Inliners International Forums General Discussion Engines Flow Ratio...

Hi y'all Inliners & Engine Builders...
I was wondering about a fully blueprinted inline six drag racing cyl head and my question is : For example.., In a Turbocharged Inline six 292 Chevy Drag engine, what's the best exhaust / intake flow ratio (percentage numbers). I guess this flow ratio is different than that for a normally aspirated inline six Drag engine. Any advice would be very appreciated. Keep inline...
MALDONADO
Brasil - S. America

Hi Maldonado,

That's a good question, and the answer is usually guarded by very large men with guns and dogs... jk

Seriously, the lower the ratio of intake to exhaust flow, the better the flow is to the turbo. If we just use LeeLites numbers from his testing, we can make some pretty good comparisons against reasonable numbers available for aspirated performance engines.

Lee got stock flow rates of 177 cfm intake and 147 cfm exhaust using an unaltered head. Dividing the two gets a ratio of 1.204 intake : 1.0 exhaust. Consider a quality racing head with numbers in the range of 300 cfm intake and 240 cfm exhaust. Those flow numbers are available for v8's and inlines if you have the money. That's a ratio of 300/240 = 1.25. The number is somewhat equal, but the ratio went UP. For a turbo, that's not the right direction.

One of turbochargings' pioneers was a guy named Ak Miller. He used a stock-head 230 with 8:1 compression in a 64 or 65 Nova with a suck-through side-draft carburetor from a Corvair, and did very well with it (even had a points dizzy at one time!). He did NOT have an intercooler, but I believe he did use water injection on the street, and claimed somewhere around 275 rear wheel hp using racing gas.

There is nothing wrong with a 1.20:1.0 ratio. If you want phenomenal performance, of course your flow numbers need to be larger than stock. You should also build a custom exhaust manifold to put the turbo within 0.5 meter of the farthest exhaust valve. A custom intake that uses about 0.15 to 0.20 meter-long approach runners to the head is appropriate (the length is AFTER THE LAST BEND), and use the biggest intercooler you can afford to get the intake charge as cool and dense as possible. A water-to-air unit (using ice water on the drag strip) is smaller and more efficient than an air-to-air type. Use fuel injection, preferrably port style. It can be gang-fired or sequential fired. There is some specific geometry to that part of the setup, and needs to be given close attention. If you can't use FI, then a blow-through multiple-carb setup should be installed with the carb butterflys no more than 10 to 15 cm from the head.

Your flow numbers and their ratio is not as important as you might think. What IS important is the temperature of the intake air, the air-fuel ratio, the ignition system capability, your base compression ratio, the cam profile, how restrictive the exhaust pipes are after the turbo, and whether you chose the correct turbo and turbine A/R ratio for the combination of items listed.

Give a holler if I didn't really answer your question...

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David
newbie #4153

Just a Six...David Hollinger,
It's a pleasure to hearing from you back again my special friend. Sorry, but I 'm so busy at this time and I didn't take a time to answer the email I 've received from you yet, but make sure that I will be sending you that as soon as I can. Going back to Flow Ratio numbers, I think I understood your reply, but I 'm talking about Exhaust/intake ratio, as follows: For example : If we have intake flow =130 CFM and exhaust flow =80 CFM, our E/I flow ratio is 0.62% percent ! Now if the numbers are changed to : Int.Flow =110 CFM, Exh. Flow =90 CFM, of course our E/I flow ratio will be 0.82% percent, therefore the question is : What of these ones (%) do you think be the right direction for a turbocharger engine ? In time...sorry but I hate fried fish,...how about a barbecue ?
MALDONADO
Brasil - South America

DEAL! Pig or cow?

The only difference between how I did the math and how you did it is which number gets divided by the other.

Using your method, a 177 intake to 147 exhaust is 0.83, but it's not a percentage yet, just a number. The way it would be written is intake : exhaust = 1.0 : 0.83 If you want a percentage, multipy it times 100, for a result of 83%

Again, using that method, the closer the exhaust number gets to 1, the better the turbo will like it. So you would want a bigger result from your math. Your first example of 0.62 is worse than the second result of 0.82. Keep in mind, though, there are practical limitations as to how close that number can get to 1, but they relate to the physical limitations of the valve sizes and exhuast passages. Lets face it, it would be tough to find or modify ANY head to have an exhaust valve/port that is just as big and flows as well as the intake.

