RotaryGeek

ok so a friend is wanting to turbo his cutlass since its no longer imca legal since the new tires (hoosiers) are too expensive for him to buy all he needs to run. he has extra manifolds and everything already flipped around with the flanges for the turbos welded on. he has the turbos and the charge piping and the intercooler from a old crx setup and most of it fits pretty good in mock up. its a .60 over 383 with dart heads and full roller valvetrain (which is why he cant afford new tires right now). demon carb. and powerglide 2 speed. now he is under the impression that a carb acts just like a fmu would under boost and he really doesnt need anything else to turbo it. he has everything to twin turbo it ( ching chong t3's and old johny racecar intercooler plus miscellaneous charge piping.) i remember somewhere on here a long time ago where we already had this descussion but i was thinking that he will need some kind of added fuel. cuz thats an expensive *** motor to be ------- up while he is waiting to buy the new tires. he plans on using this for 8th mile tracks locally with stock tires. rear end is a ford 9 inch dont remember what gears are in it but they are good for dirt track racing. he just wants to bolt it on and go cuz the whole turbo kit is free and he wants to drive. can someone point me in the right direction for this? oh the carb i beleive is a 250 single pumper per regulation imca. i have a 750 double pumper at my house he might use.

Smith-02

block the power jet, add 10-12 to the jets, hell, i'd knock the primary jet 10, maybe 12, and go like 11-15 on the secondary. better to run rich, save the motor, than scatter it, detonate it. vac advance, under boost, will retard. mechanical however, will not. make a box, put the carb in it, and run. small leaks from throttle cable won't really hurt anything.

can also tap the top of the fuel pump, if mechanical, with a fitting, plug boost into the top.

RotaryGeek

ok so im not really carb literate so can you put tha in lamens terms for me? but it is ok if he rejets it right?

Smith-02

rejetting, and blocking the power valve, will take care of the carb. you still need to address boost referencing, and 1:1 rising fuel pressure, carbs work on hvlp, (high vol, low press) not hvnp (high vol, negative press), keeping like, 4psi (good # for holleys) ABOVE intake pressure, is what you need to do.

carbs are not like fuel injectors, they are only a means to draw air into the engine at a somewhat metered rate. the jets are like fuel injectors, the shape of the carb inside (venturi) is what draws fuel out. jets just restrict fuel from dumping at gallons per minute into the intake.

power valve delete = you need to up 10-12 jets depending on engine, if its somewhat rich, oh well, they usually are. if you have #72jets for both, that would mean you need to go to around a #82-84 primary, #82-84 secondary. you really dont boost at lower rpm's, so you can just jet it 10 over whats in there (since there is no powervalve to add fuel in higher throttle applications), or jet like 6 over. secondaries will let air dump in enough for boost, you can go up to like a #84, or even upwards of 90, depending on how much boost you run.

either way, use a ------- wideband, or at least, a narrowband to see if you're even remotely close to .47v

or read plugs at throttle ranges.

carbs are great IF you spend the time AND you have patience to set em up right. most carbs are over-jetted, and usually too big for most engines people put them on.

lean running with 300 some natural hp won't slow you down, nor will richness in boost.

RotaryGeek

ok well the motor makes a estimated 500 to 600 hp n/a as it is and it high compression. only low boost here, maybe 4 or 5 pounds. so if i understand you right, i should just be able to rejet the carb and use a wideband to test how rich or lean he is then adjust accordingly? will the small *** carb be a problem? i dont remember exactly what it is but i know its small. would rejetting it be the same as just using my larger carb? or am i still missing something?

sohcpwr

use your carb, as not to ---- up the one he races with. A rising rate regulator is the best way to go. If you dont put the entire carb in the box, you can make a carb hat with a few more modifications to the carb itself. i tried finding some web page i remember reading about boosting carbs- something like hangar18 fabrication

Smith-02

yes to wideband, small carb wont have the airflow

RotaryGeek

since this carb is a little moe than double the size of the one he is running will i even need to rejet it? im kinda nervous about tearing it apart becuase i have never done it and dont want to ------- it up. everybody i know who knows carbs say its pretty easy but there is some delicate stuff in there and i dont exactly have lady fingers lol

Smith-02

whatever jets are in there, should be pretty close if it's a 750, you can always drill jets out.

holley carbs, to play with jets. you will need: 1/2" wide, thin, GOOD edged profile flatbit. hit it with a file if you have to.

whatever tool to remove fuel line

take line off. take out 4 screws holding float on. enjoy all the gas that dumps out.

set screws, float, and float bowl aside.

remove jets with 1/2" flatbit, replace left with left, right with right. jets may be marked wrong, yet similar, and be different sizes. 60 and 060 are supposedly different, dont mix em up, even though they seem the same.

RotaryGeek

ok ill give it a shot with the holley 750 and if i dont ------- it up ill hopefully have some pics to post of it sometime soon.