BakedPotato65

I'm looking to up the power on my 1974 350 L48 engine since it can't keep up with a minivan accelerating normally. With two people and luggage while driving to car shows, it's at half throttle in order to cruise at 55mph so I figured that its time to replace parts for better overall power and get a better, easy to tune carburetor. Right now I've discovered that the valves aren't opening all the way after having to tighten a very loose rocker. I figured I don't really need to mess with adjusting the valves if I'm going to get new valvetrain and I'll plan on having these new parts correctly adjusted instead of tuning old factory parts I'll swap out very soon . So far my plan is to get a Holley 650 carb #4175, a new Edelbrock performer intake manifold non EGR #2701 with kit, low price Hooker longtubes (can't see them very well in the engine bay and I'll wrap them with header wrap so I don't need any pretty looking headers) #2456HKR, and either a Comp Cam XE268H or the XE274H with a kit. I'm leaning toward the XE274H for choppier sound because I would like to have electric cutouts for open headers and this cam still has about the same street manners with plenty of vacuum compared to XE268H (from what I've read). I may plan to get Vortec heads during this upgrade or later down the road. (I think getting better heads would be a safer option with these upgrades).
I'm asking to get opinions of this parts combo. Please feel free to let me know if this will not work or is a bad idea with a stock L48 or if there might be a better parts combo. I have never upgraded/swapped out large parts on a V8 before, only on smaller engines like four wheeler/go kart engines so anyones input with knowledge of engine experience with be very helpful. The stock L48 has sat for around 20 years and could use a tune up badly but it runs good enough for going to car shows and slow cruising, all I have done is set the timing and adjust the carburetor idle screws because I knew I would upgrade the engine later on.

Thanks for the help

TimAT

If you've had to adjust the valves on a stock L-48, it's a pretty good bet that just swapping the heads and cam, while maybe a Band-Aid fix, won't be a long term cure. I would be suspecting that it's time for a full on rebuild. And you can drop an under warranty brand new 383 in there for what you'll spend rebuilding and refurbishing, especially if you get the original engine opened up and find you need to bore and have the crank turned.
As far as swapping the parts, if you decide that's the direction you want to go, it's simple take bolts out and put bolts in.

BakedPotato65

I have considered getting a crate engine because of warranties and performance guarantees but I think I would like to have the experience of building up an engine and have a rewarding outcome. Once I settle on which parts I'm going to buy I will probably start to take apart the engine to see whether it's ok to start ordering new parts or if the engine needs to come out for a rebuild or crate engine.

TimAT

I understand-- I'd talk to someone like Chris Straub about a cam, and go with a hydraulic roller. Nothing wrong with Comp Cams, just may be able to get a better performance outcome with a custom designed cam for your desires and application. The added benefit of a roller cam is no worries about the cam going flat and easier cam break in.

jb78L-82

As others have stated, by far the cheaper, quickest solution and easiest answer, is to buy a crate engine (and yes a 383 in this case). I am not a fan of 383's in general for C3 corvettes simply because they never came from the factory with one and a properly built 350/355 can easily match most of the crate 383's but that is another discussion but in your case with a base engine 74, maybe, but even here I would hesitate since a 74 is really the last C3 making decent power before the emissions onset in full force in 1975 and the last year of the 454 C3's...just me.

If your current engine is in good shape (but probs not), you could change the heads and cam and really wake the engine up. Going this route there are many choices with inexpensive cast iron heads (vortec's) that would flow MUCH better than the stock heads and increase the compression by 1 point to 9.0-9.3:1 which is a good number. In combo with a decent Flat tappet cam (crane,lunati, comp cams) or a retro roller cam, you would be happy with the transformation.

In my case, with my 78 L-82 4 speed car, I went the rebuild route for a number of reasons. My car is relatively rare (even when new-50,000 78 C3's, 12,000 L-82's out of the 50,000 C3's, and only 3,300 that were L-82's AND a 4 speed....How many of those are left? Not many). I wanted to keep as much of the OEM L-82 as possible...numbers all match.

I removed the engine myself in 2014, had a local builder who I trust pick up the engine from house partially disassembled who did the bottom end of the motor. The builder did the machine work-bored it .030, new JE Forged racing pistons, reconditioned the OEM forged L-82 crank, reconditioned the forged L-82 rods, NO DECKING THE BLOCK to preserve the block numbers, reassembled all, and returned the short block to my house.

I completed the assembly with AFR 180 aluminum heads 64 CC, Howards Roller cam, reused the L-82 aluminum intake (mildly ported), new clutch kit etc.

