Q-jet

[Sirshredalot]

That looks pretty trick Nick....that even rhymes.

But oneproblem that the Q-jet has is a low bowl capacity. Especially in hard corners or high G acceleration.
The bowls in the carb body just dont have enough there to support as much volume as say a holley or an edelbrock.

what you will want is a very high volume fuel pump with a bypass regulator and a lower fuel pressure setting like 4-5psi.

Ive been hesitant to reply because we got started on talking about Q-jet mods and I am having trouble remebering the details.
Needles and Jets are pretty straight forward but alot of the circle track guys and thestock cclass drag racers would do some drilling with a very fine drill bit and for the life of me i cant remember where they drilled.

That intake looks nice....like a torker series intake....theyre good but I dunno how well they like spacers....but you shouldnt need one...
I have also used the Weiand IMCA approved intake which are also very similar to that and i have had very good results...I even had one on my 4000lb pickup with a cam about the same size as yoursand could pull anything I wanted.
With less cubes itll seem like a bigger cam and with a long rod to stroke ratio youll lose some bottom end anywho.....but it shouldnt be killer in such a light car with deep gear ratios in the T5.

Ive got a buddy I can ask about the drilling....well see if Im not retarded.

God bless
-Shred

[NixVegaGT]

I've been reading the same thing. I read about some modifications for circle track so they could take advantage of the larger capacity of the Q-jet. Most of those classes allow 2-barrel or Q-jets. Most guys pick the 2-barrel for the larger bowl capacity.

This article was saying the first thing to do was ditch the fuel filter in the carb body and get a high volume in-line filter instead. I could get one with a bypass built in for a return line. On that note do you think I should use a low pressure electric pump or a stock mechanical pump?

Anyway the other mod they did was drill out the needle valve a tiny bit I'm pretty sure. I'll search for it again... Thanks Shred.

The block is away getting sleeves pushed in and then I'll be officially assembling.

[Sirshredalot]

as far as fuel delivery goes...I always go for overkill...just to be safe.

Ya wanna make sure you have enough volume at the right pressure.

Id use a high pressure electric pump (14PSI) and a bypass regulator to bring it down to where you need it. The additional fuel volume will never hurt you and any un-needed fuel will be returned to the tank.

Hard thing is finding a bypass regulator that will let you set pressure at 5psi...most only go down to 6....but if youre upgrading your needle and seat assembly than you could use the extra fuel pressure too.

The modification that i was thinking about had somthing to do with drilling emulsion jets or somthing....also drilling passages from one section in the "bowl" to another...But i dont remember which ones or where so I dont wanna give out any dangerous/incorrect data.

God bless
-Shred

[NixVegaGT]

That's cool. Any piece of advice I get I, as a rule, get a third opinion. Any information I get helps me research the topic to get a few different sources to corroborate the information. It does help me research the topic though... SO thanks! I'm going to see what I can come up with.

[hoffbug]

When we were kids we used to flip the lid of the air cleaner upside down so you could hear that good ole' Quadrajet roar!

You'll most likely have to tighten up the secondary air door. On our size engines 750 is way more CFM than you'LL ever use.. even with the ethanol.
The dyno sheets on my 406 only showed around 736 CFM and that was at 533 horsepower. Ive made some modifications since then and am still only running a Holley HP modified to give a TRUE 780 CFM.

[NixVegaGT]

I was thinking the same thing about the secondary doors. I think we technically only use like 535 cfm theoretically.

I did the same thing with my Olds 455 in High School. LOL! That car was a real stump puller.

[NixVegaGT]

OK so I thought I'd post an update. I finally got my hands on an early-style '68 Qjet. It turns out this one is less desirable than the early-style '70's with the APT for tunability. I decided to go with it because it is the smaller 750cfm casting and is in pretty good shape.

The carb I went with looks like it's had a budget rebuild... by a serious monkey. There are all kinds of stupid mistakes. It's clear that it leaked out the top because the air horn is warped like crazy. It's off at the front corners (where the bolts go through the body to the intake) around 0.080. GEEZ. I can just see it. "It's still leakin'.... crank it down more!" DUH>

Anyway so the main body is warped a little too (≈0.020) but it's pretty slight and apparently the Qjet can handle a certain amount of warping. I can grind the air horn flat to take out the warping there and it should work pretty good.

I had also run across a late '77 Olds 850cfm carb. Warped like a pretzel! WOW! I didn't know you could do that. Anyway I got it for $20. I'm glad I did because it has been invaluable for spare parts… Well I'm sure there's a value but it'd take too long to outline. LOL!

