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Discussion Starter #1
Here are my flow bench results for my new ported and polished heads. I am getting them installed this coming Thursday. Anyone know of a flow bench for the COBB heads?? I would really like to compare numbers. :)
 

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I really appreciate the work you've put into this, but I hafta say the numbers aren't really legible. If you'd like, I can put them into excel spreadsheets for you, or send some blank ones for you to fill in. I wish I could read those better, and I could let you know what a few of them mean. From the looks of it, anything past .450 lift is diminishing returns. Even .400 lift looks promising, which I didn't expect. Were these phase 2 heads? Do you have cams selected? Thanks again for the info! Mel
 

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Discussion Starter #9
Yes, once again, sorry for the small print. I made the pics as big as I could. The site will only let me go so big. I am thinking of scanning them to a .pdf file so it can be viewed in Adobe Acrobat for better ledgibility. These heads are completely redone, cams and all. The cams that are in there are 23HP over stock at redline!! I am very excited to get them put in. The valves are redone, cut to perfection, and also re-angled. It was a very expensive job, but I am sure I will see where my money went. :) Thanks for you interest. If anyone is wanting more information on where I got this done whether it is the heads or the cams, go ahead and PM me, more than happy to help!!! :)
 

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Don't let yourself get sucked into the numbers game. A flow bench is good for seeing how easy, and how much air can flow through an orafice. An intake port that can flow 1,100 CFM will not be a good choice for our little engines. But that doesn't mean that a smaller intake port is better either. After a little porting and polishing, normal heads generally have the best all around performance. Its when you start hogging out the heads that you start to loose low end and increase the high end. Its all about the tune which heads will be the best choice for which cams at which RPM. Your heads might perform worse with stock cams than the stock heads. When its all put together, the only thing I care about is the intregral power. Flow bench numbers are nice to see what you did before you put it on the engine, but they don't tell anybody about what charecteristics you brought out of the heads, and peak HP is something to tell someone who doesn't know much about cars, so you don't have to explain a whole lot to them. ;)

Greg
 

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Section 8 said:
Don't let yourself get sucked into the numbers game. A flow bench is good for seeing how easy, and how much air can flow through an orafice. An intake port that can flow 1,100 CFM will not be a good choice for our little engines. But that doesn't mean that a smaller intake port is better either. After a little porting and polishing, normal heads generally have the best all around performance. Its when you start hogging out the heads that you start to loose low end and increase the high end. Its all about the tune which heads will be the best choice for which cams at which RPM. Your heads might perform worse with stock cams than the stock heads. When its all put together, the only thing I care about is the intregral power. Flow bench numbers are nice to see what you did before you put it on the engine, but they don't tell anybody about what charecteristics you brought out of the heads, and peak HP is something to tell someone who doesn't know much about cars, so you don't have to explain a whole lot to them. ;)

Greg


^
What he said... well put, Greg.
 

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Discussion Starter #12
Thanks for the info Greg. I know what you mean. I really dont have any low end anyway. All of my power is 3000RPMs and up. I have read in many posts that you loose a lot of low end if you get rid of backpressure from the exhaust (getting rid of cats). I have no cats, so I am assuming that the gains will be in the same area that I get all of my TQ and HP anyway. That is just my guess. We will see next Thursday!! :)
 

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Oh yea.

Cobb used to have a graph on their site with stock heads from different Sube models, with their stage 1 heads i think. I don't know how much good it will do, but maybe you can compare a little.

Are you planning on raising your redline at all?

cheeRS,

Greg
 

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About the cat removal. I have always been intregued by this. Some cars gain HP by eliminating them, others lose HP and others have no change.

Here is my theory. I think that the cat reheats the exhaust and it stays hotter down the pipe because of it, and hotter is less dense and flows easier, this is why some cars lose HP when the cars are removed. Other cars gain because of the removal of a restriction in their system (or just an old, getting plugged cat).

