Your right about the LS-5. Who cares if you run a quick E.T. shifting at 5500 RPM or shifting at 7000 RPM, it's still a quick E.T. The 454 Corvettes are what they are, a "smaller" car with a huge motor. Weight is not an issue. Check the weight of the new Challengers, Camaros and GT500s. A well tuned stock 454 Corvette will run 13s all day. For '73-'74 only the Super Duty was faster. I think it was Car And Driver that tested every Corvette available for 1971. The high performance LT-1 with a red line of 6500 was running the quarter in 14.5 sec. at 100.55MPH. with a 4 speed and 3:70 gear (curb weight 3300lbs). The air-conditioned, every available option luxury cruiser LS-5 (curb weight 3600lbs.) with a red line of 5600RPM was running the quarter in 14.2 sec. at 100.33 MPH. with an automatic and 3:08 gear. The LS-4 is not much different than an LS-5. Slightly less compression and slightly smaller cam. This LS-5 also had a top speed of 141MPH. This is all you need to know about being fast. Relax, your in good company. If this isn't fast enough for you think of the potential. You have a really big motor with great heads, dual exhaust and no need for catalytic converters. Breathe on it a little bit and it will go!

 

I am very happy with the performance of the car, and it is more than fast enough. I started this thread just to make sure the carb swap I did was not keeping the engine from revving. From the post I am comfortable that it did not.

 

I assume this is a spread bore 650 Holley bolted to your factory intake. I would not be satisfied with this carb. The factory Q-jet will spank it performance wise and get better gas mileage to boot. For you to see a performance increase tells me your Q-jet was in need of some attention. If I were you and was determined to stick with the factory intake, I would dig out my old Q-jet and give it another chance. If its been fooled with and you need to get it back to factory stock (assuming your motor is factory stock), you can google search and find out exactly what jets and rods should be in your carb. This will ease your mind that you have the right tune and you will have close to max performance without dyno or track time. Speaking of track time you need to take your car there and see how close you are to running what this car ran when it was new. It won't hurt it.

 

1971 LS-5, yes, a 650 spreadbore, bolted to the stock intake. The rest of the engine is stock as well, except I put on an HEI distributor.

The car did not come with the original qjet, so I purchased a '73 qjet, 7043200 (pretty close to '74 qjet but actually 6% more jet area at WOT), rebuilt it per the Lars papers and installed it. The performance was good, and I like the sound of the vacuum secondaries, but they are A LOT slower to come on then the mechanical secondaries of the Holley. I am going to adjust the top plate spring to see if I can get them to open quicker, but from this and other threads I do not think it will make a difference over the Holley.

Floor the throttle at 2500 in first, and the tires chirp/spin (265/70/15 - M21 - 3.36 diff), which is actually kind of dangerous if you are not paying attention going around corners. Floor the throttle at 2500 in second and the tires chirp. The qjet was not even close to spinning/chirping the tires.

Another thread I was following was all about the great mpg of the 6210. I do not drive the car enough to care about mileage. I got crappy mileage with the qjet (12mpg), but besides flogging the car every drive I spent so much time tuning it in the driveway that I never got a fair read.

I have not run a full tank on the Holley yet. Based on all the WOT runs I have been making I am guessing it is going to be crappy too.

When you say "The factory Q-jet will spank it performance wise", have you run the Holley as well and had bad results? I have not found very many who have tested both on this forum, so any results would be appreciated.

Your track comments are on the money. I am not interested in getting that last half second off the quarter mile, but it would be nice to have some quantitative evidence comparing the two cars. I am pretty sure I am running at worst in the low 14s, but I am tuned better than stock with .045 plug gaps and 52 degrees of total advance. Car runs great.

Another local forum member posted a bunch of info from dyno runs he did. I sent him a PM to find out where and how much. I think that would be the most accurate way to get the data.

