Who does short IDA manifolds?

[John Maher]

Ratto,

You have a very solid point about the shorter manifolds. For street use they work great. The taller manifolds are without at doubt better for Drag Racing.

Rick M

Short manifolds enhance power production at high rpm.
With a longer manifold, peak volumetric efficiency (VE) occurs further down the rpm range - better for all round driveability on the street.

It's not just about the manifold... you must take into account OVERALL length of the entire induction system: from top of velocity stack to valve seat.
Running a short manifold, very tall velocity stack combo has almost the same effect as long manifold with short stack - if OVERALL combined length is the same.

There's a very narrow rpm at which optimum wave tuning boosts volumetric efficiency (and therefore torque and power).
Overall length of the intake tract is used as a tuning tool to maximise power output at a specific point in the rpm range.

High rpm engine .. use shorter overall length
Low rpm, street use .. go longer

A set of short manifolds doesn't restrict tuning the system to high rpm... fit taller velocity stacks to increase overall induction length.
Testing on the dyno shows peak torque drop down the rpm range as length of intake system increases.

Couple of examples, with optimal length tuned between peak torque and peak power rpm (measurement is from valve seat to top of stack)

2110cc street engine, 150bhp @ 5900rpm ... 16"
2332cc n/a race engine, 268bhp @ 8300rpm... 11"

You're robbing Peter to pay Paul but if the area you're robbing from is seldom used, it makes sense to tune things to your advantage so performance is improved in the area you spend most time.

Some modern day engines use variable induction length to increase torque across a much wider rpm range than is possible with a single fixed length.

If you're building a large capacity, low rpm, torquey street motor (i.e. you know you need a fairly long intake length), better to start with a taller manifold from the off, as opposed to short manifolds and very long stacks...
the further away the butterfly is from the valve, the better chance you have of feeding well atomised fuel into the combustion chamber.

Most engine bays limit the type/length of induction system.
We're also restricted (in most cases) by what's commercially available.
And some will ignore all the physics and go for what looks "cool" 

[Jon]

I think the VW is the only motor that has its Webers over 6" away from its cylinder head. Look at 911, any inline 4 running DCOEs, Italian V12, etc

does that tell us something?

Yes, perhaps it means that engines with big capacity don't need extra torque low down as its already built in? So they go for more top end?

[Jon]

the further away the butterfly is from the valve, the better chance you have of feeding well atomised fuel into the combustion chamber.

Interesting, and the opposite of what I thought, can you explain this?
I talked with Johannes at JPM about this some weeks ago and he said to tune for 3rd pulse up to a certain duration, and 2nd above this... have you found this to be true also?

[bugkeeper]

John Maher says it right; it is not about manifold length it is about the induction length. It is about the whole length from the stack to the intake valve.

The best way to describe it is if you imagine blowing into a beer bottle  . If you blow in a full bottle the tone you hear is a high tone, meaning a high frequency. If you down the beer  and blow into the bottle you will hear a low tone, meaning a lower frequency.

The rpm of the engine has an effect on the frequency your intake valve is opening. The higher the rpm the higher the opening frequency of the valve, the shorter the induction length must be for an optimum filling of the head.

So if you plan to run a FK10 with huge valves and 48 IDA you expect a useable power band at high rpm meaning at a high valve frequency. This calls for a shorter induction length than if you build the same engine around a FK43. This induction length has something to do with the weight of the air waiting at the gate (valve) to enter the head. It improves the dynamic compression ratio and improves torque over rpm.

Modern engines nowadays have variable intake length to tune the engine at all rpm’s.

One of the reasons to opt for short manifolds is the ability to use longer stacks without making the total induction length longer.

Longer stacks are necessary because the higher the rpm of an engine the further up the enrichment area moves in the carburetor where fuel gets bound to the air. That is why a Dellorto 48 DRLA TriJet is the better choice in carburetors compared to the 48 IDA. Tri Jet means it has an additional progression area all the way up in the carburetor (actually above the carburetor by means of a feeding tube) which comes into play at very high rpm. This needs long enough stacks.

[Diederick/DVK]

i reread this topic today. trying to figure stuff out. i flipped through both testaments last night looking at engines. and the many short manifolds that were ran back in the day. albeit the early 70ies, late 70ies cars seemed to run tall manifolds.
but the only conclusion i can come to is to "compensate" for the cam. i looked at the specs as well, and most of the short manifold engines ran a w-110 engle. whereas, the guys running a w-130 or w-140 ran tall manifolds. so, i take it the tall manifolds gave some bottom end power to the higher strung engines? and vice versa on the relatively mild choice of cam on the 1700 engines...

could this in any way be correct? oh and by the way, naturally they all ran the same velocity stack, so no fooling around there

[peach_]

i reread this topic today. trying to figure stuff out. i flipped through both testaments last night looking at engines. and the many short manifolds that were ran back in the day. albeit the early 70ies, late 70ies cars seemed to run tall manifolds.
but the only conclusion i can come to is to "compensate" for the cam. i looked at the specs as well, and most of the short manifold engines ran a w-110 engle. whereas, the guys running a w-130 or w-140 ran tall manifolds. so, i take it the tall manifolds gave some bottom end power to the higher strung engines? and vice versa on the relatively mild choice of cam on the 1700 engines...

could this in any way be correct? oh and by the way, naturally they all ran the same velocity stack, so no fooling around there

Hmmm thats interesting never thought of that, maybe an engine guru could shed some light on this?

