IROCZTWENTYGR8

iTrader: (1)

This has been a much talked about subject on this board that always turns into BS instead of actual information. I'd like to make this an informative post to tell others with the same questions that might not have seen it to look for.

IMO, on the 11-12 sec. 350 TPI buildup I want to do, I think the 1 3/4's will be better, while I still want to keep the lower RPM TQ of the TPI thats so good for the street, I don't think it will do anything to that when u have like 190cc heads and a good TQ converter. Post!!

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89 RS

STILL Looking For:
An 87 IROC-Z28 350 TPI

five7kid

iTrader: (14)

Buddy here runs corrected 13.3's with a highly modified ZZ3, Bigmouth TPI, through the ZZ3 iron manifolds & factory dual cats (his 2000 stall convertor is probably holding him back more than anything else).

He keeps talking about getting 1-3/4 SLP's, but he'll probably wait until after the Motown 434 shortblock is delivered...

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. 2-1/2" cat-back, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker 2055 headers, 3" Catco cat & 3" cat-back, Spohn SFCs).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

Sitting Bull

Hi again!

Apeiron, one of the posters on this site, has an 84 Z28 with a built up 350 that runs in the mid 14s here in Calgary. (I know, we were at the track with it here a month ago.)

This mid 14 ET probably translates into high 13s once an altitude adjustment is taken into account, we being at 3600 feet.

He runs 1 1/2 headers

JETHROIROC

There is a big difference between a high 13 car and a 12 second car like 20GR8 asked about. Get the 1 3/4" and forget about it.

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, MSD coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley module, ported plenum,
Ported Daytona Yellow stock base, Moroso valve covers, other stuff,
Streetdampr, Ruger P95DC, hot wife, new oil filter, !cats, !TBC, !AIR.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

Sitting Bull

Jethro,

I agree, if you are just now buying your headers and plan on running in the 12s, then go for 1 3/4 headers. But if a guy already has 1 5/8 or 1 1/2 headers he'll still be alright. Not quite as fast but nothing to get really wound up about unless you are in a championship hunt. Most of us are just street folks

I have a 305 making about 240 hp and I use 1 5/8 Hooker Shorty headers because I want to be set for any engine I might build later, even up to or higher than 400 hp.

I would have better low rpm torque response with 1 1/2 headers but it is certainly good enough for me with 1 5/8!

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1986 Camaro Sports Coupe
T-tops, variable wiper, power hatch, rear defrost, third brake light
LG4 305 V8, Hooker Shorty headers, 3" pipe into Walker Quiet Flow dual exhaust
Edelbrock Performer intake, "Damonized" Q-Jet 4bbl, K&N filter, Crane ignition kit
TH700R4 transmission and Derale cooler
Air conditioning, power windows, power door locks, power brakes, power steering, tilt wheel
Kenwood cassette, Pioneer 6x4 component front speakers
Rockford Fosgate 6x9 Punch rear speakers
Z-28 front and rear sway bars
Aluminum slots and Goodyear 225x60R15 Eagle HP tires
243 horsepower and 326 foot pounds of torque at the flywheel
_ _ _ _ _ __ _ _ _ _ _
Yea verily, and he smote the smog heathens from his small block Chevy, even from the air pump to the converter ...
Todd 1:1

AJ_92RS

I saw a formula that was shown by David Vizard on how to determine the least size primary needed.

Square root of (CFM x 1.27)/FD.

CFM - The ex. flow of the heads being used at max lift on the cam that is used.
FD - Flow density. For a race engine use 80 PSI. 85 PSI for a performance street engine. 90 PSI for a regular street motor.

I'll use my "soon to be" set up as an example. I am going to use Vortec heads. The cam I'm using is .465" on ex. The flow # for .450" of lift on Vortec heads is 155 CFM. 155x1.27=196.85 I'll use 85 PSI for a performance street engine. 196.85/85=2.316 Then press the old Square root button on the calculator equals 1.521" primaries.

My engine is only going to be putting out 330HP so it may fall under the area of regular street engine. 196.85/90=2.176. The square root being 1.475" primaries. Even if I put it under the area of a race engine, then I would still only need 1.569" primaries. That may justify using 1 5/8" headers. Especially if I would later move up to a higher lift cam or better flowing heads.

IROCZTWENTYGR8,

If you are refering to use AFR 190 heads and a cam with .500" lift or more, by using 85 PSI for the formula above, then you need 1 3/4" headers. The formula reveals 1.744". So get the SLPs. They will let your engine breath!!!!

Using the race engine category still leaves them at 1.79", so 1 3/4" headers will work great.

Good luck,
AJ

JETHROIROC

Sitting Bull, I definitely agree with you that if you already have 1 5/8 then it wouldn't be wise to switch up unless your mods will support it.

