I have a L48 1974 corvette. I have owned it for over 30 years. I have replaced all the bushings, front springs and rear leaf springs and the shocks. Now I am looking to improve the hp. I am looking for advice on a carb engine with approx 300 rear wheel HP. There are lots to choose from. The dilemma I am having is to find one that will fit under the 1974 hood.

Originally I was going to redo the Qjet carb to get more air for the approx 350 HP at the flex plate. I was looking at 64cc heads, a low rise dual plane intake, a mildly aggressive cam and of course long tube headers and bigger exhaust. I don't want to rebuild the entire engine.

I am actually not the one doing the work. So the cost of figuring out if the bottom end is good enough and putting top end parts on versus just putting a new modern engine in is where I need help. The new engines concern me with the room for the front accessories and the air filter /hood clearance.

I am not a mechanic and the vacuum lines are a another dilema.

If anyone who has some advice on this topic can help. I finally have a little money I can put into it. Luckily the body is in good shape and itdoesn't need a paint job $$$THANKS

 

L48 performance upgrades by the numbers

 

GM crate motor. 300hp 350. Everything goes back in the same place. 2 year 24,000 mile warranty

 

But a 300 HP engine won't get him 300 at the rear wheels.
And of course which transmission he's running will have a lot to do with that.

 

This crate engine will, With the correct intake, you can even put a carb on it*, and run an MSD 6014 ignition controller. With a bit of luck on Facebook Marketplace, you can find a running LS engine and complete the swap for little more than a pair of SBC heads.

https://www.gmperformancemotor.com/parts/19435110.html
https://ls1tech.com/how-tos/slidesho...-motors-500688

*You might need an L88 hood to fit a carbed LS engine, as the available intakes are all pretty tall.

 

A brand new Gen 1 sbc OE replacement GM powertrain L31 Vortec 5.7L motor --- with a cam swap w/ one of summits Pro sbc rollers at under $290 + an alexs valve improvement kit at under $150 should readily meet or exceed 300 rwhp. Best warranty in the biz; legally enforceable throughout USA & Canada.

 

Modifying any replacement engine like a camshaft swap will void the warranty. Installing a GM replacement engine in a vehicle that did not come with that engine originally is considered a engine swap and will also void the warranty. Replacement engines must go into the vehicle originally designed for it in order to maintain the warranty.

 

That may or may not be so; much of that may depend upon users' relationship with their local GM dealership's service manager and his area manager.

Regardless, that new L31 option will do the job with proven reliability and, with a cam & springs like suggested, it certainly can perform per OP's stated wishes.
Jeez Louise, OE L31 with a good carb & tune and OE cam & springs makes some 315 gross HP at flywheel.

Oh, dang it, perhaps I should apologize for not recommending the BPE option ? I remain in the GM camp.

 

Sounds like an oxymoron.

 

Sounds like you look for and act on an opportunity to poke the bear. Perhaps stop with that ?

Regardless and once more, that new L31 option will do the job with proven reliability and, with a cam & springs like suggested, it certainly can perform per OP's stated wishes.
Jeez Louise, OE L31 with a good carb & tune and OE cam & springs makes some 315 gross HP at flywheel.

I suggest this, majority of content (both bulk and dollar-wise) of that 341 is Not from North America. The reverse is so in regard to new replacement L31. For some that makes no difference; for others, it matters.

 

Who here has a good working relationship with their GM dealership service department? And if you do that probably means you are there a lot and that can't be a good thing...

I don't care who buys what crate engine, blue print has been good to me so I'd probably go that route based on my personal experience. And I also would not go with the GM for the same reason, their service departments suck, and when your GM crate has an issue you have to take it there for them to diagnose. I seriously doubt the majority of them even have a guy who has ever worked on a Carbureted 350 and they are going to determine what's wrong...I hope their WiFi is working so they can google it...

Any of the big three crate engine builders can produce a dud, do enough of them and it will happen, and there a poor reports on all of them on the net. Having a local guy do it is an even bigger crap shoot and they often don't have the resources to properly address an issue...

60

 

I pulled my 74 l48 and popped in a craiglist 350 with flat top pistons and 10:1 compression along with 64cc dart heads for less than a grand but I assembled myself. Something to consider is what this car is for and how it will be driven? Is this a track car? A 4 speed or auto? Because if it's an auto car that's going to be mainly just a street driven car you want more get up and go for, then you may belooking to address the wrong issues. The l48 actually has more torque and power when accelerating from a stop sign than the l82 this is why car and driver reviews favored the l48 c3 on the street. The rear gearing and th400 gearing is not ideal for a fun street car that transmission also consumed like 35% of the power in parasitic drag which makes it a huge loss for delivering the power to the tires. For this reason I swapped my transmission for a 2004r with a better first gear ratio along with my rear end for a 3.55 rear gear which allows me the best of both worlds. Lower highway rpms with less drone from the sidepipes and better fuel mileage along with better torque delivery to the wheels with the better gearing. I also am now swapping in a 406 with a cam that will deliver 500lbs of torque... This will likely result in me having to replace the 2004r with a beefed up version down the road but so be it.
My point here is an engine designed for higher HP at higher rpms gives up power at lower rpms where it's used on the street 90% of the time so don't fall into the trap of being underwhelmed and having to change the rest of the car to get the feeling of power your after when you actually drive it.

