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Ideally that would be the way to go... However, seems like those future plans always end up taking longer than you think, not to mention changing as your finances dictate. I'd opt for the best heads you can afford with combustion chambers sized for the cam you're going to run on the 350. High end aftermarket heads like AFR, Brodix and Trick Flow have really thick decks, and you can always either have the combustion chambers cnc'd for the bigger displacement later when you do finally upgrade, or you can always go with dished pistons. It's not like you would need that big a dish - probably just fly cut the valve reliefs a little. Anyway, it's domed pistons you want to avoid if you can. Dishes don't impede the flame front. You just have to make sure the piston has a decent quench pad that matches the combustion chamber.
I get it. In the grand scheme of things it can be done. Depending on how wild of a cam goes in the 350 it may not even really need the compression. If the heads are good it will still be alright. I've driven plenty of 350s with 9:1 and decent cams and they're still fun. The only thing I hate is that those pistons get expensive quick compared to normal flat tops and such. It gets you no matter what lol.
I get it. In the grand scheme of things it can be done. Depending on how wild of a cam goes in the 350 it may not even really need the compression. If the heads are good it will still be alright. I've driven plenty of 350s with 9:1 and decent cams and they're still fun. The only thing I hate is that those pistons get expensive quick compared to normal flat tops and such. It gets you no matter what lol.
Yep. Pay now or pay later... I learned that when I was trying to decide between nitrous or a blower...
I’d definitely rather pay now so that it “seems” cheaper for me later even tho it’ll still probably cost more lol.
I think I’ve definitely decided on AFR 195’s and probably the comp port version with 75cc chambers to support my mods later on. I was however told I’d have to have my pistons fly cut for it to work and would really rather not do that but if I have to then I do.
Once I make a final decision Then it’ll be into picking a cam along with rockers, push rods, and lifters.
I really like comp cams ultra gold 1.6’s but that’s still up for debate as well since there are so many dang options.
Call Lloyd Elliot, and visit his website too, to review cam and head packages. As an acknowledged LT1 guru, he can discuss your current and future situation to provide great advice. Also it is tough to beat his prices.
not sure about flycutting...had mine decked with a 62cc chamber to be .015 in the hole, .670 lift domed pistons plenty of clearance. They are all different though
Call Lloyd Elliot, and visit his website too, to review cam and head packages. As an acknowledged LT1 guru, he can discuss your current and future situation to provide great advice. Also it is tough to beat his prices.
I have an L98 in my car but I imagine the difference are fairly minor for heads and cam combos. I’ll look into it and give him a call.
Originally Posted by cv67
not sure about flycutting...had mine decked with a 62cc chamber to be .015 in the hole, .670 lift domed pistons plenty of clearance. They are all different though
Yeah this was coming from Tpi421vette so I’m pretty positive he’s correct as he’s pretty damn smart when it comes to building these engines.
Here’s what I’d be looking at with my stock 350 if I went with the AFR 195 75cc comp port heads. Please let me know if anything looks wrong on this calculator. Had to do some digging for the proper specs.
Honestly the 8.5 compression ratio seems pretty low for now but I know it’ll work and be fine. Just curious about yalls thoughts on that. I know ideal is normally 10.5:1 or so. I know the same setup with the 396 as planned would roughly be 10.5:1 which is a great setup for the future.
Honestly the 8.5 compression ratio seems pretty low for now but I know it’ll work and be fine. Just curious about yalls thoughts on that. I know ideal is normally 10.5:1 or so. I know the same setup with the 396 as planned would roughly be 10.5:1 which is a great setup for the future.
Maybe I'm missing something, but I think that's crazy. You know the stock 1996 LT1 came with 10.4:1 CR, and the LT4 was 10.8? Why would anyone voluntarily go with way less than that for a n/a engine?! And you won't be gaining two full points of CR when you go from 350 to 396: more like 1.2 increase. So you'll end up with a 396 that's got less than 10:1 CR. If I were you I'd be aiming for 11:1.
Also, I feel like the online CR calculators are all over the place on results. I know that previous owner of my car, MSR, was not able to get 11:1 on the 396 build even with (well ported) stock LT4 heads. They went from a measured 58cc down a few CC by cutting the decks (maybe as low as 52cc), and he still had to source special head gaskets that had a short compressed thickness. Granted that was with custom 13cc-dished pistions. But even with flat tops you'll have a few cc for valve reliefs. I think you're way out in left field looking at heads with 75cc chambers. At the very most, you should be looking at 65cc chambers with flat-top pistons, and you'll still be below 11:1 on the 396 with any normal head gasket.
Last edited by MatthewMiller; Aug 22, 2020 at 10:58 AM.
Maybe I'm missing something, but I think that's crazy. You know the stock 1996 LT1 came with 10.4:1 CR, and the LT4 was 10.8? Why would anyone voluntarily go with way less than that for a n/a engine?! And you won't be gaining two full points of CR when you go from 350 to 396: more like 1.2 increase. So you'll end up with a 396 that's got less than 10:1 CR. If I were you I'd be aiming for 11:1.
