Carbs or PI?

[Robb]

Has anyone experience of the relative advantages/ disadvantages of carbs (SUs) compared to PI?

My engine was converted to carbs early in its life (1970s) and so far as I'm concerned runs well and has adequate performance. Am I missing out on something?  
I'm not really concerned about authenticity, so wouldn't convert just for that reason.
I have most of the original PI parts but as these have probably not been used for over 35 years, I guess would be of little use as not unleaded compatible.

[andy thompson]

Carbs - easy to tune , cheap , simple - unlikely to fail completely stranding you
Carbs - Not effected by altitude
Carbs - readily tuneable by several rolling roads

Summary - fit and forget

PI - will give about 10 bhp more and better low down torque if swapped straight onto a engine previously running on carbs
PI - Induction sounds great
PI - Very reliable if well set up and looked after
PI - tends to use more fuel although if well set up should be equivalent to carbs
PI - injectors can misbehave giving misfires - generally due to o-rings in MU breaking up - usually easy to fix but annoying
PI - Bosch pumps on many TR6's not tolerant of low fuel levels
PI - Pump or metering unit can fail stranding car (rare)
PI - technical support availible and all parts availible

Summary - classic, effective but idiosyncratic system requiring reasonable understanding of its quirks for best results

[JohnD]

lagerzok wrote:

Carbs - easy to tune , cheap , simple - unlikely to fail completely stranding you
Carbs - Not effected by altitude

Above 2000 meters that is, or 6000 ft, so not a problem in the UK!

lagerzok wrote:

Carbs - readily tuneable by several rolling roads
Summary - fit and forget

Really?  How ofetn do carb owners fiddle with needles, or 'retune' their cars?

lagerzok wrote:

PI - will give about 10 bhp more and better low down torque if swapped straight onto a engine previously running on carbs
PI - Induction sounds great
PI - Very reliable if well set up and looked after

See above - fit and forget!

lagerzok wrote:

PI - tends to use more fuel although if well set up should be equivalent to carbs
PI - injectors can misbehave giving misfires - generally due to o-rings in MU breaking up - usually easy to fix but annoying

See above, carbs are not free of component failures!

lagerzok wrote:

PI - Bosch pumps on many TR6's not tolerant of low fuel levels

Depends where you mount the Bosch

lagerzok wrote:

PI - Pump or metering unit can fail stranding car (rare)

You mean that the pump on a Carb-car will never fail?

lagerzok wrote:

PI - technical support availible and all parts availible

Summary - classic, effective but idiosyncratic system requiring reasonable understanding of its quirks for best results

But fit and forget, unlike carbs!


John

[andy thompson]

Personally I would always fit PI but I tried to give an unbiased account of the relative merits - if you use SU's tuned to the application and with good new or recon components and you don't fiddle for the sake of it they will give many years trouble free running - so will PI - but it will cost the average punter more and usually give more issues. Btw where is Tim Bancroft on this?  ;-)
One more plus for PI btw - you can run a big cam and its way more tractable :-) and the '150'cam was designed for injection

[Richard B]

lagerzok wrote:

- so will PI - but it will cost the average punter more and usually give more issues. Btw where is Tim Bancroft on this?  ;-)

Still saying "I must get my PI running on injection"  ;D

Having converted my MkII PI Estate to carbs, and then converted it back again because the engine was lacking in performance; I know which setup gets my vote.  8)

[TRTOM2498]

PI is great.

I have 3 PI cars, driven to Lemans x2, X3 RBRR, X2-10CR,20HCR, track days, with no issues, in all diferent climates, and different altitudes.  All cars run Bosch pumps :)

[gilesdive]

My car runs with the PI system. Yes I have cursed at it in the past. I have been unable to get it going when the little plastic drive dog snapped. I have grumbled when an injector failed and I was running on 5 cylinders.

When it is going right, it is so right, sounds the part and goes pretty smartly too.  If I was happy with the carb set up, would I go to the effort and expense of swapping over? Maybe not, but I do love it. (mine has Bosch pump too)
Had experience of SU carbs on various Spitfires etc.....pretty good, but not without their moments of embuggerance at times as well.