Blow Through Carb Turbo


Active Member
Feb 4, 2004

I tried searching but I really didn't come up with much that seemed to answer the questions I have. Never dug into the carb stuff either so I'm in need of help.

I'm looking at doing some LSR stuff and the class I'm interested in doesn't allow EFI so I'm stuck with carbs. Slow spool isn't that much issue since traction will be limited off of the line. I'm trying to start simple as well so I have room to grow on my own and to keep it cheaper.

Basic idea is an 81 Regal with a carbed turbo 231 to start with. Was thinking later 231 block, i.e. a 109 with 8445 heads. I've read the B4B info on converting to a blow through with the LC2 headers and turbo but it was lacking on what to use for an intake and carb? From what I've read, a Q-jet will be finicky proposition for a blow through, so I'm guessing Holley on a KB or Weiand intake?

Since it's almost obligatory, how much power do you think an NA 109 with a basic refresh, new cam, LC2 headers and turbo, decent intercooling, and the "recommended" intake and carb would make on pump gas? Follow on "pick my turbo" question is there more to be had in the turbo, pistons, or somewhere else?

if you use the weiand and a holley for the set up you will be good.

Honestly I feel you will make just as much if not more power then the Stock LC2 can.

You will have the advantage of less intake manifold distribution problems. but the combo as a whole is important to see all of the benifits.

The bonnet you choose for blowing thru the carb is one of the most important pieces. the better the bonnet the easier it is to tune.

Tuning is where the hard part is. the carb is well a carb.... and it will have its temperments just like the LC2 EFI.

Choice in the carb is important too, mech secondarys, larger accel pumps, altered PCVR channels and boost reference all play into the bill if you are going for maximum power. a raising rate regulator will be needed also.

there is very little different otherwise then the LC2..... just that its feeding a carb. The one thing you may have a problem with.... is if your using the LC2 style bracket system setting up the ignition. if you can get the original BMS stand alone ignition thats simular to the LC2s you will be fine since it uses a cam sensor, otherwise you will need a distributor and the brackets wont clear the ignition assembly.
From my research, it's got to be a distributor as well as a carb, no BMS (or a similar system made out of Megasquirt parts) coil packs. Since it's a race car, manual steering, no AC, and no emissions means I'll have to run the alternator and the water pump. I'm sure I can mount them clear of the turbo and distributor.

I'm not planning on starting with a real LC2, just an NA 109 with a refresh so a LC2 cam would be needed unless it's the same as the NA motors which I doubt. I want to start low buck and work my way up to better parts as the money and tuning experience build. Are the "off the shelf" blow through carbs worth the cost of less fustration to get it dialed in as opposed to a modified normal Holley? What about carb enclosures vs. the bonnets?

Holley sells a boosted application carb but it's a bit pricey to me. You can use a standard Holley and modify it without to much headache. The person to talk to here would be Warwagon (Adam) since his is a blow through car.;)
If your running it in a real basic form then it should be easy.

The NA cam isnt far off from the stock LC2 cam... so not much would be needed to upgrade there if its fairly healthy. but from expericance the NA slugs wont hold much detonation. but they will work for awhile!

As for off the shelf carbs, since your not looking for maximum power they really wont offer much advantage over building your own until you really start pushing the boost or go after max power. A regular holley can do the job fine after some minor mods. the key mod being the sealing of the carb and boost referencing.

enclosures vs bonnets.

enclosures work great if your keeping the power level on the low side, they work pretty good to a point and can even let you use a vac secondary carb inside if streetablity is a concern. the downside? they suck for tuning... open and closed a billion times to get to jets, air bleeds, etc etc become a pain. Also at higher power levels they will induce distribution problems. as the air moving around the carb and into the throat swirls and tumbles alot. this is #1 reason most moved away from the box. cures for the problems include screens and spark arrestors to help "smooth" the airflow. But these are crutches to the real issue. perfect starting point if the carb work seems to much.

Bonnets work the best for the all out blow thru system, as the throat of the carb is the only thing the air enters. Shape of this bonnet is important, as many cheap ones cause the same problems as the enclosure. good example of a bad bonnet would be the square Turbonetics one. Best bonnet out there at the moment is the Extreme Velocity piece. my last blow thru small block chevy used this bonnet and tuning was sooooo much easier then others.

Totally talk to Adam about it, he has had some amazing results so far with his blow thru car. its really amazing to see how the carb could easily keep up with the EFI stuff.
I started reading his post on off of a search for info on the Weiand intakes so I'm working my way through what he's done. Great to see someone having tread the ground before!

