Intake runner length

kachok25 · #1 12-23-2005, 12:21 PM

OK I am a little confused, I have always heard that shorter intake runners are better for high RPM hp while linger runners are much better at low RPM torque. Well I found an online pulse calculator and it said that for peak torque at 4000RPM I would need intake runners 35 inches long! That is way longer than I have ever seen on the streets, and that is a real high powerband, for 2000RPM (a more streetable peak torque) I need 60 inch runners. I know longer intake runners are better for fuel atomazation, and thus power, so is there any reason other than conserving space that causes us to use little bitty 8-12 inch setups? I guess the whole fuel atomazation thing is a whole post in itself, so I won't ask it here.

beef_bourito · #2 12-23-2005, 12:48 PM

the reason you want a certain length intake runner is because of the pulses of the valves opening and closing. when a valve closes it sends a chockwave, kind of like a sound wave, back into the tube, when it hits the end it bounces back. you want that dense part of the wave to hit the valves when they open.

The reason these lengths are the best for that rpm is because the wave gets to the valve at the right time. but if you get a shorter one it can bounce multiple times so you can use a smaller length and take up less space. it's only a portion of the wave that gets bounced back so evey time it bounces there's less compressed air. you will get optimal power with the ideal length but you won't really notice a big difference with the smaller lengths.

kachok25 · #3 12-23-2005, 01:53 PM

Quote:

Originally Posted by beef_bourito

the reason you want a certain length intake runner is because of the pulses of the valves opening and closing. when a valve closes it sends a chockwave, kind of like a sound wave, back into the tube, when it hits the end it bounces back. you want that dense part of the wave to hit the valves when they open.

The reason these lengths are the best for that rpm is because the wave gets to the valve at the right time. but if you get a shorter one it can bounce multiple times so you can use a smaller length and take up less space. it's only a portion of the wave that gets bounced back so evey time it bounces there's less compressed air. you will get optimal power with the ideal length but you won't really notice a big difference with the smaller lengths.

Yea I know how it works, I was just woundering why they did not try to build them to the ideal length. I know it would take up more space, but the difference in pulse strength, and fuel atamazation, should make a substantial differnece after you advance the timing, increase the CR or boost. I heard that F1 cars lost 150hp when they were forced to reduce their intake runner length. The difference in the different pulse strengh between the pulses is 110% pressure on the second pulse 107% pressure on the third pulse, and 104% pressure on the fourth pulse. That being said I don't think that most autos even get their fourth pulse because even with 10 inch runners you don't get the fourth pulse intill 6,800 RPM! That is way high and alot of cars won't even see it. Even with incredably long 18 inch runners you wont see your fourth pulse untill 3,800RPM and your second pulse is all the way up to 6,800RPM! I would really like to see what a car can do if it really optamized it's VE I have heard that it is possable to reach 130 percent VE that is like running a 4 psi supercharger! That would be SWEET

beef_bourito · #4 12-23-2005, 02:13 PM

when you use forced induction it doesn't really make much difference. it will affect it a bit but not much. it's good to tune for low end torque when you have a turbocharger so that when you're off boost you still have a bit of power.

The reason people usually don't build them that length, i think, is space and money. tuning it for the perfect length for a certain rpm takes time and money, and it might not even fit. also, car manufacturers aren't always interested in getting the most power possible out of their machines. if you look at cars today, you can replace the exhaust system to get more power, you can replace anything to get more power, that just shows that they aren't interested in spending that extra time and money to tune them.

curtis73 · #5 12-23-2005, 03:00 PM

Tuning for pulse scavenging only works in a very narrow RPM band. Great for race, not so hot for the street.

Tuning pulse scavenging at 15,000 RPMs is pretty easy since its such a short runner. You get the benefit of the pulses plus the velocity of a "normal" length intake runner. Tuning it for low RPM street applications as you found out means a very long intake runner which will take it beyond the area of feasible velocity.

Chevy's TPI worked this way, but their tuned runner was 1/4 of the length of the optimal. You still get an overtone pulse, but at about 1/5 its potential intensity. They're tuned for about 2000 RPMs, so you get that 1/4 pulse at 2000, a 3/5 pulse at 4000, and the full intensity pulse wouldn't happen until 8000.

kachok25 · #6 12-23-2005, 04:12 PM

What kind of peak VE do they get?

curtis73 · #7 12-23-2005, 08:11 PM

Which one? Chevy TPI? Maybe low 80s. Their VE was destroyed by cam and heads.

Race engines with very high RPM tunings can easily see over 100% VE, maybe as high as 115% in rare occasions.

beef_bourito · #8 12-23-2005, 08:22 PM

or you could get formula one engines that produce over 800hp from a 3L V10. Next season a 2.3L V8.

kachok25 · #9 12-23-2005, 08:51 PM

I heard that the F1 caes have long intake runners is that true? I heard they use the long runners because they provide better fuel atomazation allowing them to use a higher compression ratio.

kachok25 · #10 12-24-2005, 10:54 PM

How does forced induction effect the harmonic pulse, since the air is denser does the pulse travel faster? Would that make even longer runners optimum? And would it also intensify the effect of the pulse since the the baseline pressure in the runners is so much more. I know that fuel does not like to make turns in intake manifolds but if I did bend them to allow maxamum length would it hurt my fuel atomazation or just my pressure after the bend? The Bowling and Gripo calculator says that the peak of the pulse is 10% stronger than ambernt air pressure, than how do Formula 1 racing cars reach 130% VE?

kachok25 · #11 12-26-2005, 02:44 PM

Does high RPM have an effect on how strong the pulse is?

beef_bourito · #12 12-26-2005, 02:59 PM

I would tend to think that it would because the air is moving at a faster speed so there's more of a shockwave when a valve closes.

kachok25 · #13 12-26-2005, 03:28 PM

Quote:

Originally Posted by beef_bourito

I would tend to think that it would because the air is moving at a faster speed so there's more of a shockwave when a valve closes.

What about when under boost pressure? Would it raise the pressure by the 10% by the atom pressure of by the boost pressure, and would the air be traveling faster under pressure, requiring longer runners for optimum torque?

beef_bourito · #14 12-26-2005, 03:45 PM

I'm not sure about when it's under boost, although i heard that when it's under boost it doesn't make as much of a difference as na.

kachok25 · #15 12-26-2005, 04:12 PM

It should the air should still bounce back just the same when you thing about it. I have even seen people build custome intakes for their turbocharged engines because they said the stock ones were not long enough. I just don't know how much difference it makes, if it works off the manifold pressure it would be posable to get over 220%VE at 14.7psi of boost. That would be awsome.

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