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Hello everyone, I know this is not a VW related post, but it's still an aircooled engine and I hope some of you might enjoy my project and give me some suggestions too.

It all starts from the little Fiat 500 (the one from the 50's-70's), I have a nice one that is now powered by a very good 750cc engine,those engine are basically a parallel twin that is kind of similar to the aircooled bug.

I'm running an EFI system and I have to say i'm very satisfied by the results: the car is quite reliable (it's basically a daily) and I've been driving this configuration for about 6000 miles with basically no issues.

The current NA has those spec:

82,5mm forged pistons / compression height 29mm (stock is 67.4mm/39mm CH) they are made for this kind of engines by CPS, an italian company.
130mm H rods (stock are 118mm).
10,2:1 CR with 35-30 valves
Custom camshaft, 7,2mm lift, 54-86 86-54, lsa 106°
Stronger crankshaft from a 650-700cc version of this engine (same stroke as original, 70mm) it weight about 15% more.
lighter flywheel
Oil jets for under the pistons
Entire rotating mass dinamuìically balanced the engine is incredibly smooth especially if you consider that it's a parallel twin.
Full race exhoust, 38mm tubes (with DBkillers...)
EFI system, 38mm TB (the head is a single port design.

The angine is extremely nice to drive, good pull from the bottom and it pulls hard to the redline (currently set to 7000rpm, but above 6000 is almost pointless to keep going (without DBkiller I get an extra 500rpm of pull and better performances, but too much noise).

As I said, I'm extremely happy about this engine especially because other than performing well, it does good 365 days per year.

Now the next challenge: building a new turbocharged engine.

I choose a turbo from the feat 1.3L multijet engine, it's a small turbo (KKK KP35 series) used on the little 1.3 liters turbodiesel engine, that produces about 75hp stock with a good torque and provides on that engine about 1 bar (14psi) of boost.

Since the biggest challenge will be archeive a good reliability from an engine that was born as 499cc with 18hp, for the turbo version I decided further upgrades:

Engine crankcase from the Fiat "141! series (still a parallel twin but with much more "meat" in order to be stronger and more rigid.

Stronger crankshaft from the lastest version of the legendary "fiat parallel twin" (a 700cc watercooled engine from the 1990's). the crank itself has the same stroke as the original but it's way more beefy, weighting about 25% more.

I will still use Hbeam rods (probably 130mm), but about the pistons I have some doubts.

I'm struggling to find turbo pistons, and the only option for specific "aircooled turbo pistons" is to ask CPS to make a custom set for me, but they need to make a batch of at least 4 pistons. Additionally, those forged pistons requires quite a big clearences (on the NA version i'm in the 0,0055inch for the 82.5mm pistons I currently have..) and especially at cold they slap badly. I have to say it's the only thing that I dont like of my current engine.

I did some researh for turbo pistons in the 82mm range with 20mm pin and a compression height of about 28-30mm, and I come out with those from MAHLE. they are used on modern 2000cc 4 cyl DFI turbo engines that deliver from 170 to 240hp stock.
They have a steel ring on the first compression ring and they also have a dish to help me to keeo the static compression low (I'll aim to 7:1-7.5:1 maximum).

https://www.cs-parts.de/Kolben-8251mm-MAHLE-0332100 here a link just to show you the item, pn is "MAHLE 0332100"

It's a cast piston, but I think it's better than many forged piston around, and maybe being cast it will require less clearence , so hopefully less piston slap. (it should be also hyperheutetic).

What do you think about this type of piston? on the VW engine is installed with about 0,002inch of clearence, but this is with a watercooled engine, on an aircooled engine I would give more gap anyway... maybe in the 0,003-0,0035 range?

Do you have any knowledge of watercooled pistons used on aircooled engines with success?

Below some photos of the header fabrication..

I don't see why the pistons you are running wouldn't work with a turbo. I've been running AA Hyperteutic cast pistons for 10 years now and they hold up fine to 23 lbs boost ......as long as you don't have any detonation.
Detonation will kill any piston. The "Good"??? thing about cast hyper pistons is they do have some tolerance for detonation in that they won't just blow to kingdom come the first time.
They will hold up to quite a bit of abuse but they develop hairline cracks that weaken then and cause excessive blow-by. Blow-by is your warning......once it gets bad enough to see the steam coming out of the crankcase vent....or oil exiting from the crankcase...(on a VW case it's the pulley side) that is your clue to swap them out before it destroys the engine.

