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Old 01-12-2009, 01:26 PM   #1
Fox Slaughter
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1996 SL2
Default Airflow Rate

Does anyone agree at 6500 rpms our cars flow 191 cfm round to 200 cfm?

Airflow Rate = cid x .5 x Ev/1728

So

AFR = 120 x .5 x .85/1728

At 25lbs of boost Pressure ratio would be 2.7 which would be 270% more airflow.

Which Airflow Rate = pressure ratio x engine cfm

So

AFR 2.7 x 120 = 324 cfm.
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Old 01-12-2009, 02:01 PM   #2
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Default Re: Airflow Rate

Yea they do but if we pass 5600rpms we lose peak power you know right? Otherwise if it's modded then you can go up to whatever yuo feel it still pulling.
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Old 01-12-2009, 02:07 PM   #3
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Default Re: Airflow Rate

Notice I said blowing 25lbs. Blowing 25lbs on a stock motor would destroy it. This would be forged motor with a crank balanced to 9000rpm.
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Old 01-12-2009, 03:54 PM   #4
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Default Re: Airflow Rate

Missed that, are you seriously going for that?
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Old 01-12-2009, 04:59 PM   #5
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Default Re: Airflow Rate

Where did the 120 come from?
The engine flows approx 170 cfm @ 5500 RPM @ 100 kPa
Don't aim for the amount of boost you're going to run. Aim for horsepower.
This way, if 170 cfm gives you 124 bhp, to get to 325 bhp (which is about 300 at the wheels) is gonna have to be 170cfm/124bhp=1.371cfm/bhp*325bhp= ~445 cfm. Breathing mods become extremely important at this point. Everything from the bends in the piping to throttle body to the size of the turbine wheel. Also, note that 445 cfm translates to approx 2.61 pressure ratio, which is around 24 psi.

Now in reality, our engines are more efficient than that. So with proper breathing mods, you might squeeze out 200 cfm @ 5500 RPM, and with a properly sized turbo, you might get away with running less psi to get the same flow rate. Some bigger turbos can flow that much at only 1.5 pressure ratio, so that's why you see 300 whp saturns who run about 15 psi. My turbo flows even more than that at 15 psi, for example, but I pay the price in boost lag (about 10 psi of boost at 3500 RPM, and full boost by 4000 RPM). But with a 9k redline, it's not a big issue. Although, I never rev that high with stock cams, I think the highest I took it was close to 8000 RPM, which was nice because it put me in peak power in 4th gear and fried the clutch.

Note that the limit of a forged block is a little over 400 whp because after that you're starting to lift the head up and burning the head gasket on the intake side of things. Don't be fooled that 300-400 whp isn't enough. Power to weight ratio says you only need 315 whp to beat a 500 whp corvette. And if you get your block step-decked, you can run really high boost and produce more than 500 whp, but you will find out once you cross the 300 mark that not having traction in 3rd gear might sound cool, but is entirely not practical in any way.
...
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Last edited by plastic torpedo; 01-12-2009 at 05:10 PM.
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Old 01-12-2009, 06:39 PM   #6
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Default Re: Airflow Rate

The 120 is cubic inches, considering our blocks are 119.7 cubic inches.
The conversion from liters to cubic inch is liter x 63. Same for cubic inch to liter. CID / 63.

I am shooting at roughly 325 - 375 whp.

Which means 247.728 hp to 366.828 hp according to a fast equation from Maximum Boost of how much hp a 120 cid motor will make at 25lbs of boost.

The equation is

Lower limit .052 x cid (boost + 14.7) = bhp
Upper Limit .077 x cid (boost + 14.7) = bhp

Last edited by Fox Slaughter; 01-12-2009 at 06:50 PM.
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Old 01-12-2009, 08:56 PM   #7
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Default Re: Airflow Rate

Didn't notice the cid part. We're using a different formula to calculate a different thing. My calculation is based on the actual air flow and is a little more accurate when applied to the saturn engine. You need approx 400-450 cfm to crank out a little over 300 whp. That has been discovered quite some time ago and applies to the saturn DOHC engine. The amount of boost you're going to run will depend on how efficient your turbo and piping/intercooler setup is, and how fast you can get the air in and out of the engine. You never want to build your engine to handle a certain amount of boost, you want to be able to handle a certain amount of power. Boost pressure doesn't mean a whole lot all by itself.

But good thing you're reading that book. It's very useful and helps you understand many things. Just remember - double the boost pressure doesn't mean double the airflow. You can flow the same amount of air and raise the pressure significantly if you put a restriction at the end. You need to account for the restriction of your system, the turbo, the engine itself, etc. In this situation, you want to maximize your flow without creating more pressure.
...
2000 SL2 5-speed turbo
MSnS-E fuel & spark controlled @ 17-20 psi (methanol injected)
Fully forged, P&P head and manifolds, FSR turbo cams, balanced to 9k RPM
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Old 01-12-2009, 09:02 PM   #8
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Default Re: Airflow Rate

Plastic is trying to rape my fragile little mind!!!
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Old 01-13-2009, 01:02 AM   #9
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Default Re: Airflow Rate

nah, i don't swing that way
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2000 SL2 5-speed turbo
MSnS-E fuel & spark controlled @ 17-20 psi (methanol injected)
Fully forged, P&P head and manifolds, FSR turbo cams, balanced to 9k RPM
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Old 01-13-2009, 02:53 AM   #10
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Default Re: Airflow Rate

