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 Post subject: EX port
 Post Posted: Sun May 22, 2011 10:46 am 
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I'm working on a set of GMPP large port vortec bowties for a 383 sbc that will be ran from 2500-6500 rpm.
They have 1.56" ex valves, and the throats are at 88% (about 1.37" diameter) for an ex port throat csa of 1.38".
The port exits (header flange) is 2.4"-2.5" csa as they came from GM. The ports are 3.5" long.
After doing the valve job and porting the throat/short turn area, I have good flow on the flow bench.

My question is, should the throat be larger so there isn't as big of difference in csa from the throat (minimum csa) to the exit (max csa) even if they don't flow as much cfm 2^ at the throat when the throat is made larger? On my test head i'm not gaining much flow going bigger, and it is certainly not at the same cfm2^.

If I had my preference I'd make the exit smaller, but welding is out and I don't trust anything else in the ex ports.

Using Patrick Hales engine pro book calculations The port meets all ex port minimum csa recomendations except what he calls the 11% of bore area rule. Bore area 12.75 x .11 = 1.403" csa. But most of his data was collected from much higher piston speed engines than this, and I can't help but think the time factor(more time to get the ex out) of a much slower piston speed wouldn't allow for a smaller minimum csa.

Darin Morgan says 400fps at the throat and 300fps at the exit. I'm close to that at the throat but well short of 300fps at the exit.

Should I make the throats larger just to make the minimum and maximum csa closer.

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 Post subject: Re: EX port
 Post Posted: Mon May 23, 2011 10:12 pm 
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I think its better to keep the csa smaller at the throat for the rpm range you're dealing with. Don't make the throat the wrong size because the exit is the wrong size.

Rick


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 Post subject: Re: EX port
 Post Posted: Mon May 23, 2011 10:46 pm 
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I would make the exit smaller by raising the floor of the exhaust flange, by welding a section in the opening the same thickness as the flange, and don't worry about blending it into the port, dead air will fill up that area. The walls and roof are the critical areas as far as being sensitive to offsets. The bottom of the exit isn't usually that active, you can play with it on the flowbench to see how it reacts. On my BBC, I welded in a 3/8" dam across the bottom of the header flange and it picked up the flow considerably.

It's easy to try and if it doesn't work in your situation, easy to undo.

Good luck, let us know what happens.

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 Post subject: Re: EX port
 Post Posted: Tue May 24, 2011 10:07 pm 
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My thinking was already to leave the throat/port as is.
I'm not sure one way would be right/wrong. It would be hard to tell without back to back testing.

I've tried filling in the floor of the ex port. I can't get it to flow better with the floor filled, but I can get a faster average port port speed.

I can't run an adapter between the header and ex port, so any welding would need to be done on the header itself.

In general, is the rate an ex port expands from the throat to the flange important?
If so when?

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 Post subject: Re: EX port
 Post Posted: Sat Jun 11, 2011 7:17 pm 
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Randy,
I like the comment about NOT making the CSA of the throat too big because of the port being too big... Keep the CSA small and velocity as high as possible. Have you taken flow velocity measurments with different backside shaped valves? You might try running a size smaller pipe on the header's first step. Open the ex valve a degree or two earlier for a higher blowdown pressure. You can accomplish that several ways without advancing the cam. RR ratio... lash & geometry adjustments. Just some thoughts....

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 Post subject: Re: EX port
 Post Posted: Thu Dec 06, 2012 12:09 am 
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Lets look at this a different way.

Say you made the ex. throat area bigger and it did flow more, but at the same cfm^2 at the throat so average port speed was higher, and exit speed was higher. Is this always a win no matter how big the throat size vs the rest of the port?

Say the ex throat is made larger and it flows more air, but at a lower cfm^2 at the throat but higher through the rest of the port.
is this always a win?

If the throat/exit and average port speed can be kept at a given speed, at what point will it be too big anyway?

When will the actual size of the throat vs the rest of the port become an issue regaurdless of what it flows?

If the engine is rpm limited so therefore cam limited, is there a time (and when) you should go to a bigger ex valve for the added window area, but keep the throat and the rest of the port smaller realitive to the ex valve size/

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 Post subject: Re: EX port
 Post Posted: Sat Dec 08, 2012 6:42 pm 
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I guess since no one else wants to give an opinion I'll give mine.

randy355 wrote:
Lets look at this a different way.
Say you made the ex. throat area bigger and it did flow more, but at the same cfm^2 at the throat so average port speed was higher, and exit speed was higher. Is this always a win no matter how big the throat size vs the rest of the port?
No, a BBC port may have great velocity on the bench, but it's too big for that engine. The engine size, power level, compression and comb. efficiency will dictate the best throat area IMO. The bench velocities tells you how well that port is designed for it's size.

randy355 wrote:
Say the ex throat is made larger and it flows more air, but at a lower cfm^2 at the throat but higher through the rest of the port.
is this always a win?
NOt always, only if the throat was too small for the engine to begin with.

randy355 wrote:
If the throat/exit and average port speed can be kept at a given speed, at what point will it be too big anyway?
See answer to the first question.

randy355 wrote:
When will the actual size of the throat vs the rest of the port become an issue regardless of what it flows?
Hard to say, but making the throat too big to fix a port that's too big for the throat is not good.

randy355 wrote:
If the engine is rpm limited so therefore cam limited, is there a time (and when) you should go to a bigger ex valve for the added window area, but keep the throat and the rest of the port smaller realitive to the ex valve size
I'd only go to a bigger valve if the throat can't be made big enough with the current valve size or if the lift is limited for some reason that the throat isn't the controlling csa in the port. Window area on the exhaust side is not nearly as important as intake and can be too big.