Kevin91Z

iTrader: (1)

I'm not running an 11 second TPI, but I have 1 3/4" headers. The car runs awesome on the street with this much torque. Larger headers dont give up as much torque as you think on a TPI engine. TPI already makes it own torque, so the larger headers help, not hurt.

I think your goal of an 11 second 350 TPI is a little ambitious, tho, unless you're planning on a power adder too.


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1991 Camaro Z28
5.7L 5-Speed (originally 305)
317 RWHP, 418 RWTQ
13.23 @ 107.62 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Member: SoCal F-Bodies
-=ICON Motorsports=-

88IROCs

AJ,

I've seen that formula before, used as a means to find minimum inside diameter. There's two problems you did not look at then:

1. Pipe is described by it's outside diameter. To find the inside diameter, you need to subtract the wall thickness x 2.
For 16 ga.(0.0625" wall) pipe here are some inside vs outside diameters -

Inside..................Outside
1.375"..................1.500"
1.500"..................1.625"(1 5/8)
1.625"..................1.750"(1 3/4)

So for your engine, where you calculate needing an i.d. of approx. 1.5" you will need to use 1.625" tube if the wall thickness is 16 ga.

However, that's only half the problem solved(which i.d. will give you the best velocity). You still need to calculate for:

2. Resonant tuning. This simply means tuning the pipe to achieve optimal flow at a given rpm. This could be your peak torque rpm, peak hp rpm, or any other rpm you choose. Once you know the rpm you are tuning for and the i.d. of the pipe you will be using, you can calculate the length of pipe you will need to achieve resonant tuning. If you do not have the room to use the calculated length of pipe(a problem with cramped thirdgen engine compartments), you can use a larger i.d. pipe to shorten the length. Yes the larger o.d. pipe will decrease the exhaust gas velocity, but tuning the pipe to the desired resonant tuning is more critical.

Hopefully from that perspective you might see why using a 1 3/4" o.d. pipe would be advantageous on your motor.

[This message has been edited by 88IROCs (edited September 03, 2001).]

IROCZTWENTYGR8

iTrader: (1)

Good information every1!! I am going to use the 1 3/4's like I said when I get this car, (still looking) because like u guys I think a high flowing 350 needs them. 91Z, high 11's-low 12's is what I want, but if I can do better than that it would be cool.

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89 RS

STILL Looking For:
An 87 IROC-Z28 350 TPI


[This message has been edited by IROCZTWENTYGR8 (edited September 03, 2001).]

AJ_92RS

That's what I said, about the minimum needed. Although I did say "least". If you can read you would see that. As far as the inside diameter, wrong. That formula is determined by using O.D.

It's almost impossible to determine the inside diameter due to the fact that so many companies use different wall thickness for their headers. FYI, 16 ga. is .065" not .0625"

BTW, I have posted that fact of the pipe wall thickness about 10 dozen times on here already. That's only needed to find the area of the pipe in square inches. That's more important to know for exhaust pipe tuning, not headers.

He's not looking to design and build his own headers, nor am I. Are you? The companies that design and build these headers do all of that work for you, at least what the chassis will allow.

That's also what I said.

You're making this more difficult than what it is. The book that I got that formula out of was designed for idiots like us. That's why you have to multiply the cfm by 1.27, then divide it by the flow density. Then find the square root of that amount. All the difficult math is done. Don't try and do more math or you'll screw up the formula that David took years to figure out for us. And even then, it's a ballpark idea. No formula ever developed for engine building is 100% accurate. They can't take into account all the possible variables that come into play.

AJ

88IROCs

- Well I won't argue with you if you say the formula is used to determine the o.d. of the pipe(Vizard's book on exhaust sytems has been out of print for a while and I don't have a copy). But why would you bother to even be concerned with the o.d. of the pipe: it has no effect on the flow characteristics inside the pipe. And as you say "so many companies use different wall thickness for their headers", which means for different gauges you will have different i.d.'s, i.d.'s being important because they do affect flow characteristics inside the pipe. The effect of this formula would be no better than a rough guess, in which case I would question why you would bother to use a formula at all.

- Yes some day I would like to build my own headers. But even if I never do, I'd like to know why one type of headers is better than another type and, which type is the best for my application.

- Btw, what would be the point of header design if not exhaust tuning? Why should anybody design, build or buy anything but one generic fits-all header?

- If your confident that the only criteria a manufacturer uses to design headers is, building the best header for the chassis, that's fine. And if you want to believe that ease of manufacture and cost to manufacture don't rank very highly for commercial built headers that's fine too. I am not of the same mind.

You're making this more difficult than what it is. The book that I got that formula out of was designed for idiots like us. That's why you have to multiply the cfm by 1.27, then divide it by the flow density. Then find the square root of that amount. All the difficult math is done. Don't try and do more math or you'll screw up the formula that David took years to figure out for us. And even then, it's a ballpark idea.

