Learn me draw through turbo systems: Fiat 850 turbo project

So in my continuing effort to find a way to blow up my new project, I found a turbo manifold for the car that means that is the direction I will be headed for making the little car faster. These things have a 903cc push rod 4 cylinder water cooled engine of all of 52 HP and really need another 50, but the only options are unobtanium Abarth parts, or mega expensive engine cylinder heads, cranks, etc... so forced induction seems to be the easier path.

So picked up this today, that gets me the manifold and down pipe and I'll likely get the Garrett T25 turbo that matches it. So the next steps are fuel and ignition. Ignition is relatively easy as a 123distributor will let me do a custom timing curve. So fuel, want to keep it simple, so a draw thru carb off the turbo would seem to be the easy button. I have seen where an SU carb is used, but would really like to use a weber since it is a Fiat. 

So what does the hive think?

Random thoughts:  Turbos are fun.  I assume you have a wideband AFR?  Don't go lean.  Be sure your ignition system has the goods.  Weak spark and boost are not a winning combination.

Draw through is a good quick way to do it.  Everyone likes to call them bombs, though.  Safety blanket around it?  Backfire prevention methods?

dherr (Forum Supporter) said:

So in my continuing effort to find a way to blow up my new project, I found a turbo manifold for the car that means that is the direction I will be headed for making the little car faster. These things have a 903cc push rod 4 cylinder water cooled engine of all of 52 HP and really need another 50, but the only options are unobtanium Abarth parts, or mega expensive engine cylinder heads, cranks, etc... so forced induction seems to be the easier path.

So picked up this today, that gets me the manifold and down pipe and I'll likely get the Garrett T25 turbo that matches it. So the next steps are fuel and ignition. Ignition is relatively easy as a 123distributor will let me do a custom timing curve. So fuel, want to keep it simple, so a draw thru carb off the turbo would seem to be the easy button. I have seen where an SU carb is used, but would really like to use a weber since it is a Fiat. 

So what does the hive think?

I'm not sure that the bolded statement is entirely accurate.  Just hear me out: While I do love me some original old school goodness, for the amount of time/effort/money it would take to make a homegrown turbo kit and get it running and driving well, I would also suggest the possibility that you could do an engine swap from nearly any modern fwd economy car and have something that is just as powerful and also more reliable, more efficient, cheaper and vastly easier to get parts for, will last far longer than a boosted 903, etc etc.  Just throwing that out there.  There is no wrong answer here, and either way I will be watching and enjoying the build!  

I had a Weber draw through system with a Roto Master turbo on my 1974 Pinto 2 liter almost 50 years ago. It was a blast to drive and most people didn't know what it was. Instead of a waste gate it had a device that restricted the air flow on the intake to control overboost. I think this caused all kinds of compressor issues. 

Draw thru would be easy enough. SU is the easy button but no reason you can's use a DGAV on an adapter.

As for engine swaps. I owned an 850 Spider for a while. I don't think people appreciate how small this car is. Think new Mini vs a real MINI.

In reply to dherr (Forum Supporter) :

Does the 123 distributor have boost retard?  Looking at their website, they have vacuum advance, but for safe boost, you want boost retard.  

Can't really help with the carb part, though.  Should work.  And should be fun.

ShawnG said:

 

As for engine swaps. I owned an 850 Spider for a while. I don't think people appreciate how small this car is. Think new Mini vs a real MINI.

IMHO. it's the perfect "car" for EM or DM, since it's at the lowest possible wheelbase, and it's legal to have a rear engined car to be "converted" to mid engine.  80" wheelbase, tiny.   Find 80" BM chassis, put on glass 850 body, and go fast.

ACVW guys have been drawthrough at similar power levels for decades. I'd look into what they're doing.

In reply to JBinMD :

The 850 is very hard to do an engine swap on.  The very tiny engine is behind the transaxle, in the back of the car.  Finding a FWD powertrain small enough to fit there reasonably would be a challenge.  

@JBinMD and @ShawnG two main issues here. One is trying to keep it kind of vintage, so nothing modern would be my recipe for this build. There is a distinct lack of space here and due to the transmission design, the engine has to turn counter clockwise. So an engine swap means using another transmission (old school VW is the easiest solution) a Kennedy adapter and then an engine, etc.... Verse just adding some forced induction to the little 903 and enjoying it while it lasts. I can always build a stouter 903 engine but it is much harder to add much more displacement to these little motors. The engine is tiny and the available space is tiny, but lots of room on the exhaust side for the turbo.  I am thinking that a DVAV or similar two barrel downdraft carb might be an easy solution.

So the guy with the 850 manifold also has the matching turbo, it is an T25 Garrett turbo rebuilt by Don's Turbo Service in Riddle, OR. . T25 inlet flange GT25/28 outlet flange Markings: Compressor - M10 Turbine - M26 AIR .63 

He said the turbo shop said the specs would be a good match for the 850, so here we go down the rabbit hole. He is sending both the manifold, downpipe and turbo and oil lines next week. He sent a picture of the manifold mounted on an 850 engine, so it will fit just fine in the engine compartment.

