i hear a lot of people on this site say our TPI cars make so much low end power because of the long runners and dont make much high end power because of the same reason. Now all the other sources ive heard say short runners are for low end power and long runners are for high end power. Whats the deal? Cars that have dual intake runners dont open the long runners till about 3,700RPMs (4.6L cobra engine for example). And what about those huge tunnel ram intakes for carburated engines that are for high end performance?

thanks

 

A little bit of searching would have revealed your answer.

Long tube runners are kings of low end performance, but suffer GREATLY on the top end. E.G., TPI. It pulls pretty good from off idle to about 3800-4000 rpm, then just dies. A short tube runner intake, such as the LT1, will pull hard from off idle to 6 grand in stock form, however it will make a bit less torque, however that's greatly offsetted by it's increased HP and ability to pull harder for longer.

next time!

 

That's a loaded question, TPI doesn't really work that great below 3000rpm...

Short runners tend to result in a flat torque curve, long runners in a peaky one

 

Think also about the diameter or cross section area of the tubes. at any given RPM, a skinnier tube will have a higher air velocity than a wider tube. at low RPM this is going to be good for torque and throttle response just as the smaller barrels in a Q-jet carb are. The smaller tubes will be restrictive at high RPMs. Bigger tubes, the reverse of that.

it is easier for me to understand the difference between wide/skinny tubes than it is to understand the theory behind long/short tubes

 

Long/short:

When valve closes you get a reflected pressure wave off of the intake charge stacking up and bouncing off the back of the valve. That travels up the intake runner and reflects back when it gets to the plenum, getting weaker every time it reflects, and consecutive reflections are referred to as harmonics (1st, 2nd, 3rd…)

The longer the runner the longer it takes for this reflection to happen, the longer it takes for the reflection to happen the lower the rpm that the harmonic happens. Runner diameter also effects this to a lesser extent, and taper weakens the harmonics till at more then about 2.5% per inch it weakens the harmonics to the point that they are not measurable anymore.

 

I think the dominant factor with the waves is the rarefaction wave that results at valve opening, piston descending. That "suction" wave travels up the tube, sees an open plenum, reverses (becomes a pressure wave, and travels down the runner again), sees open valve pushes some air in (not much), reverses (suction wave again going up the runner), sees open plenum reverses (pressure wave travelling down the runner) sees valve, and pushes a little extra air in before the valve closes and counteracts the piston traveling upwards wanting to push air out.

Then a pressure wave travels up the runner, does some back and forth as 83 CF TA says, and reaches the valve just as it opens, giving a little assistance to start the column moving again.

Of course this is off the top of my head right now, and I'll check my books soon to verify/retract this statement, hehe.

 

very interesting...i read in a hot rod magazine that the "long runners" on 5.0 EFI mustang engines made lots of low end power...

 

Magazines will tell you just about everything at some point in time, question is how much of it is worth the ink used to print it?

 

Here's a good write up...

http://www.grapeaperacing.com/GrapeA...ionsystems.pdf

 

Are you sure you don't have everything exactley backwards ? TPI is all bottom end ....and shorter runner setups are usually all up top ( i.e Stealthram )

 

I agree with what he said. So do my books. The advantage of short runners at very low rpms is not significant, and may not be a net advantage if done wrong. It's just that the pulse tuning becomes DE-tuned at low RPMs (the wrong pulse sign arrives, or it's pushing air out the other end of the runner).

Another important factor is Exhaust tuning. If done right, it'll yank some exhaust out, intake in, and amplify the suction pulse up the intake runner - which reflects and pushes air in later.

But again, if all things are tuned to one specific RPM, they usually don't work well at less than half that RPM.

 

You won’t see the beginnings of any major “tuned” effect or benefit from the TPI’s long runners until around or over 2,000 rpm,,, and that’s going to depended on the cid, head flow, cam used, and whether or not the TPI system is modified. Since the runners are long and bent and the entrance angle on the TPI base intake is not very good, the TPI system is not as efficient as a shorter straight runner and therefore the shorter runners could (depending on the differences in length and cross-sectional diameters of the two systems) produce a slight power advantage until the tuned effect of the longer runner takes effect. Meaning,,, as stated above the shorter runner could produce slightly better off idle response up until around 2000 rpm (for this example).

However, how hard you get in the throttle and the stall speed on an automatic plays a large role in the perception of your off idle throttle response and how much of the slight advantage you might feel. Once the tuned effect comes in (and it comes in fast) there is no comparison to the power levels between the two intake systems - the longer runner system will produce much more midrange power, but there is no free lunch. Because of the length and bends of the runners, as RPM increases and the tuned effect starts to decrease the power advantage goes back to the shorter runner intake system.

On most mild smaller engines, with head flow under 240cfm,, not running a lot of cam, gear, or converter, a ported TPI base and after market runners can typically make enough midrange power to offset the higher rpm power advantage of a shorter runner system. The larger your engine and the more wild the combination, typically the shorter runner intakes can make more upper end power to offset the drop in midrange power – especially in lighter cars with higher stall speeds and more gear. Which single fixed length runner system is better will depend on what you’re looking for and what compromises you want or need to make.

 

wow, this is some great info. There are many factors to consider when discussing this topic...