I'm not going for maximum power but I have a goal of reaching 150mph on the 3 mile course with scaling for what that would end up being on a paved mile such as the Ohio Mile at Wilmington, OH. This doesn't seem like it should be too hard considering that a stock 87 T-type would do 100mph in the quarter and the '81 would have basically the same body minus the spoiler (can't find any pictures of the spoiler on an 81 but it might not help either since I did read MotorTrend's review of the car saying it was getting light in the front end). If I break a set of NA pistons I have a set of rods with ARP bolts that I could just find a set of LC2 stockers or used forgings and do up a new rotating assembly on the NA crank or find a rolled fillet piece at the same time. If I really wanted to go crazy I always have my full forged 4.1 from my Riviera that I could convert over to a carb and distributor and go hog wild but I'd rather leave that one alone and enjoy it as is. Doesn't mean I might not steal the turbo off of it though.
Finding an intake may be an issue but I do have a KB and later weiand intake sitting around.;) Both are hi rise style but I think the KB is the tallest.
Jumping in...
The factory 4.1 v6 4bbl intake manifold is a player on the blowthru scene, It's a single plane design don't count it out. I think it's the best for stop and go drive-ability and anything still needing some power down low. My first choice going aftermarket intake manifold is the Weiand followed by the Kenne-Bell #1. Most cost effective way for the blowthru carb is to modify your own or purchased Holley 4150 or similar square bore carb. Buying a carb that is "setup"for boost is nice but it won't change the fact that you will still tear it apart more than 30 times dialing in the fuel curve, better off putting that $$ toward some reusable gaskets and a jet, accel pump, pump cam, squirter and air bleed kits. As mentioned before by my Friends A.J. and Charlie, the carb bonnet plays a big part in tuning and fuel delivery. Going the distributor route for Ignition requires either: a older vacuum/mechanical advance dizzy, a working ESC computer controlled dizzy with 7 pin module OR a gutted computer controlled dizzy converted to a 4 pin module. With a decent tune 320+whp 450+wftlbs can be had with the blowthru 3.8 setup using STOCK long block and lc2 cam on 7-8:1 compression and small 49 or 44 turbo on pump gas no alky. I'm leaving out tons of info... but we will get to it as needed.
I've not abandoned this, infact I'm looking at what engine I should build and what parts are available. Since this is an LSR motor I've got to make power for a long time, 12x as long as a typical drag car since I'll be on a 3 mile course. I'm looking into the possiblity of running a 7-pin HEI with a Megasquirt to custom control the timing and see if I can't do things like add in Oil Temp as a parameter for the timing since that's going to be related to piston temp as well. Since I can run a much smaller motor than stock and skip down to an open class with a 181cid and since no one really likes loosing displacement high quality aftermarket parts are actually cheap. Stuff like 6.5+in rods and tiny <2.66in cranks.

How big of a deal is cam selection as I'm likely starting with an NA 109 block and upgrading the parts needed? With stock LC2 pistons I'm guessing that the LC2 cam is a good one but is it worth the upgrade over the NA cam or should I get something like the Edelbrock flat tappet? If I go up in compression, start looking at the Comp and TA higher lift units?

How much do the stock and then the aftermarket 4bbl intakes run in general?

General question but how far is it safe to deck a block 109 and the older open valley units?

I've decked a 78 block .030 but it was an NA set up. Don't know if that would work on a turbo engine but it worked great on the NA.
as for controling ignition through a 7 pin, it is possible. but dont expect to get to far, remember the HEI has an RPM cut off point, and will fail to deliver the amps needed at higher RPMS to fire a turbo motor.

im not so convinced that a (.00) deck is all the important. on my superduty, my slugs are almost .026 in the hole. granted, compression is still way up in the low 10s.

id avoid decking an already thin and flexy deck. in fact, if it were up to me, id have it hard blocked to add meat to the deck. shoot for pistons to control compression.

as for the cam, nuthing more exotic then whats currently available for most hot LC2 builds. there really is very little difference between the builds by going blowthru. so maybe a 212-212 or hotter, what really needs to be determined in order to choose cam, is final drive RPM. choose the cam to make the most power at this point, as you will be in that rpm range the most for a long distance high speed run.

intakes vary alot, stockers run for like 50-75 bucks. aftermarket depending on the choice, 100-250.
Ok, I've settled on a displacement and classes to run in. Basically its a 3.0L destroker using an OEM 2.66 stroke crank with the journals turned down for BBC rods and a set of TRW flattop pistons.

On reading over at it looks like I need a small double pumper carb for this application to keep the signal to the boosters up with the reduced pressure differential across the boosters under boost. Think a 600cfm double pumper is a good starting point for ~300hp?

I'd really like the Smokey intake since it's an "Air Gap" style unit. I'm guessing they are at the top of the price range though. How does the performance compare between it and it's sibling Weiand or even the stock 4.1 unit?

The 600cfm is going to be too big for a 3.0l look for 450-500cfm. Might get away with a 600 cfm with annular boosters and running a lot of jet to get it to pull fuel, the engine will respond better with the smaller carb. I see no difference between the original Smokey Yunick Weiand intake and the Weiand Excellerater. They look the same on the inside as far as the volume and runner shape . Only differences are no egr, airgap and a different water outlet on the original Smokey Yunick. I reference the original Somkey Yunick intake because Weiand has sold the excellerater intake With Smokey's name on it but its just the single plane excellerater.
Hmmm... 600 is the smallest double pumper I've seen. This is not a street car but I do need to be able to get it moving quickly (though the salt is not a great traction surface). What kind of carb do you suggest? Work over a 450cfm 4160 with dual feed and a secondary squirter?
Don't forget a throttle return system/spring that counter act your boost pressures...Or you'll get a Run Away Car! The boost pressures will force the butterflies open an keep them there....(seen it happen on a twin turbo SB 70 nova )
Hmmm... 600 is the smallest double pumper I've seen. This is not a street car but I do need to be able to get it moving quickly (though the salt is not a great traction surface). What kind of carb do you suggest? Work over a 450cfm 4160 with dual feed and a secondary squirter?
Yes, your own suggestion is good. Converting any smaller 4160 to a 4150 style. Holley has quite a few smaller 4bbl carbs.
Wow thats super nice and already setup with tunable air bleeds and straight boosters. I'd get it do or have the mods done to it and bolt it on. Nice classic finish on that too.
Wow thats super nice and already setup with tunable air bleeds and straight boosters. I'd get it do or have the mods done to it and bolt it on. Nice classic finish on that too.
What kind of mods are you talking about? The gaskets or things on the "Hanger 18" list like jet extensions and the like?

Any opinions on the hp limitations of it?
I dont think the 390 is going to pass enough fuel under power unless you really open up those PCVR channels and when thats done, it will be crappy crappy at low rpm. whats your thoughts Adam? this is almost a case of where a CO2 driven power valve in front and a regular boost referanced rear power valve in rear would be needed.... atleast in my nutso opinion.