More blower psi vs same + ported heads

[Viper488]

Okay, here's an engine with hypereutectic AL pistons traditionally said to be limited to 5psi from a DIY blower kit using a Whipple blower.

Setting that aside for the moment, but keeping that limitation in mind, what's the difference between adding ported heads to that 5 psi s/c engine, or just changing to a smaller 7psi rated pulley with unported heads?

Net operational psi is a function of backpressure/restriction in the head isn't it?

I haven't heard any objections to adding ported heads to a 5psi s/c setup, but that would have to raise cylinder pressure wouldn't it? - At the same time lowering measured blower (back)pressure.

Yet a '7psi' pulley blowing through un-ported heads isn't putting as much volume into the cylinder perhaps. Depends on whether the s/c vendor has designated that 7psi pulley from testing on an engine with ported heads or not I guess eh?

How different is a psi pulley on ported heads from a 7psi pulley in unported heads??

 

[patprimmer]

With a truly, 100% effective, positive displacement blower, porting the heads might drop boost, but won't increase the mass of air in the cylinder. The same mass will simple move at a different speed and density, as the mass of air is set by the blower as it is positive displacement.

The qualifiers on this are:-

The blower rotors do not seal perfectly, so there is some leakage. This increases at higher boost, so there is marginally more air flowing into the cylinder.

the blower absorbs more power to drive it at the same volume and higher pressure.

The higher pressure generates more heat in the manifold, but less in the cylinder to give the same final heat at the same final cylinder pressure with small deviations for leakage etc

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Viper488]

Well...on these particular blowers, whatever negative side efffects there are, more blower speed does yield more horsepower. You seem to be saying because of the qualifiers it all balances out to zero gain...(?)

That aside, I'm still wondering what the difference is in the end for this example between a 7psi rated blower atop unported heads vs a 5psi rated blower atop ported heads.

Is there any? The tuners didn't reply much, I think they want to sell $2500-$4400 heads more then $125 pulleys..

 

[JayMaechtlen]

Porting the heads may reduce restriction, reducing power required to drive the blower at any particular speed. Hence it should yield somewhat more power than without poring, due mainly to the reduction in power absorbed in the blower.

Increasing blower speed will increase airflow, power, and physical/thermal loading of pistons.

methinks if you want to make lots of power, an intercooler, more blower drive, and better pistons would be the way to go...


Jay Maechtlen

 

[Warpspeed]

Yup, as Pat says, the blower speed/displacement virtually fixes the airflow through the engine regardless. But if you port the heads it will decrease boost pressure, decrease blower discharge temperature, and decrease blower drive power. It will also give you a bit more detonation headroom. Lower total exhaust back pressure will do all of this as well.

Quite likely there will be a small power increase, but it will reduce the stresses on all the various parts. What you can then do is to increase blower drive speed to get the boost back to where it originally was. That will definitely increase airflow and power.



.

 

[Viper488]

I feel like this keeps straying off course... The facts are that on this engine with this particular blower, the blower with the 5psi pulley and ported heads, makes more rwhp than without ported heads-lots more. The blower with the 7psi pulley with unported heads make more power than the 5psi blower with unported heads.Without dissecting that any further and getting into blower seals and other non-pertinent infinite variables/suggested optimum setups etc, I'd just like to assertain why/how achieving the same rwhp by simply going to a smaller 7psi pulley is somehow a no-no with the hypereutectic pistons if the end result is the same rwhp and same apparent increase in cylinder pressure as the 'safe' 5psi pulley with ported heads. If they're both making 650hp how is one any less stress on the pistons than the other? Why should I opt for the heads instead of turning up the boost? I mean looking strictly at the cost comparison... 650rwhp is 650rwhp either way. People opt for dialing up the boost all the time, so don't send this off on a tangent with engineering class absolutes please. Add 60rwhp with $4000 ported heads or with a $100+ pulley. The choice seems obvious. Thank you : )

 

[patprimmer]

Viper488

JayMaechtlen and Warpspeed have interpreted it correctly.

If you leave the blower drive the same, you will get slightly more power at slightly lower boost and at lower stress on the engine and with less chance of detonation.

If you port the heads and up the overdrive to maintain the boost, you will get a bigger increase in power, but with more stress and more chance of detonation.

If you run the original heads, but more boost to equal the HP of the ported heads and original blower over drive, you will put more stress on the engine.

As Warpspeed says, supercharged engines really respond to big exhaust systems. They also like strong valve springs on the inlet valve and wide lobe centre cams.



Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[richdubbya]

The 7psi setup will have a higher cylinder inlet charge temperature, so even with theoreticaly the exact same power and cylinder pressure the pistons will have less intake cooling and run hotter. Is it enough of a difference to cause the pistons to fail? I dont know.

 

[Viper488]

Anything over a 5psi pulley on this engine is strongly discouraged by those who know due to the pistons.

A higher cylinder inlet charge temperature mostly because the air's heated up more by the increased blower speed, or by increased compression in the intake after the blower?

As designed, this particular blower setup is picking up just tons of radiant heat as inlet air travels the 2.5 feet or so from throttle bodies to s/c inlet at the back of the blower. The 3"OD or so metal tubes are at the most 1/2" away from the 150deg valve covers all the way.

 

[patprimmer]

Then you might gain a significant advantage by insulating the inlet duct.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.