The key to a good turbo system is being able to manage the fuel, IAT's and ignition advance. Being able to adjust advance/retard for any driving scenario is the recipe for a long lasting engine.
I've found 130* F. to be the limit for a turboed engine to tolerate before detonation will set in without aggressive ignition retard.

It does depend on how you tune your engine.....keeping ignition advance on the edge for max power leaves very little room for safety. Being conservative on advance leaves power on the table.
I feel W/I is a great value for the safety and performance you gain. Having running it for 50,000 miles it's been a great system for taking 100* F. out of the boosted air temps. The small intercooler I added a few years later takes another 40 plus degrees off the IAT's for almost near ambient temps at boost.
It is especially effective in fall, winter and spring when the ambient temps are a little lower. I've seen as low as 87* F. at 26.5 lbs boost in 40 degree ambient temps.

wow that's very intresting, you mentioned 130°F iat right? it's about 55 celsius and yes, I have been thinking about something like that.

On my system I can set quite a lot of corrections, so i'll make a 3d ignition map to provide the correct timing based upon RPM and load (boost).

I'm also setting an intercooler system that should work well, it has a decent sized core (it's one of the intercoolers of the modern 695 Abarth, it had 180HP so I assoume only one of the stock intercoolers should be ok up to about 90hp...).

I will be heavely limited with boost anyway, by construction the engine has a weak crackcase/crank, and the gearbox is the next weak point too, so I think I will be the happiest man on earth if I will be able to get 70-75 real horsepower (it0s just shy of 100hp/liter).

I think in my case I will use 7-10psi, (about .5-.7bar). because those engines have limited capability to dissipate heat. I have a big finned oilpan anyway to make the crankcase more rigid and it increases the oil capacity to 2,5 liters to 6 liters. On the current NA 750cc engine I have never had oil temp issues.

here some pic of the NA 740cc build

On the 130* F. issue. That is where you will definitely see some detonation if timing isn't retarded......at least in my case where my normal cruise IAT's are between a low of 70* F. to 100* F. depending on the time of year.
Remember that a turbo will increase IAT's about 20* F. just by going through it at cruise not including boost. Mine is pretty consistent at a 20* F. rise over most any ambient temp in all four seasons.
With that said....summer is the most critical and I have my ECU retard timing anytime the IAT's rise above 100* F. no matter whether it's cruise or boost.
I live in the Pacific Northwest so most of my tune is set for lower temps than what I see when traveling to California in my buggy.
I ran into detonation issues when the ambient temps went over 106 degrees and my cruise IAT's would be 130* F. with no boost.
All of this is based on 50,000 miles of driving in local and long distance experiences with a turbo.
You will need to set your engine up to handle the ambient and boost temps you see in real life driving.

yes it make sense....

About the IAT timing corrrections, here some basic numbers I already have in my map....
keep in mind those are Celsius and not FArenheit, and also "coolant" is the temperature of the cooling air right after the head (it react quite well I have to admit).

On the NA engine I have never seen IAT higher than 100-105°F (37-40°C) dwhen moving, of course when I'm stuck in traffic the IAT goes up t(with some effort I have seen up to 120-130°F in the most extreme conditions/worst case scenario..) but as soon as I move again the IAT went back to the normal 100°F range. you can see from the photo that when the hood is closed the airfilter can only breathe hot air..

And what about timing Vs boost? I have the possibility to set a chart with RPM/MAP, and I can fully customize the values, so I was thinking about creating a linear rule "something like 1 degrees of retard every PSI of boost" What values you suggest me as starting point?

It's really difficult to recommend advance/retard values to any other engine than what i myself run. You need to give your engine what it wants. My advice could easily be incorrect and cause damage/issues if I base your numbers off my engine which is only the same in that they are both aircooled.
I advise running a good muffler to keep the exhaust quiet enough that you can instantly hear detonation and be able to lift the throttle as soon as you detect it.
Your boost temps are affected by the ambient temps...here is a great calculator to figure out what temps you might see in your ambient/boost conditions.

https://rbracing-rsr.com/intercoolers.htm

Just remember that you need to base your non boost temps off your IAT sensor after the turbo but before any cooling is added.
I see cruise temps between 80-100 degrees normally.....If I drive in cold weather I might see 60-70 degrees but I'm getting too old for that crap. I will drive to work until it gets below 40 degrees but I'm not paying attention to my temps much...just getting to work without freezing is usually the goal.
I do have my digital dash this season so I'm gathering more data as time goes on. I haven't logged on the computer much lately because it drives so nice right now.
I do have the ECU set to auto log for 10 minutes any time it sees more that 100 kpa but I rarely check the logs in the summer because I don't boost the crap out of it due to higher ambient temps and not wanting to hurt the engine.

