How long to make my ITB runners?

[billjam]

I am fitting Triumph Daytona 675 throttle bodies to a 964 engine and need some guidance on intake runner length.

In the pics below, the runners are over-length and will be trimmed to suit.
The minimum length I can handle between the flange and the ITBs is 110mm due to the bends. This gives about 240mm from the head to top of velocity stack as a minimum overall length.
The injectors fit just below the throttle plate.
The TBs are 44mm ID right through, which matches the ID of the 50mm alloy tube I am using for the runners and will soon match the head ports once they are opened out from 42mm.
Engine will also have DC21 cams, coil-on-plug ignition and Megasquirt engine management and is for street use.

So ... how long should they be???

[psalt]

Bill,

You may be interested in research that Jaguar did in the 1950's on intake runner length while experimenting with their slide throttle injected XK engine. It has a similar hemi head with a 70 degree VIA. Every different pipe length (valve to atmosphere) will produce a pressure wave at a different rpm that will boost the torque at that rpm. The graph has been reprinted ad nausem in many books, I'll try to find one online. Roughly, a 32.5" pipe gives a peak around 3500 rpm, a 21.5" pipe around 4000 rpm, a 15" pipe around 4300 rpm and an 8" pipe around 5000 rpm. The long pipes also have an increasing negative effect at high rpm, which is why MB and others started to experiement with variable length pipes, ultimately leading to the rotating drum intake manifold.

[safe]

What's the restriction of your engine bay?

Are you going to use an filter box of some kind?
You will need at least 1 diameter of free air above the air horns.

Do them as long as you can, but I doubt you can fit runners much longer than your minimum length.

[911pcars]

You may have some fuel delivery, engine response issues with the fuel source (injectors) so far from their respective combustion chambers. Any way you can move them closer?

Sherwood

[Henry Schmidt]

Back in the IMSA GTU days (2.5 liter engines) dyno runs showed that you needed a 6 inch runner change to even notice a difference.
Varioram intakes make a 24inch + change.
Make them any length you want and concentrate on cams to determine your engine characteristics.

[tctnd]

Sherwood is correct. The injectors should be close to the ports with the bulk of the runner upstream. There is potentially some power to be gained by mounting the injectors upstream as the cooling effect of evaporization can give a denser charge, but the price is the likelihood of fuel collecting on and running down the runner walls and the low speed and drivability issues that ensue.
regards,
Phil

[Flieger]

Porsche played around with this sort of thing back in the early days of racing 911s. They had high-butterfly, low butterfly, and slide-valve throttles. They had an injector placement up at the top of the velocity trumpets and an injector placement down at the heads where the stock MFI injectors go.

The factory seems to have mixed and matched in experimenting with these parts. From the period photos I have seen, it appears they settled on the low-injector placement though the trumpets still have the mounting provisions. I believe they found no rearl difference in ultimate horsepower and that they reported better drivability with the low injectors. The MFI fuel shutoff on the over-run was partly developed for this reason. Some cars had the injectors above the throttle, which would cause pooling fuel when coasting/engine braking and then a rough throttle response when accelerating again as the fuel dumped rich into the cylinders.

The high butterfly system was meant to allow the air to smooth out before getting to the cylinder- it was thought to flow better by having the turbulence and restriction further away. It was farther from the intake reversion, too. The low butterflys had the advatage of snappier response at low engine speeds because of the smaller volume to fill. The high butterfly system came in at high engine speeds when the flow was fast enough so that the added length did not add much time to the engine speed response.

[billjam]

Interesting input from all, thanks.

Magnus, this engine will be going into a 911 or my 930, so typical height restrictions apply due to a/c condenser under the tail.
I was planning to use a K&N filter same as used on other triple TB applications (E-3671, 12” long x 5-1/4” wide x 3-1/4” high) in a custom housing. The base of the housing will be raised up about 1” from top flange of TBs to clear fuel rails and to give about 2” clearance over the air horns.

"Make them any length you want and concentrate on cams to determine your engine characteristics."
Henry, from what I’ve read on this, I’d have to agree. I sort of came to the conclusion that with the runner length I have to play with it wouldn’t be critical, especially for a non-racing application.

