Blow Through Q-Jet?

[LemansWagon]

Anybody here run a blow through system with a q-jet?

 

[mat231v6]

you need to talk to adam (warwagon) he has a blow thru system.

 

[LemansWagon]

Is he running a q-jet? I am curious as to what mods he made to make it work. I've always been a q-jet fan and have years of experience tuning them but it's all N/A. I've always preferred them to holleys and have a shelf full of over a dozen of them. I was going to use a Holley in my blow thru until I saw a 455 Pontiac in a magazine pushing a ridiculous amout of hp through a single qjet.

 

[charlief1]

Is he running a q-jet? I am curious as to what mods he made to make it work. I've always been a q-jet fan and have years of experience tuning them but it's all N/A. I've always preferred them to holleys and have a shelf full of over a dozen of them. I was going to use a Holley in my blow thru until I saw a 455 Pontiac in a magazine pushing a ridiculous amout of hp through a single qjet.

No, he was runnig a Q-jet. He's switched out to a Demon or a Holley. Where you at Adam? To do a Q-jet you have to build a box for it so the boost is constant and not screwing it up.

 

[WarWagon]

Is he running a q-jet? I am curious as to what mods he made to make it work. I've always been a q-jet fan and have years of experience tuning them but it's all N/A. I've always preferred them to holleys and have a shelf full of over a dozen of them. I was going to use a Holley in my blow thru until I saw a 455 Pontiac in a magazine pushing a ridiculous amout of hp through a single qjet.

I've seen/read this article myself and a lot of the info is bogus and I wrote to them and called them out on it They were like "oh you have done your R&d then"
FACTS: The accelerator pump will leak with boost over 10psi(it leaks under that too just not as much, still dangerous)
They don't like a whole lot of fuel pressure but this is mandatory for blowthru:
7psi base fuel pressure+10psi boost=17psi fuel pressure inside the carb with a 1:1 boost referenced fuel pressure regulator.
They are very finiky with keeping a constant AFR while in boost.
It's hard to seal the carb up tight for boost, it can be done but it's that much more work that other sqaure bore carbs don't require.
The internal mods are not hard to perform to get it to flow the fuel needed, It's more challenging to get the throttle shafts sealed up and get the secondary side set up to not go lean in boost.
I have built a good blowthru Q-jet and it worked quite well(to a point) and I'd be more than happy to show you pics, tell you the places to drill and smooth and point you in the right direction if your serial about building a blowthru Q-jet, but I just know you will upgrade after you say many choice curse words about the Q-jet while tuning.
After going this route and been there, I'll keep the square bore carb over my Q-jet for blowthru.

 

[mat231v6]

I've seen/read this article myself and a lot of the info is bogus and I wrote to them and called them out on it They were like "oh you have done your R&d then"
FACTS: The accelerator pump will leak with boost over 10psi(it leaks under that too just not as much, still dangerous)
They don't like a whole lot of fuel pressure but this is mandatory for blowthru:
7psi base fuel pressure+10psi boost=17psi fuel pressure inside the carb with a 1:1 boost referenced fuel pressure regulator.
They are very finiky with keeping a constant AFR while in boost.
It's hard to seal the carb up tight for boost, it can be done but it's that much more work that other sqaure bore carbs don't require.
The internal mods are not hard to perform to get it to flow the fuel needed, It's more challenging to get the throttle shafts sealed up and get the secondary side set up to not go lean in boost.
I have built a good blowthru Q-jet and it worked quite well(to a point) and I'd be more than happy to show you pics, tell you the places to drill and smooth and point you in the right direction if your serial about building a blowthru Q-jet, but I just know you will upgrade after you say many choice curse words about the Q-jet while tuning.
After going this route and been there, I'll keep the square bore carb over my Q-jet for blowthru.

can you post how you converted the qjet over. i really want to know how to do that.

 

[charlief1]

Start a new thread on the mods Adam. We really need the tech in here. There's so little info in this section for mods.

 

[LemansWagon]

Interesting. I found the article online here:

Turbo Pontiac 400 Build - Ken Crocie's Turbocharged 489ci Stroker Pontiac Makes 744 LB-FT - Car Craft Magazine

Was this the same article you saw?

I would definitely like to see how you did it but given your info I have some thinking to do. One thing that would make me lean towards the q-jet despite your warning is I won't be running a ton of boost. It's a low buck setup with an n/a cast piston 3.8 I got of ebay for $9.99 locally. I would also have to buy a Holley. The only other 4bbl I have besides the shelf full of qjets is a 450 cfm Holley 4360 "economaster". I was considering using it because it's small (450 cfm) which I thought would match a 3.8 well and has mechanical secondaries which is good for boost. Then I found out it doesn't use standard holley jets that are difficult to obtain. Plus you have to remove the whole top to change the jets...pia for tuning.

