Blowthrough turbocharging a carbureted Fiat twin cam.

[Scorpion77]

Hello Mr.Croft,
I'm a huge fan of your very informative and huge site. I'm writing to ask your opinion on the turbo system I am currently putting together for my 77 Lancia Scorpion. As you know, these cars need all the help they can get to go as fast as they look. I also have a stock 1981 Fiat 124 Spider, Texas car...mint condition, I'm leaving that one alone. Anyway, about the Scorpion, I've been flirting with doing a blow-through design similar to that of an early Lotus Esprit Turbo system. I have a Mitsubishi TD-04 turbo from a Dodge Conquest I used to have. I like this turbo because it spools amazingly fast, has its own adjustable wastegate and can make anywhere from 5 to 20 psi boost. It is also is of relatively small size and is capable of oil and water cooling to the center section. The Scorpion has a unique air filter system that has a gasketed circular container over the carburetor with a tube welded to it. It seems almost to easy to me to build a exhaust manifold with mounting flange (already done) slap a K&N at the front of the turbo, install coolant and oil lines, fabricate PVC piping from the turbo to the carb cover and fabricate a new exhaust right out the back. As you know, the car is mid-engine and has a ton of room in the engine compartment. The engine is your standard twin cam 1.8 Fiat design with turbo friendly 8 to 1 compression. Now for the I don't knows...I only want to run around 7 psi as I understand, barring detonation problems, the internals should handle that. However, do you think that the carb can handle the pressure? (Crushed float bowls, general leakage) A blow off valve should handle any back pressure problems. Of course the carb has been rebuilt, in fact, I have two of them. My other question is air/fuel ratio. Do you have any suggestions on how to make that work, (rising regulator) or just tune the carb rich and suffer a bad idle condition but get enough gas under boost. Of course, all functions will be monitored via the usual stuff, A/F gauge, boost gauge etc. Before I go any farther I'd figure I'd consult someone with plenty of experience. If Lotus did it, it has to be doable right? Please give me any advice (apart from going with fuel injection) or thoughts you might have on the subject. Any help would be greatly appreciated. Thanks much,
Best regards,
Gene Carlone
Cleveland, Ohio
USA

[Guy Croft]

Gene, hi

I am reasonably confident that at low boost of 7psi max the production pistons will hold up, although the US spec n/a motor doesn't have the thicker crown and heavier top ring lands that a turbo motor should, thus - to avoid any creep towards detonation - you'll have to be careful on inlet charge air air temp, (40 deg C max, you must intercool), engine temp (75 deg C- 80 deg C max) fuel octane (97RON min, no idea what that is in the USA, sorry), jetting (esp avoiding lean condition) ignition timing (needs boost retard). A stronger turbo-type head gasket and new head bolts are vital, I can't see the old gasket lasting long.

First main point reagarding your well-written post is that you cannot blow thru the standard carb, you need a turbo-type carb with a sealed float chamber - the std unit is open to atmosphere. All Dellorto sidedraft carbs are sealed, some Weber DCOE models and the rare carb from the Lancia Delta Turbo 1600 TC - if you can find one.

You need to find someone who offers add-on boost retard - this manifold pressure sensing black box retards the ignition by a preset level (I suggest 8 deg) at high manifold pressure and produces a mirror curve to the MC standard one at a lower level.

That's what you need to find first, the rest is straighforward to deal with.

GC

[Kev Rooney]

I am currently working on a similar project which should be up and running within the next couple of weeks. It's a SOHC Ford 'Pinto ' unit fitted with a synchronised Weber , 38DGAS.

I have read that standard Weber carb seals ,with all in good condition, will take 5 psi boost. I've rebuilt mine and have also added 'air seals ' to the throttle spindles by redirecting boost from the inlet manifold to tubes let into the carb body.I have also fitted the black plastic fuel floats which won't collapse under moderate boost unlike brass units. For higher levels of boost the floats can be drilled out, foam filled and then resealed.

With a well sealed carb 'hat' the open fuel chamber shouldn't be a problem as the fuel pressure incoming will always be matched to the boost on the chamber itself ( see below ) . I will confirm that is so once it's running.

As a safety precaution it is wise to 2 pack epoxy seal all blind drillings on the carb that are nornally sealed with lead balls.

The other thing needed is a petrol pump capable of supplying standard presuure with boost pressure added on top. You can boost reference a manual pump or , as I've done, add a high pressure electical pump with a Malpassi turbo regulator which can be set to standard supply and then automatically allows the rate to rise in line with boost.

Unfortunately that's the best I can offer until I'm up and running as that's when the real world experience will come into play.

[Kev Rooney]

I should also add that I'd highly recommend the book Turbomania ,by Bob Tomlinson, available from CB Performance in America . Whilst aimed primarily at VW aircooled engines it covers many aspects of blow through turbocharging and also dispels a lot of myths.

One of my friends builds performance aircooled VW engines using their components and tech back up. His best 2.1 unit to date produced 385 rwbhp on a blow through set up.

