If you have a pipe that is 2.25

and

You have a pipe that is 2.38

How much larger in % is the 2.38 pipe over the 2.25 pipe.

Remember guys this has to do with a circle and there is a formula to figure this out.

area = 3.14 (~pi) * radius^2, where r = diameter/2

Perhaps I have oversimplified this one Dre, but if you want the percentage difference between the two diameters, here it is.


A: 2.25

B: 2.38

Difference:

2.25-2.38
------------ = .054621%
2.38

We can check this buy multiplying our numbers:

2.25x .054621% = .1228, or .13 Which means 2.25+.13 = 2.38.

So 2.38 is .054621% larger then the 2.25 Diameter Pipe.


Now if you are looking for % differences in area, like above formula, then that can be worked out using same formula:
A-B
---- = % Difference
B

Dudeman

I am assuming you want to compare the area. As diameter doubles circumference doubles. Area quadruples when diameter doubles. Area would be more accurate way to compare the two pipe sizes. Diameter and circumference separately don't tell the whole story.

area of 2.25 = 3.14*(2.25/2)^2 = 3.9740625 sq in
area 0f 2.38 = 3.14*(2.38/2) ^2= 4.446554 sq in

3.9740625 is(*) what %(x) of (=) 4.446554
3.9740625*x=4.446554
x=4.446554 /3.9740625
x=1.1188938 or 11% larger area ( the 1 represents 100%)

check: 3.9740625 + 11% = 4.446553

Aside from area, its a word problem. I detoned the key phrases with their mathematical equivalents.

I just wanted to let you all know that I can not be any help what so ever due to the fact that I hate math. :biggthump

The reason I ask this question is.

A 2L motor with Freaks headers makes more WHP with a 2.25in collector Vs the 2.38 collector.

So I am trying to figure out if the motor size goes up to 2.3L which is 15% larger than a 2L would stepping up to a 2.38 collector keep everything in proportion.

So we can agree that a 2.38 in collector has 11% more air flow capacity than a 2.25 in collector

Fluid flow is a volumetric measurement... therefore the percent in cross sect. area and flow is not directy linear... but since you only have two numbers for the 2.0L and and the 2.3L is the variable you can't really graph the numbers and make a rule of thumb until you have a few numbers for th 2.3L w/ the different collectors... w/ the dyno data you could plot some points and create a best fit line in excel to help make a rule of thumb for displacement vs collector size (while keeping primary size constant). It would still be rough because it depends on the individual engine build/compression/mods/etc. but it would still be something good to go by. But the dyno time is expensive...

Has anyone done any research with the larger displacement motor besides you Andreas?

Do they have those expandable collectors used for dyno testing?

The % difference between the numbers given 5.5%

as stated above i don't believe you can use just the percent difference to calculate how much larger you need. Since the flow volume and velocity will change disproportinate to the 5.5%

I Honestly think your best bet is to consult burns they have a list of merge collectors that really work out well. The header number i gave to Kesi way back when spoke directly to the fact that a 2.25in merge collector is more beneficial than something larger. Let me find the post.

here's what i asked for
ok, here is what i want and why
Slotted flange holes so that i can shift the header up and down on the exhaust ports to see if i can pick up a little midrange or if i need a little top end.

Exhaust primary length to be between 27 in long this is for a header that is tuned between 6500rpm and 7000rpm. generally road going motors are tuned at max torque and race motors are tuned at max HP this is a compromise

I would like the inner diameter to be between 1 3/4in and 1 4/5in this is calculated from the primary length above to get maximum velocity

I would like the step to be up to a 2in pipe 12in from the exhaust port this will hold good bottomend while still allowing good power above 8200rpm

it would be cool to do another step about 6in from the collector up to about 2 1/8in piping but it's not necessary

keep your collector the same but have the final piece bolt up to 2.5in cat

Then you could run 3in cat back


Let me know if it's doable and if so how much?

Here's what Kesi came back with
Ok here is spec's of header I have .1 3/4 inch stepped to 1 7/8th inch
10 3/4 inch from flange to 1st step. 20 ¾ inch from 2nd step to collector. The collector cone down to 2 ½ inch then flairs back up to 3inch. It will be two piece headers. I can get the holes drilled so you can shift header to tune the header on dyno for maximum hp and torque. If you want 2 inch secondary pipes he might have to order a new collector Ill get back to you on that. I am going to his shop this weekend to drop off a header he already built. After Christmas ill drop of an engine to make sure it fits perfect.

Area is the best way to determine how much larger a pipe is. Yes the % difference is 5.5, ie. the diameter is 5.5% more. Measuring the % change using a straight line (diameter) is not as accurate. This gives a better idea without using a fluid dynamics formula. A fluid dynamics formula is as long as my arm. Reread above post.

already stated that the percent difference won't do jack but that is the answer to his question

I understand everything you guys are saying I am just tryingf to get a starting point to start from.

I keep trying to think about ways to make a relationship for ya, but there's really no true way to determine an optimum size for max HP w/o actual test data... even if there was an equation are too many variables needed.

Like the exhaust gas' velocity...

Do you maybe have the numbers for head/exhaust flow for each motor? I think you might be able to come up with a decent relationship with those.

edit:

if you have the flow from the 2L and the 2.3L you can calculate a rough gas velocity at the collector w/ the diameter... and try and use that velocity to come up with a generic size for the 2.3L using its flow number. It would be rough, but at least it could be a starting point... especially if the primaries are the same and all you change is the collector... Iono.

God, welll... chalk pressure, density, assume length of pipe is the same, and assuming the ideal velocities of both are optimum... assume the gas temperature is the same... assume the headflow=exhaust flow... um... :rofl: I give up. There are books on this nonsense

All we are doing is trying to get a starting point looking at a 2L which we know works great with a 2.25 collector and trying ti figure out an optimum starting point for a 2.3L motor.

Could you possibly find a comparable size motor project that makes around the same power/tq and maybe use what they used to start? There has to be someone who has a comparable story on the net somewhere.

If your still using the 1+3/4" primaries, then I suggest keeping the 2.25" collector as the primaries will be your restriction which will improve mid range torque and then use the primaries length to dial in the top end power with some experimentation. Should make for a very streetable package.

I have a couple of formulas in front of me for the math wizards.
1. air velocity in a pipe
2. air volume discharged from a pipe
3. Theoretical Horsepower to compress air
also;dry air Specific heat & sound velocity @ 1 atmosphere, various temp.
if you want them i will post them?
here:
http://www.sr20forum.com/gallery/data/971/thumbs/100_4563.JPG (http://www.sr20forum.com/gallery/showphoto.php?photo=11965)
Ed

^^

That's what I'm talkin' 'bout, baby! Physics = Life!

Formulas are so much fun :-)

rex is right that area matters, but if you want to keep the "reynolds number" the same then you have to compare the radius/diameter of the two pipes raised to the 4th power and not squared.

So it would be a flow difference of about 25% and an area difference of 11 %
does that help.

Mike

the size of the pipe for flow will be different, but depending on the RPM range and the size of the cams and the overlap also plays a huge part. my friends 2065cc B series motor makes the most power with a 2 1/2" collector with one header but thats 10,000rpm...

I rest my case,

Max power at about 8200 rpm with 2.25 collector times 1.25 gives 10,000 ish.

Mike