I hear what you say about the fuel pressure issue. Without a fuel pressure regulator I used to leave the stock fuel filter in there to regulate things even though it's restrictive. With an aftermarket higher pressure fuel pump (even mechanical) and no stock filter I found that it would blow the needle off the seat and over flow the bowl even with a pump that was supposed to be less than 6 psi.

 

[Regallager(CND)]

Rochester Qjet in my 80"

I've seen/read this article myself and a lot of the info is bogus and I wrote to them and called them out on it They were like "oh you have done your R&d then"
FACTS: The accelerator pump will leak with boost over 10psi(it leaks under that too just not as much, still dangerous)
They don't like a whole lot of fuel pressure but this is mandatory for blowthru:
7psi base fuel pressure+10psi boost=17psi fuel pressure inside the carb with a 1:1 boost referenced fuel pressure regulator.
They are very finiky with keeping a constant AFR while in boost.
It's hard to seal the carb up tight for boost, it can be done but it's that much more work that other sqaure bore carbs don't require.
The internal mods are not hard to perform to get it to flow the fuel needed, It's more challenging to get the throttle shafts sealed up and get the secondary side set up to not go lean in boost.
I have built a good blowthru Q-jet and it worked quite well(to a point) and I'd be more than happy to show you pics, tell you the places to drill and smooth and point you in the right direction if your serial about building a blowthru Q-jet, but I just know you will upgrade after you say many choice curse words about the Q-jet while tuning.
After going this route and been there, I'll keep the square bore carb over my Q-jet for blowthru.

Was reading this thread and I was thinking from what you said Adam about the leaking fuel under boost kinda scares me since im running a Q jet are there anyway to check for that problem without doing a Ace ventura with the hood up! was the Q jet the stock carb that came with our carb turbo cars? any more input on these carbs would be great thanks. Mark

 

[LemansWagon]

This web page shows a method for sealing the accelerator pump as well as some other pointers on how to set up a qjet for blow through.

TurboForce - Project, Update 1
TurboForce - Project, Update: Pressure Bonnet

I also contacted the guy from the article in car craft by email (sent the email before I posted here) and he got back to me with essentially the same tips save for the fuel pressure issue. One interesting thing he mentioned was that blow through bonnets designed for holleys don't work well. He suggested modifying a single snorkel air cleaner or something else with a large plenum area. He didn't say why but my guess is the flow of air is directed right against the secondary air doors with the standard design carb hat. That could really cause havoc with them.

 

[WarWagon]

This thread is good enough

Your on a roll now with the H-O mods. Been there done something similar: For the first go around I used a MR gasket carb adaptor then machined it to be a 1" spacer to give the stock 4.1 manifold even more volume. Used a different later Q-jet before ccc, side inlet, no fuel filter in inlet housing, no choke but has a choke tower.
Those methods listed on Turboforce/H-O don't work for the accel pump I've tried many configurations and went through a lot of top plates trying to cure it.
Alot of info is left out of Car craft's article and the web sites.
I dug the stuff out a took a couple pics. "the gasket skin is still stuck on the spacer/adapter" I scraped a little off.
I'll get the info up later I'm done tonight

 

[LemansWagon]

Nice. I am thinking like you it seems. I had already been on ebay looking at carb spacers after I looked at the turboforce page not wanting to mangle my intake. I would love to see what you did for the accelerator pump shaft seal.

I have a wide array of qjets to choose from, including a few 800 cfm'ers, but for what I'm doing a 750 should do. I was leaning towards using the one I had on my 396 before I switched to efi. Did you increase the accelerator pump size on yours? I knever did on the one on my 396 because I knew I was going to efi soon but it had a flat spot just after leaving the hole that was probably caused by too little pump volume. I was only putting 300 to the wheels at the time but I don't plan to get that much hp with this...at least not initially...doubt the cast pistons would last very long and would need a turbo upgrade.

 

[WarWagon]

The accel pump is the stock diameter with the upgraded piston. The pump shaft is smaller than the orifice in the top plate, most later top plates have a rubber seal sandwiched in the top plate for the pump shaft but it don't seal well. The rubber boot is discontinued that H-O used to try and seal the accel pump up, I've used a similar style but it swelled with fuel and broke into millions in pieces to plugged the carb up. Then I tried all sorts of O rings, A/C O rings and even a boot on top of the pump shaft, nothing sealed it up, plus the more stuff you put on it to try and get it to seal up, the spring wouldn't return the pump up when the throttle was released. Eventually I used boost pressue to help redirect fuel and keep it sealed. The fuel pressure is what makes the leak not BOOST.

