High RPM intakes are more appropriate for boosted motors? Why?

[ratio411]

I don't believe the possible internet myth that boost on a small displacement motor makes bigger than normal and/or short runner intakes like the TFS-R w/ box, the spider, victor, or Hurricane more preferable. I'd like to see any documentation or logic explaining why boosted motors of a given displacement will see better results with high flow/high rpm intakes than their non-boosted counterparts.

All boost does is compress and heat air. The volume of air moved in a given amount of time is still identical to the non-boosted motor. The only argument that even seems viable to me is that air velocity is reaches the sonic threshold when compressed and doesn't when not compressed because the threshold may be lower with compressed or heated air. I'm not sure that this is the case, and would need details to support this.

I might be wrong, but from what I understand about boost is that it makes the intake a non-factor. In other words, generally speaking, a long runner intake will act exactly the same as a short runner intake when they are set up identically and boosted identically. Same is supposed to be true for plenum volume, and to a lesser extent the port size. However, port size can be too big or too small on a boosted engine.

That is why I say 'generally' speaking intake specs are a non-factor when boosting.

That's my .02 and the way I understand it.
My .02 is worth exactly what you paid for it!

 

[FastDriver]

I don't believe the possible internet myth that boost on a small displacement motor makes bigger than normal and/or short runner intakes like the TFS-R w/ box, the spider, victor, or Hurricane more preferable. I'd like to see any documentation or logic explaining why boosted motors of a given displacement will see better results with high flow/high rpm intakes than their non-boosted counterparts.

All boost does is compress and heat air. The volume of air moved in a given amount of time is still identical to the non-boosted motor. The only argument that even seems viable to me is that air velocity is reaches the sonic threshold when compressed and doesn't when not compressed because the threshold may be lower with compressed or heated air. I'm not sure that this is the case, and would need details to support this.

 

[FastDriver]

I'm the OP, but he responded to my post in another thread before I opened this one to keep from hijacking the other thread. I moved his post over to this thread. That's why his response is above my post. Sorry for the confusion.

 

[FastDriver]

I might be wrong, but from what I understand about boost is that it makes the intake a non-factor. In other words, generally speaking, a long runner intake will act exactly the same as a short runner intake when they are set up identically and boosted identically. Same is supposed to be true for plenum volume, and to a lesser extent the port size. However, port size can be too big or too small on a boosted engine.

That is why I say 'generally' speaking intake specs are a non-factor when boosting.

That's my .02 and the way I understand it.
My .02 is worth exactly what you paid for it!

Maybe so. Can you, or anyone reading, provide documentation on how boost changes the helmholtz effect? If what you're saying is true though specifically "the same as" then there would not be any advantage to either intake.

For those that aren't following, helmholtz effect = pressure waves that result from the shock of the intake valve closing that have a slight supercharging effect when timed correctly by changing runner length so that the pressure wave hits the valve as it opens.

 

[ratio411]

Maybe so. Can you, or anyone reading, provide documentation on how boost changes the helmholtz effect?

Helm-whatz? Hey, I am just a gearhead, not an engineer.
My teachers only passed me my senior year so they didn't have to see me again! I don't know 'greek'. (just being fun, not critisizing your use of $10 words)

If what you're saying is true though specifically "the same as" then there would not be any advantage to either intake.

Yeah, that is what I have gathered in my bench racing and general life of gearheadedness... When boosted, the intake manifold is 'generally' a non-factor. Again, 'generally' means within reason. If you go from a 2" diameter intake runner to a 4" diameter intake runner, that doesn't fall under 'generally'.

 

[Boosted92LX]

Maybe so. Can you, or anyone reading, provide documentation on how boost changes the helmholtz effect? If what you're saying is true though specifically "the same as" then there would not be any advantage to either intake.

For those that aren't following, helmholtz effect = pressure waves that result from the shock of the intake valve closing that have a slight supercharging effect when timed correctly by changing runner length so that the pressure wave hits the valve as it opens.

Doesn't the Helmholtz effect pertain to n/a engines only? It does (from what I've read about it) have to do with "packing" the cylinder with more air when runner tuning is applied.

But, I would think with compressing the intake charge beyond 1 bar, this theory would go out the window since the cylinder is being "packed" way beyond what any velocity tuning would achieve with natural aspiration.

This being the case, wouldn't runner length be a moot point?

 

[ratio411]

Did I say that I could have 'heard' wrong and may be full of it?
I'm like you, I'd like to hear (in layman's terms) what is and isn't.