[johnl]

Ratto,

You have a very solid point about the shorter manifolds. For street use they work great. The taller manifolds are without at doubt better for Drag Racing.

Rick M

Short manifolds enhance power production at high rpm.
With a longer manifold, peak volumetric efficiency (VE) occurs further down the rpm range - better for all round driveability on the street.

It's not just about the manifold... you must take into account OVERALL length of the entire induction system: from top of velocity stack to valve seat.
Running a short manifold, very tall velocity stack combo has almost the same effect as long manifold with short stack - if OVERALL combined length is the same.

There's a very narrow rpm at which optimum wave tuning boosts volumetric efficiency (and therefore torque and power).
Overall length of the intake tract is used as a tuning tool to maximise power output at a specific point in the rpm range.

High rpm engine .. use shorter overall length
Low rpm, street use .. go longer

A set of short manifolds doesn't restrict tuning the system to high rpm... fit taller velocity stacks to increase overall induction length.
Testing on the dyno shows peak torque drop down the rpm range as length of intake system increases.

Couple of examples, with optimal length tuned between peak torque and peak power rpm (measurement is from valve seat to top of stack)

2110cc street engine, 150bhp @ 5900rpm ... 16"
2332cc n/a race engine, 268bhp @ 8300rpm... 11"

You're robbing Peter to pay Paul but if the area you're robbing from is seldom used, it makes sense to tune things to your advantage so performance is improved in the area you spend most time.

Some modern day engines use variable induction length to increase torque across a much wider rpm range than is possible with a single fixed length.

If you're building a large capacity, low rpm, torquey street motor (i.e. you know you need a fairly long intake length), better to start with a taller manifold from the off, as opposed to short manifolds and very long stacks...
the further away the butterfly is from the valve, the better chance you have of feeding well atomised fuel into the combustion chamber.

Most engine bays limit the type/length of induction system.
We're also restricted (in most cases) by what's commercially available.
And some will ignore all the physics and go for what looks "cool" 

Here is a link to the Chrysler 300 Club International, Inc that talks about intake lengths and what it will do.

 http://www.chrysler300club.com/uniq/allaboutrams/allaboutrams.html

I own a 1960 300F wth long rams and have driven one of the seven built short ram cars that ran at Dayton.  Major differences.

[Jim Ratto]

pick up a copy of the book "Scientific Design of Exhaust and Intake Systems" by Philip H Smith. If you can get through the dry, boring text and info, it's a good reference for understanding what manifold length does to power characteristics.

How about a "tunnel ram" type manifold for each 48IDA (one on each side)? each runner tapers from plenum chamber under BOTH 48mm butterflies... at any given time, each cylinder can grab air from BOTH venturies. Idle and bottom end might suffer..... but how about top end? What do you think?

[Sarge]

My head's starting to spin

[Jim Ratto]

My head's starting to spin

usually a good reason for that.... not my fault.

 

[OgCalLook]

Ratto,

You have a very solid point about the shorter manifolds. For street use they work great. The taller manifolds are without at doubt better for Drag Racing.

Rick M

Short manifolds enhance power production at high rpm.
With a longer manifold, peak volumetric efficiency (VE) occurs further down the rpm range - better for all round driveability on the street.

It's not just about the manifold... you must take into account OVERALL length of the entire induction system: from top of velocity stack to valve seat.
Running a short manifold, very tall velocity stack combo has almost the same effect as long manifold with short stack - if OVERALL combined length is the same.

There's a very narrow rpm at which optimum wave tuning boosts volumetric efficiency (and therefore torque and power).
Overall length of the intake tract is used as a tuning tool to maximise power output at a specific point in the rpm range.

High rpm engine .. use shorter overall length
Low rpm, street use .. go longer

A set of short manifolds doesn't restrict tuning the system to high rpm... fit taller velocity stacks to increase overall induction length.
Testing on the dyno shows peak torque drop down the rpm range as length of intake system increases.

Couple of examples, with optimal length tuned between peak torque and peak power rpm (measurement is from valve seat to top of stack)

2110cc street engine, 150bhp @ 5900rpm ... 16"
2332cc n/a race engine, 268bhp @ 8300rpm... 11"

You're robbing Peter to pay Paul but if the area you're robbing from is seldom used, it makes sense to tune things to your advantage so performance is improved in the area you spend most time.

Some modern day engines use variable induction length to increase torque across a much wider rpm range than is possible with a single fixed length.

If you're building a large capacity, low rpm, torquey street motor (i.e. you know you need a fairly long intake length), better to start with a taller manifold from the off, as opposed to short manifolds and very long stacks...
the further away the butterfly is from the valve, the better chance you have of feeding well atomised fuel into the combustion chamber.

Most engine bays limit the type/length of induction system.
We're also restricted (in most cases) by what's commercially available.
And some will ignore all the physics and go for what looks "cool" 

Here is a link to the Chrysler 300 Club International, Inc that talks about intake lengths and what it will do.

 http://www.chrysler300club.com/uniq/allaboutrams/allaboutrams.html

I own a 1960 300F wth long rams and have driven one of the seven built short ram cars that ran at Dayton.  Major differences.

A lot of good info here and great article John!
I always knew longer manifolds and ported heads increased air speed (port velocity) which makes torque at lower RPMs,
and now I have the math to analyze it. I'm just not sure I want to go thru the brain damage to calculate it out.
Sarge said it.... My heads starting to spin