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I’ve read & reread Leigh's thread “L48 performance upgrades by the numbers”. I value your and others expertise and guidance. I have a 1973 L48 convertible, 60,500 miles, July 1973 build date matching block, intake, Q-jet, OEM 945 paint but painted resale red. Engine & transmission rebuilt in 1996. I don’t have knowledge and skills to do L48 upgrade work so I took it to a shop for an evaluation. Short list is seals are leaking, bushings front & rear deteriorating (~30 years old), differential clutches are yelling, engine missing under load, plus unknowns.
I believe it’s clear the path I have is a crate engine vs. a L48 upgrade. But, I haven’t got the estimate yet on the L48 engine diagnosis, diff and chassis items and upgrade cam, heads, intake, headers, and 2 ½ inch exhaust/mufflers. Believe it will be very expensive having the shop do it...
If I go the crate route, do the L48 engine castings have any market value today given they are still in good enough shape?
Any thoughts on this are very appreciated, thank you all!
I’ve read & reread Leigh's thread “L48 performance upgrades by the numbers”. I value your and others expertise and guidance. I have a 1973 L48 convertible, 60,500 miles, July 1973 build date matching block, intake, Q-jet, OEM 945 paint but painted resale red. Engine & transmission rebuilt in 1996. I don’t have knowledge and skills to do L48 upgrade work so I took it to a shop for an evaluation. Short list is seals are leaking, bushings front & rear deteriorating (~30 years old), differential clutches are yelling, engine missing under load, plus unknowns.
I believe it’s clear the path I have is a crate engine vs. a L48 upgrade. But, I haven’t got the estimate yet on the L48 engine diagnosis, diff and chassis items and upgrade cam, heads, intake, headers, and 2 ½ inch exhaust/mufflers. Believe it will be very expensive having the shop do it...
If I go the crate route, do the L48 engine castings have any market value today given they are still in good enough shape?
Any thoughts on this are very appreciated, thank you all!
It sounds like your car is an original equipment clean survivor and your looking to change that.
I purchased a basket case to avoid this very dilemma. Save whatever you take off the car if you care about the resale as the next owner as most classic car enthusiasts prefer original equipment.
I removed the l48 on my 74 and it sits in the corner of my garage taking up space for the last 13 years. now the th400 will sit under it as I just swapped in a 2004r overdrive trans.shops were too expensive so I just watched you tube videos and read here and learned how to build an engine and have done all the work myself.installing a 406 now which will replace the 355 I built and installed 13 years back.
BTW the rear clutchpack issue is normal if the fluid hasnt been changed in forever and new limited slipp additive hasnt bee added. I added a bottle of this without even changing my fluid and it completely resolve the chatter/popping for years until I could get the new 3.53 rear end in. https://www.summitracing.com/parts/a...4aApoPEALw_wcB
Last edited by augiedoggy; Sep 19, 2025 at 02:26 PM.
Regarding the differential easy fix to change out the fluid.I thought i had major problem with all the noise I had in mine.Changed the fluid took less than an hour was totally fine after that.
Do you have more information on the rebuild done in 1996? Was there any machine work done or components changed out? That will definitely help determine what to do with it.
Having never touched a carburetor or worked on an engine in my life 2 years later I did a heads, cam, and intake swap on my ‘76. This website, as well as other forums and YouTube were invaluable in learning how to do it correctly. It’s really not that difficult, but you have to do your homework.
Shops can take you to the cleaners with the cost of parts and labor these days. If you’re physically able I would try and learn as much as you can before parting with your money. You might be able to get your current engine running better with an ignition tuneup alone.
Last edited by Piersonpie; Sep 19, 2025 at 03:09 PM.
That is impressive! Info I have on the 1995 engine/400 rebuilds: Bore & Hone .020, Crank clean & polish, R&R 8 pistons, ck rods, 16 new valves, 16 new Z 28 springs, 16 new guide liners. Absent any detail, I have to think the same cam and same type of pistons were used. I did see a note there were used heads installed...my suspicion is they are probably not date correct.
That is impressive! Info I have on the 1995 engine/400 rebuilds: Bore & Hone .020, Crank clean & polish, R&R 8 pistons, ck rods, 16 new valves, 16 new Z 28 springs, 16 new guide liners. Absent any detail, I have to think the same cam and same type of pistons were used. I did see a note there were used heads installed...my suspicion is they are probably not date correct.
