Short version: are you satisfied with your blow thru, and why?

Long version: Having a blow thru carb for the past several years has made me realize, the blow thru is really just a number of compromises for the sake of having forced induction with a "mechanical" fuel system.

First there is the fact that you cannot have a choke, so cold starts require a bit of patience and usually driving with both feet until the engine has warmed enough to idle in gear without throttle being added.

Second, the circuits must be utilized slightly different than originally intended, primarily the power enrichment or "power valve". Many boost reference the pv so it's not pushed shut or fluttering under boost but that means you will need to jet up the primaries so you don't get a lean stumble under sharp acceleration i.e. what the pv was intended for.

Aside from those things, my carb personally seems finicky needing frequent attention to idle acceptably. Maybe I'm not just willing to settle with "good enough" results, I feel that with as much time as I've put in on getting it to run "best as my tuning capabilities are" I simply cannot get the results I want.

So what are your blow thru experiences like? Have you got your carb so well tuned you've almost forgotten what the engine bay looks like? If so, how long and what did you do to get there?

I suppose I'm looking for efi like results from a carb. Is that asking to much? I have a family, friends, many other projects, and a job that frequently requires overtime. I simply do not have the time nor interest to constantly be changing and adjusting a fuel mixer. Besides, I would really like to drive the truck at least as much as I work on it.

 

About to start on my 7th turbo build....

I have had the best results with smallish(for the application) straight leg booster carbs with nothing more than main jetting at cruise and opening pvrc's slowly for boost. In stick shift application's, using "the cure" to hold the pv open on low rpm boosted shifts handles the fuel. No iab, hsiab tuning at all. They start, idle, cruise, and boost great. Accelerator pump and various cams can usually help transitions but are generally not needed.

My whole theory with "the cure" and carb building is to make it simple so anyone can do it with just a few jets and drill bits, and most importantly, do not undo what holley has gotten right. The last build we went around the block to dial in a 450 tunnel ram carb on a 3.8 v6. Made it to the first corner, turned around, rejetted, cruise and full boost was done...that quick(got lucky)...not my first go around with this stuff tho.

Started playing with bigger carbs, e85 metering blocks, bigger than necessary turbo housings on my 300 inline, and started pulling out my hair. A stock(mod for blow thru of course) conservative sized holley has been the best bet in all of my experience.

Added fitech 1200hp power adder model to the v6 3.8 has worked very well with e85. Fuel and timing transition into boost well, and when the menthol comes in. It does what it should within a safe margin and is a very good bargain. The ecu had failed recently but was replaced free of charge under warranty. I am VERY mechanically oriented and tend to shy away from electronics for this reason. Have been reconsidering efi lately though, as my next turbo build will be a weekly driver with cold winters.(-20F) With the fitech(or other efi systems) I can switch to e85 in the summer in one minute with just a few afr/timing adjustments.

I feel your pain, with the 300 inline I pushed the envelope on everything because the head is poor. Installed a stock 351w h.o. in it's place with the same carb(pulled a ton of jet), turbo,(half blocked turbine housing now) in it's place, and everything is cake being more appropriately sized.

As I get older, I want port efi for cold starts and idle only with a holley carb on top handling the rest!

 

My C&S carb has been a lot of work to tune, like you I would like to shut the hood and drive it. Fitech is coming out with mpfi, I am looking forward to possibly getting it. I have been told FAST is the way to go but the Fitech would be half the price.

 

afterfire, I haven't been on forums much lately and don't know your build, but your headache symptoms sound a lot like lack of heat on the bottom of the intake manifold. Also, 2 bbl holley's have been awesome on the low power builds, so keeping the secondaries closed down on the 4 bbl's has been key to great streetability for me as well.

A good test on intake floor temp is to warm/drive til everything is up to temp, keep hood closed and shut off engine for 10 minutes. If it flood starts, not enough heat. After it starts if it revs and idles better, not enough intake heat.

