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Old 06-29-2000, 06:26 PM   #1
Trey
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Smile Tech Section Update - SOHC Engine Build

We finally had an opportunity to put the second part of our SOHC Naturally Aspirated engine build online. Sorry it took so long but we've been rather busy and it took quite awhile to write all that text!!

You can get to it directly by following this link:
<a href="www.cobbtuning.com/tech/sohc/page2.html" target="COBB">www.cobbtuning.com/tech/sohc/page2.html</a>

Enjoy,
Trey
CobbTuning.com
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Old 06-29-2000, 07:22 PM   #2
ColinL
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for some weird reason, trey's link is attempting to launch as part of this thread's url.

go here: http://www.cobbtuning.com/tech/sohc/page2.html

Just finished reading it, damned cool.

[This message has been edited by ColinL (edited June 29, 2000).]
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Old 06-29-2000, 09:17 PM   #3
cOz
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Wow the article is excellent. I cant wait to see what the final outcomes are. Moreover I cant wait to get some Cobb tuning on my sube.

Keep it up

cOz
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Old 06-29-2000, 10:23 PM   #4
Eric Gagnon
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Nice research Trey. I can't wait to see where this goes.

Eric
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Old 06-29-2000, 11:08 PM   #5
8Complex

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"And as we saw before, the late model Phase II SOHC cylinder head uses a high-tech mechanical roller rocker design. This means the end of the rocker arm that comes in direct contact with the camshaft has a rolling cylinder on it." - Trey

Then why do they make so much damn ticking noise?!? I don't think it's just mine cause everyone's seems to tick like that from what I've noticed.

Nice article. As always very enjoyable to read since I can actually go through and comprehend everything I'm reading.
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Old 06-29-2000, 11:53 PM   #6
rrsettgast
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Does the flowbench measure using a steady-state flow, or a pulsating flow?
Please be sure of your answer. What is the flow source?

Randy
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Old 06-29-2000, 11:57 PM   #7
Pilot
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Wink

Uh oh Randy is at it again.

[This message has been edited by Pilot (edited June 29, 2000).]
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Old 06-30-2000, 07:00 AM   #8
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Lightbulb

Very interesting. Trey says the intake manifold is very good, and not worth messing with. While Simms claims the manifold they are developing will give 20 HP increase.

They Can't both be right.
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Old 06-30-2000, 07:36 AM   #9
boxerman
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Trey,
Thanks again, this is great. Nice news about the stainless valves. 7200 gives us a lot of room to play with. Depending on how the bottom end looks, with uprated springs the EJ-25 could be a real screamer.

8Complex, I thought the ticking was valves also. It is a noise every 911 owner is familiar with.

I was told by the dealership tech at my 2000mi checkup, that the noise comes from the injectors.

Tim.
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Old 06-30-2000, 07:53 AM   #10
Jay_UK
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Cool

Good write up.

I think a redesign on the intake and primaries would be required to gain some benefit..

anyone want some tubular ones ? custom length primaries ?

The STI's run bloody strong valve springs... but even these need uprating to go upto 10K revs...

There does come a point where the returns dont justify the cost... especially on relatively low vloume cars.

I think I passed that point ages back..

J.


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Old 06-30-2000, 08:07 AM   #11
Trey
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Randy - Here are some links that help explain what a flow bench does. This might clean some questions up for you. We use the Superflow benches which are considered the industry standard in the performance automotive world.
http://www.superflow.com/support/sup...wbench-faq.htm http://www.superflow.com/flowbench/sf600.html http://www.superflow.com/flowbench/flowcom.html

That's a FAQ and a product data sheet on the style of bench that we use w/ the optional FlowCom.

On the intake manifold, it's not that there are no any gains to be had but I don't see how you can get 20HP out of the thing unless it's at some outrageous RPM. I'm not saying I'm 100% right either... I'd love to be proven wrong and find yet another way to get more power out of this engine.