Now this part of your response goads. If you're content to blindly plug numbers into a formula you don't understand, I'm happy for you. You're easy to satisfy and you'll probably buy whatever anyone has to sell you.

I'm not so easily convinced. I want to know how the formula was derived, what information the inputted numbers relate and how changing the numbers can affect the outcome. I don't appreciate someone who is only looking for easy answers, and refers to himself as an idiot, chiding me for wanting to know more and trying to gather the info to make an informed decision. I simply don't buy into the line of thinking that says "That's the way we've always done it, so shut-up and do it our way, because any other method is invalid." Also please note: even though I disagree with your method's and conclusions, I don't feel the need to assert my superiority by questioning your intellect.

- A lot of formula's are meant to be very accurate, even 100% accurate. But the accuracy of the result will only be as good as the data you put in(something you don't seem to concerned with). But once you identify and accurately account for all variables you'd be surprised how accurate the results can be.

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"The reasonable man adapts himself to the world.
The unreasonable one persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man"
--George Bernard Shaw.

AJ_92RS

OK, then what is the purpose of multipling the CFM of the exhaust port by 1.27? Do you know? I don't. Obviously David does. Do you wanna call him and ask him? Do you really think that he'll answer you? If he wanted us to know all that, then it would have been printed.

When did I ever question your intellect? I never did. The "idiot" comment about myself was just a joke!!!! Lighten up!!!!!

And way to many of you *** -U-ME way too much. I never said anything meaning "If your confident that the only criteria a manufacturer uses to design headers is, building the best header for the chassis, that's fine." I never said that was the only criteria. I said "...at least what the chassis will allow." I'm sure that they would like to be able to design the perfect header for every car. But they can't due to this very fact. That's what I was saying.

As far as your last comment about the 100% accuracy. You made such a bold statement, however earlier you stated "....and how changing the numbers can affect the outcome." There are way too many determining factors to come up with a "100%" accurate formula when talking about engines or any mechanical object.

Take the formula for calculating the area of the movement of a piston for example. The first thing you figure is the area of the cylinder bore by using Pi. Pi is an infinite number that, if used accordingly, would give an infinite number for the area of that bore. How can you possibly come to a 100% accurate determination by using that method. You can't. It's just an approximation. (BTW, also ask David how Pi came about. )

Then, as another example, there's the CFM of the ex. runner in the head. While the head is in the car, that will be determined by the heat and density of the exhaust gases, which will always vary from situation to situation due to intake air temp, timing advance (which is constantly changing), as well as the humidity, barometric pressure, and contaminates in the air itself. How can someone possibly use a formula to determine all of that? Then say "That head will produce exactly ____ CFM of flow" when all of those things are constant variables?

Put away your calculator and realize that formulas started out as theories that we have accepted as truth due to our lack of proving otherwise. MOST of them are approximations. "100%" and "Perfect" do not exist. At least not on this planet.

One more thing. "Also please note: even though I disagree with your method's and conclusions, I don't feel the need to assert my superiority by questioning your intellect." This comment alone shows that you ARE doing exactly what you're saying you AREN'T doing. There is a contradiction of your own statement within itself which is shown solely by the purpose of you writing it. How **** can you be?

AJ

the roc

LOL, let the roc help you guys out a little. “David” charted a linear relation between exhaust port flow at the maximum valve lift and the INSIDE pipe diameter. As with most of “David’s” formulas, he derived this one from flow bench tests and dyno tests (using different pipe diameters). The graph is based on the premise that the INSIDE pipe diameter needs to be sized appropriately for optimal exhaust velocity, and that velocity is influenced by the exhaust port flow. Since a chart/graph is kinda hard to carry around with you,, he gives a formula to derive pipe size from the square root of (CFM x 1.27/FD) in which CFM is the exhaust flow at 28” at the maximum valve lift and FD or flow density is 80 CFM per square inch for a race application. And AJ thinks the roc knows nothing. Shame on you.

You may want to email “David” at Publisher@motortecmag.com he’s the technical editor over there and you never know,, he might respond back to you. I would however suggest you address him as Mr. Vizard as opposed to “David”.


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People who think they know it all,,, are especially annoying to those of us who do.

nblanchard

My 2 cents..... I believe header sizes depend totally on the amount of mods that have been done. You don't want your engine to breathe more out the exhaust than the intake, or visa versa. If you have a too free flowing exhaust, your bottom end power will suffer but top end power will increase. And the opposite with a restrictive exhaust. Myself, I am going for the 1-5/8 headers for 2 reasons... 1) they're the smalles ones out there, 2) I don't plan on too many mods... accept for maybe nitrous (ie cheater gas)... but I think staying smaller is still the way to go with nitrous, casue I don't want to sacrifice the performance with out nitrous.

Anyway, my 2 cents, more like 2 bux... LOL

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1988 Pontiac Trans Am GTA L98 5.7L, Black on Black with 113,000kms