The flooding is from condensed fuel on the floor that vaporizes when the engine is off and heat from the engine rises into the intake. The smooth/crispy running after the 10 minute shutdown/wait is the intake being warm enough to keep the fuel vaporized as it comes through the carb. This will fade away as the very reason carbs make great power(cooling effect of fuel vaporization) as it cools the intake floor if not enough heat is present. Ghost flooding during cold engine start up is a symptom also when you swear that the choke is stuck(not happening without a choke!) or needles are sticking but it's really just a cold floor.

Most gasoline powered engines will overcome the cold intake floor with all the heat underhood/coolant circulating after awhile etc...which leaves a guy scratching his head even more. E85 will let you know very quickly about intake floor heat...unless you have a very efficient intake with small plenum to keep velocity up and/or high idle to keep liquid fuel on the floor moving.

There is a little heat on the floor in the homemade intake on the v6 3.8. I can't help but laugh as the engine lopes(sounds big and bad) on a cold start up even when the afr's are at 13 with e85 and efi. Once the intake is warm(no fuel puddles getting sucked around) it idles glass smooth. For reference, it ran 11.09 at 122 with a loose converter and more boost available.

 

It does seem like the farther away from stock and fancier, the more work things are sometimes!

Have considered the fitech port aslo. The good part about port is velocity being high in the port even at idle, doing away with fuel puddling. The plenum area is too generous on the v6 3.8, but the tbi fitech does well other than cold start lope, which doesn't bother anyone.

 

I made my blow thru 267 as cheap as possible, and I used an Edelbrock 500, which then later made it a 600

It was a royal PITA to get mostly right...however I LEARNED alot about manipulating carbs and their various circuits.

Having a wide band o2 makes all the difference. I could not imagine attempting any of it without it.

I was going to go FiTech but then i bought a different motor, and was about to pull the 267...now Im putting it back together...Im going to put a real (ish) fuel system in (electric pump and a regulator) and push my combo harder with a carb...will I stick with an Eddy or go holley? No idea.

Ran a best of 13.90...i want 12.90s next year

 

There are a few things that bug me about having a carb (csu). I have thought of going efi in the future but for now I will put the money into other things. Under boost the carb is awsome! Cold starts are no prob for me, its hot starts that bug me, after 10 min, fuel evaps and fills the intake pluming, takes 6 seconds of cranking to flush it out to start the motor, then the smell of gas vapours fill the air. Took quite a bit of tuning to get the off idle& transition circuit decent, and heat soak affects the carb after driving the car a while. Still intend on building some heat sheilding to keep some heat off the carb and intake manifold. Now that its winter here a lot of those issues have gone away for now.
If I did it all over I may have tried a fitech setup, but that system was not out when I put my car together. It is nice being able to buy carb parts at your local parts store though.

Living with a blow thru,, LOL.

 

You have fuel leaking out of the carb into the intake plenum. Find the leak with the bowls full and the carb removed.
It starts good cold because of the extra fuel in the intake.
Evaporating fuel has no way to travel down into the intake.

 

Thanks for your suggestion, I will look into it more closely to see if that is in fact the problem.

 

Well, my blow through is finicky... weather really plays a role.
Like you notice- when it's cold, it's kind of a pain. It cranks and starts then immediately dies and then it'll start again requiring me to hold the throttle to maintain 2000rpm for about a minute before it will idle on its own.
Idle afr depends on ambient temp- it'll idle at 14.5 on a nice spring day and idle at 12.0 on some days and then some days it'll idle at 15.5... I don't care cuz it's idle and under boost it's fine.
Next problem comes with transition throttle- from cruise to full throttle- it goes pig rich causing the truck to stumble until boost builds... it then holds 11.0-11.5 afr through boost- where I want it- and is fine, it just stumbles and goes 10.0 on transition- I could mess with squirters more but idc.
Cruise ispicky too- if I go 65 on flat ground, it'll cruise at 14.5-15... if I go 70- it goes to 13, if I go 55 it's 12.5-13... it is kind of a pain.
So living with blow through- I say eff it because under boost she's fine and that's all that matters in a "race" vehicle


@Pro

 

Interesting input from everyone. It seems like everyone agrees that there are quirks that must be tolerated.

