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#2401407 - 11/10/17 12:01 PM Re: Boost and HP [Re: Dr Dave]
Cab_Burge Offline
Too Many Posts

Registered: 08/23/03
Posts: 29559
Loc: Bend,OR USA
I don't know of any good formula for boost versus HP gained shruggy
When you compress air it heats up and expands so if you don't cool it before it enters the motor you have a increase in manifold inlet temps. which can and will lead to detonation which kills HP and motors work
The last roots boosted Hem motor I dyno made 924 HP at 7300 RPM with 33 degree total timing with it 12 % under driven making 7.0 lbs. of boost on CA pump swill. We switch the pulleys around so it was 13 % over driven and retarded the timing to 25 degrees total which let the motor make 1027 HP at 6500 RPM with 12 Lbs. of boost with 110 octane race gas and it went into detonation shruggy The manifold inlet temps went up also but I don't remember how much, we added two degrees timing on the next pull and it went into detonation sooner puke shruggy
IHTHs


Edited by Cab_Burge (11/10/17 12:03 PM)
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#2401452 - 11/10/17 01:36 PM Re: Boost and HP [Re: GTX MATT]
TRENDZ Offline
pro stock

Registered: 01/24/05
Posts: 1500
Loc: Milwaukee WI
Originally Posted By GTX MATT
Originally Posted By TRENDZ
This from your last post GTX MATT...

"I'm looking at it from the standpoint of the valve is closed, there's some # of boost in the intake manifold behind the valve, now the valve opens and its going to fill the cylinder in less time that at 0 PSI."

Hard to argue if that's where it ends. But that's not where it ends. What is the pressure on the other side of the intake valve when under boost?
The cylinder has only two points of reference, the intake charge, and the exhaust. As boost increases, so does exhaust pressure. It doesn't matter if it's supercharged or turbocharged... exhaust pressure gets very high under boost. So at best, cylinder fill starts to happen (loosely)at around a 1:1 pressure ratio.


Correct, I would say it absolutely doesn't end at the intake valve, once the intake valve starts to open the pressure wave is now going through the cylinder and into the exhaust path (assuming overlap) until the exhaust valve closes, then its ending at wherever the top of the piston is. Pressure on the other side of the valve changes depending on where the piston is of course, cam specs, etc. But it should be less than the pressure on the intake side of the valve under boost.


Ok. so what part of this has created the increased port velocity? What part of any of your description is different than a typical N/A engine?
I use to purposely put in a narrow lsa cam to allow me to run exclusively on pump gas. My typical exhaust to intake ratios were around 2:1. This would intentionally contaminate the intake charge under boost to give me an EGR effect. I can post pictures of the intake port carbon stirthepot During overlap airflow was literally backwards. Flow happens from differential pressure. So how did I get exhaust to flow into the intake?
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#2401611 - 11/10/17 07:18 PM Re: Boost and HP [Re: TRENDZ]
Blusmbl Offline


Registered: 01/20/03
Posts: 7341
Loc: Plymouth, MI
Originally Posted By TRENDZ

Ok. so what part of this has created the increased port velocity?


The backpressure increases the trapped mass in the cylinder (and is where any motor will see reversion) but the port velocity is consistent for most of the duration of the intake valve being open when the exhaust valve is closed. The 2:1 pressure ratio across the engine you're seeing isn't reducing the port velocity because the pressure differential isn't 2:1 anymore once the exhaust valve closes and the cylinder volume is changing as the piston is traveling down from TDC.
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#2401735 - 11/10/17 11:07 PM Re: Boost and HP [Re: Dr Dave]
TRENDZ Offline
pro stock

Registered: 01/24/05
Posts: 1500
Loc: Milwaukee WI
So, the swept volume RATE(piston speed) stays the same (because RPM stays the same) and we have double the density in the port compared to a no boost situation. If those two things are constants between our boosted and non boosted examples, then you would end up with an engine capable of doubling the horsepower by doubling the density without any increase in port speed. We can get pretty close to that in the real world.
Now take those constants, and add speed. I don't know how but let's throw that in there. Now you have twice as many molecules, moving faster than my previous example(pic any rate faster that you want but explain please) Then wouldn't any increased port velocity make the engine breath more and more efficiently as density rises. So 100% more air would make more than 100% more horsepower???? Sort of a perpetual motion type thing? I don't buy it, but I'm willing to keep an open mind. So far I'm not convinced.
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#2401801 - 11/11/17 08:18 AM Re: Boost and HP [Re: Dr Dave]
dizuster Offline
master

Registered: 01/12/05
Posts: 3288
Loc: Oakland, MI
Do a little math on Cab' s example. They added 5psi, and it picked up 103hp. Almost exactly 20hp/psi.