Head dates and casting numbers are easy to see by removing the valve covers.
Without knowing the current state of the engine as far as compression, cylinder leakage, and oil pressure and consumption, you're just kinda throwing darts while blindfolded. If your engine remains tight, then I'd put the money into upgrades rather than a crate -- unless you are looking to go wild where a complete rebuild would be necessary to equal the crate.
You can dump thousands into that L48 (especially if the shop does it for you), and have a tired old engine that no one wants. Yank it, and let it haunt your garage until you sell the car (the next owner won't want it, either). Or you can sell it on Facebook Marketplace for a few $hundred, and put the proceeds towards a crate engine.
It seems the only real dilemma is which crate engine. Another Gen 1 SBC, a used LS, or a crate LS3 new from GM.
Are you equipped to do an engine swap yourself? Even honest mechanics will charge many $thousands to do this, since it will tie up their lift until it gets done.
I had a leak down done a couple years ago and was told, "very good leak down percentages" but I did not get pressure read outs on each cylinder. My plan is to again have a leak down test completed along with a dye test as there is something going on in the engine as it backfires under hard acceleration.
I don't plan on making any decisions certain re: upgrade top of engine keeping OEM heads & intake vs. crate engine until I get these 2 tests completed as results will dictate which direction to go.
Your comments though tell me there is value in the OEM L48 parts and something I would look to retain if I go in that direction...
Thank you for your comments, much appreciated!
Might not need a rebuild given the low miles. Needs some attention but major surgery is not likely IMO.
Flat tappet cam might have lost a lobe or two. Need to consider replacing the bad parts and be sure to use better oils.
Pulling the engine looks daunting but it is actually very simple.
You can buy the basic hand tools to do the job and rent the engine hoist.
It takes time to do a good job, but if it takes all winter you can easily be back on the road.
As stated above, first rent some tools to do the leak down and compression tests.
Pull the air cleaner and post us some pictures of what you have so everyone can help you better.
You may need nothing but some new parts and refresh the engine without removing it from the car.
You’ll learn from everyone here as long as you ask questions.
Another great asset we have here on the forum is Gary Ramedei GTR1999, He is the worlds best at determining what your rear differential may need and he can answer all your questions.
I had a leak down done a couple years ago and was told, "very good leak down percentages" but I did not get pressure read outs on each cylinder. My plan is to again have a leak down test completed along with a dye test as there is something going on in the engine as it backfires under hard acceleration.
I don't plan on making any decisions certain re: upgrade top of engine keeping OEM heads & intake vs. crate engine until I get these 2 tests completed as results will dictate which direction to go.
Your comments though tell me there is value in the OEM L48 parts and something I would look to retain if I go in that direction...
Thank you for your comments, much appreciated!
the issues you describe could be caused by a dirty carb passage or incorrect timing as well... honestly its likely they have nothing to do with the bottom end but a wipes cam lob is certianly possible especially if your not running a flat tappet oil with higher zddp. a mechanic can inspect for this though without pulling the engine. I replaced the cam in my engine while in the car a few years back not a big deal for someone mechanically inclined. If I were you id find a good mechanic and have him diagnose whats actually going on before you jump to conclusions.
I had a leak down done a couple years ago and was told, "very good leak down percentages" but I did not get pressure read outs on each cylinder. My plan is to again have a leak down test completed along with a dye test as there is something going on in the engine as it backfires under hard acceleration.
I don't plan on making any decisions certain re: upgrade top of engine keeping OEM heads & intake vs. crate engine until I get these 2 tests completed as results will dictate which direction to go.
Your comments though tell me there is value in the OEM L48 parts and something I would look to retain if I go in that direction...
Thank you for your comments, much appreciated!
Only If keeping OE heads:
suggest find & open convo with Local Circle Track engine builders that are accustomed to hotrodding class-mandated OE big chamber, iron heads. They KNOW how to make those so-called boat anchors make lots more power. This is especially so if your '73 L48 has c/n 3973487 heads. Those 487 are sturdy, not thin, not crack-prone and can be safely milled to 70cc (T-shirt). Do Not let anyone goad you into replacing entire valve guides; expensive, wrecks port flow & risks cracking heads. Instead, specify Only guide Liners. Most of the later OE iron heads are not so desirable.
