Exhaust Port Size Differences

[Boost+k]

yip, on turbo's that would be the way to go.

On an NA K engine if you open the ports up it can kill power, that doesn't mean don't clean them up, but don't get carried away.

 

Have seen one head where the guy changed them from square to round and killed the engine power. :P

 

i fully agree, i think I'm stuck in the turbo mindset :(

 

but yea even on a turbo i would only tidy up the bend and port the exit a little and taper it back down to try and let those exchast gasses expand as they escape

 

although on a 7k you may run into trouble getting the ports to stay sealed, its hard enough on a turbo 5k

[Taz_Rx]

although on a 7k you may run into trouble getting the ports to stay sealed, its hard enough on a turbo 5k

 

+1 :P

 

They're a good gasket. BUT.... look how much gasket there is around ex ports 2 & 3. :( Atleast with a permaseal EM30 the gasket itself is bigger than it has to be.

[Taz_Rx]

you were talking about opening up the exhaust ports in an earlier post, don't!!!

the exhaust shouldn't flow any more than 80% of the inlet.

If you don't believe me try it!!

 

 

solid lifters in a 7K is not as easy as changing lifters, push rods and adjustable rocker gear.

7k blocks are taller and there isn't any K series push rods longer enough, and i wouldn't think Datsun ones will be any better.

saying that it has been done, recently too.

 

if you are that keen to put solids in talk to Teddy.

 

I have said too much!!!

 

Hate to be a stick in the mud TRD ke70 but have a look on page 6 under:

STAGE III

1. CYLINDER HEAD

(1) Main modifications

 

Enlarge the intake and exhaust ports to improve the intake and exhaust efficiency....

 

http://www.vhtracing.com/TRD_K_ENGINE_MANUAL.pdf

[Felix]

Taz, they also say on that page to increase the valve size.

 

No point opening up the ports to far without installing larger valves.

[Taz_Rx]

Taz, they also say on that page to increase the valve size.

 

No point opening up the ports to far without installing larger valves.

 

So it does. I just browsed over it (and missed that valve bit) and posted the link.

 

Some good engine building info there, I might go and chuck it in the FAQ if its not already there.

[KE55PIG]

So is it worth getting ure head ported if u don't upgrade the valves or not ? is the 3k-b heads valves different to the 4k head ?

As ive purchased the 3k-b twin cars there port/runners is around 31mm but the 4k head is oly 28 ish mm

shorely it would be worth getting them to the same size. Same as the exaust ports make them same size as extractors

[TRD ke70]

Yes i see your point TAZ, and they are right to a point.

What i'm was thinking of is a street/weekend track engine.

Those trd engines were rev'ed to 10,000rpm, where they did need to open the exhaust up.

But the TRD manual is old technology these days, there has been some big improvements in understanding how gases flow through an engine.

 

It's hard to explain without the aid of pictures, but if anyone has heard of anti reversion, they will know what i'm talking about, that's with the exhaust.

 

Velocity/air speed over volume is more important, as air speed will create volume.

 

Very hard to explain this!!!!

but hope everyone understands. :lolcry:

[KE55PIG]

so TRD KE70 YOU ARE SAYING there is no need to get the exaust ports ported to the same size as the extractors in my case running a 286 deg cam ? wouldnt the more flow help ?

[seabiscuit]

so TRD KE70 YOU ARE SAYING there is no need to get the exaust ports ported to the same size as the extractors in my case running a 286 deg cam ? wouldnt the more flow help ?

Yes. By all means port your head, but to a certain extent. As he said: Velocity/air speed over volume is more important, as air speed will create volume.

ie, the quicker the exhaust flows the better, not just how "wide" the ports are. I'm assuming that the more you port it the less easily the air flows in the direction

that it needs to go. Like TRD KE70 said, it's very hard to explain lol.

 

I may be completely wrong, but I think this is what TRD KE70 is trying to say (metaphorically).

