Single throttle for PIs.

[shenderson]

Hi

Rather than hijack Nick Jones's thread I thought I should start a new one on this topic.  After 20+ years running a PI and knowing all the issues, I am thinking of upgrading my father's estate.

I have seen that a number of PI people have ditched the 6 separate throttle butterflies and fitted a single one at the end of the intake plenum.  As well as the obvious advantages, this should also improve breathing (if we were bothered about originality, this was also the concept envisaged by the original designers).

Ideally the throttle would be at the centre of the plenum but I can't see a way of doing that without major modifications.

What I need to know is:

where can I source a suitable throttle and linkage?

has anyone done this to an auto? (kickdown cable arrangement etc)

anything else I need to consider?

The car is a pre-facelift MK 2 with BW 35 auto, otherwise unmodified.

Any advice much appreciated.

Cheers

Steve H

[Lord Sorbington]

It's interesting to note that when BMW created their high performance M series 6 cylinder engines they fitted a butterfly per cylinder, dispensing with the single (central) butterfly of lesser models.  Some of their lovely V8s also have a butterfly per cylinder - they obviously think it's better than a single throttle butterfly?

[James]

But a PI isn't a high performance machine and they use a single throttle body on everything but M cars.

They are also far better made than the Triumph throttle bodies that all need aligning and balancing.

After seeing Peters Ridgetone racer I'm pretty sure there is no performance hit and for sure everything to gain from everything being in balance.

Not sure about the kickdown arrangement.

[Nick Jones]

Single throttle per cylinder has advantages when using wilder cams (wild cams + plenum = ragged idle and poor low rpm response) and gives a slightly sharper throttle response.

Two main problems I see with the standard PI setup is firstly the rather oversize butterflies which means an awful lot happens in the first few degrees of butterfly movement and secondly the rather poor linkage which makes balancing the butterflies tricky and frequently impossible.  The balance is more critical at small openings and as already mentioned, due to the size of them most driving takes place at small openings.

Single throttle body choice is not that critical.  You'll need something between 55 and 65mm and it wants to be fairly compact as there isn't that much space.  Connections upstream of the TB for breathers and such are a bonus.

I've used a Nissan Primera 2.0L one from a mid 90s car.  That's 62mm bore and very compact.  It came with a throttle position sensor which is good for me when the EFI goes on but irrelevent if staying mechanical.  It's not well equipped with vacuum or breather connections though.  It comes with a simple cable throttle connection and that has an extra connection point which I presume is for a kickdown cable.

I think James' was from a 1.8 K series (MGF?) - that's a bit smaller, but big enough and comes with it's own throttle cable outer bracket and a better selection of vacuum and breather connections.

Others have used Landrover ones.  What ever comes up, has the right size and configuration will do.  Watch out for those with integral idle control valves.  You don't really want those if sticking with PI.  My choosing method is to go along to my favourite scrappy and have a nosey at what looks like it will fit.  Get a good choice of trunking, vacu pipes, brackets, cable ends etc - I find that piles of random bits confuse scrappy pricing policy and improve value!

When fitting, it's worth cutting the plenum back slightly at an angle to point the TB more directly at the cut-out in the from panel - it'll make trunking and air filter arrangements a bit easier.  I also squashed the end of the plenum slightly and positioned the TB as far out as possible to stop the incoming airflow being sent behind the air horns.

You'll note from my PI thread that the vacuum signal from the standard take-off point is affected.  It appears to be strengthened giving rise to lean running, I think because the original sensing points are within the sweep of the original butterflies causing the rise in measured pressure to be amplified over the first few degrees of throttle movement.

I've moved the vac takeoff point to just behind the new throttle, which has helped alot but is still not perfect - although variations in performance lead me to think that there is another factor at play as well.  Peter (Ridgetone) seems to have found his worked ok at the standard takeoff but I think he also ran a more aggressive cam which will have given less vacuum and may have inadvertently balanced things out!

Cheers

Nick

PS I'll take a better pic closer up and showing the new vac connection point

[James]

The K-series throttle body I am using comes in 48, 52 and 56mm sizes- approx rated 130, 150 and 180bhp.

I like it because it has it's own cable bracket and also includes an outlet for the air idle valve.

[Nick Jones]

Closer up picture of the Nissan TB

Nick

[Lord Sorbington]

jcarruthers wrote:

They are also far better made than the Triumph throttle bodies that all need aligning and balancing.

The mechanically controlled versions of the BMW M series throttle bodies can suffer from wear, lack of alignment and poor synchronisation too.

I think low service and manufacturing costs are the main reason they use the more basic single throttle body arrangement on their run of the mill volume production cars.  Maybe not the ultimate solution though?

As I say, they obviously think the more complex arrangement is worth the extra maintenance and build costs when it comes to the M cars.

[James]

I assure you a set of M throttle bodies is a work of art compared to the Triumph PI ones.