Cheers,
Trey
CobbTuning.com
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Old 06-30-2000, 11:18 AM   #12
Catfish
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Great writeup Trey. Nice mix of technical info and "novice" language. People aren't just getting numbers thrown at them, they're also learning what they mean. Great job.

On a more technical note....
Those flow numbers are extremely encouraging. Other than the exhaust port design, it seems as if we have a great cylinder head. With a little porting and possibly stiffer springs, we can easily see 7000rpm N/A screamers.

Anxioulsy awaiting more results.

Cheers,
Dave
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Old 06-30-2000, 10:26 PM   #13
doripreza
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sick.
i've heard of several people revving there motor up to 7500 and not getting any valve float on stock springs
i think thats pretty amazing..

i would rather fork out the cash for stiffer springs than risk destroying a nice new engine setup.

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Old 07-01-2000, 01:10 AM   #14
wolve80
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ya, the ticking is a char of the flat engines. Porsches do it, and when un silenced, our cars do sound alot like the 911. vrooom... ok, I gotta go out and drive now.
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Old 07-02-2000, 08:41 PM   #15
rrsettgast
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Trey,
The reason I ask about what "type" of flow the flow bench uses, was to determine whether you are considering dynamic wave effects in your statement:
Quote:
What we found was that the intake assembly (manifold and throttle body) flows 236 cfm which is slightly less than our intake port at max lift, but not enough of a difference to be concerned about. This is only a 1% difference in flow at the intake's max lift which means there's not much to be gained."
From the information in the links you provided, I would guess that the answer to my question is no.

At this point, I will have to whole-heartedly disagree with your quoted statement. From what I have read about intake manifold design, it is pretty clear that long primaries will increase volumetric effeciency (flow) in the mid-range, while killing VE in the top end. This is what I think is going on with our intake manifolds. If you jack up your redline to 7k, and stay NA, you may be unpleasantly suprised at the results.

On the other hand, what does this mean for our turbo RS brothers... Well it is all good news for those guys. The intake manifold provides high VE at low/mid range rpm, which is were the turbo is not helping. But when you get to the top end, the poorly tuned manifold doesn't matter due to the fact that the compressor is now cramming air into the manifold....which will increase VE.

I don't really want to discuss the validity of my opinion, since I KNOW that I am right . If you have questions about this subject, I suggest that you check out this book:

Design Techniques for Engine Manifolds:
Wave Action Methods for IC Engines
By: DE Winterbone and RJ Pearson
ISBN: 186058179X
The BEST book that I have found on the subject.

Randy
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Old 07-03-2000, 09:45 AM   #16
Trey
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Perhaps I should better explain my statements about the intake manifold as they seem to be a touchy subject for many.

When you flow a manifold on a standard flowbench, like the one we're using, it's impossible to duplicate the dynamics occuring inside. It is, however, possible to flow the manifold at a steady state to get a general idea of how much of a restriction in flow the manifold is. If we have a manifold that flows 50% of what the cylinder heads flows at a steady state, then we can have an idea that the manifold is better suited for the trash can instead of the engine bay.

When I stated that the intake manifold isn't something to be concerned about, I'm speaking in context to our project. Remember, we're working on a naturally aspirated STREET motor. I still think we might be able to make a few extra horsepower (due to better VE at high RPMs) by redesigning the intake manifold but the costs involved would outweigh the benefits for a street car. Had I wanted to build a naturally aspirated RACE motor revving to 8000 RPMs, I would definitely focus more energy to the intake manifold.

We will be testing a variety of manifold designs for future projects but I wanted to point out that the stock manifold isn't quite as bad, for the street, as some of us assumed it was.

You're also forgetting the fact that everything has to work together as a system. Your disappointment at 7000 RPM is just as likely to be from a poor exhaust port, poor header design, camshaft profile, et cetera. I'm trying to point out that laying all the blame solely on the intake manifold is incorrect.

