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Old 08-19-2014, 07:35 AM   #11
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Quote:
Originally Posted by amc49 View Post
FYI, oversize valves are a waste of cash unless the head is reworked in valve seat area to allow the oversize to work. The bowl throat has to be enlarged to complement the oversize. Otherwise the oversize part flows worse.
I would hope that any reputable shop wouldn't install OS valves without working the throat and seats. As you said, that would be completely pointless. It's not a DIY sort of mod. I really wish that more retailers would state that in the parts description.

INSTALLING OVER SIZE VALVES REQUIRES EXTENSIVE MACHINE WORK. THIS IS NOT A DO-IT-YOURSELF, PLUG AND PLAY INSTALLATION... lol

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Originally Posted by amc49 View Post
And while lots buy this..........

'The key in the exhaust is to maintain velocity.'

.....I'd like someone to explain why pretty much ALL engines make more power when you dump the exhaust quick into a low velocity situation, or atmospheric with unmeasurable open area (drag header), or how you can SLOW velocity down and make far more power in several different scenarios (expanding bell megaphone) or even worse stop the flow and make it go BACKWARDS (reverse bell on the rear of 2 stroke expansion chamber). How about crossover pipe joining both sides of dual exhaust, it certainly messes up laminar flow there and the doubling of area would lower velocity greatly. Yet more power is the result.
The area of low velocity, or low pressure that I think you were referring to, actually creates higher velocity at that point of exhaust gas exit. Where it goes after is meaningless... Unless it is required to continue for another 12' through another resonator and a muffler. That is where maintaining flow and velocity is key...sorry I should have added that we are talking about a complete exhaust system.

I don't really have time to get into fluid dynamics and 2 stroke vs 4 stroke.... that is like apples and oranges. I think we can agree that I should have added flow into my original statement along with velocity and for that I apologize.. Sorry you had to type all that out!
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Old 08-19-2014, 10:21 AM   #12
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Doesn't the SVT run 10.2:1 on 91? Crazy that you can't get 11:1 on 93 with these things!
The SVT runs on 91 but does not make its best performance on 91 because timing has to be lowered for 91

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Old 08-19-2014, 10:23 AM   #13
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FYI, oversize valves are a waste of cash unless the head is reworked in valve seat area to allow the oversize to work.
For 95% of the guys modding the Zetec engine over size valves are a waste of money even if installed properly , NA mod like this OP is talking about ported head and larger valves wouldnt add vs the cost

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Old 08-19-2014, 07:06 PM   #14
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Yep big money to do that right. 1 mm. not big, need to go bigger if one is to do that much work.

'The area of low velocity, or low pressure that I think you were referring to, actually creates higher velocity at that point of exhaust gas exit.'

Actually, it is the higher velocity ahead that creates the lower velocity behind, the dense plug creates a lower density behind it as it continues down the pipe (think a hypodermic needle piston drawing in fluid), the leftover that got missed spreads to thin out. At the exit, when the positive exits a positive then pops back in pipe from the attempt to even pressure up and then that positive races back up pipe backwards, if timed right it acts as a stuffer to prevent overexhausting at the end of overlap.

I like to type.

No difference in 2 and 4 stroke once you mess with them enough, BOTH are air pumps. You don't see it commonly done but you can even put the reverse bell on a 4 stroke exhaust and it works well. Only difference is the double up in firing events and half the time to do all events in.

I wish someone would make cheap throat boring tools that didn't cost an arm and leg.............
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Old 08-20-2014, 04:19 PM   #15
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http://www.yellowbullet.com/forum/sh...43&postcount=2

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Originally Posted by Chad Speier View Post
The biggest thing people need to remember is the flow rate is the same from start to finish. What changes in a cylinder head is the velocity due to area.

I explain it like this. Think of a river. The water that passes through a tight area "looks" like it's moving faster rate, it's not. The pressure from that area being smaller is changing the velocity and the water rate is the same.

The velocity of air moving through the port is primarily a function of how much air it is moving vs. the port's volume (more accurately, its cross-sectional area). As the port moves more air, the velocity goes up; eventually getting so fast it blows the shortside turn and separates. When that happens, the port will typically stall, flowing no more or even less air no matter how much further the valve is opened. How much air will get through before the shortside separates and the port stalls will depend on the shape of this critical part of the port. The shortside form is a key limitation on the ultimate flow potential of a head. All ports aren't created equal, and some have a decent shortside form while others are handicapped in this area. A large radius turn represents a better flow path than a tight one. Some heads are compromised in how much material there is to work with, while others are generously endowed. Race heads are often designed with raised intake ports and a large meaty radius to help get the air around the bend without separating.

Take the general from that quote.

Flow does not change over the course of a valve event. Only Pressure and Temperature do (combined, this is Density).


There is another term that should always accompany Bernoulli IMO, never does. It's call Mean Free Path -> http://en.wikipedia.org/wiki/Mean_free_path
If you understand MFP, you'll understand that it is a proof for Bernoulli's theory.
Velocity and Density become easier to understand.
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Old 08-22-2014, 10:03 PM   #16
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That was actually interesting, never heard the term before.

I do know that using the description above of port stalling across the short side radius, that a cure for it can be to actually bring the roof of port down lower to make a venturi at the valve guide area, that tightening up of space then allows the mixture to make it at higher flows around a less than adequate short turn radius. The port becomes lower in overall height but has same width. Honda has done that numerous times in their stuff. I used to open up the port there until I realized I was making things worse doing so. The faster flow then allows the short turn flow to happen with less turbulence at higher gas speeds, the separation cannot happen as easily.
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Old 08-22-2014, 10:16 PM   #17
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Thats why you rarely mess with the short turn radius

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Old 08-23-2014, 01:03 PM   #18
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The short turn radii in the 2.0L DOHC Zetec head is terrible. Probably why they don't seem to respond as well to bolt-ons, or make as much power, as other similarly sized engines. SOHC or DOHC......

In these heads I have found that you have to add material to the ports.... As is mentioned in that article... However, that is as far as I am going to take this topic for now. I will post up some numbers and details in a few weeks once I get everything put back together and tuned.
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Old 08-24-2014, 12:18 AM   #19
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Hmmm

That'll be interesting to hear about.
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Old 08-25-2014, 07:12 AM   #20
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Man I mess with short turn radius in everything I lay a grinder on, it can pretty much always be made at least a bit better and often much better. Boring throats for bigger valves makes it even better. Nothing major, simply smooth it up and if possible set up a bit of straight approach to back of valve. Depends on if you are trying to set up a cone flow (2V) or across top of valve to tumble into chamber like some 4V (heavy pentroof) like so much.

When I referred to opening up port I meant the roof around valve guide(s) to open that venturi up, a mistake in some cases. Depends largely on the angle switch from port proper to bowl. The flatter the intake port and more angle in degree change at short turn then don't touch it. Some of it depends on the approach and area of rest of port.

If one adds material add it to floor of port, if epoxy, well, we ran every brand you could think of and a real chore getting it to stay in place once the engine heat cycles a few times. Welding on aluminum is the thing to do there but carries special problems too.

We actually pulled some stunts trying to influence flow percentages in Honda MC heads when the port was severely offset and one valve got more flow than the other at high speed, putting different sized intakes in each pair. Messing with port divider bias was a useable tool as well.
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