It is interesting you say "ease your mind". That is exactly what this is about, and I really need to let some of this go and start focusing on other areas of the car. In particular the AC is not working, and that is something that will keep me from driving the car in about 5 months.

 

Another item that controls the air vlv opening on a Qjet is the choke pull-off. That little plastic item has a diaphragm and spring connected to the air vlv. So knowing your engine vacuum parameters my help with a fix for the slow(er) response. BTW the Qjet had what they call secondary pull over similar to what Holley's pump shot does. I recall u can drill these pull over holes larger for a bigger shot as the secondaries open.
That's another tuning trick u will find in Dave Roes book and the Ruggles Qjet "How To" book but i'm not sure how well he explains it. I think that pull off has a designed leak and is tricky to measure.
Just another item to look at when tuning a Qjet. The Qjet learning curve is much steeper than other carbs.

Hope this helps,
cardo0

 

First, keep in mind that the Q-Jet is not a vacuum secondary carb. The last section of my paper describes the difference between vacuum secondaries and the Q-Jet secondaries - be sure to read that to get a good understanding of the system

You can quicken up the secondaries in two ways: Easiest is to slightly loosen the secondary airvalve spring. 3/4 turn windup is a good, safe setting, but many large engines can take it as loose as 1/2 turn.

If the airvalve is still slow, you can drill out the restrictor hole in the choke pulloff. It's good to have a couple of spare ones on hand if you do this. This will relax the pulloff more quickly. Enlarge the hole about .002" at a time, since a slight change makes a significant change in the pulloff's response. Also, when the airvalve opens quicker, you get less of a "pump shot" from the secondary pullover circuit, which can contribute to a stumble.

Lars

 

When you say "The factory Q-jet will spank it performance wise", have you run the Holley as well and had bad results? I have not found very many who have tested both on this forum, so any results would be appreciated.

Mandingo, I say the Q-jet will be better for you performance wise because I believe your engine will use the extra cfm the Q-jet has over the Holley. A smaller cfm carb will feel more responsive around town but there comes a point in the distance you are racing that it does not make as much power if it can't flow adequate air for the motor. You've only got two choices in carbs if you stick with your factory intake. You do not want to use an adapter plate! And of those two choices for your motor the Q-jet is best. I think that Q-jet can deliver the same tire spinning performance as the Holley. You've got Lars attention he can definitely help work this out.
Track time is cheaper than dyno time and slightly funner. Since your motor is so close to stock you can be sure your jetting is pretty close. Find out how fast it really is with that Holley then try your retuned Q-jet. The one that gives you the best MPH is the one you keep.
I was very happy with the Q-jet that came on my car but was not happy with my intake. That stock intake is a beast! Very heavy. So when my car went on a diet it lost the intake in favor of an aluminum square flange. I replaced the Q-jet with a Holley 780cfm vacuum secondary. Really no change in performance. Due to choke clearance issues I have since replaced that carb with a chokeless Holley HP 750cfm vacuum sec. This is a great carb and is definitely the best performer of the bunch. Here in Ohio, I miss the easy starting of the Q-jet though.

 

I've run over 700 different carbs on my 357 cid 64 Vette when I was doing actual carb testing/rebuilding a few years ago (I'm not kidding). Here is my factual seat-of-the-pants observation regarding Q-Jet versus Holley versus BG versus vacuum secondary versus mechanical secondary:

Properly set up/jetted/tuned, I can tell no difference at all in performance between a 750 cfm Q-Jet and 750 Holley/BG carbs. If you correctly tune the vacuum secondary on a vac sec Holley/BG (get it to open quick enough), the vacuum carb will perform very similar to the mechanical secondary carb ("double pumper") with the exception that the secondary side is not as easily tunable due to the metering plate on a 4160. Bigger carbs (I've run carbs up to 1050 cfm on my 357 small block) don't neccessarily produce more power above a certain size, yet they can be set up and tuned to run just fine on the small engine with no stumbles or mixture problems. I can tell the difference in performance between a 750 carb and anything smaller -top end power does drop off with the smaller carbs. I have found no drivability or loss of low-end by running 750+ carbs, but you cannot slam a big mechanical sec carb into the secondaries at low rpm without the car falling on its face - you have to use good "racecar driving" technique with big mechanical carbs.