And if you think that a poorly tuned intake manifold doesn't matter when you run a turbo, then you need to get a new book. I've spent enough time dyno testing high HP turbo cars to know that.

I appreciate the comments though. I don't mean to seem defensive to Randy or anyone else. The more you all question everything, the more we all learn. I try not to put too much techno-babble in the articles to make them more readable for everyone so feel free to ask something that I might not have made clear enough in the article.

Thanks,
Trey
CobbTuning.com
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Old 07-03-2000, 10:40 AM   #17
Pilot
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Talking

Ok guys lets get technical about intake design.


I haven't read any books on the subject yet (vehicle dynamics are my hobby)

Anyway lets talk about what makes a perfect theoretical manifold for several SPECIFIC engine types.

Here are the rules this engine will displace 2.5 liters, be able to rev to 7000 rpm, have four cylinders, and it is a street motor.

1)If it was a low pressure turbo (6-8psi) what would be the best manifold type?
Low volume plenum, medium length primaries?

2)If it was a low pressure (6-8psi) Supercharged car same as above?

3)High pressure turbo?
Short primaries and low volume plenum?

4)High RPM N/A Motor?
Short primaries and a dual stage intake plenum, so that the long track is used during low RPM conditions?

5)N/A torque motor?
Long primaries, large volume plenum?

Remember all of my thoughts are just thoughts, you guys correct me (I'm not even close to authoritative on this subject), also remember we are talking about a theoretical motor here so don't get into port design and exhaust. We will assume for the sake of arguement that other factors are appropriate for the given theoretical engine type. In another thread we can discuss cylinder heads and other things.


Ready set go

EDIT: For theoretical engine clarification.

[This message has been edited by Pilot (edited July 03, 2000).]
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Old 07-03-2000, 12:08 PM   #18
Marco
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This sounds just as much speculation as talking about the new generation WRX. In theory, we can say all we want. I'm yet to see anyone (other than Trey) take the time and labor to disassemble an RS engine and do the research needed to see what works and what doesn't.
I can read all the books in my bookshelf, but unless I take the time to work on the specific engine I'm interested in, all I know is pure theory.
My point is: why in hell are people so keen to jump on somebody's post based in actual data to say that his or her book says otherwise.
I've seen so many posts about people complaining for no having any real data about our cars, yet we post about "book" data being god's word!!!
Why don't we make up our minds? Do we want real hard data about Subaru performance, or do we want to base our upgrades on our city's library data?
I'm not against people offering their opinions on the subject, on the contrary, but doing so by citing books is really cheesy to say the least!!!

[This message has been edited by Marco (edited July 03, 2000).]
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Old 07-03-2000, 02:22 PM   #19
Marco
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Pilot,

First of all, my post was not intended to offend anyone. You stated in your post that you had read no books, so my post does not apply to you in any case.
I don't think that books are cheesy, in fact, I had my share of books and I always enjoy reading technical books.
My point, once again, is about people objecting a post based on what their book said about a paricular subject. I think Trey is doing a wonderful job with the RS, and he's doing so hands on, so I think stating "my book says this, or my book says that" is indeed cheesy!!!
Unless you can get your hands dirty and come up with some "real world" data, it is better to listen, or read in this case!
If you want to be educated, as you stated:

Quote:
Anyway, I'm currently looking for theory. Right now I don't care what precisely will work on the RS motor, right now I want to understabnd egnine design from a broader veiwpoint.

Remember that Trey is building a street performance, normally aspirated engine and this topic IS about “precise work on the RS motor”. I think you can get a wealth of information on this board, but if you want to get a broader viewpoint, perhaps you need to open a different post, in which you can get a broader viewpoint. Let’s keep this topic as a “narrow viewpoint”, as it was intended, shall we?
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Old 07-03-2000, 04:39 PM   #20
rrsettgast
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Trey,
Thank you for the clarification of your statement. I just wanted to make sure that you were not neglecting the dynamic effects of the manifold. (It sounded that way to me, but no matter now) I agree with your assertion that a manifold redesign would be a bit soo expensive for most people. I don't know what kind of RPM you are planning on going up to, but I would like to see some VE measurments taken on the stock system, as well as your modified systems (to determine when the manifold starts sucking bad, and which componenets are at fault). I think that such measurements would be extremely helpful in validation of your work.