Sherwood & Phil, thanks for comments on injector placing.
I was taking the easy way out by just using the existing injector ports in the TBs. I hear what you are saying regarding response and driveability, but I’m currently driving a 930! Whatever response this engine has, it will be light years ahead of the 930. Can the difference in response be that much different with the injectors 100mm lower?
I can see that fuel collecting on runners could be a problem – that might be the bigger issue I think.

I would be interested in seeing some typical overall height dimensions from head to top of filter for TBs or carbs if anyone can measure for me.

[Steve@Rennsport]

Some musings,......

Short runners will be fine with your mild cams,....its only critical when high-RPM, max power output is required.

Your bent runners on those outside cylinders are a bigger concern when injector placement positions them above the changes in direction. Generally speaking, fuel-air mixtures don't like to turn corners (even gentle ones) and that affects fuel mixture. With carbureted engines, we make small corrections to ensure each cylinder has the same AFR, however EFI is a different matter altogether since each injector usually gets the same width injector pulse.

Food for thought.

JMHO, of course.

[billjam]

QUOTE=Steve@Rennsport;5131367]Some musings,......

Your bent runners on those outside cylinders are a bigger concern when injector placement positions them above the changes in direction. [/QUOTE]

Yes Steve, I have already had a rethink on this based on earlier comments.
I am now going to fit the injectors immediately above the head flange. This has the added cost advantage of allowing me to use the standard 964 fuel rails and hoses.

The TBs I am using don't easily lend themselves to being spaced wider (by 35mm). It introduces other complications with linkages and filters, so I am prepared to live with the trade-off of bent outer runners.

[Mark Henry]

Conventional wisdom is the runnier/TB length should be about 9 inches
I've been looking at the slot plenum design with a single TB and I was about to do it on a T4 but then i got the 3.0 /6 to put in my 914
I may still try this but with a NA set-up:

Photo Album
Or for a bit of light reading on this:
Turbo Intake Plenum - HybridZ

[safe]

Try and get a measurement from another car. This is the absolute maximum height I could get (on the right side) to fit under a whale tail, don't rememeber the height i used.

[billjam]

Looks v nice Magnus.
What TBs are they?
Can't quite work out what filters you have there either. More details/pics?

[safe]

Originally Posted by billjam

Looks v nice Magnus.
What TBs are they?
Can't quite work out what filters you have there either. More details/pics?

Thanks, the TBs are from a (3) Suzuki TS1000.

The air filters I made myself, didn't find any that fit good.
Made the fuelrail and manifold too.

[psalt]

I've been looking at the slot plenum design with a single TB

The Audi manifold is an interesting design, but since +950 hp has been made with a single plenum sheet metal manifold on a 911, how much benefit could there be ? Maybe there is a different issue on a rally car, none of the F1 turbos used a dual plenum with a slot.

[billjam]

Magnus,
The lack of suitable long oval filters was a problem for me as well. I didn't really want the extra work and cost of using six smaller round ones either. That's one of the reasons why I have decided to keep the Triumph TBs at their original 83mm centres and use offset runners.
I will be making up a GRP or CF filter base and top cover in due course. (waiting for filters to arrive first)

One of my next problems is to design the throttle linkages. I was planning to adapt standard Triumph throttle cables or something similar. You also seem to have used cables. Do you have any pictures of how you made the connection to the existing throttle crank on the engine?

[billjam]

For those interested, here are a few more detailed pics of the Triumph Daytona 675 throttle bodies.

[PFM]

Billjam,

Consider Pipe Max software to get the tuned length right for both intake and exhaust. As for injector placement down low for smog and low RPM, up high for HP. If the ECU can do it staged is the answer. The high EFI location is good for quite a few HP if everything is correct.

PFM

[billjam]

Originally Posted by PFM

As for injector placement down low for smog and low RPM, up high for HP. If the ECU can do it staged is the answer. The high EFI location is good for quite a few HP if everything is correct.

PFM

"high EFI location is good for quite a few HP" ... can you quantify?

[PFM]

Billjam,

The numbers below are for a very tuned Cosworth 2LTR 4 valve 4 cylinder. The numbers are TQ on the dyno. The EFI is MoTec agian very tuned. The increase is in the 10% range, nothing to sneeze at. Not my motor just some info from a tuning acquaintance.


rpm low inj high inj
4500 82 108
5000 108 130
5500 145 162
6000 150 165
6500 148 163
7000 148 158
7500 145 155
8000 140 146

PFM