Want to spruce up my 165 h.p. L-48 convertible. Maybe 225- 250 h.p.
Don't want to blow the back of the car off. Anybody in the same column?

 

1. Recurve your distributor. Look under tech tips or for Lar's tech paper.
2. Lars has an excellent paper on timing, tuneup and troubleshooting these older systems.
3. A mild cam change.
3. Different manifold. Edlebrock Peformer series for example.
4. Exhaust - 2 1/2" dual with crossover pipe. I'd keep a cat.

 

No one ever seems to mention shaving/milling the heads...
a mill and a good valve job are cheap... your probably going to
change/freshen the springs with a cam swap... so it's logical to just
perform all of this at one time..

Raising your compression with a head shaving will make your cam
choice easier and more effective..

my suggestion

1. Mill heads/valve job
2. GM 327/350hp Corvette Cam
3. Free Flowing Exhaust
4. Carb and Ignition at optimal

 

with the above, but a decking won't get him the kind of compression bump that a nice set of used Vortec iron heads and a Vortec manifold would.

A set of cheap Vortec heads with a small bit of work and a cheapo Summit cam and lifter combo will get you right around a full point of compression and another 75-100 HP IF you can get the exhaust out faster.

Combine the head, intake, cam, lifters for a single swap and you're looking at a solid gain.

 

Here's an article on L-48 upgrade. It's the one that says "Horsepower goal".

 

No offence to anyone on this but I don't agree with any of these suggestions. You've got a 33 year old car that no matter how well maintained it's been is going to have inherent wear and weaknesses. If you start messing around with changing heads, increasing compression, changing camshafts, you may open up a can of worms. You'll get away with it for a while but sooner or later it just might come back to bite you and fail. You can buy a real nice GM crate motor with a warrentee that's engineered to perform perfectly for less money than you might think. In the long run it may be the cheaper more reliable way to go.

Bill

 

Good point but I would have assumed a bottom end rebuild would be looked at when everything was torn down. A compression test prior to decommission and a check of the bearings.

I guess we should have mentioned that before our suggestions...as it seems to be a step we know to do but forget to bring up.

 

Huh??? re check the above statement... milling the heads
makes the combustion chamber smaller.... so you can achieve
the same compression ratio as swapping on 64cc heads.... like Vortecs
and no manifold change or head purchase.

He also stated he only wanted to get up to 225hp.... so why suggest
he spend $$ on a head swap... the 327/350hp and 350/370 corvette
engines used stock unported heads.... a good competition valve
job on his current heads after milling will have them flowing well enough to achieve 350hp or 370hp with cam/tuning.

 

How much compression can be gained from decking? I wasn't aware it was that significant.

 

It's very easy to turn 76cc heads into 9:1 to 10:1 street heads with
Milling...

Milling and Decking are 2 different operations...

Milling (sometimes called shaving) whittles metal off of the Head's mating surface. The Head's CCs will be reduced after milling.

Decking takes metal off of the block's mating surface.

Most of the time you can have your heads milled for less than
$100...

Mill, Valve Job, Cam, and Tune.. you could very well bump the
power up to 350hp using a Factory Corvette Cam... that's 15hp less than a 200hp increase for less than $500 if the L48 is putting out 165hp.

 

Caution: If you mill the heads you might have to mill the port face of your intake manifold to match.

 

The heads on this engine are probably 882 castings. These are one of the worst Chevy heads as far as cracking goes. Milling the heads enough to make a meaningful difference in the compression ratio will make a bad situation much worse. I would never mill a set of these any more than necessary to make them flat. Milling is a very bad idea with these heads.

The thread starter needs a conservative combination that will retain the low end torque yet improve the top end breathing. That being said you must keep in mind that even in modified form this will always be a low speed motor. The compression ratio will limit your options as far as camshafts go. A mild camshaft will run out of steam at about 5000 RPM. If you are able to raise the compression ratio substantially (World Products has some 58cc heads with 170cc intake ports) you will be able to run more camshaft and wind the motor higher.

If this car were mine I would first do the exhaust (headers and a good 2 1/4" dual exhaust). You could probably do this for $400. Then I would do a street port job on the original heads. This can be done for $125 if you do it yourself. Use the thinest head gaskets you can get. These two modifications will be good for 35 to 50 HP. Then comes the camshaft. I would not go over 210 degrees duration at .050" lift. You need to maintain low end torque. If you put too much camshaft in it you'll screw it up. The camshaft will probably get you another 20 HP or so when used with the other mods.

Then I would start saving up for some heads.

BigBlockk

Later.....

 

Yooooooo bigblock

Your answers seem the best. I know this just a crap motor but I want to keep the original #s. I'm think new pistons, cam and vortek heads.

MarkeyMark

 

I doubt you'd need new pistons, because switching to 64cc heads will up the squeeze with your existing pistons to approx 9.2-9.5:1, about the most you will want to run with iron heads on pump-gas:
choose a cam that will work-well with the heads, yet not turn so-many RPMs the 'junk' bottom-end can't handle it (5600-5800? ), and I think you'll be happy.

 

In my earlier post I was under the impression that you did not necessarily want to rebuild the engine but simply jack the power up a bit. That is why I suggested a couple things you could do quickly and relatively cheaply that would give a good return. If you are in the mood for a rebuild, that can simplify things a little, though it will be more expensive.

Have your engine block checked to make sure it will clean up with a .030" over bore. Speed-Pro makes a hypereutectic piston that will give 10.2/1 compression with a 76cc head. Now you can use more camshaft. Crane makes a modern version of the old 350/350 camshaft. It has 222 degrees duration at .050" lift and will make good power to 5500 RPM, maybe a little more. This will give you a good foundation for the rest of the engine.