Either way, I cannot stand the stink of a catless car. Gives the term "fart can" a whole new meaning. :stinky:

"back pressure" is sort of a myth. It has nothing to do with the operation of an engine. It is the RESULT of a constrictive exhaust, not the catilist, or reason for torque or power. Lets say that you have a large bore exhaust on your NA car. We just put a three inch system on your car. You say, What the HELL!!!??? I have no back pressure! This thing has NO TORQUE!!! Well that is only partially correct, the larger exhaust allows the hot exhaust to cool to much, and it travels down the pipe slower than the smaller exhaust, and is able to scavange less exhaust from the cylinder when the exhaust valve opens. So we yell at the exhaust techs, and they put a better size on there, smaller than the 3 inch system. Now we have more torque due to the better ability to scavage the cylinders, and teh exhaust velocity is higher, and it stays hotter farther down the pipe. HOWEVER, this is where the term "back pressure" has been abused. You do not say that the engine has some back pressure so it runs better, you say that at higher RPMs the smaller exhaust has some back pressure compared to the larger exhaust. OVERALL the torque curve is better, but peak power might be lower than the large exhaust.

instead of saying that an engine needs a little back pressure to run well, say an exhaust with a little back pressure will give the engine a flatter torque curve.

cheeRS,

Greg
 

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Discussion Starter #15
Once again, thanks for the info Greg. :) I understand what you are trying to say, but I was just wondering if you have ever driven a car w/o cats before? I have actually done the process of taking and removing my straight pipe at least 3 or 4 times just to try and compare the two. In my opinion the straight pipe is WAY better. For some reason when the cats are on there, you can just feel the congestion in the exhaust through the power band. I actually have a place where I live that I test all of my work, nice 3 mile straight road. With the cats on I can reach a bookmark on the road going 100-102MPH. Without the cats I can reach 111-115MPH. I have done this test a number of times. There are many things that you have to take into consideration. Some people just dont have enough modifications done to their car before they put the pipe on, and that is why they are not getting the power they are looking for, but other people do, and they see the power. I myself see the power w/o the cats. There are so many factors that are in play when trying to get HP gains. For one altitude. At one point I talked to a guy that said he gained 4 more HP just by moving from one state to the next. What I am trying to say is that I am in total agreement with you about everything, but the cat removal. I love no cats!!! :) Thanks again for the info.
 

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I was talking generally. Not just about the Impreza. Our cars benifit from it because of the creepy design of the exhaust manifold and cat placement/shape.

Yes, I have driven a car with out cats before. I can't stand the stink for a couple of HP at most vs. a good highflow cat in a freeflowing exhaust design.

Talking strictly Imprezas now-

Almost anything is better than the stock set up. Comparing stock exhaust manifold with stock cats to a straight pipe with no cats is a foolish comparisan. Put a high flow cat in your straight pipe and see how much difference there is. I bet you cannot tell one way or the other from the drivers seat. You will need a dyno for that and it will tell you that you have lost a negligable amount of HP. A gain with out cats, sure a tiny bit, but I hope that you live where there are no emmisions laws.

Generally again-

Magazines have done tests with cats and without on identical exhausts, and the concensus is that gains are minimal at best, sometimes none. One test showed a gain with the cat. Take it for what it is worth, but don't compare the stock cat section to a straight pipe section. After the muffler, the stock cat section is the most restricive section in the system.

cheeRS,

Greg
 

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Discussion Starter #17
Agreed. :) I just hope that my setup is ideal for the ported and polished cylinder heads and cams. Need to get my fuel system recalibrated.
 

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Are you using a header? What kind? With higher overlap cams, I would think that equal length headers would provide the best scavaging of the cylinders. There are very limited selection of equal length headers though. If it is not "perfect" it will be fine, with the cat pipe corrected and a free flowing exhaust, alot of things will work. This is where the limited avilability of parts hurts us. :( There must be close to 200 different headers for Fox bodied mustangs. We have Brullen, MRT and Subaru Yakama Motor Sports. (Cobb is suposidly coming out with one later this year).

For people who don't have a header, If an equal length header is important to you, ( and you are looking for something different than a Brullen or MRT header) I was concidering swapping the cross menber, water pump and thermostat from a turbo car an use one of the equal length headers from the turbo car. Of course they come in 4 to 1 and 4 to 2 to 1 variations and a large variety of manufactures and prices. :curse: Oh well.
 

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Discussion Starter #19
Yeah, I do have a header, Borla. It is not equal length, but it does the trick. I am thinking of maybe buying the SYMS equal length in a few months. ??? I am not really sure what the specs are on the cams. I got them from Yoshio. ??? :)
 

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>Subaru Yakama Motor Sports.

hey section8, who is that company?

thanks
-aaron:)
 
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