crow with 265 deg duration ( 228 @ 50 thou) and 9.5mm lift.

I havent had the car run with these yet but wanted to start this thread to see what are other people running / thoughts ?????

I know stew ( old corollas ) is toying with the idea of runing longer duration on the exhaust side - there is lot of ways of heading !!!

**kingmick** · 04-07-2006, 07:42 AM

thats not bad as a cam choice, i take it you have shim under setup? looking at your lift.

**chris davey** · 04-07-2006, 09:17 AM

for reference, the crower group buy had [email protected]" and 9.1mm lift for small core and 10.1mm lift for big core.

This idles very well, almost like stock.

HKS 264 for 1jz are 221 and 220 @ .050 and 9.0mm lift.

Unsure what the 272's are in duration but they are 9.3mm lift, so you may have something similar to HKS 272's.

The group buy was going to have the "stage 1" cams at 218deg but that was too large jump from 189 stock they thought.

**Joshstix** · 04-07-2006, 09:27 AM

Originally Posted by 30psi 4agte

I recently purchased some cams from crow with 265 deg duration ( 228 @ 50 thou) and 9.5mm lift.

Those cams are very simmilar to the Wade Billets I run in my 3S. My cams are 268 advertised and 228 at 50 thou with 10mm of lift. They idle quite nicely with 12 inchs f vacuum at 950rpm and flow well enough in my setup to make 200rwkw at 10psi.

I'm not too sure I agree with your theory on needing short duration to stop blowing your intake charge out the exhaust though. The pressure in the exhaust manifold is pretty much always going to be higher than the pressure in the intake manifold since you have to turn the turbo with the exhaust gas in order to make pressure on the intake side. This higher pressure is going to stop the intake charge from getting out the exhaust in my opinion. In a supercharged setup short duration is definately the go for this reason though.

**kingmick** · 04-07-2006, 09:42 AM

its overlap you need to watch, to stop the charge going out the exhaust!

**30psi 4agte** · 05-07-2006, 01:08 AM

Originally Posted by kingmick

thats not bad as a cam choice, i take it you have shim under setup? looking at your lift.

almost have ordered em but dont have em yet

**Nim** · 05-07-2006, 01:24 AM

Originally Posted by Joshstix

Those cams are very simmilar to the Wade Billets I run in my 3S. My cams are 268 advertised and 228 at 50 thou with 10mm of lift. They idle quite nicely with 12 inchs f vacuum at 950rpm and flow well enough in my setup to make 200rwkw at 10psi.

I'm not too sure I agree with your theory on needing short duration to stop blowing your intake charge out the exhaust though. The pressure in the exhaust manifold is pretty much always going to be higher than the pressure in the intake manifold since you have to turn the turbo with the exhaust gas in order to make pressure on the intake side. This higher pressure is going to stop the intake charge from getting out the exhaust in my opinion. In a supercharged setup short duration is definately the go for this reason though.

Is the pressure in the exhaust really that high? I would have thought it was higher than an NA, but not higher than the other side of the turbo...

And if a little bit of your boost escapes to overlap, is that a big deal? It'll just get rid of some of the expended exhaust gas wouldn't it? And also spin your turbo more.

I dunno, I'm just making shit up.

**oldcorollas** · 05-07-2006, 03:56 PM

Originally Posted by Nim

I dunno, I'm just making shit up.

yes.. yes you are....

Is the pressure in the exhaust really that high? I would have thought it was higher than an NA, but not higher than the other side of the turbo...

usually, the pressure between cylinder and turbo is higher than in the intake manifold..., however, overlap occurs when piston is near TDC, and there is already momentum in the outgoing exhaust.. it is still critical for overlap.
it is possible to have lower exhaust manifold pressure than intake pressure. (think about turbine diameter)
i think you are getting confused.....

And if a little bit of your boost escapes to overlap, is that a big deal? It'll just get rid of some of the expended exhaust gas wouldn't it? And also spin your turbo more

why do cars with big cams run shit down low? overlap.
longer overlap does a few things.. it reduces dynamic compression = less power/torque. it reduces the volume of charge retained in the cylinder = less exhaust gas to spool turbo.
you know that when air/fuel burns, the change in the number of molecules is only about 5% greater?? most of the energy comes from the 3 or 4 times increase in volume due to temp. having cold gas going thru into exhaust manifold will not help...

how much overlap you choose depends on where you want your torque band to be.
on NA it is critical for exhaust scavenging and drawing in the intake charge, BUT not letting it escape.
on SC it is critical for not letting charge escape, as well as a bit of exhaust scavenging (and dead gas is just along for the ride.. although it could reduce detonation

)
on turbo it is critical for both the above reasons... off boost low rpm, you need something like an NA type effect, to maximise exhaust gases. on boost, you need to reduce the loss of charge, as well as possibility of exhaust reversion....

it's a bit complicated, and there are too many factors to be able to give an exact number, as different goals and different setups require different specs...