Single is for simplicity as you say— as our standard engines are simple I reckon it's enough. If I get bored of my setup and run out of other stuff to do I may try ITB

[Andy Borris]

jcarruthers wrote:

If I get bored of my setup and run out of other stuff to do I may try ITB

Or pop round to mine and help me! ;D

Andy

[Ridgetone Triumph]

Nick_Jones wrote:

Closer up picture of the Nissan TB

Nick

Hi Nick,
Your single butterfly conversion seems  different to mine, the vacuum and and outlet pipes are 1/4 the size I had, when I first got the single unit installed I had the problem were it would not rev properly from idle, but had reasonable revs through the range, it wasn't until I connected a rubber pipe between the idle control valve and the butterfly that it made all the difference, it idled sweetly, revved up instantly and it would freely rev right up to the limit. When I fitted the single butterfly unit on the car in 1997 the engine was not modified  apart from a freeflow manifold.
The single throttle body came off my 1975 Mercedes Benz 280se, this was mechanical injection therefor having the right vacuum outlets and sizes required. Once the engine was modified 2 years later the meeting unit had to be adjusted to allow for more fuel to feed the engine. But it did run well, 14.87sec 1/4 mile  on budget road tyres ;)
Regards
Peter  

[Nick Jones]

Interesting comment about the size of the vac line.  Could be a factor as it often feels like the MU is not responding fast enough, possibly restricted by the small vac take-off not flowing enough air.

Nick

[Ridgetone Triumph]

Nick_Jones wrote:

Interesting comment about the size of the vac line.  Could be a factor as it often feels like the MU is not responding fast enough, possibly restricted by the small vac take-off not flowing enough air.

Nick

It is possible that you need to make the pipes larger, also the breather pipe from the rocker cover on the race car was nearly 2" diameter. I had to drill out the old breather outlet and weld in a much larger one, all in the name of better breathing so they say  :)

I just seen I wrote the wrong 1/4mile time, it should have said 14.187sec ;)

Regards
Pete

[andy thompson]

14.1 1/4mile What cam / exhaust are you running !? i thought 14.8 was good for a lightened Mk1 - that is in a different league!  ;D ;D

[shenderson]

Thanks everyone, I will start gathering bits.  I don't suppose anyone has a spare PI inlet plenum?

Steve H

[Ridgetone Triumph]

lagerzok wrote:

14.1 1/4mile What cam / exhaust are you running !? i thought 14.8 was good for a lightened Mk1 - that is in a different league!  ;D ;D

It certainly browned off Porsche and Lotus owners on the track ;D
The cam was a Kelford F12/13-6-8. Camlift .291" in, .302" out, valve lift at 1.5 .437" in  .453" out,  duration at .050 234 in  244 out, cam timing at .050", in o 11btdc in c 43   ex o 52  ex c 12, the  timing was 12 to 16 degrees depending on fuel. The exhaust was custom built from the head all the way.  42mm freeflow 6-2-1 and 64mm from the collector pipe and one 21" coby muffler (the rear subframe was modified as was the floor pan to allow for the larger exhaust, all very discreetly ;))
The old girl was a bit of a jaw dropper, did a street sprint  in her and was 3 seconds off the top Subaru Rallys cars on a short fire tarmac circuit, and quicker than the not so top ones :)
We just ended up with a great combination of mods which worked well beyond what we even expected. I could rev it off the rev counter  ??)(I didn't try that too often)

Hope this helps a little.
Regards
Pete  

[shenderson]

Another thing - did anyone sort out a fast idle linkage for the single throttle?

Steve H

[Nick Jones]

I'm using an electromechanical Bosch extra air valve plumbed in to bypass the throttle.  This is as used on the older K-jetronic mechanical systems (common on older Volvo 240, Golf GTI mkI and early mkII and others).  This in preparation for the EFI, but it simply has a 12v ignition controlled feed, no ECU connection.  Has had very little testing in PI environment but works really well on the Vitesse.


Nick

[TroyBeros]

My old supercharged arrangement had a central inlet, and a v shaped deflector inside the plenum so that the inbound air / fuel mix didn'e just go to cyls 3 and 4!

The throttle was on the back end of the s/charger.

[Nick Jones]

That's a compact install  :)

Did it ever blow the rubber pipes off?

Nick

[TroyBeros]

No. The plenum had a couple of extra barckets solid mouted to the inake manifolds to hold the whole thing together. The rubber pipes between the plenum and the intakes were clamped at both ends as well!

If I did it again, I'd mount the supercharger on the other side of the engine, and run it through an intercooler.

It was efi as well, so maybe it looks less bulky without all the extra fuel hoses too.

[andy thompson]

Go on you know you want to - used to go pretty well didn't it  8) - really needed forged pistons and a USA head with about 7.5CR - and an intercooler would have been a very wise idea.

[TroyBeros]

If I was to do it again, here arer the things needed to prevent me from pulling my hair out...

1. Rust free fuel tank. Check!
2. Better fuel pump. Check!
3. Bigger pulley for supercharger belt. Check!
4. Much better ECU. Check!

Oh dear, it looks like the planets are re-aligning. lol

[TroyBeros]

(Have a look at my avatar - that's what happened most of the time...)