SPI vs. MPI

dcioccarelli

dcioccarelli

Dominic Cioccarelli
Hi all,

this is probably a theoretical discussion for most as the SPI engine was never available in the US, but it may be of interest to some.

We all know that from around 1980 the X1/9 in the US came with a Bosch L-Jetronic based multi-point injection system (MPI). This same engine was used in Europe for the Fiat Uno 75ie but only until 1987. Late model Mk1 and all Mk2 fuel injected Unos got a single point fuel injection system. The power output remained the same with the SPI system, which looks more like a carburettor. It seems strange that the MPI would be replaced with an SPI system, although I assume that cost and the less stringent emissions controls in Europe had something to do with it.

By many accounts though, the SPI system has a higher performance potential than the MPI and has similar running characteristics to a carburettor with the reliability of a fuel injected system.

I came across this discrepancy in the Uno 75 i.e. when buying an ECU, which I'd originally assumed to be simply an updated and repackaged version of the L-Jetronic:
s-l1600.jpg


In reality it is a Bosch Monojetroic system which is designed for the SPI system below. Internally it is much more advanced than the L-Jetronic (which is essentially an analogue computer) and contains a micro controller. The corresponding SPI injector system is shown below:

9874-mono-jetronic-uebersicht-1-jpg


There is plenty of info here (in German):

https://www.fiat-uno-ig.de/forum/thread/1873-mono-jetronic-spi-bosch-motorsteuerung-bilder/

The conversion from a carburettor to the SPI system is apparently much easier and yields better results than converting to a (standard) L-Jetronic MPI system. Apparently a few people in Germany have already done this to meet emissions requirements in city areas (Euro zones):

https://www.project-x19.de/forum/viewtopic.php?t=9533

https://www.project-x19.de/forum/viewtopic.php?t=7749

https://www.project-x19.de/forum/viewtopic.php?t=12900

Sorry, all in German but Google translate is your friend...

Whist you are improving your German, here is a cool 80's video explaining the system:


I recently purchased the SPI manifold, more out of curiosity than with any serious intent of using it:

s2.jpg


I'll post more when I get it and have had a chance to see how it works.

Cheers,
Dom.
 
Interesting info Dom.

I'm not too surprised a newer design SPI works better than some older MPI systems, not necessarily specific to the models you mention. Technology changed a ton during that period. Older water cooled VW's had a similar evolution with various Bosch injection systems.

I suppose it depends on your goals. If ultimate performance is desired then none of the older systems are worth using and a aftermarket standalone ECU with a mix of modern components is best. In a case like that starting with a MPI is a benefit due to the ability to use newer injectors in the old holes/injector mounts (which SPI does not have).

On the other hand if you are looking to throw together a simple inexpensive system that will drive well, then the SPI would be beneficial, mostly for its simplicity. Especially on an engine that did not have holes/mounts for multi injectors (e.g. a converting a carb engine).
 
A simple inexpensive system would, of course, be a nice two barrel Weber carb.
The hot rod crowd have access to all manner of four barrel throttle bodies that look like an old four barrel carb. A two barrel version would be an interesting alternative for an X but they would probably be way too big. Sad, because they seem to be complete kits and if you believe the TV shows they are simple to install.
 
carl said:
The hot rod crowd have access to all manner of four barrel throttle bodies that look like an old four barrel carb. A two barrel version would be an interesting alternative for an X but they would probably be way too big. Sad, because they seem to be complete kits and if you believe the TV shows they are simple to install
Click to expand...
That existes and one of our members has installed it on his X. Not sure if he has documented it other than mixed throughout this thread:
https://xwebforums.com/forum/index.php?threads/what-did-you-do-to-your-x1-9-today.10638/page-141
 
Hi all, I needed some information, where does the tube that says "tankentlüftungs dampf rücklauf" go? What is it related to? The picture was posted higher.

EDIT: oh, now I understand where it goes, but I don't have the canister, can I do without it? Because the pipe in question absorbs air, I don't want it to enter the combustion chamber.
 
Carbs have disadvantages?
Would you rather maintain a Fiat with a 40 year old carb or a 40 year old FI system?

Just yanking chains as I usually do.
 
I think the answer to this question is simply in the UNO’s model designation “Uno 75”!
Carb X1/9 had 85bhp.
It was simply a cost and emissions compromise for the Uno price point.
 
The original purpose of the SPI systems were easy progression to FI from carburators, lower costs, improved emissions performance and reduced service requirements (improved reliability).

GM used its "throttle body injection" systems for years with good success. I had a van with a TBI system on it. It was quite reliable if not a particularly capable performer. It did require regular doses of fuel injection cleaner to keep power levels and fuel economy up to par.
 
I saw the throttle body injection system on a buddy's Dodge Omni. After looking at all the hoses and wires going to it I wanted to run away screaming. But then the last years of carbs were pretty hideous too.
 
