Intake length vs torque, Just anthoer carb question No boobs Options

[Joe Ricard]

So I have short intakes and 44IDF's with 38mm vents
How long of an intake should I go to with these carbs or should I go to 40 IDF's with 32 vents or 28 vents.

I have some longer runner intakes and the other carbs so it's just a matter of experimenting I guess.

2.0L Bus piston Web 86B portmatch intake and exhaust ports. 13lb flywheel Bursch SSI Car weighs 1965 lbs 205/50-15 Kumho V700 R tires.

Thought of playing with spacers under the carbs to get more length and maybe spacer under the common length velocity stack. Looking for more torque down lower to pull harder exiting the apex of tight AX turns.

Whose got the experience of really been there done that? great gains or don't bother.

[Dr. Roger]

Please correct me if i'm wrong anyone.....

Generally speaking most torque engines use a low rise intake. More low end and quicker response. The closer to the head the better the response. At low RPM's the air velocity is slower unless the intake runners are more narrow. If I was you i'd go more narrow tubes. Shorter intake. The trade off. Smaller intake runners = less top end. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Top end motors use high rise intakes and velocity stacks to optimize air pulses stacking one on top of another.

My intake is a high rise and runners are very open all the way to the ported heads. I am trying to avoid lots of low end torque in my setup. The oposite of what you want. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[SirAndy]

QUOTE (rogergrubb @ May 18 2005, 02:25 PM)

Generally speaking most torque engines use a  low rise intake. More low end and quicker response.

actually, it's exactly the opposite ...

longer intake runners will promote more low end torque/hp, shorter runners will promote more high end torque/hp ...

what size is your engine? 38mm vents sounds awefully big for a smaller engine, you'll have *no* air-speed to speak of.

i use 32mm vents in my 44s (on a 2056 with a hot cam) and that really helped with the low end grunt and the top end HP and the transition between idle jets and main jets.
with the old 36mm vents, it would fall flat on it's nose at around 4k rpm ...
now i have power all they way up to 7k rpm ...

(IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif) Andy

[Tom73]

QUOTE (SirAndy @ May 18 2005, 02:03 PM)

QUOTE (rogergrubb @ May 18 2005, 02:25 PM) Generally speaking most torque engines use a  low rise intake. More low end and quicker response. actually, it's exactly the opposite ... longer intake runners will promote more low end torque/hp, shorter runners will promote more high end torque/hp ...

Nope, the other way. Short = torque; Long = HP.

tom...

[Aaron Cox]

pull a move like mazda racing did-

varying intake length on the fly! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[SirAndy]

QUOTE (Tom73 @ May 18 2005, 03:18 PM)

Nope, the other way. Short = torque; Long = HP

are we sure about this? (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif)

because that's not what i have learned and that's also not what the factory had to say when i was researching the numbers on the various air-cooled porsche intake designs ...

everything i dug up said that longer intake runners will promote low end torque.
but i was only looking at porsche factory FI setups, maybe carbs operate differently?

but then again, what do i know?
(IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) Andy

[lapuwali]

Andy's right. Long = low-end weighted power curve, short = top-end weighted power curve. In both cases, with all else equal, the area under the torque curve is the same, you're just moving the peak up and down the rev band.

Picture a typical dyno chart, with RPM rising left to right, and HP and torque rising bottom to top. Move the torque peak to the right (higher revs), and you move the HP peak UP as well as farther to the right. Make the intake tract longer (no other changes), and the torque peak moves left (better low-rev torque, but less HP at peak). Make it shorter, and the torque peak moves right (less torque at low-revs, but more HP at peak).

Same thing happens with exhaust header lengths.

[Dr. Roger]

Well actually it's got way more than just intake length and size.

Skinny intake=higher velocity.
Larger intake = slower velocity buy higher max volumnes of air.