Well in that case I still want money for the psychological distraught I could have endured. The money will go to my turbo build so it will go to a good cause.
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Old 01-13-2009, 02:13 PM   #11
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Default Re: Airflow Rate

It just so happens I flowed a stock Dohc head a couple of weeks ago.
I mounted the #2 chamber, and just used the stock cam to lift the intakes to max lift as installed. The first reading netted 106cfm at 10@ of depression (Superflow 110).
I then removed the head and scrapped the carbon form the left and right walls( there wasn't much), the floor and ceiling were clean, I got 116cfm @ 10@.
Now these numbers seem low, I was only testing a 10" of depression. I did check at 15" and the reading was almost 140cfm@15". If you apply the math, that would also equal around 200cfm @28" of depression.
When people start throwing around flow numbers, they mean absolutely nothing with out the amount of depression.
I now have to make a fixture to be able to test the other cylinders and to actuate the valves.
There are a lot of ways I could have done these test, this example was just a off the cuff, just for giggles, not looking for anything in perticular.Take it for what it's worth.
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Old 01-13-2009, 03:10 PM   #12
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Default Re: Airflow Rate

Quote:
Originally Posted by laser3kw View Post
It just so happens I flowed a stock Dohc head a couple of weeks ago.
I mounted the #2 chamber, and just used the stock cam to lift the intakes to max lift as installed. The first reading netted 106cfm at 10@ of depression (Superflow 110).
I then removed the head and scrapped the carbon form the left and right walls( there wasn't much), the floor and ceiling were clean, I got 116cfm @ 10@.
Now these numbers seem low, I was only testing a 10" of depression. I did check at 15" and the reading was almost 140cfm@15". If you apply the math, that would also equal around 200cfm @28" of depression.
When people start throwing around flow numbers, they mean absolutely nothing with out the amount of depression.
I now have to make a fixture to be able to test the other cylinders and to actuate the valves.
There are a lot of ways I could have done these test, this example was just a off the cuff, just for giggles, not looking for anything in perticular.Take it for what it's worth.
I've seen your post on 6thsphere. By depression, you mean vacuum? If so, the numbers don't look that bad.
...
2000 SL2 5-speed turbo
MSnS-E fuel & spark controlled @ 17-20 psi (methanol injected)
Fully forged, P&P head and manifolds, FSR turbo cams, balanced to 9k RPM
http://www.cardomain.com/ride/3070918
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Old 01-13-2009, 09:12 PM   #13
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Default Re: Airflow Rate

yes -depression can equal vacuum. Basically it is really "differential pressure". The vertical manometer is used to monitor and establish a "differential pressure" between two plenum's. In a intake valve case the plenum under the valve is "lower" pressure then the reference which is ambient atmosphere. Then the flow is measured by the difference in pressure across a know orifice which is between the lower plenum and the prime air mover. that can be translated in to cfm.
So to turn up the power on a motor, you do the same as a flow bench - increase the differential pressure between inlet side of the valve and the cylinder.

PS - I am dying to try Megasquirt and maybe turbo it later on.
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Old 01-13-2009, 09:14 PM   #14
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Default Re: Airflow Rate

Quote:
Originally Posted by plastic torpedo View Post
But good thing you're reading that book. It's very useful and helps you understand many things. Just remember - double the boost pressure doesn't mean double the airflow. You can flow the same amount of air and raise the pressure significantly if you put a restriction at the end. You need to account for the restriction of your system, the turbo, the engine itself, etc. In this situation, you want to maximize your flow without creating more pressure.
The bolded is one of the most misunderstood aspects of forced induction there is. I've seen a lot of cases where seemingly small issues had drastic affects on flow, often at fairly low (15lbs and less) levels of boost.
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Old 01-13-2009, 09:34 PM   #15
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Default Re: Airflow Rate

Quote:
Originally Posted by Fox Slaughter View Post
The 120 is cubic inches, considering our blocks are 119.7 cubic inches.
The conversion from liters to cubic inch is liter x 63. Same for cubic inch to liter. CID / 63.
uh...
1 L = 61.0237441 cubic inches
1.9 L = 115.945114 cubic inches
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Old 01-14-2009, 02:40 AM   #16
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Default Speed Kills

Wouldn't the piston speed at 9000 rpm (almost double stock) be fatal ?

Last edited by POS; 01-14-2009 at 02:40 AM. Reason: tiepoe
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Old 01-14-2009, 03:11 AM   #17
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Default Re: Airflow Rate

The stress would be ridiculous, but no not fatal for a forged motor. It's not exactly useful either. There is currently no cam that will make it reasonable to rev that high. 7500-8000 is about the reasonable limit to make any power. I sometimes have to rev that high if I'm racing a faster car, it gives me a longer gear and quite a bit of an advantage, but most of the time I don't do it, it's not necessary. I would definitely not rev it to 9k, even though there are people that do.
...
2000 SL2 5-speed turbo
MSnS-E fuel & spark controlled @ 17-20 psi (methanol injected)
Fully forged, P&P head and manifolds, FSR turbo cams, balanced to 9k RPM
http://www.cardomain.com/ride/3070918

Last edited by plastic torpedo; 01-14-2009 at 03:23 AM.
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Old 01-14-2009, 08:47 AM   #18
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Default Re: Airflow Rate

Quote:
uh...
1 L = 61.0237441 cubic inches
1.9 L = 115.945114 cubic inches
My bad the number is 61 not 63.

1.9 x 63 = 115.9
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