The worst thing about the 882 heads besides the cracking issue is their large combustion chambers. Because we have gotten around that by using the pop up pistons I think we'll keep them. If the truth be known these heads actually flow fairly well. When you do the street port job on them you don't want to get carried away and reshape the port. All you want to do on the intake side is blend the sharp corners and knock off casting flash and ridges. Sometimes the most important thing is to know when to stop grinding. Go easy. You want to leave most of the cast finish untouched. Do not polish the intake ports.

Now the exhaust ports are a little different. You will grind the entire length of the port to blend corners and ridges and to remove the cast texture ONLY. Do not reshape the port. You want the surface smooth but you don't want to change the shape of the port. Now some polish the exhaust ports. I don't think that is necessary. All you want is the cast surface smoothed out.

Top this off with a good three angle valve job and back cut intake valves and you're good to go.

With this long block, just about any descent intake manifold, carburetor and exhaust system will be right at 350 HP.

BigBlockk

Later.....

 

My old combo made 220 rwhp: HE260H cam, Performer intake, hooker headers....everything else bone stock L-48 with TH400 tranny. Cost about $1,000.

My current motor made 250 rwhp with stock bottom end (pistons/rods/crank) with TH400 tranny: Performer heads, performer intake, hooker headers, chambered exhaust. Cost about $2,000.

You don't have to touch the bottom end for your hp range (unless it needs attention). Do a compression check (leak down is better) before you go tearing the motor apart. Good luck.

 

I certainly don't begrudge you being angry about 882's cracking..
especially if it's happened to you...or someone you did some work for...
That being said I'm going to have to respectfully disagree with
you about them being a bad choice for milling operations...

882's are VERY popular for Dirt Track use in Claimer Engine and
Stock head classes... the 76cc chambers make them ideal to perform
ANGLE milling proceedures on... deshrouding the valves for increased
flow.... Angle milling, not flat milling is the proceedure which causes
a manifold mismatch.. Most of the 882's in Claimer/Stock head classes
are attached to motors making 450hp plus....
My take on 882's cracking is similar to my take on problems with electronics or batteries... if they are going to go bad... they usually go bad early in their life... I've seen angle milled/seasoned 882's last 5 seasons atop Dirt Racing engines with narry a problem...

882's flow as many CFM's as Double Humps when they've had
a decent valve job.... plenty of flow to hit 350hp plus.

 

I'm sure 882 heads do very well in claimer racing. Like I said in my earlier post, they actually flow pretty well. I never said I was angry about them.

I don't think you can compare street service to claimer racing. I don't know of too many claimer cars that are driven to work when it is 10 degrees outside. Then driven home that afternoon. For weeks on end. This is called thermal cycling. It along with age and over heating, crack more heads and blocks than any other single cause. Street engines experience far more of it than do racing engines. This, and the fact that the 882 is a thinner casting than some of the more performance oriented heads can give you trouble. This is why I say if you cut away substantial amounts of iron it will weaken the head. Now, take this head and run it through the heating cycle twice a day for several years. I don't think race motors are run that much.

Anyway, the thread starter has indicated that he may do a rebuild. He will have to buy new pistons anyway so an extra $5 per gets 10.2/1 compression without milling the heads. What does milling a set of heads go for now, $100 maybe. If he goes with the pistons he will be $60 ahead. Now that I think about it, that is probably why Speed Pro makes that particular piston.

BigBlockk

Later.....

 

882 and 624s are junk.
Vortecs are a waste.
But to save some money get your factory heads checked out and a valve job, then use some GM .029 head gaskets from Summit, then port match to the gasket, but not $1000 for street heads like AFR or Brodix.
An aluminum intake such as a new Weiand stealth or Professional Products line.
Drop the Q-jet and get a 750 Holley and this will be your biggest gain in power right there.
Add a decent cam like the Comp Cams 470/270 magnum and some roller rockers(anytype other than stock) and a new $100 recurved distributar and wires along with your new headers and 2.5 exhaust with 40 series Frowmasters and you will be satisfied for a street cruiser!
(PS) for a cylinder head swap, I suggest a set of Dart 194/150 iron Eagles for low end grunt and power, 202/160s are for high end bigger cammmed engines and a waste on the street other than bragging rights!

 

Everything is a compromise. You need to decide what you want to give up.
Right you now you have a gutless motor but its very quiet and idles smooth. Once you start to mod the motor, cabin noise increases and depending on the cam, idle quality suffers. To retain the L48 longblock, you will definitely have a compromise, especially if its an automatic with tall gears. We modified a 76 L48 automatic with all of the typical bolt-ons. Dart iron heads, a Comp Cams 262 cam, Edelbrock intake, open element air cleaner, headers and dual exhaust. Going down the road, if it downshifted, it would scoot along OK but from a standstill, the stock torque converter and 3.08 gears killed it and it took a while to get rolling. The same overall acceleration could have been had with a rear gear change, but then you run into high rpm's on the highway.
I just swapped out my 1975 L48 yesterday and dropped in a Goodwrench crate motor. It has a mild cam and an aluminum Holley intake. I am still using the factory crappy exhaust and manifolds because I want the car to be quiet and I didn't feel like fabricating AC brackets. The car would definitely make more power with headers and duals, but I don't want the noise.
An engine is just a big air pump, and it has to have an "insa" and an "outsa", when they are matched, you make power.