Longer runners allow air to be "stacked". Stacking meaning.... As the column of air is drawn into the cylinder the intake shuts.
At this point pressure ouside the intake is building up for a milli second. Depending on the velocity of the air and the runner size it can maintain a positive pressure for either a shorter or longer time. You might want a shorter time for low end engines and a faster time for high revving engines.
Ideally I want the pressure waiting outside the intake valve to still be there when the valve opens again. Kinda' like a blower but not.
If I take a straw and fill it up with water. Then remove my finger from the end and then stick my finger back on the end I feel a vacuum for a quick second. The water is acting kinda like the air column in the intake runner. Air has mass and inertia. Having air going in a straight line and making these pressure surges at the right times of intake and exhaust is the key to REAL tuning of the intake and exhaust systems.

the same canbe done with the exhaust system. Using the air rushing out the exhaust valve, a negative pressure can be exerted in the exhaust stroke more effectively ridding the cyl of exhaust gasses. Kinda like on my 500cc 2 stroke dirt bike. =-)

[lapuwali]

Nice theory. Wrong, but nice-sounding.

You can the below (which is very imperfect, and diagrams are really called for), or you can just look at any motorcycle made in the past 20 years. Bikes make power (lots of it: 150-160hp per liter is common) at high revs, and very little power at low revs. They also have very short intake tracts. The typicla car engine makes power much lower (like half the revs or less, and most don't break 100hp/liter, many are below 80hp/liter), and have long intake runners. Short = high revs, long = low revs.

Now, the treatise:

Intake length is tied to valve timing and engine speed, and only really makes a big difference with cams that produce lots of overlap.

You open the exhaust valve as the piston is about 80% of the way to BDC on the compression stroke, as you've extracted all of the useful energy from the pressure rise after combustion. This sets up a positive pressure wave out the exhaust and down the pipe. This wave keeps going until it hits a major change in section (like in the collector), where it reflects back up the pipe to the valve as a negative pressure wave. This helps to suck the burnt charge out through the open valve. The wave reflects again, and keeps travelling back and forth, diminishing in strength, until the next time the valve opens. A similar thing happens on the intake side, except the "sign" is reversed. A negative pressure wave travels up the tract until it hits open air (or the airbox), when it gets reflected back as a positive pressure wave.

In an engine with lots of valve overlap, the wave coming back in through the exhaust can travel across the cylinder and try to push the new charge (and some residual exhaust gases) back through the open intake valve. With ideal timing, the reflected intake wave will arrive at the valve just in time to prevent the exhaust wave from pushing any charge out the intake valve. With bad timing, the charge will be blown all the way back up the tract, only to be reflected back. With really bad timing, the intake valve will close before it can re-enter.

If you look at the torque curve on an engine with lots of overlap, you'll see a series of hills and valleys. The hills are where the timing works out, the valleys are where it doesn't. Typically, each hill is higher as the revs rise, and each valley is deeper, until the torque peak is reached, which generally happens about when the speed of sound is reached by the charge attempting to flow through the intake valve.

Less overlap smooths out the hills and valleys, but at the mean between the peaks and troughs, not at the tops of the hills. Changing the length of the intake and/or exhaust tracts also shifts the height and position of the hills and valleys. With variable length intake tracts, variable valve timing, and variable exhausts, you could conceivably smooth out the bumps with the torque peak up near the peaks, rather than at the mean. This is the sort of thing current F1 engines are using, which is how they're managing to get any kind of drivability with 20K rpm 3.0 engines. Most motorcycles do not have these kinds of toys, but they have the severe cam timing, so you end up with peaky engines that make very little power below (say) 5K rpm, and huge power above 9K rpm. Motorcycles make lots of power at high revs, and have very short intake tracts. Short = high revs. Long = low revs.

[jwalters]

Yea, Andy is correct--

A good view of this in work is any 80's-early 90's chevy small block FI---you can see how chev wanted as much TQ as possible by incorporating those really long runners from the plenum--they are about 14 inches long from plenum side to the vavle head--made a real stomper of a motor--but lacked any real serious high end power. 205 HP but over 300 TQ

The antethesis of this is look at any modern crotch rocket--absolutley no intake runner length at all from throttle plates to the valve head--this makes unreal screaming power on top, and a real deficit of TQ just about everywhere. 165 HP but only about 70 TQ

i.e.--the shorter the length the more top end power ( cammed properly )and less TQ
the longer the length the more TQ generally across a stock rev range--but,
really hurts breathing abilities up top where most of the HP is made

[Tom73]

So why to make HP on a V8 do you use a "high rise" intake manifold? That increases the runner length and that is done to build HP. Right?