I get there is a lot of math behind it all,and there are always exceptions... but in the 500-1000hp normal guy horsepower range, the rule of thumb is usually pretty close.
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#2401830 - 11/11/17 09:13 AM Re: Boost and HP [Re: Dr Dave]
Blusmbl Offline


Registered: 01/20/03
Posts: 7341
Loc: Plymouth, MI
So what our examples are missing a couple key points:

1. air density linearly decreases for a given pressure as temperature increased. Using the same turbo or supercharger, doubling the boost will likely come with increased air temp, so you aren't actually getting double the density. The temp can be calculated from the compressor map, too.

2. The backpressure, which you touched on earlier, actually hurts more due to work lost to pumping losses. The indicated power keeps going up but the engine is wasting more horsepower just overcoming the backpressure.
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#2402336 - 11/11/17 11:23 PM Re: Boost and HP [Re: Dr Dave]
hemi-itis Offline
I Live Here

Registered: 12/29/07
Posts: 10147
Loc: Great Neck,LI,new york
With my roots 1071 went from 13.3 UD to 6.1 OD and only picked up 2 lbs of boost.Car went from 140 MPH to 147 mph @3700 lbs
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#2404819 - 11/16/17 11:58 PM Re: Boost and HP [Re: TRENDZ]
GTX MATT Offline
master

Registered: 06/05/06
Posts: 4744
Loc: CT
How did the narrow LSA help you run on pump gas?

I'm assuming your talking about a turbo application, which has different back-pressure than supercharged.

At any rate, your intake ports had carbon in them because the engine had reversion, which I'm sure you know, just like lots of engines with lots of overlap. Being boosted doesn't help reversion at idle and most of the other conditions that engines are susceptible to reversion. So you ran a cam with a lot of overlap and saw evidence of reversion, nothing surprising there really.

I'm keeping an open mind as well. But we know that flow is initiated by pressure drop. When the intake charge begins to fill the cylinder we know the pressure in the intake manifold is greater than the pressure in the cylinder. So, now lets say the intake manifold pressure is 14.7 PSI (just 1 atmosphere) and the pressure in the cylinder is somewhere around 0. What do you think happens to the air/pressure in the port right around the valve once it starts to open? And what does the air behind it in the rest of the port do?
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#2404841 - 11/17/17 01:35 AM Re: Boost and HP [Re: Dr Dave]
FastmOp Offline
master

Registered: 01/20/03
Posts: 4906
Loc: Hot Rod Ridge
Lmao. My IAT are below ambient.
My cam has no overlap and I run a 440source crank.
I'm doing it all wrong but my car flys. My research backs up TRENDZ but I stopped researching and moved on to running my car.

Y'all worry about flowing air.

I'll worry about flowing fuel !

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#2404855 - 11/17/17 05:42 AM Re: Boost and HP [Re: Dr Dave]
TRENDZ Offline
pro stock

Registered: 01/24/05
Posts: 1500
Loc: Milwaukee WI
MATT: How did the narrow LSA help you run on pump gas?

ME: Goggle "What does EGR do?"
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#2404901 - 11/17/17 08:44 AM Re: Boost and HP [Re: Dr Dave]
GTX MATT Offline
master

Registered: 06/05/06
Posts: 4744
Loc: CT
Decrease combustion efficiency I'm guessing? Did it make a noticeable difference? I'm interested...
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Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street

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#2405024 - 11/17/17 12:48 PM Re: Boost and HP [Re: Dr Dave]
TRENDZ Offline
pro stock

Registered: 01/24/05
Posts: 1500
Loc: Milwaukee WI
Yes, you are correct. Counter intuitive to making power right?
EGR slows down the burn and decreases combustion temps. Pluses... Still bad though, right?
Yes and no. You need to increase the boost to make the same power, but the engine is tolerant of the increased boost. Timing is also RADICALLY different. You can run dang near any fuel without detonation at power levels well beyond what "stock blocks" are supposed to ever be able to handle, because you aren't rattling the engine. Ever.
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