I have a 73 L48 I’m looking to build a 300hp clone I know that the ran a different intake and I know they were 186 or 187 64cc heads but does anyone know if the L48 and the 300hp 350 ran the same piston

 

In 1973, I dont think the piston on the L48 and the L82 was the same. That said, making 300HP even for an L48 shouldn't be too difficult. A pair of 64cc heads and a cam/lifter kit would go a long way to putting some of the power back into that SBC.
Also, set your distributor and ign timing up using Lars Grimsrud paper on ignition timing. That alone made a huge difference in my 73.

 

The L48 should have dished pistons. The L82 forged flat tops.
L82 also had a 4 bolt block. Stronger crank and rods.

 

When I rebuilt my L48 I bored .030 over, went with flat-top pistons, and changed to Eldbrock 64 cc heads and intake. I used a Crane cam with 440/454 lift. Later I went with the Magnaglow 2-1/2” exhaust and Ramshorn 2-1/2” manifolds. On a chassis dump that put me about 300hp.
If I do it again I would probably look into a different cam.

long story longer, before I changed the heads I had just kept the original heads with stronger springs. The rocker studs are pressed in not threaded and they started coming out from the extra lift/spring pressure.
Getting to 300hp isn’t that difficult but it still takes some thought.

 

The earlier 1970 era 300 HP engines would have flat tops and small valve 186 heads for an "advertised" 10.25 CR. Truth was probably closer to 10/1 CR. L-48 type pistons would be a little lower in compression than that with the 186 heads. Maybe ~9.75 or so CR.

How accurate of a clone are you shooting for?

 

No, the pistons were forged flat tops on the performance motor. l48 is the cast station wagon engine with 2 bolt block, dished cast pistons and cast internals . That said, the cam pistons and heads ( I think the valve sizes were different but not sure) were different. You can bump the compression on the l48 with 58, 62 0r 64cc heads and add a good cam for the combo and EASILY make over 300hp reliably. you will need to unrestrict the stock exhaust too for best results. If your looking to make it all appear stock than options are limited and keep in mind the earlier hp rating system inflated all those hp ratings compared to later engines if your not already aware. the hp was rated at crank with nothing bolted to engine.

First thing I did was pull my 74 l48 and set it in the corner of the garage. I then found a machined 355 block and built a new engine aiming for 350+ hp... hyper flat tops 64cc heads and a 218-224 duration cam took me there. facebook is full of these engines cheap some with no or little use as people change directions for LS swaps and the like. Hell I may be in that same bag selling my 355 roller engine with edelbrock alumium heads and intake and comp roller cam soon since im putting together a 406 to replace it now. Nothing wrong with the engine, less than 10,000 miles on it. I just want something bigger with more torque for my new overdrive transmission.

 

ttt, thanks....

 

The high compression L48 (1969 ZQ3 or 1970-71 L48) used "flattop" pistons (actually 4 eyebrow -6 cc). To recreate the earlier high compression L48 from a later low compression L48 you would need to change pistons.

If all you want is the 300'ish hp level then it would be far easier to just swap to heads with a smaller chamber. 1960's 64cc cast iron heads and C4 58cc aluminum heads (128/113 castings) can be found inexpensively.

When I rebuilt my '69 ZQ3 restomod back in 2015 I used a 1980-82 aluminum intake and C4 aluminum heads (as well as '81-82 stock tube manifolds). I did this for weight savings and not performance, although, I did add 1.6 rockers. The L48 cam duration and overlap isn't awful but the lift is super low @ .390/.410. 1.6 rockers help a L48 cam a lot.

 

Agreed.

Your ZQ3 probably had 4 bolt mains, right? I know the one I built 2 years ago came with them. Cast iron crank.

 

Yes, "442" cast crank but the desirable "2482" nodular 4 bolt caps. Also had 8" balancer.

 

The 73 l48 in question here would have dished pistons though correct? I know my 74 l48 did.

 

yes.

 

ZQ3 cast flat top 4 eyebrow pistons.


1970 ZQ3 350/300 stock short block.