hey guys i am new to the ls forums i am putting a 2004 lq9 6.0l in my 1989 z71 pick up. i have decided to go with the carb set up for certain reasons. so i have been doing some research and i am completely lost on what carb to go with now. some say i need a 750 and up some say a 750 will be way to big for a stock 6.0l to start with. what would you guys suggest right now i am looking at a holley 750 double pumper with mechanical secondaries

 

I like the quick fuel lineup of carbs, if it's your daily driver and can get cold where you're at, nothing wrong with the ones with electric choke. 750 will be fine on that motor, might need a wideband tune while you figure out jetting but no big deal. Overall for the price I'd get the quick fuel over anything else on the market, put probably 10 of them on various cars and always happy with the performance.

 

If this truck is for street use I wouldn't use a 750 double pumper. I am a strong believer in vacuum for the street, especially if MPG's matter to you at all. The engine can take what it needs and if tuned properly you probably won't give up anything in performance.

 

yes, I agree, bigger is NOT always better.

I did a bunch or research for my LQ9 and finally settled on a Holley 670 - I jetted it up a couple sizes and it screams.

 

No way a 750 is too big. My guess probably 90% of the guys on this site running a 6.0L with stock to street type cams run a 750. Carbs are rated on airflow, not displacement. LS motors make more HP than the first gens and for that reason need more air flow. Think of a stock lq9 as more of a 400 cube first gen or even a 396.

Mechanical vs. vacuum, sort of up to you. There is one guy on here who says no one should ever use a vac secondary since they never fully open. Opinions vary. I run mechanicals since my car is light and geared pretty low.

I have a double pumper but again, from what I have read, you might be better off starting with an HP type carb. They seem to work better with LS motors right out of the box than the classic double pumpers. DP's work fine with some tuning, but it takes a bit of effort to get rid of the off idle bog. Especially if you run a single plane intake. There have been a number of guys who said they didn't have that problem with an HP carb.

Also keep in mind that LS motors have full roller valve trains and high flowing heads that like to rev. A smaller carb will make it easier to tune in the low to mid range, but it will give up top end. Especially if you go up in cam size.

You also need to decide between choke and no choke. If you are summer only or live in a warm area you can get by without a choke. You just might need to nurse the throttle for a bit till it warms up.

 

750 will work great. You just have to decide vacuum or mechanical secondaries and if you want a choke mechanism. Unless you are racing or trying to squeeze out the last ounces of power, the vacuum secondary with electric choke is the way to go.

 

Good advice. I was more opposed to the double pumper idea for the street. I think there was some confusion in my previous post. I'll have to be more clear next time.

 

Comes down to the car/truck usage....DP carbs are great for going fast VS carbs are good for DD's.
If the use would be as a DD with towing,hauling,and such get the VS.

A 750 VS carb IMO would be great for that truck.

 

I think Car Craft got 1 hp and 1 ft lb going from a 700 cfm Holley to a 750 cfm Holley. Lost power with an 850.

 

Another consideration is if you ever have to sell it. 750s sell quickly.

 

hey sorry i guess i should have given a little more info. first of all i do live in ontario canada so it does get cold here. this is not my daily driver it will only be driven in the warmer months of the summer on weekends kinda thing. i have spent alot of time and money restoring this truck so i want to get this right not having to buy 3 or 4 different carbs. i as well am not really going for fuel mileage. i guess you through that out the window when you have 4.88s going to be lifted 6" running 35s and putting a carbed 6.0l in. i think i want more torque and off the line because lets face it this isnt a drag truck or car. but still would like it being able to scream at 6000 rpm and not miss a heart beat getting there. right now i am looking at buying the 750 holley dp with mechanical secondaries(0-4779c) on monday from summit and the performer rpm intake and msd box i think i am going to go with the dual plane intake oppose to the single.

 

89 sierra, Ontario Canada eh... How close to Ottawa are you? We might be in the same area.

I'd be temped to go with a good 850-950, I purchased a 770 street avenger but second guessing now. Especially after talking to Geoff at EPS when I ordered the cam for my LY6. He said I was under carb'd... He said "think of it this way the truck 90mm throttle bodies are 900+ CFM." then he talked about air speed etc etc...

I wish I woudl have went with the 870 instead, but I will keep this one for now and access on the car is on the road. I will also most likely use the 770 fro the street and maybe get a race carb for the track. Have the street one setup nice for lots of power for the street but get decent fuel mileage and swap to the race carb at the track.

I've seen it done around here, so might be an option as I will be driving the carb almost an hour to the closes track.

Bruce

 

I too am doin a ls series swap but into my 1986 grandprix, its currently got the stock 305 and im swapping in an lm7 (carb), what fuel pressure do ya'll think would be sufficient? For now im just dropping my stock quadrajet on there to get it up and running

 

Whatever Size you decide to go, Let me know. I can give great prices on Quickfuels and Proforms for LS1tech members.