 

[ratio411]

On the subject of runner diameter, or more precisely runner efficiency, a smaller (or less efficient) runner will increase boost, simply due to the 'back pressure' caused by restrictions. While larger (or more efficient) runners will decrease boost because the air is more free flowing. Make sense?

 

[NIKwoaC]

What I've come to understand, and what I've read from people who know a lot more about it than me, is the more efficient you can make a motor N/A, the better it will perform on boost. Simple as that. With a supercharger or a turbo, all you're doing is increasing the number of air molecules that get into the cylinder when the valve opens. All the same rules of making performance apply.

 

[FastDriver]

I honestly have no idea if it applies, Allen. People seem to think it doesn't, but I'm not convinced.

I agree with what you're saying ratio, but that's regardless of boost, really. I'd say that since you're moving the same volume of air at a given RPM in either engine, then a runner diameter that's too small will be too small on an n/a motor or a boosted motor.

 

[Sharad]

I know better than to get into another physics debate on this site but for the record, Richard Holdener has written some interesting articles over the years dyno testing every kind of combo imaginable... and his general philosophy is that you match your H/C/I combo, and the boost just multiplies the power. He tends to like long runner intakes (like the Holley Systemax) on all but the most radical builds.

When I built my current combo, I called Ed Curtis at Flowtech Induction and said "I want 1000rwhp, but streetable". He supplied the cam and told me which heads & intake to use.

People can make general suggestions (long runner, short runner, etc) but when you want to maximize your particular combo, there are really only a few people in the country that I think are experts in the field.

 

[FastDriver]

Yep! That's exactly the way I feel, Sharad.

 

[85_SS_302_Coupe]

The way i always understood boost is that it's pounds per square inch, and so 8psi in a smaller space vs. 8psi in a larger space, you're moving more air in the larger space even though the boost number is the same, because your square inches are increased. High rpm intakes generally breath deeper than others, so you get more volume. Am i wrong?

 

[FastDriver]

The way i always understood boost is that it's pounds per square inch, and so 8psi in a smaller space vs. 8psi in a larger space, you're moving more air in the larger space even though the boost number is the same, because your square inches are increased. High rpm intakes generally breath deeper than others, so you get more volume. Am i wrong?

Yes. Average air pressure at sea level without boost is 14.5 psi. If your assertion can be applied to a car with boost, then it can be applied to a car without boost.

Another way to think of it is that you really don't have a larger space if the motor is of identical displacement. The volume in the cylinder remains the same.

Further, to be clear, you're moving more air mass under boost, not more air volume.

 

[5.0Torx]

Further, to be clear, you're moving more air mass under boost, not more air volume.

If I may...


Its the mass that matters most. There's a reason cars have a Mass Air Meter, not a Volume-Air Meter...

Boosting an engine is similar to increasing its displacement. However much airflow your H/C/I/E (the "top end") can flow, a smaller-displacement bottom end will "max out" that airflow at a higher rpm than a larger-displacement bottom end. Case in point - 2v 4.6s (espeically non-PI) vs. 5.0 HOs. Same ****ty-flowing airflow numbers from the heads, same restrictive exhaust, etc etc... The 4.6 revs higher because the bottom end is of smaller displacement. They make more power because the peak hp is at a higher rpm, but they sacrafice the low-end response...

Ever put a vacuum gauge on a relatively stock 5.0 HO? Floor it, and the gauge will read 0" Hg until about 5000 rpm, then it starts backing off because the bottom end has essentially "maxed out" the available airflow from the top end. Boost will simply move that point lower. You'll get ~6psi (for sake fo argument) until a certain point, and then the boost pressure will start to rise. Too much top end restriction.

There's no magic in all of this. Its silly to say that intake parameters "dont matter" when you bolt a blower on. In basic terms, which seems to be what everyone here is after, boost simply lowers that rpm point that has "maxed out" the top end airflow. Hypothetical question to prove my point - Whats the peak hp/tq points on a stock 5.0 HO before and after a blower? Now, how about taking a factory-boosted car, such as an STi - how much higher would you have to rev it (in N/A form) to attain the same 300 hp?

 

[FastDriver]

If I may...

By all means... I'll play devil's advocate, if you don't mind.

Its the mass that matters most. There's a reason cars have a Mass Air Meter, not a Volume-Air Meter...

Total mass of air that makes it into the cylinder will determine how much fuel has to be pushed in. Mass absolutely matters the most, but let me ask you this: will a motor under boost be less volumetrically efficient?

Boosting an engine is similar to increasing its displacement.

Except that with displacement the mass of air making it into the cylinder is increased by a corresponding increase in Volume instead of density.