I did not mean to imply that the intake manifold is the only component to blame for bad top-end performance. It is obvious that the entire engine is not designed for top-end performance. I is just my opinion that the intake manifold is a very large contributor to the problem. I think that we still disagree on this, but at least I know that you recognize that it matters.

As far as your statement:
Quote:
And if you think that a poorly tuned intake manifold doesn't matter when you run a turbo, then you need to get a new book. I've spent enough time dyno testing high HP turbo cars to know that.
So where should the "sweet spot" be in a turbo intake manifold? It seems logical to tune your manifold to give the best off-boost performance, which will lead to quicker spoolup as well, but I could be wrong.

I appreciate your somwhat civil response...although that last quote kinda seems a little insulting, but I don't mind. .

Randy
PS
Hey Marco,
Why don't you keep your critisisms and personal attacks to yourself. Ingnorant people always discount the importance of a good theoretical understanding of a topic...don't be that guy. I never said that Trey wasn't doing a good job, or that "real data" is not important. Being a scientist, I think that "real data" is the most important part of any endeavor, since you never know if what your theories tell you is true without it. I simply took issue with a poorly posed statement.
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Old 07-03-2000, 04:43 PM   #21
Pilot
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Marco,
Narrow viewpoints are worthless, narrow scope is not that is why I specified a 2.5 liter 4 cylinder in my above post. It doesn't matter if it is the RS motor or another motor, the principles are the same.

Trey thinks the manifold is okay for a street motor spinng 7000 rpm, Randy doesn't. Why? and Why not? Those are broad theoretical questions relating to a specific engine.

What I am trying to get out of them is what type of manifold is good for certain types of engines and if you are building a certain type of street motor that is 2.5 liters and four cylinders what type of manifold will be best it may not be the stock manifold, or it make be perfectly okay. I want to know WHY?

If Trey and Randy keep discussing it then we will know WHY and that is not a narrow veiwpoint.
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Old 07-03-2000, 05:19 PM   #22
shiv
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From my hours of mapping the TEC-II's VE table on Alex's NA MY99, I'm left with the opinion that the stock manifold is tuned for 3700-4000rpm performance while the cams are tuned for 5200-5750rpm performance. If this is true, and if I'm not getting mixed signals from a bunch of variables, it would partly explain the "double hump" 3-dimensional VE curve I'm seeing with my fuel mapping. In these two rpm ranges, the NA Phase II EJ25 loves to suck down fuel and make power. A sure sign of good VE. Above, below and between, the engine seems to be a little less torquey, needing less fuel to acheive a/f ratio targets.

Interestly enough, the 3D VE curve on my turbo Phase II EJ25 is (as Randy would probably expect) much much more linear, with almost no dips or humps anywhere.

shiv
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Old 07-03-2000, 06:07 PM   #23
Marco
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Thumbs down

I'm not trying to start an argument (I don't have the time and they never end in a friendly manner). If you guys feel that I have done so, or offended anyone, my apologies.
I'll follow Randy's advice and keep my "critisism" and "personal attacks" to myself although I don't remember attacking or critisizing anyone!

Let's go ahead and talk about my Honda NT 650 hawk engine and how to improve its performance. I think that'll keep the subject broad enough!
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Old 07-03-2000, 06:56 PM   #24
Pilot
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Shiv,

That is interesting.

Possibly Subaru was trying to create power in different rev ranges by making different parts of the engine efficient at different rpm.

Any commments?
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Old 07-03-2000, 07:13 PM   #25
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Hey Shiv,
How can you tell which VE hump is from what component?? Is there a third VE hump for the exaust manifold?

Thanks
Randy
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