[Scorpion77]

Thanks to GC and Kev for the info,
I am in my third week of rehab for a rotator cuff surgery, so right now I'm kind of playing the sponge and getting as much information and theory as possible so I know what I can use, such as carburetors and manifolds and what kind of boost is realistic. I once put a 79 Buick V6 turbo engine in a Triumph TR7. I kept the chassis manual and have been perusing how that system worked. (Now that I have lots of time to read) The turbocharger bypassed the carburetor and went right into the intake manifold. I wonder if this method combined with a customized manifold might be a better way to go. This could take the carb damage problem out of the equation. This method did work in conjunction with a black box similar to that mentioned by Guy earlier that adjusted timing in accordance with boost. I wonder if any of these parts could be used in my or Kev's application. I have read about guys who did use the standard Weber carb with low boost and got away with it. Guy also brought up a good point about head gaskets. The Buick had lots of problems with those. Guy states that we should intercool which will be difficult for me because of my engine configuration, but not impossible of course. Going back over the Lotus system again, I found that the first couple years the Turbo Esprit was built, it wasn't intercooled. I also heard that these engines earned the nickname of "grenade" probably for obvious reasons. Once again, engine configuration does not lend itself to sidedraft carburetor installation on my car so I will be interested in seeing how your carburetor mods hold up. The fuel pump mod is already in progress as I had fuel delivery problems when raising the boost in my 86 Conquest, but that car was injected and a completely different story, but it did teach me that the engine must have more fuel at boost and of course the more octane the better. Should be able to resume work on mine in about three weeks. Hopefully by then I'll have a plan on paper, know which parts to get, and like Guy said, it'll all be straight forward from there. Besides all the blown up parts and lessons learned along the way, but thats what makes it all fun right? Good luck with the Pinto and please keep me posted on your progress. Once again, thanks to Kev and GC as any and all help is always appreciated.

[Guy Croft]

MODEL THREAD!

Well done indeed, Gene, Kev

GC

[Scorpion77]

More info for any and all concerned,
Carburation and turbos work just as well as fuel injection and turbos. Don't anybody tell you different.
The trick is to balance out everything. First of all you need a fuel pump capable of making at least more fuel pressure than 3 psi + boost. Webers don't like more than 3 PSI. So at 7 lbs of boost you would need at least 10 PSI of fuel pressure. That way your not forcing fuel backwards under boost.
So far the only pump I've been able to find that will give me such a pressure is from Advance Auto parts. I don't have the part number in front of me but I think I still have the box. This pump makes 20 psi dead head pressure and flows around 130 gallons per hour. Best I can recall it's for a GM throttle body/ Diesel pusher pump. Make them get their book out and look it up if I can't find the box. The one I tried from AutoZone had too much flow and the regulator couldn't handle the flow so the fuel pressure would spike under boost.
You'll need a fuel pressure regulator for a carburetor (1:1) ratio.
Mine came from a Maserati bi-turbo but if you'll do some digging you can find these in the United States from places that specializes in air cooled VWs. It's made by Malpassi.
Next trick is to run a hose from the carburetor bonnet to the fuel bowl. This equalizes pressure across the jets and tricks the carb into thinking it's still operating under atmospheric pressure.
The jetting: The carb I'm using DFTA is similar to the one (DMTR) used on the Beta/Scorpion so the DMTR is OK to use. The only reason I didn't use a DMTR is because it had some issue where I couldn't adjust the idle mixture and it has no throttle return mechanism. It's basically the same carburetor.
In fact I pulled the jets/emulsion tubes and air correctors from a DMTR because it works well on my setup. Probably because I estimate I'm making about the same HP as a stock 2 liter Beta motor.
Really you don't need to mess with the primary mains, just the secondary main because the turbo won't really kick in until the secondary is open. I did have the mess around with the idle jets because it seemed the engine ran too rich under idle. Most likely because I disconnected all the smog equipment so the engine ran a bit rich under cruise and idle.
As far as the float is concerned. The DFTA uses a plastic float which is ideal. From what I've read a brass float will collapse under pressure. I can't remember what the DMTR uses. But you could call you Pierce manifolds and see if they have any plastic floats. If anything you could use a float from a DFTA which is also used on the Fiesta and Escort. I think it's the same float.
One problem I have right now is I had to retard the static timing on my Yugo by 5 degrees because at 5K rpms the engine pings. The distributor has too much advance.
I got an idea to see if I can mount a second magnetic pickup in the distributor about 18 degrees off from the main pickup. And use two GM HEI control modules and toggle between the two with a pressure switch. If possible. That way I can get a little bit more fuel mileage under cruise. I get between 17 and 25 mpg on my Yugo. But I've went from a 1100 cc engine to a 1500 cc engine and doubled the cars HP so I think that's pressure good. I think the original 1100cc Yugos got 31 on the highway.
On my Spider I'm using a Jacobs boostmaster. But they no longer make it! So I can run 17 degrees BTDC under idle/cruise/off boost and the control unit will retard the timing about 10 degrees under boost. I found that SOHC don't mind some static retard but DOHC run like inappropriate under cruise with the static timing retarded. Then run better with the static timing advanced about 7 degrees.
MSD and Crane makes the next best thing if you want to contend with having a big box.
Don't worry too much about carb leakage. Mine leaks a little bit and this is not much of an issue as far as fires are concerned. I haven't figured out how to seal the throttle shafts. On a DOHC it's even less of an issue because the exhaust manifold is on the opposite side. Any fuel leakage is minimal. It's not like it's going to be spraying fuel all over the engine compartment.
I'd also recommend a blow off valve to prevent the fuel pressure spikes between shifts. This will prevent blowing the needle off the seat between shifts.