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Exhaust / intake flow ratios on LS heads

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Re: Exhaust / intake flow ratios on LS heads

Postby Newold1 » Wed Jan 11, 2017 2:48 pm

lets think about the question this way. Lets take a moderately modified LSX engine of lets say 427 cubic inches with a 4.125" bore and a 4.00" stroke with lets say a static compression ratio of 11.0 to 1 with a camshaft spec. of a 242 degree @.050" intake duration and a 256 degree@.050" exhaust duration and intake and exhaust valve lifts of .650", a 108 degree LC and a 112 degree LSA. This camshaft was optimized to work with one cylinder head with a flow potential of 315cfm @.650 on the intake side and an exhaust port with a flow potential of 240cfm @.650". The camshaft is installed straight up at TDC.

In this first case the intake is opening .050" at 13 degrees BTDC and closing at 49 degrees ABDC, the exhaust is opening at 62 degrees BBDC and closing 10 degrees ATDC, overlap is 23 degrees. Lets say peak power is made at 6300 rpms.

Now we install on the same block a different set of LSX heads that flow 385cfm@.650" lift on intake and flow 250 cfm @.650" on exhaust or ONLY 10 CFM MORE THAN THE ORIGINAL HEAD USED. Now lets say to using the higher flowing intake port we use a duration of lets say 252 degrees @.050" and we use a 278 degree @ .050" duration on the exhaust to account for the not much greater flowing exhaust port (crutching the exhaust port) with the LC of still 108 degrees and an LSA still of 112 degrees. Now the intake is opening at 18 degrees BTDC and closing at 54 degrees ABDC. The exhaust is now opening at 75 degrees BBDC and closing at 23 degrees ATDC with a 41 degree overlap. The peak power is now made at 6800rpms.

I think the above examples are pretty close approximation of how the 427 cubic inch LSX engine could be configured and based on what I see on quite a few dyno tests of similar builds this is about the camshaft specs, that have been used effectively for good power numbers at those top HP rpms.

My question again is based on a similar real world result if we could shorten the exhaust duration necessary for a better flowing exhaust port with 280 cfm @.650 lift lets say by 18 degrees and use an exhaust duration of 260 degrees @.050" which would mean the exhaust valve could open at 66 degrees before BBDC (this would allow possibly more of the power stroke to lengthen by 9 degrees of crankshaft rotation and have an exhaust valve close at only 14 degrees ATDC which would allow the intake stroke to pull on the intake charge 9 degrees more on the intake side.
My thought here is that should produce more power and torque under the curve on this type of LSX build.

If my assumptions or thoughts are wrong here please explain where I am going wrong. If this is possible then why would aftermarket LSX cylinder head designers and makers not want to try and make some larger improvements on the exhaust flows in these heads?
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Re: Exhaust / intake flow ratios on LS heads

Postby Stan Weiss » Wed Jan 11, 2017 3:57 pm

If we look at the geometry of the rotating assemble we see that the rod and crank arm are at 90 degrees to each other between 70-75 degrees crank rotation ATDC and by 90 degrees crank rotation we are not gaining very much extra power per degree we keep the exhaust valve closed.

Let me ask you this. Why do I care about peak exhaust flow when the piston is well on the way up at that point. Why are we not looking at low lift flow where most of the heavy work is done?

Stan

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Rotation_Time_of_crank_per_rev_in_Milliseconds_______9.2307692
Crankshaft_rev's_per_Second__________________________108.3333333
Crankshaft_Degrees_at_which_Rod_and_Crank_are_90_Degrees_71.8473

Crank______Piston_____Crank_______Rod____Cylinder___Cylinder
Angle______Travel______Rod_______Bore_____Volume_____Volume
Degree_____Inches_____Angle______Angle______CI_________cc
-ATDC_
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Re: Exhaust / intake flow ratios on LS heads

Postby groberts101 » Wed Jan 11, 2017 4:29 pm

I don't understand what you don;t understand. lol

Seriously though. Too many preconceived notions or variables involved to even touch on all that.

I would say this to help you wrap your head around it. As others have said many times now.. the intake side is sooooooo much tougher to get the mass moving efficiently/starting, stopping than the exhaust side. Just part of the physical world we live in until you start forcing it into the combustion chamber and/or tricking the chamber into thinking it sucked in more o2 than it actually did on that last stroke. Then a small exhaust port becomes an impediment.