 

[mat231v6]

how much does it cost to convert the carb over?

 

[WarWagon]

It cost TIME and perhaps whatever the amount of a rebuild kit is



.....If you have two carbs to start with preferably a big block tuned Q-jet and one for a 4.1-350(5.7) swap some parts out of the BB Q-jet.

 

[LemansWagon]

You never went with the larger 2bbl pump? Good to know. I doubt I'll need it for this application either.

And you never really solved the accelerator pump issue huh? Hmmm. Do you know at what boost level the fuel pressure got high enough to cause the issue? I'm sure you were running more boost than I will be. Like I've said before this is low buck so boost will be low. If I ever decide to go big with it I will likely go with a spare chevy TBI set up I have. I have the software to burn the chips, but initially I just want to get it running for some cheap fun and use it as a daily driver perhaps. I have other projects that are getting the money right now. If it goes 14's I'll be happy. I'm looking to get between 200 & 250 hp. Motor is an 84 Regal non turbo that I will put a GN cam in. I picked it up from someone local on ebay for $9.99 ha ha. It was a running car that he was making into a roundy round car.

The carb from my big block is from who knows what. It came with a 350 I put in the car before the 396 and wasn't original to that. I just tuned it for the 396. I have a huge collection of hangers, rods, etc. I've been picking up qjets for parts at swap meets for years. My friends always laugh when I return to the spot with another $5 qjet in hand.

 

[WarWagon]

I never had a accel pump shot issue with the small primaries it was always good. I ran 10psi boost max with the Q-jet, anything over that and it would go lean. Around 10psi fuel pressure the accel pump would leak. It's a bad design for an accel pump in a pressurized chamber, great for NA though. No-one has solved the accel pump leak using a Q-jet for blowthru. If I'd dive into it again I'd rig up a diaphragm style accel pump like a Holley or Demon, then it wouldn't leak.
I built a 4MV and As I recall "I'm not tearing this carb back down" I put 70 primary jets in the base. The power piston spring was swapped out for a 1.35 length one. and I swapped out the primary metering rods to ones that have a .020 tip taper. On the secondary side I cut the tips clean off the metering rods (DA) to have full flow. Tightend the secondary air door to slow down and only open 2/3 (makes a lower pressure area and helps pull fuel). The secondary cam is unchanged, and the secondary jets are pressed in and can't be changed. Sealed the bottom of the main well with epoxy. I did a variation of H-O's take to seal the throttle shafts. I cut all the dividers out of the mainwell so it's all open inside. Upgraded needle and seat to .145
There is an external fuel bowl vent the used to go to the charcoal canister. I supply that vent with boost pressure an cap off the vent in the carb throat(it's uncapped in the pic, I used this for an NA app before I stored it)
I sprayed all of the gaskets with tack n seal to make then more fuel proof to leaking with boost.

 

[LemansWagon]

Wow, that is a lot to get it to work. How much hp were you making? What do you think of draw through? Even worse? I have a couple draw through carb adapters from a TTA and a 3.8 that I could modify to attach a hose to.

This is certainly making me reconsider using the qjet and going back to the 4360. Except they only make hollow floats for them.

 

[LemansWagon]

You know, I'm beginning to think it might be easier to do the TBI thing afterall. I already have all the parts except the harness and big enough injectors, including the software/hardware to tune the old GM chips. I have a 2.8 V6 truck ecu, 5.7 TBI unit, and 4bbl-TBI adapter from a 454. I figured a carb would be cheaper and easier than getting the harness and some 90 lb injectors. Not looking that way anymore.

 

[Street Lethal]

I figured a carb would be cheaper and easier than getting the harness and some 90 lb injectors. Not looking that way anymore....

A carb is cheaper and easier. There is really no need to go with a quadrajet though, just get yourself a Holley w/spreadbore to squarebore adapter plate. It is way easier to run an enclosure if you want to run the carb w/vacuum secondaries, than it would be to run a carb hat. Most people agree that w/vacuum secondaries, 15-psi is usually the limit, and if you want to run anything higher than that, then mechanical secondaries is the way to go. Contact gearheaddib from the following link over on the turbo forums if your interested in vacuum secodaries....;

Vacuum Secondaries for Blowthru

Here is some more reading about vacuum secondaries....;

Anyway to make Vacuum Secondaries 750 Holley work?