Also, I feel like the online CR calculators are all over the place on results. I know that previous owner of my car, MSR, was not able to get 11:1 on the 396 build even with (well ported) stock LT4 heads. They went from a measured 58cc down a few CC by cutting the decks (maybe as low as 52cc), and he still had to source special head gaskets that had a short compressed thickness. Granted that was with custom 13cc-dished pistions. But even with flat tops you'll have a few cc for valve reliefs. I think you're way out in left field looking at heads with 75cc chambers. At the very most, you should be looking at 65cc chambers with flat-top pistons, and you'll still be below 11:1 on the 396 with any normal head gasket.
A 396 with flat tops, (5cc of dish roughly) and 65 cc chambers with total quench around. 045 will be 11.4 to 1.. roughly. Its just math... as long as the numbers are right comp ratio is a open to compressed ratio... initial volume over final volume... running the numbers you provided and assuming a .025 gasket yours should be 11.9. Unless I've missed something... Granted actual ratio is never that but still. I don't know. Its a number that actually doesn't mean much
So basically go with the 65 cc chambers. Sounds like the 75 cc’d will lower the ratio too much to be efficient, even with the 396. The car is NA and currently I have no plans for forced induction but who’s to say I won’t get bored and throw nitrous in it lol probably not but who knows.
Like I said, good with aircraft engines and basics for cars. Never had to do much when it came to aftermarket upgrades such as new heads and everything else so it’s a learning process haha
I just wanna get the correct stuff now to make life easier later.
A 396 with flat tops, (5cc of dish roughly) and 65 cc chambers with total quench around. 045 will be 11.4 to 1.. roughly. Its just math... as long as the numbers are right comp ratio is a open to compressed ratio... initial volume over final volume... running the numbers you provided and assuming a .025 gasket yours should be 11.9. Unless I've missed something... Granted actual ratio is never that but still. I don't know. Its a number that actually doesn't mean much
I plugged those numbers in two online CR calculators and both gave me 10.4 if you add .045 quench and .025 gasket height, or 11.05 if you just use the .045 quench height (.020 deck clearance + .025 gasket thickness). By definition the latter is the correct way, I believe, as quench height is defined by the distance from the flat top of the piston to the head's deck surface. If the block is .030 over, then it jumps to 11.2. Regardless, 11.2:1 is not too much for any typical cam spec and premium pump gas, and if you want to drop it a couple tenths you can play with gasket height and/or piston deck clearance. OTOH, going to 75cc chambers drops it to 9.9:1, which is going to be hard to make up without ugly domes.
Last edited by MatthewMiller; Aug 22, 2020 at 04:57 PM.
I plugged those numbers in two online CR calculators and both gave me 10.4 if you add .045 quench and .025 gasket height, or 11.05 if you just use the .045 quench height (.020 deck clearance + .025 gasket thickness). By definition the latter is the correct way, I believe, as quench height is defined by the distance from the flat top of the piston to the head's deck surface. If the block is .030 over, then it jumps to 11.2. Regardless, 11.2:1 is not too much for any typical cam spec and premium pump gas, and if you want to drop it a couple tenths you can play with gasket height and/or piston deck clearance. OTOH, going to 75cc chambers drops it to 9.9:1, which is going to be hard to make up without ugly domes.
I think you are correct. Strange how results vary wildly. So you are not a .03 over. Most 396s ive seen are that so thats where I based my math off. It makes a substantial difference despite only a little visual...
So basically go with the 65 cc chambers. Sounds like the 75 cc’d will lower the ratio too much to be efficient, even with the 396. The car is NA and currently I have no plans for forced induction but who’s to say I won’t get bored and throw nitrous in it lol probably not but who knows.
Like I said, good with aircraft engines and basics for cars. Never had to do much when it came to aftermarket upgrades such as new heads and everything else so it’s a learning process haha
I just wanna get the correct stuff now to make life easier later.
Adding my two cents. With modern combustion chambers and correct quench height you can run 10.5 compression all day even on crappy 91 octane gas from the Stop-n-Rob. I'm currently running 10.4 compression on my supercharged car. I'm only using 6 lbs of boost, but still... there's absolutely no reason to go that low on compression with modern heads like the AFR's.
Adding my two cents. With modern combustion chambers and correct quench height you can run 10.5 compression all day even on crappy 91 octane gas from the Stop-n-Rob. I'm currently running 10.4 compression on my supercharged car. I'm only using 6 lbs of boost, but still... there's absolutely no reason to go that low on compression with modern heads like the AFR's.
I’ll take the 2 cents wherever I can get it! I’ve been doing a lot of reading and video watching so that when the time comes it will all be as seamless as possible. I can rebuild and teardown/build engines no problem. For me it’s learning the science and other stuff for actually upgrading and making more power with certain combos. I’ve only ever done stick rebuilds before.