So, just because the intake side works more effectively on one motor over the other doesn't necessarily mean that you automatically want to give up duration. Maybe you need that higher duration for the extra rpm/blowdown volume? A lot easer to tax an exhaust port design at 6,800 vs 6,300 rpm, right? Maybe the weaker exhaust port, relatively speaking, was not fully optimized for the engines parts combo/powerband, or could have gotten by with a smaller valve/port combo and picked up torque below peak?) or the cam was not perfectly tailored for its capability to begin with. Maybe, maybe, maybe... :wink:

Now that same "weaker port" comes alive and shows its truer potential when you pump up the volume coming in with the bigger flowing intake design and higher rpm. In that scenario, just growing the exhaust valve/port won't add any additional power and may end up causing you to change the cam in another direction. IMO, not hard to get a motor to work better with its respective camshaft choice because many/most?.. are undersized/oversized/at least slightly mismatched in the first place. Because of that haven't seen many that don't like higher compression ratios either. Then just when you think you got it right.. the chassis or the rest of the drivetrain combo needs to be re-tweaked to fully realize it at the rear tire. If it was easy.. everyone would be good at it.. we'd be much further along in 4 stroke technology. Too many physical limitations for an antique design. We're scratching for the last %'s at this point.

Look at all the most advanced head designs in the world. Designers ALWAYS trade exhaust valve size for maximum intake valve real estate when maximum power at higher rpm's(relatively) is required. And they always go up to the point that the peak exhaust flow potential starts becoming the limiting factor for any further gains coming from the intake side. The bore is only so big, moving and canting valves will only get you so far, and cam timing will always be some form of compromise. Compromises upon compromises.

Think about this. Many big diesels have exhaust valve sizes/curtain area's approaching the size of the intakes. We don't always need to use bigger intake valves or higher inertial tuning/rpm to ram more into the combustion chamber. Not to mention that bigger bores don't need the same piston speed and can be inertia tuned as well. Point is that it's always good to keep in mind that heavy reductions in pumping losses have a major effect on usable torque production too. Go too far and you just kill any additional scavenging possibilities that may have been leveraged. Not that I'd kill intake size on either of those motors just to stuff a bigger exhaust valve/port("Flowz" more on a bench) into the equation.. only that there are 4 cycles going on here.. well, 5 if you count inertial tuning. You can only optimize a few cycles at a time, simple cause and effect rule cancels out some potential gains that may be had when treating the induction and exhaust tracts as if their improvements are separated and then added together in full cumulative fashion at the end. Like all the bolt on parts "xx horsepower increase" claims.. they will never fully add up.
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Re: Exhaust / intake flow ratios on LS heads

Postby groberts101 » Wed Jan 11, 2017 4:43 pm

Stan Weiss wrote:If we look at the geometry of the rotating assemble we see that the rod and crank arm are at 90 degrees to each other between 70-75 degrees crank rotation ATDC and by 90 degrees crank rotation we are not gaining very much extra power per degree we keep the exhaust valve closed.

Let me ask you this. Why do I care about peak exhaust flow when the piston is well on the way up at that point. Why are we not looking at low lift flow where most of the heavy work is done?

Stan


True true.

To take a shot at that question I'd guess it has to do with the general fact that higher mid-lift flow will almost always tightly follow peak flow capabilities. You can independently form/adjust the bottom of the flow curve with seat/throat angles a bit.. but the top of the flow curve is too heavily tied to mid-lift to see an exaggerated flow spike. At least I've never witnessed a port that flows mediocre at mid-lift and then took off like a rocket to reach some ridiculously high peak flow number.

It's like the age old argument of why do I need more valve lift beyond the point that my cylinder heads max flow is reached. Because it improves the available flow window and lets you take advantage of it in other area's of the engines design.
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Re: Exhaust / intake flow ratios on LS heads

Postby turdwilly » Wed Jan 11, 2017 5:21 pm

If exhaust port flow is completely irrelevant, why in the hell the Ford pro stock guys spend so much time/money doing this in the 1970s? Just didn't know any better back then?

cleveland1.jpg
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Re: Exhaust / intake flow ratios on LS heads

Postby Stan Weiss » Wed Jan 11, 2017 5:29 pm

While these numbers are from sometime ago. In a drag race application they were making around 2.8 HP per ci on a 400 ci small block.

RDP-SBX Australian Pro Stock Darin Morgan SF600 Alum
Exhaust = No Pipe
2.27
0.20000 - 146
0.30000 - 231
0.40000 - 314
0.50000 - 384
0.60000 - 426
0.70000 - 450
0.80000 - 465
0.90000 - 473
1.00000 - 482
1.6
0.20000 - 92
0.30000 - 151
0.40000 - 188
0.50000 - 238
0.60000 - 271
0.70000 - 286
0.80000 - 296
0.90000 - 297
1.00000 - 299

Stan
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Re: Exhaust / intake flow ratios on LS heads

Postby Stan Weiss » Wed Jan 11, 2017 5:31 pm

turdwilly wrote:If exhaust port flow is completely irrelevant, why in the hell the Ford pro stock guys spend so much time/money doing this in the 1970s? Just didn't know any better back then?

cleveland1.jpg


Because the original shape was just ugly. It had a very small radius turn in the port.

