Highlander1732

Thinking way ahead here as I'm still working through suspension and cosmetic issues with my numbers matching L48 1975. That said, I'm hoping to hear options for increasing the performance from my stock L48. The AIR pump has already been removed and true dual exhaust w/o cats has been installed by PO. So I'm looking for anything that will let me keep the stock intake/block/carb/exhaust manifolds. Am I limited to cam change and maybe re-jetting carb and porting heads? Can I stroke the stock block to make a 383 maybe? Again, just a wish list at this point. My goal would be to boost performance keeping the same "look" as the original underhood appearance.

Dynra Rockets

Camshaft and block bore/stroke are undetectable from an external view. Most people will also not notice a head swap.

I had a similar goal to keep it looking factory but mine was more about weight loss than power. I used an aluminum intake from a 79 painted orange and aluminum heads from an 87 (with adapter plates for earlier valve covers). Only 2-3 die hard Chevy people have noticed when I pop the hood.

You need to define exactly what you want your "power boost" to be and where the power band will be. A factory style intake manifold and exhaust manifolds will be the bottleneck to higher rpm numbers.

I took about 150 lbs off the front of my car with everything I did. At 6 lbs per hp that is the equivalent of me adding 25 hp. The 58cc heads helped a lot in the torque department also.

mobird

Why worry about keeping it looking stock? Unfortunately, a numbers matching L48 isn't really worth anything more than a non numbers matching L48. This is coming from a guy who has an L48 by the way, so I'm not hating on it by any means! Just hate to see you limit your performance to stock intake and cast manifolds for a reason that won't really change the value. If it's for your own personal pleasure of having a stock looking engine that performs better, then by all means go for it!


And if that's the case, and you REALLY don't want to put on aftermarket heads that aren't original, then you can have the stock heads shaved and ported and polished and install a better camshaft. Should be able to get a little more power out of that L48, but you will be majorly limited by the stock exhaust manifolds, poor flowing stock heads, and (to an extent) the stock intake.


If you aren't willing to change the heads or the exhaust manifold, I wouldn't bother with a 383. The stock heads and exhaust just don't flow enough to get much benefit out of a few more cubic inches.

jb78L-82

I did the same with my numbers matching L-82 4 speed in 2014 when I rebuilt/upgraded the OEM motor and wanted to keep/be as much as possible looking like a stock L-82 but with MUCH more power. The L-82 now is a 355 (bored.030), has aluminum AFR 180 64 CC CNC ported heads, roller cam, 10.2 compression, but I reconditioned the L-82 forged rods, reconditioned the L-82 forged crankshaft, cleaned and mildly ported the aluminum L-82 intake (L-48's do not have forged components or intake) and reused the OEM dual snorkel GM cold air intake, reused the L-82 high performance oil pan, L-82 valve covers, etc. If I had painted the AFR Heads, you would be really hard pressed to look at the motor and know that the engine now makes about 425+ Gross HP.

NewbVetteGuy

To get any meaningful power improvement at this point, you're going to need new heads. Do NOT waste the money porting those heads, that are going to crack as soon as you port them, anyway.


An L82 intake isn't a bad way to go, especially if you can find one for cheap on here. Personally, I'd ditch the stock rusty manifolds and get headers while you're at it, but that wouldn't be a stock look, obviously.

You'll want springs and a cam matched to those new heads, too.

The Jegs brand SBC heads are available in 185cc and 195cc intake ports and are rebranded Profiler Heads; on a sale you can get 2 of them for $998 shipped; I believe they have a version with springs appropriate for a flat tappet.

This winter, I'm going to be pulling off my L82 intake, heads, and cam and will be looking to get rid of them; maybe you can find someone on here who's already done this and who would be willing to sell you the L82 intake and CAM.

I'd also recommend making sure your deck surface is super flat and going with the Felpro 0.015" thick gasket to up your CR by 1/2 a point.


A stroker kit is definitely an expensive way to get there, particularly if your bottom-end is good. Spend $1000 on some good heads and pickup a nice aluminum intake and call it good.
Hmm... I've only been considering parts for my L82, but mobird probably has a good idea with 58cc chambers instead of 64cc with an L48 -get a good pickup in that abysmal CR. -Anyone run the numbers on what the CR would be on a typical L48 with 58cc chambers and a 0.015" Felpro gasket?




Adam

Highlander1732

Thanks all for the comments. Maybe I am barking up the wrong tree considering a lot of the originality has already been removed via the smog equipment. I know '75 L48's are the dog of the Corvette world given low factory HP, but it's a pretty well optioned convertible so I was thinking it may be worth something more down the road more original than less and am thinking I should try to walk the fine line between keeping as much of the numbers matching allure as possible and get some more oomph out of it at the same time. At the end of the day, it's never going to be an NCRS car and I'm getting it together enough to be driven without worrying too much about "correctness" or being a trailer queen. Maybe I'll just go chevy orange crate motor and keep the numbered block under wraps

Dynra Rockets

I have that that combo. my 58cc heads with a .015 head gasket on a 4 eyebrow flattop piston (7cc) is about 10.75:1

Assuming stock deck height a later L48 has slightly larger cc dish is at about 16cc so figure just about 10:1.

The 1990 L98 with the factory .015 head gasket and 12cc pistons was factory rated at 10:1.

stingr69

You need better heads for sure. More compression and more flow is required.

Cam selection depends on your exhaust system. Cast iron exhaust manifolds/restrictive mufflers need less cam duration/overlap while headers can handle more overlap and a higher RPM powerband. Even the 350/350 L-82 cam runs much better with more compression even with the cast iron exhaust manifolds. Not real sexy but it does work.