Whichever way you flip or flop, good luck w/ your project.
Thanks stingray69, we did mention the possibility of the cam. I've owned the car for 19 years and it was probably driven pretty hard to require an engine rebuild in 1995/96. No mention on the documentation I have as to the mileage at that time.
It sounds like you had a rebuild in the not too distant past, had very good leak-down percentages, and not too many miles since.
With that in mind it sounds like the short block (block bore, rings & pistons) could likely be sound and reuseable.
(Gaskets don't count they just dry out)
Misses are just cam or tuning.
With that in mind I would do a leak-down test again, and pull a sparkplug and take a picture of the top of the piston head with a cheap borescope camera.
Post that here and someone can help you figure out basic piston type & compression.
And inspect the bore for rust, and a good hone pattern.
If that and your oil pressure is good, I would just do head & cam bolt-ons, and just upgrade the L48 shortblock. That would be the least expensive.
If your goal is greatly improved performance and roughly 375-400 max HP, then a good set of AFR or Trickflow heads on that shortblock will get you there.
If your goal is past 375/400HP then a crate engine would likely be the cheapest/easiest way to go.
Last edited by leigh1322; Sep 22, 2025 at 12:36 AM.
Thanks Leigh for your input and guidance. I really appreciate your and others technical knowledge and the fact you and others are willing to share it!
I have no desire to go over 375-400 hp as I think that would beg chassis upgrades, etc. and my plan is to gift the car to my son and grandson and I don't want to knowingly gift a > 400 hp vehicle as I remember when I was 16 years old...
Much appreciated!
Regards, Joe
Great!
You can easily add 50-75-90 HP to an L48 with those mods.
You likely have ~260HP (gross HP) now.
350HP feels like a different engine.
Most of it is in the heads, and un-corking the exhaust.
Buy the best heads you can afford.
But you may wind up with only 9:1 CR. (You only have ~8:1 now)
That is OK, but if you do have that, just go really mild on the cam duration choice. (under 220*)
9:1 CR does not like big duration cams. You need 10:1 or more for that.
And I certainly would not spend another 1k to 2k $ just to get 1 point more compression if your short block is good.
You'd think by this stage of my life I'd know better than to go for dirt cheap scope...finally got a borescope that works and attached are couple pix of piston and cylinder wall 3.
Also discovered on the 1995/96 engine rebuild, the used heads installed are: 3970126. Difficult finding the specs on...most info says these were heads used on base 350 in 1969-1972 on Caprice and like for 2 bbl configurations. Researching valve size info I've found: 1.72 or 1.94 in / 1.5 out.
Interested in everyone's thoughts on if those are OEM speced pistons or something else and #3 wall...
Thanks much!
Joe
If I had to guess those are 9.0 to 9.5 CR pistons, not the 8:1 big dish style, but not true 10:1 CR flat-tops either
I would say that short-block is very sound.
Throw the heads away, and put some 64cc heads on it and you will have 10:1CR
Stay mild on the cam, or you will go over 400HP very quickly! LOL
AFRs are the best, trick flows are very good for the $.
You need to pick the port size and the cam based on your intended driving.
So what rear gears do you have?
And how are you going to drive it?
Last edited by leigh1322; Oct 1, 2025 at 01:23 PM.
If I had to guess those are 9.0 to 9.5 CR pistons, not the 8:1 big dish style, but not true 10:1 CR flat-tops either
I would say that short-block is very sound.
Throw the heads away, and put some 64cc heads on it and you will have 10:1CR
Stay mild on the cam, or you will go over 400HP very quickly! LOL
AFRs are the best, trick flows are very good for the $.
You need to pick the port size and the cam based on your intended driving. But likely 175s. $1700 to 2300
So what rear gears do you have?
And how are you going to drive it?
Thx Leigh, very happy and surprised to learn about the pistons :-)
Pretty sure they're 3:08 but not certain. I'm really after a driver at this point blending low end with highway. Also don't want to gift significantly >400 hp absent chassis engineering upgrades to son/grandson. I remember how I drove on country roads in Southern Illinois when younger...
1973 L48 dilemma