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Re: Exhaust / intake flow ratios on LS heads

Postby 4vpc » Wed Jan 11, 2017 6:01 pm

CGT wrote:
Warp Speed wrote:WTF is "Flowz"?!?

I think it may be Latin or an Olde English spelling of FLOW. :D


Old English was Flowan before being shortened to flow, 'flowz' is just Meercan gangsta speak.
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Re: Exhaust / intake flow ratios on LS heads

Postby 4vpc » Wed Jan 11, 2017 6:40 pm

groberts101 wrote:
4vpc wrote:
groberts101 wrote:
Thank you. Goes right back to the "no need to try and get out what couldn't get in".


What if someone designed some clever gadget that forced more air in, what would that do to the in/ex valve ratios? :-k



Easy one. It would be akin to making the old OEM Cleveland 4v intake port smaller via stuffed floors or port plates. Run that with a factory exhaust port and even the old cast manifolds and what happens to the power curve?

Would you actually think.. "exhaust port don't need any improvements in "flowz"" to help maximize the now larger amount of mass that gets squeezed in"? Or just rob peter to pay paul back some added duration and/or lift over on the exhaust lobe and call it the same end result anyways?

An engine is just a huge series of compromises to get towards the end result. All I'm getting at with all this is that if you trivialize or underestimate one piece of the puzzles worth or ability to affect some other piece?.. then you're downplaying the physics involved with the whole deal. I come here for the first hand experiences and physics breakdown of it all.. not diluted comments like.. "doesn't matter.. you won't be able to measure it or feel 5 horsepower on the street anyways". I can get that crap on some other sites.

From the standpoint of properly timing and sizing the flow window.. which is what all this needs to be dwindled down to anyways.. there are various ways to go about moving the same mass. You can vent larger amounts of mass out the cylinder throughout the low/mid range and limit lift and slow the valve down all the way to max lift. Or you can lower the low lift flow capability of same port(smaller valve size with 55° seats?) and increase the valve open speeds(rocker ratio/roller lobe?). Purely rhetorical but which combo is best? Best for what?.. valvetrain life.. higher rpm? Do both combo's make same power under 6k? Optimized cam requirements wouldn't change either?


It's 'flow'.
Maybe we're a bit off-topic with turbo stuff and i'm sorry, but I have no idea what a old OEM Cleveland 4v is or was.
With a turbo you can stuff more in, but it's still got to find its way out the same way as any N/A so why not make the in/ex valve ratio a bit closer?
As per your reply above, not everything is a compromise, sometimes (yes I know it's rare) we can have a win-win situation.
In answer to a post claiming bench testing an ex port is pointless well no, you just have to know what to do with the info you have collated. Lets say an ex port stops flowing at anything above .39 then you could say you've got your max valve lift. Afterall what is the point in going any further? You just use a bit more duration, keep the velocity up and use it to draw the inlet charge in.

So, getting back to my earlier point that high output turbo engines have slightly different rules, yes I have increased the ex valve size and seen big improvements, this is dependent on how far they were out to begin with of course.
If you have any Dyno sim programs then give it a go and see what it gives, from what I have seen it's win-win all the way.
What are they doing with thoroughly modern engines? Check the valve sizes here: https://lmr.com/products/2015-Mustang-E ... 4-Cylinder
Or a second (different) opinion here: http://www.full-race.com/articles/ecobo ... train.html

Although we don't know the port flow, the actual ex valve sizes are pretty big compared to the intakes.
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Re: Exhaust / intake flow ratios on LS heads

Postby Ken_Parkman » Wed Jan 11, 2017 7:19 pm

Obviously the exhaust flow is important - no question. But under what conditions??

When the flow is sonic at low lift and high delta P? Or at high lift and low delta P?

The 1400 degree gases at sonic velocity are unrelated to what a flow bench can possibly simulate, and sonic flow has completely different characteristics. Like the velocity increases with increasing area, opposite to sub-sonic. It's a completely alien set of conditions, so is a flow bench the right tool, especially when you consider all the variables? I don't think so, but what do I know. I think the exhaust is way more complicated then a flow bench can begin to simulate. Not saying you can't learn something from a bench though.
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Re: Exhaust / intake flow ratios on LS heads

Postby novadude » Wed Jan 11, 2017 8:48 pm

groberts101 wrote:Upsize the exhaust valve and run more rocker ratio and you will see the results in the first half of the throttle pedal travel.