Bottom line: Run the carb you are most comfortable tuning. Within a given cfm range, any of the carbs will run the same if they're tuned correctly. Big disadvantage with the Q-Jets now is the complete lack of parts availability other than jets and some early-series (4MV) primary rods - unless you have a Q-Jet parts stash, you can't tune a Q-Jet properly.

Lars

 

Lars thank you for the clarification. I read the section (actually all) of your excellent paper about the qjet not being a vacuum secondary carb, and understand the difference between the air valves being "sucked" opened by vacuum vs. mass airflow. I even appreciate the Texas reference. Since you took the time to explain it I should be more careful about propagating the misunderstanding on this forum that the qjet has vacuum secondaries vs. air-valve controlled demand mechanical secondaries.

Right now I have the qjet set at 3/4 turn past where the spring hits the tang. Going to 1/2 a turn past does not feel much different (on the bench, not on the car). Setting it when the spring hits the tang lets the plates hang partially open, so I am assuming that is not an option.

I will try the 1/2 inch setting and see how that runs.

1/4 past also keeps the plates closed so I was going to see what that did. All I am risking is a secondary tip-in stumble, correct? As long as the adjustment will not cause gas to be sucked into the engine at idle, or something else bad, I thought I would try that.

I am worried about the abbreviated "pump shot". For all the talk about how the Holley gets better mileage, looking down into the carb when you hit the throttle it looks like it is flooding the engine with gas. I think that is a big part of the response differences in the carbs.

My last concern on the qjet is the choke pull-off. I do not want to risk screwing up the whole air-valve shaft by drilling out the restrictor hole, but how fast should the pull-off be relaxing? I put a new one on as part of the rebuild and it takes two seconds from fully in to full out.
That time alone would make it less responsive than the Holley.

Thanks for the help.

 

Lars
I'm so glad to see you chime in!
Too bad we don't both live near the same track! It would be great racing!
650 cfm 6210 challenges the 750-800cfm qjet
.
mandingo
don't confuse carb noise with HP. My 283 w/6210 would go 121 mph WOT WAAAAAAA Then i'd let up until the back barrels closed, very quiet and maintained 121 mph. Front only 1/2 open. Cool Sound made it sound faster, but it wasn't.

 

I seem to remember my 73' LS4 laying down @ 4000/4200 rpm stock , but I can tell you those old smog motors can come alive . You will be lucky to have 7.8-7.9 to one STR with that motor. I woke mine up a bunch ( even with 7.9 ) M21 with a 3.70 gear , 1 7/8" headers , LS6 cam , 3310 (780 with modifications ), also used 950 hp , Holley street single plane (very old ) and now an EDELBROCK RPM AIR GAP. HEI small cap dist. , and took 300# weight out . TOTALLY different car but still very mild mannered . Pulls hard to 5600 rpm before it lays over . I am putting a pair of 702's pocket ported and runners cleaned up with 95cc chambers an 2.19/1.88 valves on it this winter. Should get to 9.3-9.4 on compression .

 

I really like my old 7044221 Q-Jet.........its never had a major overhaul and parts are worn and shakes like a well worn winchester 30-30. I'm scard if I make it pretty and have it totally rebuild , base rebushed, ect, then it won't run right.....I guess if it an't broke I'll leave it be.
took a peak at a 72 454 auto that has sat for 10 years in a garage or shed really and hasn't been started, dame owner25years It has the original motor and transmission...... sitting on 4 rotted flat tires , dead apint and rattle can sprayed interior ..... home to mice . I think it might make a nice car with ALOT of TLC.

I like 72 and up , the VIN IDs the car.