[Dr. Roger]

That's right... (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

Quote from CarCraft Magazine...

Smaller carburetors are commonly suggested for building torque, because their smaller venturis keep air velocity high to promote good fuel atomization. If you want to broaden the power band to retain good torque at the low end and extend power at the top end, you can make a case for a larger carburetor if it is teamed with the appropriate mix of components. The primary reason for keeping venturi size small is to maintain air speed through the boosters. This is especially critical with single-plane manifolds and larger cams, which generate weak booster signals at low rpm and the resulting loss of atomization quality and metering accuracy. This results in reduced torque output and poor driveability, but correcting it with smaller high-speed venturis may limit power at the high end.

Bottom line. there are trade-offs. U need to decide what's best for you. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[Dr. Roger]

OHHH CRAP! Why-oh-why will my hood not fit right??? It did when it came off the car!! SHEEEEEEZE!!!

And a little tip...

Before removing any lid's, make an outline with a pencil around where the hinge bolts to the lid. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

I learned that in Mr Batterman's auto shop class. 1979.

[Joe Ricard]

Well I knew all that. But what I want is the recipe for the engine I have. I am going to try the 40 IDF long intake with 46mm opening at the top vs 50mm short intakes.
I may make up some spacer shims to lengthen the intake even more. Also try to lengthen the velocity stack. can't go to much as I have short filters.

I'll let you know how it goes.

By the way this motor does rock above 4500 RPM doesn't fall on it's nose ever I have seen 7200 RPM in 3rd. Co- driver thought I was going to scatter it all over the track.
That was 10,000 miles ago. I put a rev limiting rotor in it now. Shift light @ 5500 makes my shift About 6000.

[Joe Ricard]

QUOTE (Tom73 @ May 18 2005, 05:26 PM)

So why to make HP on a V8 do you use a "high rise" intake manifold? That increases the runner length and that is done to build HP. Right?

You nailed it right on the nose. High rise itakes straighten out the air flow increasing velocity.

[SGB]

Personal experience:

really really short intake = never get below 3400 rpm or there is no torque. Motor just flys from 4000 to 6000
really long intake = plenty of pulling as low as 2000 rpm, not as quite as quick getting to redline.

For street and AX, I'd use long ones. For runoffs or high speed tracks, use shorties. I would also use the 40 idf carbs for higher flow speed to get quicker on/ off throttle response..

Put those phenolic heat resistant spacer 'tween head & manifold, too. I left 'em out when I switched and under the right (wrong) conditions ( 200 miles, then stop, then restart 3 minutes later) I had some burbling and gurgling from the vapor mix in the hot manifolds, Seems to have been eliminated by heat spacers.

[bondo]

What I don't understand is why runner length would affect anything at full throttle. With the butterfly all the way open, shouldn't it all be runner? Couldn't you get the best of both worlds by putting "runners" on the intake side of the throttle valve?

Hmm, I just realized... that's exactly what velocity stacks are for. Talk about reinventing the wheel!

[scotty914]

i did get a ride in joe's car, with the exception of starting i dont think he was ever below 2500 rpm, he was amazed at the pull my engine had in 4 th gear at 1500 rpm.

joe the only thing i would try with your car is MS with a stock fi runner and throttle body, you want torque and the stock system made gobs of it. heck even subaru made the same basic intake setup and you saw what that is like

[Joe Ricard]

Yea Scott, Your motor in my car and it would be pretty damn scary fast. HOWZ it going with the Transaxle? Figure out 3rd gear yet? I'll bet you are hitting the plastic bushing before getting full rod motion.

[scotty914]

nope i have done almost nothing... got a new battery... pulled the alt

i just charge the battery and go driving, sometimes with 3rd some times with out i can go an hour with no alt with out worring. i just need to rebuild money before i look at it too much