The completed rebuilt/upgraded L-82 355 was a lot of work, took some time (3 months), cost more than a crate engine (but I used superior parts to any 383 crate engine) and is a blast to drive and I have the satisfaction that I did it mostly myself...My cost for the builders participation and top parts, all in...$6,000.

The motor looks and runs like a stock L-82 until you get hard on the gas...It will out run easily most 383 crate engines...but speed costs money......The AFR aluminum heads are pretty much the best you can buy for flow throughout the rpm range, not just leak flow, and the retro roller cams are about 3X as expensive as a FT cam kit but it too will provide superior area under the curve power to a traditional FT cam. I would do it again in a heart beat...was out with the L-82 355 yesterday for about 2 hours cruising and I still cannot believe the power it makes...............68,000 original miles and counting...........

REELAV8R

If you want to try to get the Q-jet to run better I would suggest this book;

I did, and modified by Q-jet, knowing nothing of Q-jet's prior to this, and got an excellent running carb out of the deal. Took Cliff's $100 rebuild kit, some tools, jets and time but in the end it runs really well.
If not go ahead with the Holley. Lots of people have them and it sounds as if they are easier to tune.

As far as the cam only change, I think both of those cams are too long in duration for your CR.
Problem is that the current heads offer little compression. GM says 8.5:1 but that is a very optimistic CR compared to what it really is. My L48 after I cc'd the heads and the pistons came to 7.6:1 CR.

A longer duration cam waits longer to close the intake valve. So the piston is farther up the stroke on the compression stroke before the intake valve closes and leaves less remaining stroke to create pressure or compression before ignition. This creates less cylinder pressure and less torque below peak torque and delays peak torque to a higher RPM. I don't believe that is the direction you want to go reading your post. You can get more HP after peak torque if you can rev the engine high enough with longer duration cams and that is the idea of a long duration cam.
Have you looked at a cam like the Lunati voodoo 262?
http://www.lunatipower.com/Product.aspx?id=2324

It may give you more of what you want.

or if you really want a comp cams maybe an xe 256?
http://www.compcams.com/v002/Pages/386/XE256H-10.aspx

Isky has some cams that may suit your needs as well.
They have a 262 cam with a 109 LSA ( this will make your idle choppier, but also reduce low RPM torque)
http://iskycams.com/cart/index.php?m...roducts_id=382

Look around and do some research on cams. It's not an easy descision and you need to know what kind of CR you will ultimately end up with. It took me many months of study to decide on a cam. Guys like Chris Straub spend years learning cams and the various applications. Calling him would be a good idea.
Isky also has some 108 LSA (even choppier idle) cams in that same range, but I'm not sure how well it will idle with that LSA and your current CR.

Dynra Rockets

The biggest help an L48 needs is compression. Throw on a set of 58cc L98 heads to get about 10:1 CR and it will wake up nicely even with the stock cam.

cv67

Trick flow 175 heads 58cc chamber..add your 262 or 264 cam headers tune that dist & Q jet it will wake up big time

Shdggsdv

My personal opinion is why spend the time, effort, and money on a new set of heads and leave the old cam in? A 268h or 274h with that compression would give a pretty good kick to a car for not too much extra expense

Also OP, I've been in the same boat as you. I'd suggest pulling the engine to do the upgrade work. I'm only just completing my engine build (274h, 64cc Aluminum heads, etc) and I would have saved so much time by not doing the engine in the car.

Also; check these out. I bought them for my build and they cost practically the same as any iron vortec, but are aluminum and shiny.

SpiritOf76

I'm looking to do the same with my '76 L48. I'm not looking to race, or even take it to the track for fun, I just enjoy a nice relaxed day trip cruise... but I do like to get on it once and a while for fun, and it would be nice if it had a little extra. Honestly, it's not BAD now, I guess its just in our DNA to take it up a notch.
I just can't decide what to do with it. My engine and trans (and almost everything else) is all original, and I'd like to keep it that way. However, I have been tempted by the crate option. I thought about pulling the original, dropping in a crate, and then taking my time to tear down and rebuild the original all nice and purdy.
At this point, I'm kind of leaning on building out the original, and upgrading pretty much everything but the lower end (cam of course), as long as the pistons look ok when I pull the heads off.
There's just so many options, and everyone you ask will give you a different opinion.

REELAV8R

If a compression bump is in the future then leak down test is a good idea to see if the current rings can support the bump without becoming an oil burner.