 

Wow, great advice and options here. I love these threads.
no clue on current engines condition.
If you have any diy skills, maybe try increasing power in stages?
these also could be used on later upgrades ..and preserve budget.
- your carb can handle enough airflow for more power. Consider a tuned rebuild from forum member Lars. You can diy this by mail.
- timing of your engine, another diy
- exhaust, diy if lifted , but this can be outsourced by your trusted mechanic as well
- this forum walked me through everything and a great resource/ people.

see how those upgrades feel.

good luck.. how about a picture of your ride?

 

Welcome to the Corvette Forum virago99A!

We are glad you have chosen this forum and are glad you are with us. This is the place for questions as Corvette troubles are solved regularly here for folks.
I come here to learn, there is a lot of knowledge here if you have the time.

Cool air and cool fuel help make power. I have insulated my fuel lines and then use a cold air intake to insure that my engine is happy.

 

L48 performance upgrades by the numbers

I wrote this thread specifically to avoid having to repeat myself over and over.
This is a step-wise approach, if like you said, "you do not want to rebuild your current engine"

The original compression ratio is very low, it likely measures at 7.8:1 although spec is 8.2
Heads, and then cam, will make the most difference.

You can tweak an L48 pretty good, while leaving the pistons in place, and get pretty decent results in the mid 300-350HP range.
If you do it right.

 

If you consider a crate motor that delivers with ALL Front accessories ALL brackets ALL pulleys. Stop right there and You Determine if it uses a very common "Long" water pump.

Fact: if you plan on keeping your C3 mechanical fan and its shroud, that Long pump Will NOT fit because it's Too Long and will push mech fan too far forward.
Fact: Your '74 L48 C3 delivered new with a "Short Corvette" pump. apples and oranges.

 

Obviously a Crate Engine is the easy/peasy way to go. Pick your power level and reputable company to buy from.......and you're done!

If the budget doesn't allow for this and you have decent short block to work with in the car already it's pretty tough to argue against any one of the many available 64cc aluminum heads out there for sale in the after market to replace the absolutely worthless emission era heads on the engine now. You'll have a still easy to live with 9:1 compression at most but it's just about enough to make flat tappet Comp cams 268 or even 280 cam work. I'd favor the 268 with a stock converter automatic for sure. It won't make as much power over all but it will work better down low than the next level up 280. For a 4 speed car or a 2,000 stall converter if you you choose to use a Comp 280 grind David Freiburger's engine master show verified an honest 1HP/ cubic inch and just so you don't have to take my word for it. They use vortecs but for very little bit of extra cash spent you can get very basic aluminum as cast heads so I'd skip the Vortecs......here 'ya go: (yes, the crappy rebuilder special they use is basically an L48 clone)

 

Harder and harder to find these days; been discontinued for many years.
OE serial production GM Aluminum heads with 58cc chambers; there's big compression bump for L48/LM1/dish piston 350.
OE on '88-up camaro & C4 vette w/ L98 motor. Also on discontinued ZZ4 crates.
Casting Number 10088113 and they're standard NOT reverse flow: for GEN 1 sbc.
Good, robust heads but Not modern fastburn chamber, flow rates uninspiring, but that 58cc will certainly bump compression

Trick Flow has some nice Al heads w/ 60cc chambers.

 

I remember the advice to use those original Corvette Aluminum heads with the 58cc chambers. It was the recommendation for many "Hop ups" of the low compression engines 30 years ago. An 8:1 engine could be pushing closer to 10:1 with those heads. As tempting as that sounds vs. the great many available 64cc options available now that will only net 9:1 it's still not worth it. They don't flow nearly as well as a set of iron Vortecs or any of the entry level "as cast" aluminum heads from companies like AFR. They're getting hard to find in no small part because the advantage of using them just isn't there anymore. Easier to find, more power potential and a pump gas friendlier 9:1 makes the new head designs the way to go vs reaching for just a tad more compression. Seems to me I also remember that some headers don't work well with those heads. You had to be careful about which headers you were working with. The exhaust ports were a little different. Maybe someone else remembers the issue on that one and what works and what doesn't.

 

As I wrote, flow "uninspiring" --- Intake port volume only 163cc
The earliest versions have Round exhaust ports.
Later versions have a D-shaped ex port (like Gen II LT4 w/ flat floor + round roof) that is Raised 0.100" requiring Fel-Pro P/N 1470 gasket (Many of todays aftermarket heads have raised ex ports).

*plenty of em on x603 circletrack crates (discontinued), where some round header flanges could be convinced to fit. T-shirt.
x603 essentially ZZ4 but "sealed" + different valve covers + breathers And a very good left turn-only pan + windage tray.
Seems most OE GM dished pistons may fall short of 10:1 w/ 58cc chambers.

Even today, ZZ4 heads, if acquired dirt cheap, can be a viable option to boost compression (and power) on dished piston 350.