That's how I see it anyway. :-k

Rick


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 Post subject: Re: EX port
 Post Posted: Sat Dec 08, 2012 8:29 pm 
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this area of testing , i think needs on track tests to validate, i think the bench has led us down some tricky paths when ex flow testing to give accurate results, when cam and rpm limited i dont see a downside to increasing valve od to help the curtain but keep the throat smaller to help low lift velocity, assuming the headder is not a gross mismatch to the combo


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 Post subject: Re: EX port
 Post Posted: Sun Dec 09, 2012 7:25 pm 
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Thanks for the replies.

Tom, I agree the flow bench may not be leading us in as positive way on the ex as it has on the in.
And I also think there would be a time to use a larger valve for increased window area in lift-limited, and/or duration-limited engines.


Rick, at what point would you accept a too big of ex port exit and keep the throat the size you want it?
Maybe an acceptable throat to exit ratio vs what you consider an optimum throat to exit ratio?

The idea of 400 fps at the throat and 300 fps at the exit leaves out the rate the port expands, and I can't help but think port expansion rate counts some. So I can see a point where the exit is already larger than you'd like, so even though it's not the first choice, the throat is made a little larger just to reduce the rate the port expands as it leaves the valve area.

There's got to be more to ex ports than xxx cfm on the flow bench is needed, or xxx ex flow / xxx in flow = ex to duration split.

I think using in flow to ex flow % to decide on in to ex duration split will lead you to ](*,) .

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 Post subject: Re: EX port
 Post Posted: Tue Dec 11, 2012 11:37 am 
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randy355 wrote:
Rick, at what point would you accept a too big of ex port exit and keep the throat the size you want it?
Maybe an acceptable throat to exit ratio vs what you consider an optimum throat to exit ratio?

The idea of 400 fps at the throat and 300 fps at the exit leaves out the rate the port expands, and I can't help but think port expansion rate counts some. So I can see a point where the exit is already larger than you'd like, so even though it's not the first choice, the throat is made a little larger just to reduce the rate the port expands as it leaves the valve area.

There's got to be more to ex ports than xxx cfm on the flow bench is needed, or xxx ex flow / xxx in flow = ex to duration split.

I think using in flow to ex flow % to decide on in to ex duration split will lead you to ](*,) .


I can't think of a time when I'd make the throat too big to make the expansion rate better to the exit. Just like on the intake side I think the throat is the most important part of the port. I agree the expansion rate is important, but I wouldn't give up throat size for expansion rate. On a 23º head you have a good amount of turn from the throat to the exit so a bigger expansion than a race head may help the air slow enough to turn.

The 400fps and 300fps numbers may work well in an all-out race head, but may be impossible to get in a 23º exhaust port and have the port sized right.

These are just my opinions.

Rick


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 Post subject: Re: EX port
 Post Posted: Tue Dec 11, 2012 7:58 pm 
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for what its worth, on the 23* 1.6 ex port im working on @28" .6 lift im getting 210 cfm no pipe conservative size <69cc> 1.375 throat not a large amount of taper, exit area is 1.825, at this size im reading 520fps at the throat- 365fps at the exit, im not inclined to open the throat a large amount just to slow it down, i have some room but i dont think i would go above 89% on the exhaust- just another tricky path down the ex road


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 Post subject: Re: EX port
 Post Posted: Tue Dec 11, 2012 11:40 pm 
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Zums,I take it this is the vortecs your talking about?

The 520fps is a probe reading?

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 Post subject: Re: EX port
 Post Posted: Wed Dec 12, 2012 9:16 pm 
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randy, yes the 062 mehico junk - the ex port is a little shorter than the vortec bowtie, all fps readings are with a pitot not calculated - just got one of the fast burn valves in, will be interesting to duplicate it at 1.6 and see what its all about....


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 Post subject: Re: EX port
 Post Posted: Fri Dec 14, 2012 8:50 pm 
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Lets take another approach.

This head could have fit a bigger ex. valve. Instead of a 1.56" ex valve should this engine have a 1.60" ex valve?

1.60" valve and leave the throat the size used with the 1.56" valve?

This engine showed signs of wanting more ex duration. Every cam tested made more power (average and peak) with the ex lash tight.

Would a bigger ex. valve reduce ex. duration requirments?

A larger ex. valve would flow more even if the throat was left the same size, so if throat speed is important, a larger ex. valve should be a plus because of increased throat speed.

So would this always be a plus?

Would it sometimes be a plus, and if so when?

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 Post subject: Re: EX port
 Post Posted: Fri Dec 14, 2012 11:23 pm 
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Wow, this thread came back from the dead!!
one comment; Did the tighter lash on each cam make more power because of seat timing, Tighter lash, or did it reduced valve gear deflection??

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