I guess what I need to say is that the exhaust needs to flow better for a turbo installation than for an aspirated motor. If you want to start out with an unmodified head, and the intake is flowing OK, then figure out how, and put your money into making the exhaust flow better.

The low static compression on a turbo motor cannot push the spent gasses out of the cylinder as well as a high-compression aspirated engine. There is also a larger volume of "dead" gasses in the combustion chamber that has to be cleared by fresh intake charge during valve overlap. To get a turbo to work at maximum efficiency, you have to get the maximum amount of spent, heated gasses to it with the least amount of resistance.

Hope this helps, and if anybody else has a question or two that I haven't touched on, jump in and ask!

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David
newbie #4153

David Hollinger... Just A Six,
Of course my good friend, 0.62 and 0.82 are numbers only, but it times 100 equal a percentage, sorry for my mistake !Each and every time a Tech Question is placed to you I learn about these engines anymore, so I was wondering you are an ENGINE BUILDER, or not ? Another subject I've seen in an earlier Topic you've got posted here, is concerning a Shaft Mounted Roller Rocker System for the inline six Chevy motors. Just let me know where I can get one ? Keep up a good work...
MALDONADO
Brasil - S. America

I build street rods on the side for fun. That includes rebuilding and modifications for any and all components, including engines. I have to contract out most of the heavy work that requires machining to a shop down the road, as there is no room in my garage for a boring machine or big lathe. I don't have a flow bench either, but the shop I use does, and the guy is VERY good at doing what I ask.

As for the shaft mounted roller rocker, you may want to just have a system built locally for your engine. I have not found anybody that offers a ready-to-use shaft for an inline 6.

There are a couple of ways to approach the issue. One of the best ways is if you have, or know somebody that has, a rocker geometry-checking setup, and then make the shaft yourself or have a machine shop mill one for you.

You are going to have to make a single shaft for all 12 roller rockers. It can be hollow or solid. It will have to be machined to the exact same outside diameter as the little short shafts that each rocker uses already. Of course the single shaft must be drilled for 12 studs, and each rocker position has to have snap rings to hold them in place. Take each rocker apart and reassemble them onto the single shaft.

Now that you have a single shaft, you will need to figure out how high to mount it. The height I'm talking about is how far above the spring seat the center of the shaft is located. Set the new shaft with rockers down onto the STOCK studs, and use the geometry checking equipment to find out how high the center of the rocker shaft needs to be above the spring seat. This requires that you have the block under the head with the cam installed that you are going to use with this combination.

Now there are a couple of brands of hi-performance roller rockers that come with pedestal bases, and the bases are something else that has to be made, unless you can find a source to buy them. I prefer to make them, so I know they are correct length and solid. The pedestals can be either round, rectagular or square. You will have to mill off the rocker stud bases just like if you were installing screw-in studs. That gives you a flat to set the pedestal onto. How much you mill off depends on how tall the pedestals are.

Finally, the pedestals should be held from rotating by a small pin installed between the base and the flat on the head.

It's a lot of work to build one of these, and for the street, really isn't worth it. In fact, I don't think it's worth it unless you want more than 2 HP per cubic inch, or about 600 HP from a 292, aspirated or turbo. Plain individual roller rockers will work almost as well up to about 6500 rpm, and often higher. And these engines should not be run at the 8000 to 9000 rpms that v8's and v6's operate at. What you don't get with individual rockers is the stability that allows each cylinder to get exactly the same valve timing as every other cylinder. For a turbo application operating for longer periods of very high boost and high rpm, a shaft system becomes desirable.

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David
newbie #4153

edit: I forgot to note that to build a shaft system, you have to start out using the type of roller rockers that have the adjustment screw over the pushrod. You can't build a good shaft system using the rockers intended for mounting only on studs and adjusted by polylocks.

[This message has been edited by just a six (edited 07-24-2003).]