JaimeVrod

I'll guess cuz there's less stress on the drive train (crank snout) and less power used to make that 13 psi boost compared to 16 psi.

Unreal

How? If you are spinning the blower at the same speed, and running same pulley, then that doesn't change. If you port the heads and boost goes from 16 to 13psi because of better head flow, you still need to keep the blower turning the same to keep the power up.

schpenxel

ding ding ding.

JaimeVrod

The energy required (work) is a function of the mass of the air moved. The mass of airovef is a function of the pressure drop across the air flow path (starting from air filter all the way thru to the tail pipe) and the blower rpm. pressure drop is inversely proportional to efficiency. An inefficient airflow path requires more pressure to overcome and that means more work.

The following is an extreme example to illustrate my point.
Put your car in neutral, rev the engine quickly to 5000 rpm, how much boost was created? None! Very little work was required to move that air because the drive train is hardly resisting the flow of air thru the engine.

Put the car in 1st gear (let the clutch out if you have one) and quickly rev engine to 5000 rpm, how much boost was created? A bunch (prob between 8-12 psi depending on your set up, maybe more). More boost was created because of the engine's resistance to air flow from the drivetrain being engaged.
Poor flowing heads and intake and poor valve timing all contribute to pressure drop and make the system less efficient.
The idea is to make your power as efficiently as possible.
All that extra required boost to overcome the pressure drop is just turned into heat, a hotter charge of air, and that's not good.

Unreal

Blowers are not load dependent like that. That works in a turbo car, not a blower car. I can go out in neutral and see 20psi if I want.

JaimeVrod

Well then yours is different than mine and probably most others. I have acboost gage hooked up and there is no boost registered unless the car is under load. I'm pretty certain you are wrong, sir.

You must have a huge impeller and hitting very high blower speeds to see boost without engine load.

schpenxel

He isn't wrong.

Boost from a blower is just dependent on engine/blower speed. It has nothing to do with load because the blower doesn't give a damn about load.. it's spinning the same ratio vs. engine speed regardless of load.

JaimeVrod

You're both wrong. Ask a good mechanical engineer.

lt1z

Because there will be less heat with less restriction while still making the more power and not having to spin the blower harder to do it. Put the same pulley combo on a stock LS2 with a cam vs an LS2 with a cam and good heads. The one with heads will make more power and have lower restriction (boost). You will also see more boost on the street then you will on the dyno even with a belt driven blower, though not as big of a difference as a turbo.

My point was a more efficient motor will always make more power with everything else remaining the same. I'm not bringing budget or spinning the blower harder and spraying more meth up as an alternative to heads because that is another conversation.

Unreal

Doesn't matter in the end. Yes making more power at less boost is great. Dropping $3k in heads to make same power at 13psi instead of 16psi is where I'm having the issue. I don't care if it is 16psi or 30psi. If it is making power and hitting the goals you want, great. If funds were unlimited then do it all, but it sounds like a budget build, so why waste the money when there are so many other things that should be upgraded first.

And just to make sure I wasn't crazy I went out to the parking lot at work, fired up the car and rev'd it up a few times. Just blipping the throttle was seeing 10-11psi. Maybe if you have a blower pullied for 5-6psi a quick blip won't show much. My setup makes 24-25psi max.

JaimeVrod

That's a huge amount of boost Unreal.
I agree in that I have a hard time spending $3000 for heads when a good cam will provide more hp for fewer dollars.
My car is pulleyed for 9 psi and it doesn't register any boost unless the engine is under load.

lt1z

I get what you are saying but there are certain situations where it does make sense. Like not being able to spin the blower any faster and still wanting more power. Now, I could see you saying buy a bigger blower instead of heads and make even more power, but not all of us want 20+ psi setups. Different strokes, and as I mentioned above , budget had no place in my efficiency argument behind using heads. I was talking about why it works not a subjective argument like "Is it worth it?".

Unreal

I get that, but that isn't realistic for most people. Might as well say do a custom canted valve head solid roller tall deck twin turbo setup.

Nothing wrong with 20psi+. People get way too caught up on boost numbers.

If the argument is "are better more efficient more power making heads better than stock heads?" then I don't think anyone is going to say no. That is like arguing if a 10lb sack of potatoes weighs more than a 5lb sack. Yes the better flowing, better heads are better.

lt1z

When I decided to do the heads on my car the reasoning was not only for increased flow but also better valves and I replaced an LS3 head with a rotocast LSA casting. The TFS would provide similar benefits to an LS2 if it is in the owners budget.

Then again you could just add nitrous too if $/hp is the name of the game

schpenxel

I was messing around with some transient fuel tuning this evening and just blipping the throttle relatively quickly, even in neutral while sitting still, I could get enough boost to make the alky control light come on.. so at least 3 psi. This is on an as basic as it gets setup.

The only connection between engine load and the supercharger is the bypass valve. You have to get close enough to 0 vacuum for the bypass to close before it will build boost. Pretty tough to do that sitting still before you're bouncing off the rev limiter.. but that doesn't mean it won't build boost without engine load. It will.

Obviously the second paragraph isn't directed at you.

JaimeVrod

Schpenxel, you are right. I did get the gage to blip and register a little boost with the car in neutral but it wasn't easy to repeat.
With respect to the bypass valve that you mentioned. Is there a way to keep the valve closed between shifts on a manual transmission car so the boost doesn't drop between shifts?

schpenxel

Yeah it's not super easy for us lowly < 20psi of boost guys to build boost sitting still, lol. If you bounce it off the rev limiter and hold the pedal on the floor, it will build boost. I have no desire to do that to prove it though.

I would imagine that it would be incredibly hard on the blower if you could find a way to force the bypass valve to stay closed during shifts (assuming you aren't flat footing it or something). I don't really know of a way to do it though, although anything is possible of course.