My 32 year old LG4 owes me no more. I am considering replacing it with the GM crate 350 290hp engine. I have been advised to not use the intake and Q-Jet carb from my LG4. Does anyone know how this might work or not work? Thanks, Gary.

 

Why not use them?

Got extra cash burning a hole in your pocket?

If they are in good shape use them and tune as you would any other carb. There is nothing about the "290" hp 350s cam that would be to wild for them. Without headers, exhaust, intake and tuning that motor wont be anywhere near 290 anyway.

 

The intake and carb will work just fine. You may have to readjust the carb fir the larger engine but it will work as it should. No, it won't be a restriction at this point. The exhaust from front to back though is ALREADY a restriction so keep that in mind. I wouldn't advise using that crate, it won't have much more power than the LG4 was capable. That crate has low compression and a terrible cam. I'd go vortec crate and just change the intake manifold to the vortec style.

 

x2

 

Thanks for your advice. I thought that the Q-Jet would need to be tuned for this engine. I am planning on running shorty (Hooker) headers and large intermediate pipe and tails to accommodate the 350. Plus...my exhaust system is a 32 year old original.

I would prefer to go the vortec route....ZZ4 or even ZZ5. It's just a matter as to how much $ I can work with (get from the family coffers...I have a wife and 4 kids... ).

Thanks naf, aliceempire and midias.

 

We got a vortec 5.7 for 900$ used with a warranty and got excellent results. We got in the 290 HP range that you are wanting, with a carb setup, to the wheels its actually closer to about 265-280. With stock l31 cam. Ive seen them on Craigslist for 500-700$ range as well. I just did this swap and definitely recommend it, but stay away from the stock intake and q jet. You cant use the lg4 intake anyway , due to the vortec head intake bolt pattern.

I am about to buy a 85 with a 305 qjet for another L31 swap. I will not be keeping the computer controlled qjet either. For 100$ or so i can find a used edelbrock carb or holley.

Im actually doing a similar swap to you except ill be slapping in a lt4 hotcam and reworking the vortec heads. itll also be easier to tune ditching qjet anyway. If you are that dead set on a qjet they are 200$ new on summit.

 

Just so we are all on the same page...

Are you talking about the 290 HP PN 12499529 correct?
Not the universal PN 10067353

If so, why not just go brand new vortec L31 replacement.

The 12499529 is around $2100-2200

The Vortec L31 PN 12530282 is $2050-2150... So in fact a little cheaper.

The 12499529 290 HP has 8.5:1 compression, flat tappet cam, junk smogger heads, cast pistons, 2 pc rear seal. 2 year/50K mile warranty

The 12530282 vortec has 9.4:1 compression, roller cam, vortec heads, hyper pistons, 1 pc rear seal. 3 year/100K mile warranty.

On the L31 vortec engine with a cam change, vortec carb'ed intake, set of headers that engine will make 330-350 HP no prob, and really 400 HP aint hard to do with that one.

 

The qjet uses the o2 sensor for feedback making it a more accurate mixture. Performance wise it's not a choke until hp goes much higher. Plus with the small primaries qjets tend to have really good throttle response and mileage.

Most carbs can be properly setup to work for good street use. To each their own but I'd personally never switch from a qjet to the other carbs for a street car. No real advantage, the originality stays closer to intact, resale stays up, and nothing gets hacked. But again, to each their own.

 

I dont know how resale on a LG4 car is ever going to be higher with stock setup over a performance engine.

I can see a 1LE or G92 being worth some scratch if all original but a LG4 car.. idk

I always hear that 'throttle response' is better on a Qjet but I think its all psychological. A proper Holley or Eddy, I could feel no measureable difference between the three to be honest.! I see that alot of guys over carb their engines and put big 750 CFM carbs on mild 350s and wonder why their low end takes a hit. Im just talking about my personal experience. Eddy\holley 600 CFM all the way on this one especially if he goes bigger and better engine because its a million times easier to tune and he doesnt have to rely on some guy out in the boonies to try to figure out how to tune that old PROM for the qjet to get that 'perfect mixture' you are talking about.