 

RednGold… yes/no/somewhat…

Yes, you are right that I oversimplified it, intentionally. Your spreadsheet pic… I’m trying to remember where I saw those numbers before, but they are just a rule of thumb… the chamber size at the end of the runners, the runner volume/cross section and the frequency generated by the engine assembly all modify where the harmonics actually happen. It’s a rather simple helmholts equation, but when trying to apply it to an engine it gets rather complicated. What is interesting about wherever those values came from (I wish I could remember) is that they had a % change/effect for each harmonic calculated assuming no taper to the port which showed why you don’t bother tuning for anything past the 4th harmonic and why there isn’t that great an argument for tuning past the 2nd and 3rd (runners have to be so long to tune for the first that it’s impractical if you’re going to get it under a hood).

Actually, one other thing, I think that those constants are way off… Like I’m next to certain that the 2nd harmonic constant should be in the range of 130000-134000

BadSS… I’ll agree in spirit, but your numbers are off also… you really don’t see any tuned effect with TPI till around 2900-3K (4th harmonic) and you don’t see anything really impressive till around 3900rpm, and then it’s all done by the mid 4xxx range because the heads run out of breath (it would still be done by about 5K, may be a little more on a 350 with good heads, too soon to really catch the 2nd harmonic), compare something like the crossfire setup that came before the TPI at 2000rpm to the TPI and you’ll see how lame the low end of the TPI really is.

The only reason that the TPI has a reputation for low end is that the relatively small runner cross section really does give nice velocity through the runners/ports which fattens up the bottom/midrange irrespective of tuning.

 

Yeah, those numbers are from the DeskTop Dyno book (I sure didn't create them). And of course, it's just an approximation. Some end effects of the runner will come into play. I've read in places that you have to add/subtract .5D to the length for some reasons (whether you're calculating the length, or using the length in a calculation), blah blah blah.

It's just an example. Tuning to 3rd and 4th is realistic on most engines, but if you look at a TPI manifold, from the valve to the plenum is about 29inches. Look at an LS1, and it's 19 inches. Now look at the RPM at which those harmonics come in, and you'll see that GM has probably been shooting for 2nd order on TPI, and 2nd/3rd/4th on LS1 - probably why it's a flatter torque curve.

 

It's correct.

 

They’re compensating for any radus at the end of the runner and any weirdness by the valve. Anything that acts like a diffuser effectively kills the pulse, so they figure that somewhere in that radius is where they length actually ends.

I’m not sure that they’re that long… most actual measurements of the TPI stuff comes out right around 25”, and I’ve heard the LS1 stuff was right around 16, but I don’t know anyone that has actually measured it. The 5.0 mustang intake is right around 19-19.5”

 

Valve to plenum dimensions on those TPI and LS1 lengths (not just intake). But that's off the top of my head again. Yeah, any end, radiused or straight, acts like a *longer* tube by about .5D according to one site.

 

I am confussed here . Not meaning this in a bad way or anything, just trying to learn In one you say TPI has no lowend the other it does. I have a modified TPI and made plenty of power bellow 3000 rpm ..... ( but not sure if this count )


PS great thread

 

QUOTE]Originally posted by 83 Crossfire TA
BadSS… I’ll agree in spirit, but your numbers are off also… you really don’t see any tuned effect with TPI till around 2900-3K (4th harmonic) and you don’t see anything really impressive till around 3900rpm, and then it’s all done by the mid 4xxx range because the heads run out of breath (it would still be done by about 5K, may be a little more on a 350 with good heads, too soon to really catch the 2nd harmonic), compare something like the crossfire setup that came before the TPI at 2000rpm to the TPI and you’ll see how lame the low end of the TPI really is.[/QUOTE]

I’ve had no dealings with the cross-fire intake and never seen a dyno pull on one, so whether or not it’s lame compared to a cross-fire,, I’d have to take your word for it as long as you’ve had both intake systems on the same engine.

However, you see the BEGINNINGS of benefit from the TPI’s long runners well before 3,000 rpm on most engines. Stock TPI engine, to stock Crossfire engine, to stock LT1 engine I couldn’t say which one has the best off-idle, or what the power levels are at 2000 rpm. I can say from personal experiences with different intake systems,,, modified TPI to StealthRam on a moderately powered 355,,, there is absolutely no comparison between the two at 2000 rpm – none - TPI hands down. Modified TPI on a 9.7:1 355,,, same heads and cam, better exhaust ,,,, 11.25:1 406 with a 7525 Wieand and 850 AD carb, slight edge to the 355TPI at 2000 rpm (giving away 1.5 points of compression and 51 cubes). Holley 950 projection on a Performer RPM – dyno numbers at 2500 rpm – Torque at 332 lb/ft, same “330 horse” 350 engine with MiniRam 339 lb/ft, stock TPI – 350 lb/ft, with ported GM base and AZ S&M runners – 363 lb/ft. 2500 is as low as reported and who knows if the timing maps were comparable, but I promise you with a 31lb/ft advantage at 2500 rpm, there was an advantage at 2000 as well.

 

It's a known fact in acoustics, that when a long pipe changes diameter abruptly (such as at the end of the pipe) that the effective length is L + radius, or L + diam/2. So it's not just someone empirically adding adjustments to the math buried in the DD software, it's because it has a physics foundation that's been proven many times. Rayleigh (an obscure reference in this forum) probably showed it in his 2-volume book over 100 years ago, so it predates the sbc by a very long time.

Adding a radius to the pipe (a belled end) reduces the effect of the vena contracta to prevent the flow from reducing in diameter, but the effective length of the pipe will still have the extra length.

 

yeah youre right