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Rb26 Aftermarket Plenums, Benefits For 600Rwkw Mark?

eXquiSit

By eXquiSit,
in Engines & Forced Induction

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bigmikespec Veteran

bigmikespec

Posted
Posted

most comments/threads say stock is fine for about 500kw, how are they making more?

It is just a manifold, it will make more power with it but won't do so efficiently.

A larger volume plenum is better as it allows the air inside to distribute itself better to each of the ports. Are you still using the individual throttle bodies?

Greddy is a nice cheap upgrade as it is over 5L which is closer to two times engine displacement.

Blackkers Experienced

Blackkers

Posted
Posted

larger volume plenum = lower velocity air for the same given airflow.

at low rpm that translates to less torque, at high RPM/boost that translates to more airflow.

around the 600rwkw mark I assume you don't care much for low end drivability so go for it. I assume it would help the top end slightly at a slight cost of bottom end torque.

Whether or not it's worth the cost is, as always, up to you

If I'm wrong someone please weigh in

GTSBoy Grand Master

GTSBoy

Posted
Posted

larger volume plenum = lower velocity air for the same given airflow.

at low rpm that translates to less torque, at high RPM/boost that translates to more airflow.

around the 600rwkw mark I assume you don't care much for low end drivability so go for it. I assume it would help the top end slightly at a slight cost of bottom end torque.

Whether or not it's worth the cost is, as always, up to you

If I'm wrong someone please weigh in

I'll weigh in just a little bit. Most of what you wrote is right, just the bit about reduced torque is worth a bit more thought.

The velocity of the air in the plenum is not much of an issue for torque. What matters for torque is velocity in the runners and ports. That is what drives cylinder fill. The plenum is exactly what its name implies - it is a plenum. It is just a volume. Its purpose is to provide a volume from which all the cylinders can draw, hopefully evenly, with the air having come from a common source beforehand. In this case the common source is the air flow metering (if present) and the turbo(s). It is of course perfectly possible to run an RB26 or any other ITB engine with no plenum at all. Of course you have to go without the turbo charging, and you have to do something other than AFM sensing, but you get the idea.

Anyway, given the above, a big plenum shouldn't hurt torque. What it does hurt is throttle response. A bigger volume that needs to be filled up with boost. Luckily though, if you keep the ITBs, then the penalty on throttle response is merely that required to fill the plenum with boost, not that caused by having a plenum at much lower pressure with a throttle at its inlet, which can be worse (bcause you get both effects at once, instead of just one of them).

The rule of thumb that the plenum should be about double the engine volume is not based on any real science. it just kind of works because once the plenum gets to about that size then it tends to be big enough to work to distribute the air fairly evenly to the cylinders without too much localised high velocity regions (near cylinder #1 for example). if you stick with a small plenum and jam a massive amount of air flow through it, the velocities go up and any problems with distribution just get worse. Bigger is better for high power but not quite as good for throttle response, as noted above.

  • Like 3
bigmikespec Veteran

bigmikespec

Posted
Posted

I'll weigh in just a little bit. Most of what you wrote is right, just the bit about reduced torque is worth a bit more thought.

The velocity of the air in the plenum is not much of an issue for torque. What matters for torque is velocity in the runners and ports. That is what drives cylinder fill. The plenum is exactly what its name implies - it is a plenum. It is just a volume. Its purpose is to provide a volume from which all the cylinders can draw, hopefully evenly, with the air having come from a common source beforehand. In this case the common source is the air flow metering (if present) and the turbo(s). It is of course perfectly possible to run an RB26 or any other ITB engine with no plenum at all. Of course you have to go without the turbo charging, and you have to do something other than AFM sensing, but you get the idea.

Anyway, given the above, a big plenum shouldn't hurt torque. What it does hurt is throttle response. A bigger volume that needs to be filled up with boost. Luckily though, if you keep the ITBs, then the penalty on throttle response is merely that required to fill the plenum with boost, not that caused by having a plenum at much lower pressure with a throttle at its inlet, which can be worse (bcause you get both effects at once, instead of just one of them).

The rule of thumb that the plenum should be about double the engine volume is not based on any real science. it just kind of works because once the plenum gets to about that size then it tends to be big enough to work to distribute the air fairly evenly to the cylinders without too much localised high velocity regions (near cylinder #1 for example). if you stick with a small plenum and jam a massive amount of air flow through it, the velocities go up and any problems with distribution just get worse. Bigger is better for high power but not quite as good for throttle response, as noted above.

This bloke! Good post.

Blackkers Experienced

Blackkers

Posted
Posted

I'll weigh in just a little bit. Most of what you wrote is right, just the bit about reduced torque is worth a bit more thought.

The velocity of the air in the plenum is not much of an issue for torque. What matters for torque is velocity in the runners and ports. That is what drives cylinder fill. The plenum is exactly what its name implies - it is a plenum. It is just a volume. Its purpose is to provide a volume from which all the cylinders can draw, hopefully evenly, with the air having come from a common source beforehand. In this case the common source is the air flow metering (if present) and the turbo(s). It is of course perfectly possible to run an RB26 or any other ITB engine with no plenum at all. Of course you have to go without the turbo charging, and you have to do something other than AFM sensing, but you get the idea.

Anyway, given the above, a big plenum shouldn't hurt torque. What it does hurt is throttle response. A bigger volume that needs to be filled up with boost. Luckily though, if you keep the ITBs, then the penalty on throttle response is merely that required to fill the plenum with boost, not that caused by having a plenum at much lower pressure with a throttle at its inlet, which can be worse (bcause you get both effects at once, instead of just one of them).

The rule of thumb that the plenum should be about double the engine volume is not based on any real science. it just kind of works because once the plenum gets to about that size then it tends to be big enough to work to distribute the air fairly evenly to the cylinders without too much localised high velocity regions (near cylinder #1 for example). if you stick with a small plenum and jam a massive amount of air flow through it, the velocities go up and any problems with distribution just get worse. Bigger is better for high power but not quite as good for throttle response, as noted above.

Thanks for the more detailed response!

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