I've done blow thru with cheap as dirt motorcycle CV carbs,turbobike.net(from memory) has loads of good info on setting it up.

Malpassi rising rate regulator and fuel bowls seeing boost signal via a pitot tube was easy peasy and worked great.

Big thing on draw through is it needs a different seal on the compressor to keep fuel out of the center section. Most modern turbos need to be modified for it 

I thought the seal wasn't so much to keep fuel out of the oil, but to keep manifold vacuum from sucking the oil out and into the engine.  Turbocharger seals aren't normally positive seals against pressure or vacuum, but labyrinth seals that use centrifugal force to make it very hard for oil to get past them in the first place.  When subjected to vacuum, this doesn't work well.

The most modern turbos I can think of that would have the correct type of seals would be from early Turbo Dodges, and from pre-1986 Turbo Buicks, as they were both draw through applications.  Both are Garrett T3 turbos but sized a bit differently.

Draw through has its advantages for simplicity as you don't need to use a fancy fuel pressure regulator that can increase fuel pressure to match increased air pressure in the bowls the way a blow through does, but at the expense of not being able to intercool, which is probably why it fell out of favor.  I've heard of people with good success using water injection to compensate, up to the point of using almost as much water as fuel in high boost applications.

Turbo I am using is a T25 that has been rebuilt, so not sure what seal it might have. I am not opposed to blowing thru the carb, just trying to keep it reasonably simple.

In reply to dherr (Forum Supporter) :

It may or may not have the right seal, but the other consideration is that the wastegate would need to be changed to one with two ports.  Normally, pressure from the turbo outlet goes to one side of the diaphragm and the other side is open to atmosphere.  While this will work, this can cause overspeeding under the wrong conditions, so you'd need a wastegate that compares pressures before and after the turbo.

This sounds like an expensive way to blow up the fiat motor.  If you only want 100 HP put an air cooled VW motor in.  That was a big thing in the old days.

Just keep the boost low. 4 or 5 lbs won't hurt it.

Nobody wants a Fiat that sounds like a Volkswagen

I agree, it will stay Fiat, no interest in a transmission or engine swap. I don't think it will be a problem with reasonable boost levels. Trent mentioned that these engines respond well to boost.  So carburation, a hot spark and a reasonable means to do boost retard on the timing and I should achieve my goals for this build.

Anyone who's owned one of these cars understands how poorly the intake is designed. Anything that helps that is a huge bonus.

If that distributor was used on cars in the smog era, there might be an advance / retard vacuum can available which would handle your spark advance-under-boost issues. If not, an MSD box might be the easy button.

With draw-thru, you won't have to modify the carburetor much if at all. Certainly not as much as blow-thru.

Pontiac and Buick used a specially ported Quadrajet and a valve on the intake manifold that sensed boost. The metering rod plunger in the primaries was connected to this valve with one side to atmosphere and one side to manifold vacuum. Under normal operation, it functioned like any other Q-jet but when manifold pressure was positive, it opened the port to the primary plunger and allowed the spring to push the metering rods to the full rich position.

In theory the carburetor -should- be fine as it's built to meter a given amount of fuel, corresponding to how much air is passing through. More air should mean more fuel but, from the draw-thru systems I've worked on, they always seem to have a way of adding a bit more, just to be safe. Being a little rich probably helps keep combustion temperatures down.

Just jetting it a bit rich may work or you may need to build some sort of an enrichment system. 

My buddy did this with is 1.0L Metro land speed car. He was using a T3, and if I recall, a 2 barrel Holley. The second barrel was jetted for boost. I set a record in the car, it was running 20psi at 9000 RPM. It worked well, the only problem was heat over the long 1 mile runs. I expect it would work very well as a daily, he said it had good manners on the street.

It looks like another option is to just use a Weber DGV style 2 barrel and as sealed plenum like the kit below and do a blow through system. With rising rate fuel pressure, this might be the easier option.

I turbocharged a 250 I6 in a Chevelle back in the dark ages. I used the turbo and Quadrajet carburetor off a 1981 Firebird. Timing was controlled by an MSD Boost Timing Master.

I only dumped 5 psi in the engine. It worked well enough to blow the clutch out of the car in short order. 

Sorry I have missed the fun of this thread. I'm currently in England.  

If draw through, I would say a DCOE is probably the most well traveled path. It is the carb of choice for VWs with an AMR500 supercharger. 

But a hat on a common DGV in blow through sounds best to me. Either way, you will be making your own manifolds.

I am astonished that you found a premade turbo manifold for an 850 somewhere.  The runner size seems a bit excessive for a 900cc motor but it should work.

I ran EFI on mine and ran 21psi. It was ludicrous