Crunchy on Sucking & Blow through +T setups

[JoKer]

if the picture or the text ever dissapears here's a copy Paste of probably what is already on these forums but thought was pretty relevant so I'm sharing here

Quote

RCIS Race Cars In Sheds (Facebook Public Group)

Steve Murch

 

Thought this might interest someone wanting to do DIY turbo/carb, wrote this sometime ago thought worth a share.

Its not a shameless plug for work i have more than enough at the moment i tell ya

To suck or blow im told im pretty good at both but this is about carby turbo charging something that has become a dying art and most know little about it which is a shame as its great to do and just as effective as injection without the cost.
Most think its limited to power levels but that’s far from the truth back in the 70’s and 80’s there was many carb kits made by people like SK japan popular for turbo Hondas making a lot of todays cars look sad on the strip.
So where to start this is a very lengthy topic for what seems to be a simple thing to do but don’t let that put you off the main limitation would be the carb that’s used ill attempt to cover as much as I can before RSI sets in.
Now before the skeptics jump in here and rattle off all the latest ecu’s on the market its well documented back in the 80’s when the Mexicans use to run low 8’s with a turbo 3t carb setup in a starlet and 280zx carbs in japan doing the same so if your one of these bashers please jump in if you’ve topped that……still waiting.
Ill cover this in a couple of parts so bare with me now where to start lets pick a Rota nice easy setup and very effective, you build the motor like any turbo motor whether it’s a piston or rota the main thing as I said is the carb the stock 12a’s used a nicky 4 barrel which is getting old and hard to find but still works great.
First advantage is you don’t need to move the carb from the stock position making throttle cable and fuel lines easy, the top hat to feed the carb needs to be around 200mm dia and 100mm thick with drw bolt in the center to hold it to the carb,I like to feed the boost pipe on a offset not dead center to prevent backfeed or a reflection.
By doing blow thru you can run a intercooler as per normal and piping etc remains the same as per EFI as per turbo choose and gate, the advantage of a 4 barrel carb or a 2 barrel mechanical secondary means you can jet the secondary for all the fuel you need under boost the primary for economy and everyday driving.
Fuel lines need to be slightly larger than EFI as we need volume over pressure so 3/8 feed and 3/8 return is needed the reason for the same size return is if we need more fuel we can run a restrictor in it after the fuel reg,as for pumps you can use a Holley red or blue I prefer a EFI pump but wait there too higher pressure?
Yes they are when using a EFI regulator but not on a carb reg do not use a Malpassi ive had motors fail with them you can use a EFI reg just change the spring remember higher pressure pumps normally increase in flow with lower pressure so job done, you need to keep the fuel pressure 2psi above boost all the time to prevent turbocharging the fuel tank.
One the most important parts of the system is plumping the wastegate/fuel reg/blow valve fuel and gate signals are feed from before the carb and the blow valve from under on the vacuum side if you don’t itll flood as soon as you backoff from under boost this also keeps the fuel pressure 2psi above boost all the time, jetting the carb is something you need to get right with dyno or a/fr meter also the needle and seat area must equal the same area as your jetting something many get wrong.
Sometimes on high fuel demands you just cant get enough fuel into the bowl to feed the getting so I remove the needle and seat drill the top of the float area and insert a tube to correct height then run a second pump to scavenge back to the tank this works faultlessly no more running out of fuel.
Now we understand some of the basic setup there’s some things you must look out for the worst is the carb types and leakage around the throttle shafts this largely depends on the carbs condition when new not so much.
Many years ago the carb was placed in a sealed box then blown into this made for a bulky installation and routing cables etc became an issue you can run a small 3mm hose to each throttle shaft from main boost supply this works well so does maching some seals into each end of the shaft.
Next is the floats some carbs come with brass floats which can take a reasonable amount of pressure the composite plastic ones don’t I have injected foam into them to add strength but not 100% reliable thus converting the carb to a second pump suction return fixes all issues.
When picking a carb youll find people like Holley and BG do blow thru carbs you need to be careful here if the carb runs a power valve it dumps too much fuel under boost so has to be removed you cant use a vacuum secondary carb as you have no control over secondary fuel is must have a mechanical setup.