I'd admit that I'm just a dumba**, but I need someone to explain this one to me.

In the first half of the throttle travel (throttled intake), you are filling the cylinder less. Why would you need more flowz to evacuate it? I realize that intake pressure will be lower during overlap, but I would think that there would be no ill effects from a restrictive exhaust port. A very late EVO might even be beneficial at low throttle openings to get the most work possible out of the expanding gasses. Why open it early and worry about good low lift / mid-lift exhaust flowz if there is less "stuff" to evacuate?

Am I thinking about this wrong? Can someone explain the science?
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Re: Exhaust / intake flow ratios on LS heads

Postby groberts101 » Wed Jan 11, 2017 9:03 pm

novadude wrote:
groberts101 wrote:Upsize the exhaust valve and run more rocker ratio and you will see the results in the first half of the throttle pedal travel.


I'd admit that I'm just a dumba**, but I need someone to explain this one to me.

In the first half of the throttle travel (throttled intake), you are filling the cylinder less. Why would you need more flowz to evacuate it? I realize that intake pressure will be lower during overlap, but I would think that there would be no ill effects from a restrictive exhaust port. A very late EVO might even be beneficial at low throttle openings to get the most work possible out of the expanding gasses. Why open it early and worry about good low lift / mid-lift exhaust flowz if there is less "stuff" to evacuate?

Am I thinking about this wrong? Can someone explain the science?


I would have to tack it up to almost being strictly related to reduced pumping losses. Basically uncorking your exhaust from the valve end first. DV talked about this and even installed a 1.7 valve after shrinking down the intake, IIRC. Think I remmber him even discussing how to "time the stop lights" to improve mileage. Always though that was pretty funny.. true.. but funny. Should dig that old tattered book out. Smokey's old book is pretty awesome too.

Dunno for sure, kinda a dumb@$$ myself.. I just keep chugging along hoping to learn as much as I can before my hands don't let me grind any more. Which seems to be quicker than I had ever imagined. This grinding stuff is hard on a body.. that's for sure. Thank god for soft aluminum but now my bits fill up when I get too impatient and greedy with sudden heavy material removal. lol

Less flow resistance like removing smog bumps.. thinning guides.. raising the exhaust port roofs exit height.. etc. Even though the engine isn't moving much mass at idle and low rpm's.. that gas still has to make its way to the manifold as easily as possible. Fewer obstructions sure never hurt any motor I ever built, helped with, or ported the heads for.
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Re: Exhaust / intake flow ratios on LS heads

Postby shawn » Thu Jan 12, 2017 1:01 am

groberts101 wrote:
Lots of assumptions being made that you can actually gather any useful data "flowing" an exhaust port.....

shawn


"Now that sounds just plain silly. The heads you see on the market today are directly the result of flow bench and dyno data. Do you know how long it takes to develop a head strictly by the "try this and then dyno it" approach? Holy Jesus.. we'd be another 20 years behind in cylinder head development right now and only rich people could afford to have fast engines. Thank's to flowbenches dyno and CNC we can open a box, bolt the parts on, and race with the bigger dogs. :wink:

It's just data. Knowing what to make of it and how to design the rest of the engine around the data is the tougher part. Has absolutely nothing to do with the data being useless."

You're missing the point. I don't know of a flow bench that exist that can simulate the live running conditions of an exhaust port. Even my custom built bench that will create more depression than anyone I've ever seen. We quit "flowing" race engine cylinder head exhaust ports to achieve a specific "number" or "ratio" years ago. Not to say that it isn't used, it's just not for that purpose.
A friend of mine that does prostock bike heads told me they haven't "flowed" an intake port for a specific flow number in almost two years. Chew on that one for a while...
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Re: Exhaust / intake flow ratios on LS heads

Postby cspeier » Thu Jan 12, 2017 8:58 am

I think a person could get rich doing seminars under the title "How to PROPERLY use a flow bench." The secrets on how to apply what your seeing.. :D

I think is so misunderstood, it's easier to stay away..

my opinion.
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Re: Exhaust / intake flow ratios on LS heads

Postby turdwilly » Thu Jan 12, 2017 9:02 am

Stan Weiss wrote:
turdwilly wrote:If exhaust port flow is completely irrelevant, why in the hell the Ford pro stock guys spend so much time/money doing this in the 1970s? Just didn't know any better back then?

cleveland1.jpg


Because the original shape was just ugly. It had a very small radius turn in the port.

Stan


....which made it.....flow poorly?
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