And if he insists on Qjet.... Non computer controlled! but like you said to each his own. I am sure he will figure out what is best for him.

 

I'm referring to resale not so much in price but as in being easier to resell. A car often looks more appealing when you think no one has hacked it up. Seeing an aftermarket carb makes me think someone didn't know how to fix it...what other things didn't they know how to fix? But that depends on the buyer and depends on how well the car was put together.

It's not psychological, it's science. Keep the air flow velocity up, the response will be quicker. Also, I believe those qjets were 750cfm. A 750 holley will have larger primaries and that's why that throttle response may be a bit softer. Nothing to do with loss of torque or low end, just loss of air speed. I'd prefer a qjet on the street because if I open the throttle just a little, the car will start moving faster without waiting for the air to catch up. Just feels more crisp. A proper size, properly adjusted Holley will do the same.

I don't know about easier to tune. They all work in the same fashion. People just have to understand them, just as any carb, to tune. Electronics aren't needed to be messed with to tune it. Tune it like a normal qjet. The prom will need to be changed if you're working on spark curve though. A lot of people switched to a L69 prom for that.

I will give this though, I'd never use a qjet on something that wasn't equipped with it. And I likely wouldn't use one to race. Much faster to readjust a holley, barry grant, etc style carb.

 

Factor into the equation that the L31 will require a new manifold and flexplate/flywheel. It will also not have the mech fuel pump provision.

Each is easy to overcome, but increases the capital investment slightly.

My opinion is that the L31 will be capable of better mileage over the long run vs. the older model 350, stock for stock, allowing a future break even on cost.

The ccc-qjet can easily be adjusted for either motor with the stock cams.

 

I just did a 5.7 vortec swap.

You can use the same flywheel you already were using on whatever old 305/350 you were using. We used the old 305 TPI flywheel with no issues.

and as far as a Mech. fuel pump provision. Youre right, but it doesnt need one. Doesnt a computer controlled Qjet use an electric pump?

We used the stock 45 psi electric fuel pump from the TPI and just used a bypass regulator to bump the fuel pressure down to 5.5 PSI. worked like a charm.

With a 600 CFM carb fuel wasnt bad, he uses it as his daily and says he puts in about 30$ a wk. however with all the extra power the vortec makes I promise, mpg will not be there for long once he realizes how much more power it made over a LQ4....

Fact is from my personal experience, it only cost us 1500$ to do a used vortec swap with a warranty on the engine. There is no way you will find an engine that can perform like that for that kind of cost. and if we bit the bullet and got the hotcam and valvesprings it would have only raised the total cost by a few hundred.... so 350+ HP motor for less than 2k$....

Granted we did all the work ourselves but in my experience most third gen guys can turn a wrench.

 

I am in the process of putting the l31 in my camaro. I have one in my vega and it hauls the mail as well as as a new 5.0 or SS camaro. The 2900 pounds helps alot.

 

32 years puts it at 83/84: two-piece rear main, new flexplate required.

Prior to 87, the LG4's were not factory equipped with the 'helper' pump in the tank, although it could have been a dealer installed 'fix'.

The LG4 still requires a mech pump on the block as the in-tank pump is not adequate to support the motor's fuel requirements. It's purpose is to prevent vapor lock between the tank and the mech pump.

 

To add , L69 cars also had the helper pump including the earlier cars.

 

Mines a 84 L69 and it has no intank electric pump or wiring for one.

 

Interesting, I wonder if it changed midyear or hit and miss sort of thing. This one had it..https://www.thirdgen.org/forums/tech...y-84-fuel.html

 

*facepalm*

The guy wants around 300 rwhp. He isnt gonna get that very easily with the stock intake and Qjet.

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