The 383 stroker is only on the table if you need to bore the block. Good modification for sure but not worth the expense if your bores are bottom end are still serviceable.

Recurve the ignition after the modifications are done.

That is all I would do. The rest you can work with what you have.

GOSFAST

Easy build, we do these regularly for a local resto shop here on Long Island. Most L/J units end up on the 383" platform.

All MUST stay 100% stock-appearing down to single-thick V/C gaskets.

All are full roller builds, cam/retro-lifters, (real) stainless roller rockers, and lower C.R.'s, in the 9.75 range. Forged pistons, L/W rings, and independently balanced. We are also able to mill the decks (if necessary) and leave all the OEM numbers intact.

We recently devised our own method to allow the late stepped-nose cams to work in these early castings, no more cam buttons OR timing cover worries.

(Add) Just delivered a '65 original 300 HP S/J 327" (not a stroker), came off the dyno with 330 HP/5900 RPM with 9.5:1.

Thanks, Gary in N.Y.

P.S. Also just finished a '72 Blazer (383") with all the above components, 9.4:1 C.R., will be above the 375 HP mark and remains 100% stock looking. Here's a 400" SB with the late (modified) thrust plate in position.

Richard Daugird

I have to agree, and I also own an L48. It'2 a '75, not a '65.

Also, I am done with iron heads. Aluminum heads ready to go that are light years better than worked cast iron stockers, for less than a grand? No question.

http://www.blueprintengines.com/inde...cylinder-heads

Highlander1732

Thanks Gary. That's exactly the kind of engineering I was wondering if anyone had pulled off before. You may be hearing from me sooner than later if I can scrounge up enough cheese!

dmruschell

I'm currently going through this on a 75 L48 that I'm rebuilding as part of bringing the car back to life. I'm also keeping the emissions controls there (the ones that most people say don't rob horsepower).

I bought Summit Racing iron heads (they're Dart Iron Eagle 165cc heads) that will be painted, an Edelbrock Performer intake with EGR that will be painted (though the ZZ4 intake might actually look a bit more factory correct since it has a GM casting number and the firing order on display), and will be (hopefully) keeping the Quadrajet and stock air intake. I'll be putting on L82 valve covers just because they look cooler.

To the casual observer, it will look factory correct for 1975, but I expect it to make around 375hp while being easy to drive on the street.

Highlander1732

I'll look at the dart iron heads. What is your plan for the exhaust manifolds?

dmruschell

The car had headers, but I bought some exhaust manifolds (2 inch outlet) and the L82 style dual exhaust from Corvette Central. The car is an automatic, so I couldn't easily do what I did with my 79 (and with the power it's making, probably won't need anything more).

On my 79 4-speed, I had headers. They kept eating spark plug wire boots and being a pain. So, I bought 2.5 inch outlet manifolds from Speedway and 2.5 inch downpipes from a 66 Corvette from CC to mate up with the rest of the 2.5 inch dual exhaust. There was no real noticeable decrease in power vs headers. But, it looks factory, saved my plug wires, and quieted things down a bit.

There is a thread about 2.5 inch doenpipes for an automatic on the forum where a certain vendor will bend 2.5 inch downpipes for an automatic C3.

cv67

grab any good aluminum intake grind the name off, paint the whole mtoor orange. noone will know
headers are acceptable almost expected on any old car...heck they make a cool looking white ceramic coat looks old school. Use a GM air cleaner with a large snorkel outlet...some 1 ton trucks have a large one use your stock lid lits lower. Stroke it you get all kinds of power and look "stock".
99% of people will never know, could be a fun project.
Fooled a lot of people many rys ago doing one in an 89 Trans Am ran stilver state classic and did well. Car ran super strong....youd never go back t a 350 again.

GOSFAST

Just to be clear, there is NO "middle-ground" here, it's either 100% stock-appearing or not, that's it.

We've been building these for dozens of years now, and you CAN get the power with the OEM heads, it's easy.

Just going to the retro-roller/roller rockers yields an add'l 30 HP by design!

(Add) Below, a couple shots of an early build, full-roller (w/o any V/C gasket)!

Thanks, Gary in N.Y.

P.S. I'll tell you the lengths some customers go to for "originality"? On many of our earlier builds they will use a spin-on filter with an adapter, when they do a judged show they change back to the cannister for the show then back to the spin-on again! We cannot even use any aftermarket intake gaskets, must look identical to the OEM's.

69Vett

keep your block and heads, use new parts everything on the inside roller cam and rockers.
with properly built heads, all new HP parts, stroked crank and new pistons.
I even kept the original intake, plenty of horsepower and looks original on the outside.
Do not deck the block which generally will remove your numbers.

Metalhead140

You can easily paint alloy heads too. All depends where your priorities lie, and which you'd prefer to compromise and by how much - performance or stock looks. My engine doesn't look stock (was never intended to), but very few people have any idea how potent it is, or notice the alloy heads and water pump, or fuel injection, when looking under the hood. In fact, nobody has so far has noticed these without me pointing them out.

7T1vette

Maybe this is a rhetorical question, but why are you worried about keeping the L-48 'looking' stock when you have modified the car by removing the A.I.R. pump (which really absorbs almost NO hp) and added dual exhaust with no CATS?

Just do what you want to the engine and make it look as close to stock as you can. It would have been easier to have bought an L-82 in the first place.

Highlander1732

Great point. That's why I'm wondering if it's even worth it to try. PO removed the air pump and installed the new exhaust. Maybe I should just paint whatever I end up doing orange and call it a day!