I run a dual ignition and fuel map switched at >100 kpa and have since about 2014 when I switched to Ver. 3.0 MS2 I continued it when I upgraded to MS3Pro-Evo.
Most all my boost retard is built into the ignition map and only if the IAT's go over 100 degrees does it retard timing.
I rarely see intake temps boosted or N/A over 105* F. unless it's really hot here or I'm in California.
My W/I and intercooler keep temps in check even up to 26.5 lbs of boost.

The mid range high load areas are most sensitive to boost depending on when your turbo makes max boost. I have seen 23.0 lbs boost at 3800 rpm and my advance map in that area drops significantly to compensate for that.....but my high rpm, high boost areas are up to 25 degrees advance.

Thanks for the link, I remember that site, I already checked some stuff there, nice info

Let's see how it goes, meanwhile I think i'll buy one of those MAHLE piston and i'll have the machine shop measure them in order to check the dimensione and other tolerances.

Knowing how delicate are the Aircooled FIAT engines, it's really amazing that you're able to pull so much boost on the VW engine, hat off to Mr. Porsche

At this point I hope I will be able to use those MAHLE 0332100 pistons, but worst worst case I'll get custom made forged piston, but as I said, I hope I will be able to use the german cast pistons with a reasonable clearence in order to avoid piston slap but also any blowby/oil consumption.

here some updates, primary headers are almost done, i'm happy about the outcome especially becaise I didnt tig weld for like 15 years, I just bought a smaller machine with the purpose of doing those works.

Turned out excellent! Great work so far

Sent from my SM-S918U using Tapatalk

Thanks a lot!!!

Anyway, the project it's a GO! I'm just back from the machine shop (Tani e Giannini in Rome, yes, that Giannini that set several record back in the days and even produced their own version of several cars..) ane here the lastest updates:

The head will have 36-31mm nimonic valves, compression will be about 7.5:1

82mm barrels with custom forged piston for turbo use made by CPS in italy (they are quite famous around here..) we decided not to go the safe route and get a reliable product. 740cc total.

130mm H beam rod

I'll hkeep you uodated.

Cool build! Nice work.

Thanks doc!!

Today I went a bit ahead with some stuff: i'm plannning to use an air-air intercooler placed on top of the cooling ducts, I will bleed some of the air thru some holes and force the cooling air thru the intercooler. Of course this is bad because I will loose some precious cooling air, but I will do some mods to mitigate this issue:

- a very good friend gave me a sed of smaller dynamo pulleys, they give about 15% more RPM, so at the same RPM the dinamo-cooling fan will spin more. with the stock ratio the dynamo-cooling fan is good up to 7000-7200 engine RPM (the stock ratio already make the fan spin more than crankshaft...) The turbo engine will have the rev limiter set around 6500, so I will be in the same size anyway (I hope).

-I will place a baffle around the cylinders (similar to the newer aircooled bug..) that was introduced to the latest version of the Fiat aircooled twin, this will help to cool better the cylinders and allow more air thru the head (those engine are famous for lack of head cooling..)

-additionally, I will make different sets of cover plates in oeder to bleed the correct amount of air to the intercooler: enough to keep the IAT low but als enough to cool the engine properly.

Even my current NA engine, was born with the carburator breathing from the stock airbox, this airbox used air from the cooling system so at full speed the engine was loosing some cooling air anyway, but I never had temp issues to be honest, not even is summer driving full speed.

The intercooler tray-baffle-support (or whatever is called…) is almost done…. I have to say I'm pleased with the results and trust me if I say that it looks better from real, the photos are not great.

The packaging is super tight but I should be able to keep it under the hood for a perfect sleeper.

Only when everything is finished and adjusted-set I will powdercoat everything.

In the beginning I will close about 50% of the holes with some high speed tape and I will monitor the IAT’s. (the intercooler tray is movable so it will be a quick process in case I want to block or open more the vent..) CHT will be monitored too so I shouldn't have bad surprises hopefully.

My friend Giuseppe sent me some pic of the crankcase getting prepared to accept the 82mm barrels and reinforced,

After some welding to create more meat before machining (normal for those engines...) he squared all the surfaces within 0,01mm (0.0004inch).

So fat i'm very happy with the outcome. the engine block will be brushed to create a semi-shiny finish. in the attachment you will see how my actual 740 looked during the assembly. i just spent some time with wd40, steel brush and lot of elbow grease.