 

Going with the dual plane is a very good choice especially on a big 4x4 with a stock 6.0. You will have more torque if you use a 650 carb. Since you aren't running the 1/4 mile or other WOT races (regularly) you will give up almost nothing compared to the 750. If you get the 750, you would be much smarter to get a vacuum type secondary otherwise you are much more likely to bog the motor if that double pumper isn't tuned just right. But, it sounds like your mind is made up...so...live and learn.

 

dual plane intake means it's only pulling from 1/2 the carb at a time due to the divider so your 800cfm carb might as well be a 400cfm. So much misinformation in this thread WHEW. If you get a bog....tune it out! If you're swapping the motor and concerned about mpg, how about keep your foot out of the secondaries? Neither a vacuum secondary nor a mechanical secondary will get better/worse mileage if you stay on the primaries. Vacuum secondaries are great for motor homes and tow vehicles, that's about it. Bogs can also present themselves if you're running the wrong converter or rear gear or flooring it at too low an rpm instead of rolling into the throttle. Just remember if you take your foot off the gas at all with a vacuum secondary, there is a delay before the secondaries will open, so if running a stick this will be very apparent.

 

I normally don't say things like this, but I think you are giving some misinformation too. Vacuum carburetors are calibrated differently than the mechanical secondary carbs and will provide better fuel economy out of the box. They will also be simpler to tune. The dual plane manifold will be simpler to tune too and will likely outperform a single plane in his application. The tow vehicle and motor home comment is completely inaccurate. Vacuum carburetors are superior for most any full weight vehicle with an automatic transmission and stock or near stock torque converter. They also work excellent on vehicles with manual transmissions and low numerical rear gear ratios.

I have a 20 years of experience on old school v8s and now recent and relevant experience on LS stuff. For OPs 4 wheel drive truck and intended usage, I think the OP can do no better than a Performer RPM and a 750 vacuum.

 

Just to clarify...when I say "bog", I am referring to being much more likely that it is (notably) out of tune...aka too much or too little fuel...especially when you slam the secondaries open. So, it may not "technically" bog but unless you are running an AFR gauge then you won't know exactly how far "out" it is.

 

hey guys dont want to start a heated debate here lol. same as the efi vs carb debate. i am 100% going with the dual plane intake. but the carb i have mixed feelings on. everyone has a different opinion with the carbs. most of the guys i have talked to are racers tho. the one guy has a drag car and performance shop recomended me a 670 avenger for low torque and said what i would lose up top wouldnt be much. another guy that runs a speed shop did a stock 6.0l ls swap in his s 10. hes running a 700 holley and said he wished he went bigger because it is starving the motor. then i have two other car guys recomending 750 double pumpers to me said they "wouldnt run anything different they are the best" lol. so i am all over the place.i do not have much carb experience thats why i turned to you guys. not to start anything but can someone please explain to me why the vaccum secondaries work better on heavier vehicles with auto trannies? i am running 4.88 gears right now which make the truck pretty useless and i am in hamilton so i am about 4-5 hrs away from ottawa

 

Debates are a good thing as long as everyone remains civil. They can bring out a lot of useful information. Double pumpers tend to try and "force feed" the motor a bunch of fuel that it may not be ready for. In a heavy vehicle the motor is having a tougher time getting the RPMs up and the weight moving to begin with and it may not be ready for that much fuel/cfm when you slam it to the floor. I hope that is a decent explanation. It was kind of basic...maybe someone can do a better job.

It is good that you are asking questions but you have to realize that most of the people you are talking to are building for Horsepower (top end) numbers. You will be better served building for Torque (low to mid range) to get those big tires rolling. I think you realize that. A smaller cfm carb will provide you with more torque and you won't lose much up top. Your performance shop guy is on the right track. Do you really expect to run wide open very much?? If you are not planning to race anyone head-to-head then I would definetely go with the smaller carb. I believe the 750 (vacuum) would be a better choice and have an advantage if you had some plans to race 1/8 mile or longer. You have to decide what is most valuable to you...a little more up top or a little more in the bottom and middle. And, if either of the (670 or 750 vacuum) carbs is tuned properly then it isn't going to make a night and day difference on one end or the other. Don't just take it out of the box and stick it on there and think its perfect. I think a lot of people have that misconception and if it doesn't work "it's the carbs fault". NO...it's the tuners fault...or lack of a tuner.

FYI...I have read that the Avenger series carbs are misleading. Supposedly, they don't flow what is stated...ie a 670 does not flow 670cfm. It is something I read a while back and I (unfortunately) did not save the link or I would share it.