So if you're working with the L48, and you reference this post from CF: https://www.corvetteforum.com/forums...mber-size.html Which likely represents a more real-life representation of the state of an L48 at this point
You plug all that into a Compression ratio calculator and you end up with ~7.7:1
If you're looking at a new top end, many people recommend getting into the 64cc chamber range to really bring that back up. But when you combine that with newer head gaskets which are typically slightly thicker than .017 stock gasket (I've seen many in the .035 range)
You still end up with only about ~8.5:1 CR according to calculations. If you're looking into a cam swap, many of the cams suggested have an operating compression ratio of higher than 8.7
What happens when your cam's operating range has a higher minimum compression ratio than your engine can provide?

 

https://www.summitracing.com/parts/f...make/chevrolet

try this gasket.

if you install a cam with lower then recommended CR it will have low cylinder pressure and poor low rpm torque.

milling a head down for better CR is not a big deal either. The pistons in there do suck for quench purposes so don’t count on mechanical detonation prevention from a tight quench.

also keep in mind bumping compression with existing rings could make it an oil burner.

 

Bottom line. If you have a stock L48. Loose the pistons, put in some flat tops. Better heads are a real good idea as the stock one's are prone to cracking not just the huge chambers.
sometimes we really are on a budget. I know. Took me years to save up for some decent heads. But putting any amount of cam in a otherwise stock L48 is kinda like pissing up a rope.

 

you need to remember once you start doing performance improvements you ain't gonna stop. so, what you buy to improve engine in its existing condition-setup better still be applicable to the next step. you buy those 58cc aluminum L98 ZZ4 heads, they will work better...for your tired L48. once you bore that block and put 383 rotating assy in there those L98 heads are gonna be a choke point. BUT if you buy 220 cc 2.08 heads cuz you are gonna go bigger some day your tired L48 is still tired. so there are compromises at every stage and with every component. shorty headers. DONT!! not enough less price or labor compared to real long-tube headers. and this is where too big doesn't hurt either. best is to decide what your bottom end is gonna be and build or buy it. it is also not that difficult to get to a power level that will do bad things to your drivetrain. you spend 5500 on 550 hp and don't dare use more than 400 of it cuz you haven't done GTR's upgrade to the rear yet.

 

Not true.....64cc heads bring the compression up one whole point, even with a composite gasket. You end up around 9.2 to 1...plenty for street work.
I do not recommend the ZZ4/L98 head......it bumps compression plenty, and they are aluminum, but the ports on these just suck. Even ported, they just don't yield big gains.

Jebby

 

Do you still have the stock bore and whats your budget? if you look for gasket shims you can find the .015 shims like the felpro mentioned and you want to pay attentionn to the quench......IF you have the time I would pull the motor and have it checked for cylinder roundness and possibly bored. If you do the "pulling the engine out and apart" labor, the shop labor prices are very reasonable. Then go with the flat top pistons and some nice aluminum heads. Best you can afford in the 195 range. If you stick to the original crank and buy a good .500ish lift cam with some 1.6 rockers you'll be happy on the street and with the vacuum. If you want to bump it up then go with a roller upgrade, the cams are reasonable, but you need to spend the money for a really good hydraulic roller lifter or go solid roller. You can mix them but you need to "want to". solid lifters have to be paid attention to.

Its all about what is going to satisfy what you want. I went through 3 cams over the last 3 years settling on a hydraulic roller/solid lifter combo this year and I hope its where i draw the line. Its big enough it doesnt pull enough vacuum and needs an electric pump to help but small enough I should not be breaking anything if I dont get too crazy. My old camshafts beat the hell out of my rear snubber mount that was an early 64 mount and wasnt reinforced like the 68 i had on the other diff and didnt notice when i swapped the diffs. And that was high 300 horse to the tire. This combo might give me high 300 to low 400 to the tire.