However much airflow your H/C/I/E (the "top end") can flow, a smaller-displacement bottom end will "max out" that airflow at a higher rpm than a larger-displacement bottom end. Case in point - 2v 4.6s (espeically non-PI) vs. 5.0 HOs. Same ****ty-flowing airflow numbers from the heads, same restrictive exhaust, etc etc... The 4.6 revs higher because the bottom end is of smaller displacement. They make more power because the peak hp is at a higher rpm, but they sacrafice the low-end response...

I agree with your premise... same components + less displacement equals the same airflow at higher RPM

Ever put a vacuum gauge on a relatively stock 5.0 HO? Floor it, and the gauge will read 0" Hg until about 5000 rpm, then it starts backing off because the bottom end has essentially "maxed out" the available airflow from the top end.

Gotcha

Boost will simply move that point lower. You'll get ~6psi (for sake fo argument) until a certain point, and then the boost pressure will start to rise. Too much top end restriction.

I think this is a non-sequiter. It doesn't follow from the premise.

There's no magic in all of this. Its silly to say that intake parameters "dont matter" when you bolt a blower on. In basic terms, which seems to be what everyone here is after, boost simply lowers that rpm point that has "maxed out" the top end airflow.

Do you have any data?

Hypothetical question to prove my point - Whats the peak hp/tq points on a stock 5.0 HO before and after a blower?

It would depend on the blower. I think a centri would naturally bring the powerband up because boost is constantly rising with the rise in RPM. I think a PD blower might have lower peaks because it's less efficient up high.

Now, how about taking a factory-boosted car, such as an STi - how much higher would you have to rev it (in N/A form) to attain the same 300 hp?

It won't, at any RPM, reach 300hp. The heads won't support that power at ambient air pressure because it's not about the volume of air being moved, it's about the density of that volume of air.

 

[FastDriver]

This guy's apparently "bone-stock" 5.0 reaches peak power higher than expected at 5000 rpm. dyno'd my bone stock 5.0 UPDATE 372hp 487ftlb on 11psi!!!

Now, my guess is that the only reason that the power is stretched to the right is because the turbo is getting more and more efficient as the mass of air it moves is increased.

 

[Sharad]

Hypothetical question to prove my point - Whats the peak hp/tq points on a stock 5.0 HO before and after a blower?

I think a centri would naturally bring the powerband up because boost is constantly rising with the rise in RPM. I think a PD blower might have lower peaks because it's less efficient up high.

When my stock 302 ran 12.91 @ 103.7, I was shifting around 5-5200. It would actually rev higher, but there was no power there. The car was fastest when I kept it below 5200. Then, when I put the ProCharger on it, I tried to keep the rpms below 6000 even though the ProCharger kept pulling harder up top. (I didn't want to run over the crankshaft with the back tires as I passed through the traps) I couldn't get the car to run any better than 11.70-80s. Then my friend told me to quit driving like a wuss and let her eat. We trailered the car to the track and we were already building a new engine, so I said screw it. I think the spark cut was set to 6400 rpms and the fuel cut was set to 6500 rpms so I tried to rev it close to 6500 on the shifts. The car responded with an 11.36 @ 122. Ran .40 quicker in the quarter by using an extra ~500rpms. Mind you, this was over 1000rpms more than my best pass N/A.

Conclusion: the stock 302 liked more rpms with the ProCharger.

Extraneous data:

I used to shift my dad's H/C/I '93 GT around 6000rpms. (13.30s @ 106) After the KB Blowzilla/Flowzilla, the car was a lot quicker, but still didn't want to wind much higher than 6000rpms. Boost was dropping around 5500. (12.15 @ 114) I can't explain the science behind this, because I'd think the blower would let it rev higher, but I guess the KB was just less efficient up there? Fwiw, I think it's a 2.1L.

My '92 notch went 10.17 @ 132 with 17psi from a ProCharger P1SC. It had a stock block, Eagle 306" kit, stock cam, Edelbrock Performer RPM heads, Performer intake, shortie headers, etc. That would be about a 5800 rpm combo N/A. With the ProCharger at 17psi, I was shifting at 7200 rpms and it was still pulling. (again, didn't want to run over the crank)

For entertainment purposes:

These are horrible passes, but they're before and after boost videos of my '89 LX.

13.31 N/A (best pass in this configuration was 12.91 @ 103.7 on a 1.69)

YouTube - ‪Sharad @ London Dragway‬‏

11.70? ProCharged (best pass in this configuration was 11.36 @ 122 on a 1.70)

YouTube - ‪Project Shocker Beech Bend Raceway.wmv‬‏