Old school carbs like Webbers work very well like a 28/36 again mechanical secondary easy for jetting im sure theres others out there that can be used just check as per above or send me a pm im sure we can work it out, that pretty much covers the carb as for sizing whatever the carbs cfm rating is multiply it x2 for 14psi of boost then divide that by 1.67 will give you its hp rating to match whatever turbo your running.
Word of warning you MUST run a blow valve and set it light you can cause lots of issues on trailing throttle if you don’t and foul plugs boost pressure can rise way above set leaves when you slam the throttle closed it has to go some where and the carbs float bowl isn’t a good place.
As for jetting start rich and come back from there also buy a good afr meter start slow and you wont go wrong youll also be amazed how well itll run first time out.
This article will really suck hopefully in a good way what would have to be the oldest way to turbo a non turbo car and cheaply but it has its limitations.
The biggest issue today is the turbo its self most modern turbos run piston ring seals on the compressor end this is pretty important due to the high vacuum generated when placing a carb before the turbo inlet thus the turbo needs to use a positive seals known as a carbon seal.
Some turbos like old Garrett t3/t2 and tdo4 and early tdo5 ran a carbon seal however I have fitted carbon seals to newer type units by making a custom back plate to accommodate one.
Anyone remember the Monster Garage VW turbo ? was very cool but they got it wrong if you can find the video when they first tested it they never used a carbon seal turbo and when they backed off it was a white out of engine oil being sucked into the motor which isn’t so cool.
Not much else needs to be done to the turbo apart from the seal and making sure there’s enough intake spigot on the comp cover to allow clamping or bolting a carb to it, the actual turbo sizing remains the same as far as power goes many say flow a fluid threw the turbo intake erodes the wheel away this may be true but were talking a far amount of time to do and let’s be honest comps wheel aren’t that expensive anymore it’s the price we pay for power.
Now one thing you can’t do is run a intercooler as the fuel can puddle inside the core and risk a big bang if there was a backfire which isn’t ideal, it’s also not really needed as the fuel turns the intake pipe to the manifold to ice cold which is great well sort of. The fuel remains a solid which hurts performance and can create puddling in the comp cover creating a a flat spot on sudden throttle opening the way around it is to wrap the piping with heated piping taken from the engines water system.
Some 10mm pipe wrapped a around it or a sleeve over the pipe with the ends welded for 200mm or so does the job nicely the stock intake manifold works fine just a custom piping to bolt to it and a flexible joiner from the turbo is all that’s needed.
Ill do this in 2-3 parts as much as its simple compared to blow though there’s a lot more work than one may think stay tuned.
The most common carb used for suck through is the old SU which by all accounts works well but getting hard to get hold of pretty much any carb can be used a Holley 2/4 barrel works well and not over priced the main thing is getting enough fuel into it under boost.
Heres a trick ive done using a single side draft of whatever size you like to have good economy and power I cut the main throttle shaft in half level the throttle end as per normal now here's the trick, for extra fuel under boost I put a old vacuum advance actuator off a dizzy and connected it to the second shaft.
By doing this and connecting the actuator to boost only then jetting the main for the fuel needed you got low down power and the secondary opens under boost best of both worlds
For the fuel supply we no longer need pressure just volume so a holley pump does the job well youll still need a reg and 1-2psi pressure at idle is tons enough then run boost to the reg to compensate as per any efi setup remember to run good size fuel lines.
Ignition can be a dizzy again as per blow through you need to set the advance curve based on compression and fuel or electronic now one thing you cant do is run a Blow Valve if you do youll fill your engine bay with fuel when it opens anyway its not needed when a carb is mounted in front of a turbo is suppress surge, there's many other things that come into this setup but its also pretty easy to do any questions ask away hope you guys get some from all this.

 

 

 

[smokin'joe]

unsure about the Mexicans part, unless the Puerto Ricans copied them.

most of the guys on the 3TC forum that Karl Skewes had a hand in starting were either mainland 'Muricans or mad Puerto Ricans

[RUNAMUCK]

Twas a good read. Bit hard to read, but good all the same.