Again, check your budget and what you want to acccomplish. A 350hp motor can be really fun and it will still have the vacuum for accessories and be decent on fuel economy, but you might just keep wanting more, so decide early. And then figure out if you wan tot deal with finding oil with lots of ZDDP and phsphous for the flat tappet cams. If you go roller, its a lot of money up front , but the roller lifters can be reused on other cams so thats a one time expense.

 

Initially I started out thinking I could put on better heads and use the existing bottom end with a better cam and get what I wanted. However the more I read and learned the more I realized that going half way was only going to lead to my disappointment in the performance i was going to get.
I got this book and read
Spent a lot of time searching all subjects on the Chevy 350 and performance/rebuild subjects. Spent plenty of time on this site and on speedtalk.
Got this book on cams
Then decided to learn what I could about the q-jet on the vehicle since I had no previous experience with it and got this book.

rebuilding the q-jet to handle the fuel delivery properly even before I rebuilt the motor was a big performance adder.
I initially went with somewhat cheaper heads instead of the AFR's I have now and learned the hard way you certainly get what you pay for most of the time in the Chevy 350 aftermarket world, or somtimes maybe less. Learned plenty bout proper valve install height and spring pressures from that experience.

So I guess long story short, decide what you want achieve early on, it will save you time and money in the future. If your happy with about 350 HP then the stock drive train will do as will the stock components in the bottom of the engine. Once you decide to go higher you need to have a better understanding of the impact that will have on the related components or you will spend more time and money on fixing broken parts in the future.

I ended up @ about 440 HP from the engine, getting a 3.55 rear diff rated for 500 HP vs the 3.08 it came with, new TH350 with shift kit rated for 500 HP along with torque converter with 3000 stall, all new ujoints along with ujoint caps vs the Ubolts it came with. Using street tires only so they are my fuse. They spin before anything can break.

 

The L98/ZZ4 matter is essentially moot because they haven't been produced in years and they've become hens' teeth ... prices rising ... and
... the OP Lemmings doesn't indicate what performance result he's pursuing.

Those heads easily support 350-360 hp as in OE ZZ4 and x603 crates (both discontinued but both still in use) ...
... If it's a street motor & goals are no more than 400hp; they're OK ... but they do Not have a modern "fast-burn" chamber.

Although limited, There ARE other heads w/ chamber smaller than 64cc.

 

Apologies yea I should have stated my goals. Right now I’m looking for a “Phase 1” to have really drive some pep into the stock motor. I think in the 350 range is my goal for now. I’m hoping this will keep things fun and exciting giving me time to work on all the other pieces to really bring the power (brakes, suspension, driveline). “Phase 2” many years down the road I’d really like to bring the heat with likely a 400+ CID small block higher end build. So for now something to unlock the best potential of the stock block until I get the transmission rebuilt, better gears in the rear, a better suspension, steering system etc.

Budget wise at this time the wife is practicing trickle down economics with the C3 budget. Maybe some of you are familiar with this . I’m looking to do as much myself as possible with essentially standard garage tooling.

This is as far as I know a stock 1975 L48. Based on research a previous owner has removed all AC components as well as bypassed the heat. There are long tube headers (needs replaced) and an aftermarket exhaust system with no catalytic converter. I don’t see any additional markings on intakes or carb to indicate those are aftermarket. I haven’t dug around the heads enough to find the casting number but it’s a fair shake I’ve probably got the smog heads. I’ll try and confirm that today.

I am concerned that with what Reelav8tr mentioned about upping the compression and turning it into an oil burner. I’m not sure I’m equipped currently to do a piston/ring swap.

 

Fix the heat & air ... she'll never like that car unless - until it's reasonably comfortable.

Step #1 ... Recurve the distributor and freshen - tune the carb ...
... send an email to CF member Lars and request his dist & carb tuning papers ... include in your email what year and engine & trans you're working on ...
.... Lars doesn't charge for that valuable expert instruction.

Lars Grimsrud
v8fastcars@msn.com

 

Agree, with your goals get Lars' papers and tune it. It'll make a big difference.