I'm also with everyone in that my carb makes awesome power at WOT.

My carb is probably too large at 750cfm to ever really accomplish the snappy part throttle I'm looking for. I may just try a 650 main body and the cure to see if I can get the performance I'm looking for.

 

Great thread as I am trying to decide which way to go on this.I currently have and AEM but way to many adjustments so I am thinking of going blow thru,however both look to be a pain.

 

I'm a die hard carb guy with a lot of experience with them. Here are some facts that may turn me to efi:

carb+boost+e85=safe, awesome power
carb+boost+e85+air temp changes=suck (wot afr's 11.5 @ 90*F, 14+wot afr's @ 60*F)
carb+boost+e85+going lean=no issues at 22 psi and low timing with one combo

fitech+boost+e85=safe, usually slightly lower power than carb, but can be made up with boost (looking at QFI 8i, have venturi's like a carb for better atomization)
fitech+boost+e85+air temp changes=no problem
fitech+boost+e85+going lean with fuel pump going soft=no problem, went 14-15 afr's at 19 psi and low timing a couple times

The point of all of this is pump e85 has proven safe and more power vs pump gas so that's what I want to run as much as possible. E85 is a ton better with efi everywhere except maybe slight loss of power up top.

Any issues with a carb and/or intake heat are usually magnified with e85.

fitech+e85 just works great on the v6 3.8, you can see it go a bit rich when the meth(blue light on the left) comes in, this was also the first time givin er the berries(extra safe tune) so it hasn't learned the meth yet. It's rich on decel too as the meth is still on for a bit(we spray a lot and love it):

[video=youtube;GT46P2feAL4][/video]

 

BUT!!! when the ecu went out on the fitech, it was showing no boost with mild acceleration, and the mechanical boost gauge read 7 psi. The part that scares me is "was it pulling timing?" probably not at the time. The owner did know that their was some kind of issue with the system at the time, so he took it easy.

The fitech can be used with or without timing control, so personally, I will run a locked mechanical on my dizzy with vacuum advance and dump T until confident in the system.

 

^^^FWIW, 80m/20w was spraying in every lean situation...went lean 14-15 afr's on a stock 351w 13psi low timing with 91 gas with out issue due to low fuel. Meth was spraying at the time and I am pretty sure it saved the day in that situation.

 

I had a 2.3 blow thru, Holley 350, efi lower intake, ported head, mild rv cam and holest hy35 boosting 22 lbs. it was soooo easy to tune and required almost nothing. Car made 330 hp and it was my daily driver. (Except when it snowed). Went to a 500 two barrel looking for more and it all changed. Ended up burning a piston trying to tune the dang thing. I'm a fan of small carb on blow thru. I feel carb signal is super important with a blow thru. Wish there were more options when it comes to small 4 barrel carbs.

 

Im in the middle of a blow thru build on my 78 pontiac sunbird, nice cad by the way not a rusted out sunturd like all the others its age but anyway, their was a lot of challenges getting it where it is now but i feel like the whole blow thru idea is superior with some drawbacks like no choke, especially on a stock two barrel carb, and hardly never starting in the cold and so on...

 

My truck is BT and I drive it everywhere. 13mpg and stupid power. It does go a little rich here and there, but that's just part of it. The very title of this thread makes me think you should go EFI.

 

I was blow thru for many years and really really really didn't want to give it up, I think it was a pride thing. Now I will never look back and its not the cost really anymore that keeps people away from efi.

 

I've been blow thru for quite some time. It has its pros and cons.. for me the thing that bugs me the most it the hot start issues after a long drive. It does have a lean bobble under acceleration but I can drive where you can't feel it but I know it's there. WOT car is an absolute animal.. the fi tech is very tempting for the price... Hell the car is done runs great, can't complain..

Jeremy