Although it would seem that MPI is a more elegant solution, I would expect that SPI might have the same advantage that carbs do over MPI in terms of supplying a cooler charge and creating more power.
 
carl said:
Carbs have disadvantages?
Would you rather maintain a Fiat with a 40 year old carb or a 40 year old FI system?

Just yanking chains as I usually do.
Click to expand...
Chain yanked; I'll let you know in two year's time, but I know I much prefer maintaining a 38 year old FI system over a 38 year old carburator...
 
dllubin said:
Although it would seem that MPI is a more elegant solution, I would expect that SPI might have the same advantage that carbs do over MPI in terms of supplying a cooler charge and creating more power.
Click to expand...
As far as delivering fuel, architechually a throttle body type SPI system works exactly like a carburator with a fuel injector replacing the carburator's venturi. In theory, if not in practice, the fuel delivered by the SPI injector is better atomized and more precisely metered.

Intake architecture is by far the biggest factor in making power. Intake runner size, lenght and taper all are factors. In theory the SPI system uses fuel to cool the intake charge in the manifold where the typical production type MPI system injects the fuel directly into the intake port and therefore doesn't cool the charge as much. However, the trade off there is the fuel is better atomized and you get a more consistent mixture at the intake valve.

The most sophisticated MPI type systems mount the injectors above the throttle plate or actually in the center of the velocity stack. It seems that doing so provides the best atomization and and therefore fuel signal in the combustion chamber. While this makes better power than any OE type system it is far from practical for production.
 
It also depends on if it is batch fire or not. Many multi port systems still fire all injectors at the same time. Many times the difference in power is very small between different type systems. Some gain fuel economy or better emissions.
 
A few episodes back on Enginemasters, they actually took a look at the carb vs. mpi question and confirmed the charge temperature advantage of the carb which should also apply to spi.

There was a recent thread here showing an mpi ITB race engine with the injectors above the throttle bodies, presumably to better cool the charge.
 
Steve Hoelscher said:
As far as delivering fuel, architechually a throttle body type SPI system works exactly like a carburator with a fuel injector replacing the carburator's venturi. In theory, if not in practice, the fuel delivered by the SPI injector is better atomized and more precisely metered.

Intake architecture is by far the biggest factor in making power. Intake runner size, lenght and taper all are factors. In theory the SPI system uses fuel to cool the intake charge in the manifold where the typical production type MPI system injects the fuel directly into the intake port and therefore doesn't cool the charge as much. However, the trade off there is the fuel is better atomized and you get a more consistent mixture at the intake valve.

The most sophisticated MPI type systems mount the injectors above the throttle plate or actually in the center of the velocity stack. It seems that doing so provides the best atomization and and therefore fuel signal in the combustion chamber. While this makes better power than any OE type system it is far from practical for production.
Click to expand...
Agreed entirely.


Additionally, the more port/wall area there is between the injector and the cylinder, the more transient fueling correction you'll need, especially if that area is made up of a log style manifold with many bends and corners. In a SPI system, the amount of tip-in enrichment needed is monumental, and it's much the same for tip-out enleanment...likely why MPI was more favorable from an emissions standpoint (due to SPIs NOx emissions on tip in, and dumping unburned fuel on tip out) far before MPI was even sequential and actually provided measurable benefit.
 
Stuartc said:
I think the answer to this question is simply in the UNO’s model designation “Uno 75”!
Carb X1/9 had 85bhp.
It was simply a cost and emissions compromise for the Uno price point.
Click to expand...
Right, but here we are talking MPI v s SPI and both variants of the Uno had 75HP. I don't think the extra 10 HP came from the fuel system in the case of the Euro X1/9 but rather the cam and exhaust (as Steve has pointed out a few times). In fact, I'm pretty sure that if you swapped a 34 DMTR for the SPI system above on a Euro 1500 carb car you would still have 85 horses.

Disclaimer: I'm currently running a 36 DCNF ;)
 
Stuartc said:
I think the answer to this question is simply in the UNO’s model designation “Uno 75”!
Carb X1/9 had 85bhp.
It was simply a cost and emissions compromise for the Uno price point.
Click to expand...
Actually the SPI must give a little more power if anything.

Here in Australia we had 2 model years of Regata 85s, 1985 was carb and called an 85S, 1986 was SPI and cat conv and called an 85Si.e., both were rated at 85hp, and the SPI engine is actually a lower compression engine (it has slight dish to the piston top compared to flat top for the carb model)

So it made the same power with less static compression and a cat conv when fitted with SPI

Uno also came with an 1300cc engine in the uno75 ... it was never offered with a 1500cc engine

SteveC
 
The point I was trying to make, not disagreeing with what’s been stated is that the SPI was more a function of emissions than power and like a lot of Fiat engine/models the power output was one of marketing rather than engineering.

I never knew the Regata 85sie existed only the Regata 100 S ie🤔 interesting! Not seen one do you have a pic of the engine Steve?

Never really had any issue with Carburettors! My Bertone starts and runs fine even after weeks of laying idle!
I think the US spec cars are more problematic because of the extra plumbing and emissions strangling.
 
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