'Specs' on L48 ('71) ?

[corvgreg]

The L48 pistons in 71 were dished with 4 valve reliefs...I don't know the cc volume of the dish though.
Yes the 487 head was used for both engines with the differences noted.
The LT1 had magnufluxed and shot peened rods, otherwise the same as the L48.
I ran a Crane HMV272, .454/.480 lift, 216/228 duration at .050. I'd recommend better valve springs and flat top psitons. This cam had a little lope at idle and was strong in the 2000-5000 rpm range. It also is legal for smog. I liked this cam and it was in a 71 L48 4 speed car with performer intake, rejetted q-jet, recurved distributor, and 2 1/2" exhaust. If you want to make a lot more power you'll need to run headers and improve the flow in the heads.

Greg

 

[pgtr]

Greg,

Thanks Greg, I'm thinking along almost identical lines as all that...eventually. I'm thinking flat tops w/ 4 'eyebrows' and mildly ported stock heads. Maybe a smidge milder on the cam than an H272 - not sure. I'm pretty happy w/ ram's horns. Anyway that's all another topic/post well down the road for me!

I'm just looking to baseline some technical specs on a bone stock 100% factory original 1971 L48.

Still looking for some '71 L48 factory 'original' specs like:

a) factory original head gasket thickness (compressed) and bore
b) compression height of factory original stock dished pistons
c) CCs of factory original dished pistons (12 and 17 have been mentioned)

thanks,

 

[black_81_vette]

Hello all! well I was reading these posts and I have something id like to share... Back in the spring of 72 my father purchased a 72 rally nova with a L-69 350 wich in all reality is the same engine as the 245 hp 350 2v available in 71.I later in life inhereted this car wich I still own.As I torn down the engine I had the heads and ect redone,I know for a fact the heads are 487X castings but the kicker is mine are the 1.94 1.50 valve size combo and are 76 cc chambers.I ended up swapping them for a set of 492s.Well to make a long story short I know the history of this car since day one and It never had any engine work done on it till it was overhauled, so how did my engine get these heads? BTW I am selling the 487X heads il never use them.

 

[pgtr]

AFAIK the L69 was a much later 5L V8 on FBodies. Did you mean the 2 bbl/145horse engine? (I don't know if it was an 010 block).

All 487 heads are or were 76cc and I'd guess the overwhelming majority were originally 1.94/1.50 valved. Were your 487 studs threaded or pressed? From everything I've learned about them - the 487 heads are pretty good, flow well and work nicely w/ modern pump gas.

From back when you swapped heads do you happen to remember if it had a relatively thin metal head gasket originally from the factory? Also the shape of the dish on the original pistons?

still looking for...
* '71 L48 original head gasket type/thickness/bore
* CCs for '71 L48 dished pistons

thanks,

 

[corvgreg]

I'd guess the dish on the L48 pistons at 15 to 17CC. I'm curious as to why this is so important to you?.

The factory used a steel shim head gasket about .015 thick. The fel-pro perma-torque head gaskets are about .038 thick. They seal better but you'll lose compression and more importantly you'll affect the quench. The pistons usually are .025 below the deck andideally you'd like the quench to be .040 to .050(quench is below deck measurement plus head gasket thickness). Fel Pro now makes a .015 steel shim head gasket that will maintain the quench and compression. The drawback is that you'll need to retorque the head bolts after it is initallly run.
Block decking is done to both true the deck to the main saddles and to bring the top of the piston to .002 or therabouts below the deck. But you'll lose the engine stampings on the front pad.

 

[pgtr]

Greg, am just working on a spreadsheet model as a baseline. I'm using a bone stock 71 L48 to see how close I can get to the factory ratio based on avail specs from the period. From there I'll be able to plug in different (actual) numbers for this or other engines and see what affect(s) they have. So far it's been darn educational too.

I knew the old steel gaskets were thin but didn't realize they were that thin (015) in 1971. I'm guessing they ran a bore of 4.1 on those gaskets?

15-17cc ballparks my calcs at between 8.45:1 and 8.6:1 respectively which is right in there. The one thing I'm not including is the small space between the piston and cylinder above the top ring. When I first started this exercise the specs I started w/ barely got it to the high 7s on CR. But it pays to have more accurate factory info.

Personally I have no desire to use an original style gasket - too thin and difficult to seal on anything but a perfect block/heads (like when new?). But it's interesting to see just what an affect a modern gasket or gasket changes have on CR!

But for now am just trying to gather all the factory blueprint specs as it left Flint in 1971.

thanks,

 

[black_81_vette]

the head gaskets were steel shim and the studs are pressed in.I still have them if your interested.Also the pistions were dished with the four valve relifes cut in them.

 

[jeff72]

PGTR,

I have a 72 350 and was wondering the same or along the lines of the L48. Is the 72 350 a 4 bolt main or 2 bolt?

thanks -jeff72

 

[pgtr]

PGTR,

I have a 72 350 and was wondering the same or along the lines of the L48. Is the 72 350 a 4 bolt main or 2 bolt?

thanks -jeff72

Yep, the L48 was the base engine on Corvettes from '69 on. Some folks may get hung up on onbscure codes on build sheets but rest assured it's an L48 also found in a number of other Chevrolets as optional engines. The Corvette version of the L48 does get a few goodies - most notably ram's horns manifolds. No it's not a 4 bolt in almost all cases though once in a while a 4 bolt from the factory may pop up I've heard as a pleasant surprise (or been converted by a previous owner). This is an ideal street engine and built for torque starting off idle - it doesn't need 4 bolt main IMHO.