I decided to stick with it though. I'll just have to do a little more primary swapping to get just the right low-end mixture. I got a great book:

http://www.amazon.com/Rebuild-Modify-Ro ... 774&sr=8-1

It was created pretty recently. It outlines some really specific modifications to make the Qjet a really great performance carb. It has a great description of the fuel systems and how to modify them for a given application. The best part is how to modify them for stout cams. The difficulty lays in the throttle position at idle for engines with low vacuum. They need more throttle plate angle to run unless there is another source of air that bypasses the throttle plates.

He outlined a cam similar to mine with a 350 and what kind of modifications to do to get idle quality back. (My compression is a bit higher at around 11.5:1 so my idle quality could work out to be pretty good without too much change to bypass air.) Then use a power valve spring that is tuned for the lower vacuum at idle/part throttle. A lighter spring keeps the metering rods from lifting too early.

SO I made some modifications to the air bleeds, idle tubes, secondary enrichment air bleeds, etc. without going too crazy. Since my displacement is small I may have to contend with some idle quality issues that may not be offset by the compression.

This is seriously fun, guys. I'll write more details about what I did tomorrow.

[NixVegaGT]

OK so I cut the "idle bypass air" ports this morning. I made them a little smaller because I'm using higher compression and my cam isn't that huge.

It involves cutting a hole through the casting in the voids between the primary barrels and the outside of the carb. There are holes in most gasket sets for this port. You use the gasket as a template and drill a hole around .160" down into the baseplate almost all the way through, leaving about 100 thou. Then you drill a small hole to meet the bigger one.

The book suggested a 0.090" hole for a warm 350 so I went with 0.060" for my 300 cid. If I was running closer to 9:1 compression I would probably made the hole similar in size because the smaller displacement is more finicky with the more aggressive cam profiles. I'm running around 11:1 or maybe a little more.

It's been fun so far.

[chevyart]

Just to give you a little hint of how good these carbs work, check this out. Alot of my friends run in the NHRA stock and super stock classes and in all the classes you have to use the carb that came on the car and keep them in basic stock form and shape, and guess what: alot of these cars run in the 9 and 10 second range with the quadrajets. Figure that out. There are lots of little, simple tricks with these carbs(I dont know nay as I use Holley carbs), but I have to respect these old carbs. There are probably web sites out there where you can easily get any info on these carbs that you may need.. Chevt Art

[NixVegaGT]

Yeah. Pretty amazing. It's a really advanced solution. They had some real thinkers back in the 60's. They were sending guys off to the Moon with slide rules! LOL!

I'm planning on building a mod page on my CarDomain site about the changes I made.

[NixVegaGT]

OH I forgot to mention I finished the carb. I plugged the secondary plugs with set screws and JB weld:




I did it on a very cold day. We weren't able to break 0º and had a high of -5º. I put the carb parts into a bucket with solvent and put it outside for 20 min. When I went to check on them the solvent had frozen! I cracked through the solvent ice to the parts. It was COLD. Then to make matters worse I needed to clean the parts with brake cleaner to take off the residue. Needless to say my hands were damn near frozen solid. OUCH. lol!

Here's a pic of the solvent ice:



Thanks for reading.

[Sirshredalot]

Sounds like what Ive been going through. I am down at my folks here writing this cause im on vacation kinda....so I drove for 61 miles in -2 degree weather with no heat in my truck....so after my feet started to hurt so bad i couldnt move them...I got ticked off enough to change my thermostat(again) on the side of the road and had heat from then on....but not before I spent over an hour outside total trying to fond working air compressors that werent frozen...So I didnt thaw out until about mile 142....lol.

Make for a chili ride.

But the carb looks like its coming along well...freezing petroleum products has to be COLD.

Those look like anualar boosters...sweet...I never noticed that on the Q-jet before...i dont think. Probably one of the reasons they work so well.

God bless
-Shred

Any ideas on that intake?....Gonna leave it alone for now?
Ive got my intake with me here...I'll try and post a picture of it later.

[NixVegaGT]

I'm going to see what happens with the intake before I modify it. It could be fine the way it is.

I was thinking I'd try to set up a Holley for the Edelbrock intake I bought a couple years ago. It's setup for a square flange carb. I thought it'd be a good comparo to make. I'm not sure where I'm going to get a spare 600 cfm to use for that though.

I'm not sure I'm going to do this but I've got both intakes so it would be interesting to see the diff. on an engine dyno. The other issue is how it actually puts power down on the street. Numbers are great and all but it's really how it acts that will make a difference.