If you are blowing through a straw, it is very "tight" and the air all seems to flow very DIRECTIONALLY in a tight stream out the other end, but is still quite hard to blow out,

leaving lots of "pressure" for you to push out. If you were to compare this to blowing through a vacuum cleaner pipe (or something bigger), the air only comes out very slowly,

SWIRLING around sloppily in the pipe, slowing down the air behind it to a certain extent, also leaving you breathless very quickly as the pressure is released a little too quickly.

I don't know how to tell, but you need to find a compromise of both straight-line flow (the straw) and diameter (thick pipe) to keep you happy. Obviously depending on the capacity

and the ability of your lungs to push air, the size of your "exhaust" will be different.

 

It is a strange metaphor I know, but to me seems quite effective.

[seabiscuit]

I just read this... Understanding Exhaust

 

"""For virtually all high performance purposes, backpressure in an exhaust system increases engine-pumping losses

and decreases available engine power. It is true that some engines are mechanically tuned to "X" amount of backpressure

and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same engine that lost

low-end torque with reduced back-pressure can be mechanically re-tuned to show an increase of low-end torque with the

same reduction of back-pressure. More importantly, maximum mid-to-high RPM power will be achieved with the lowest possible

backpressure. Period!"""

 

EDIT: Another metaphor. If you have an orange gun with a 0.5L chamber and a 30cm wide barrel it will be useless, same goes

for having an orange gun with a 10L chamber with a 1cm barrel. Think about it... what to do to make a good orange gun? :lolcry:

Edited October 22, 2009 by seabiscuit

[Evan G]

oh the good old backpressure arguement is starting to brew!, ive been have this arguement with people at work for a couple of weeks now.

 

"the customer put a 3inch straight through and a 4 inch cannon and wondering why its running like a pig?"

 

my tafe teacher said "you don't need exhaust back pressure!"

 

but a tech of 40 years says "you need back pressure especially for engine braking"

 

another tech said "you need back pressure a good example of this is the v8 supercars, 600hp N/A, see them slowing down to take a corner and blowing flame? thats backpressure"

 

 

I'm f@$king confused

 

can taz_rx make a poll out of this ???

Edited October 22, 2009 by Evan G

[rob83ke70]

all backpressure is bad full stop. but you need to maintain exhaust gas velocity or else you will lose power.

 

thats how I understand it but EVERYBODY thinks they understand that differently.

 

if you look into wrx exhausts, you will find that they achieve best power with 3" from the turbo to two thirds of the way down the car then they step it down to 2.5" for the last third including the muffler, leave it all straight through, and this is a tried and true wrx performance modification. You need to keep the exhaust gas speed, and as the hot gas cools down (the further it gets from the engine) the pipe diameter needs to decrease to maintain the same amount of gas speed.

 

Feel free to correct me on anything, it is a free country :lolcry:

 

Robert.

[Felix]

TRD ke70 wasn't really talking about back pressure.

 

What he was getting at is that if the exhaust port is smaller than the extractor tube, you have a lip that helps prevent exhaust reversion waves travelling back into the cylinder. The reversion waves hinder the ingress of the next air/fuel shot on cam overlap.

 

Exhaust reversion is more common with bigger cams. Ever seen carbs spit fuel or have fuel standoff at lower revs? Exhaust reversion is the main culprit.

 

 

My understanding of back pressure in an NA motor is: You should ideally match exhaust flow to induction flow. Induction flow should be matched to the motors requirements at peak power RPM.

[KE55PIG]

so still havent really got the answer i wanted should i port my exaust ports to the size of extractors

or

What I'm thinkin is go around 1-2 ml smaller then extractors but have a _/ sorta thing so it angles out to promote it to flow out yea ??

_\

Edited October 22, 2009 by KE55PIG

[philbey]

Q=v.A

 

I would have just said match your exhaust port to the extractors.

 

Any change in diameter will change the velocity of gas flow, be it up or down.

 

Your suggestion will slow the rate of gas flow, as per the equation (which is also a link). Q (volumetric flow) is constant, so if A increases, v decreases, thus slowing your exhast gasses down.

 

If your extractors are scavenging properly, then keeping the speed up should make scavenging more efficient which in turn will improve volumetric efficiency of the engine.

 

That's just my thoughts on the matter.

Edited October 22, 2009 by philbey