383 vortec high compression

Preston Tucker · Feb 10, 2021

I am building myself a 383 Stroker and wanted to run a set of 96 L31 Vortec heads on top of it because I have heard they have way better flow than any other stock head for a small block. Its supposed to be a cheep hp swap
(around a 40 hp gain I have been told)

Well I have been doing some research and the vortec heads have 64cc or combustion chamber which makes the compression a little high, around 9.5:1 on a 1996 5.7 liter. I have heard if I stroke it my compression with these heads will be around 10.6:1!

I wanted to run it on pump gas or run an octane booster to keep fuel in the cheep.

I could run inverted dome pistons to bring down my compression but that also increases my combustion chamber (less hp) Or is the compression estimate not accurate? Im still learning about engines.
I found a few crate motors that are 383 with Vortecs and they say the run at 9.5:1 compression, however it doesnt say anything special about the pistons and I just want to find a recipe that works on 87 octane or a little higher.
Shes gonna be a daily driver so I want it to be a powerful but practical build.

https://www.speedwaymotors.com/Blue...ngine-Vortec-Heads-Flat-Tappet-Cam,61943.html

Bextreme04 · Feb 10, 2021

You can adjust the piston or the head gasket thickness to get the CR where you want it. If you want an iron truck engine that will be doing truck stuff on the street, I wouldn't go too far outside that 9.5:1. There are numerous compression ratio calculators available on the interwebs. I used the one on summit to check my quench and compression ratio setup.

Preston Tucker · Feb 10, 2021

Bextreme04 said:
You can adjust the piston or the head gasket thickness to get the CR where you want it. If you want an iron truck engine that will be doing truck stuff on the street, I wouldn't go too far outside that 9.5:1. There are numerous compression ratio calculators available on the interwebs. I used the one on summit to check my quench and compression ratio setup.

Ok thank you kindly. With a .090 head gasket and a .025 deck clearance my compression ends up around 9.9:1 for a ratio, is that too much for 87 octane?

hirschdalechevy · Feb 10, 2021

Quench is what you are after, what pistons say on the box for compression means nothing. If you bolt vortecs on a stock small block with stock heads, (as long as the stock heads are 72cc) they will bring compression up and flow better and have a way better combustion chamber than old stocker heads which means more power.

Since you are building a new motor all that changes. If you are after as much power as you can get, buy as much head as you can afford. Afr 180cc or 190 cc will blow vortec's out of the water everywhere.

Quench,
You need to go to a compression calculator online and put your spec's in to figure out where you want to be, With good quench and the right cam I shoot for 9.5 with an iron head and 10 for an aluminum head for 91 pump gas.

On a stock 350 the pistons are down in the block deck .020 to .025 , then you add your head gasket thickness , (anywhere from .015 to .060) and that's your quench area. You really want to shoot for a quench of (.039 to .042). Then the head cc and type of piston comes into play for the compression that you are shooting for.

You will need to know your piston's compression height, your blocks deck height, your head gasket thickness and bore, then head cc to get a close compression number.

Example,
A 350 chevy has a 9.020 deck height and a 5.7 inch rod and a 3.48 stroke, and lets say your piston has a 1.560 compression height. So you cut the crank stroke in half, (1.74) add you rod lenght ,(5.7), then add your piston compression height, (1.560) and that equals 9.0 , so your piston is down (.020) in the block and if you run a common , (.039) mls head gasket your quench would be ,(.059), no good, you want to be around, (.040)

So what you can do is deck the block to bring the pistons up close to 0 deck and run the .039 heads gasket and there you have it, (.039 quench)

Then you take the head cc, (64cc) and you take the inverted dome pistons, (16cc, 22cc, 25cc etc.) to bring your compression where you want it at.

Then the cam comes into play, (cylinder pressure) , but thats a longer story.

The better the quench the more the motor will handle compression on pump gas.

Hope this helps you out a bit.

Remember , if you want to go all out buy as much head as you can afford. Stock Vortec heads can only handle so much valve lift as well so that limits your cam choice.

Bextreme04 · Feb 10, 2021

Preston Tucker said:
Ok thank you kindly. With a .090 head gasket and a .025 deck clearance my compression ends up around 9.9:1 for a ratio, is that too much for 87 octane?

A .090 headgasket is going to kill your quench area and probably make the change to Vortecs counterproductive. Do you already have an assembled shortblock? 9.9:1 with Iron heads is premium territory, especially with a huge quench like that. Make sure you are entering the correct numbers in the calculator... a dome piston will be entered as a - number and dished pistons as a positive. Deck clearance will be the same... - number if piston "deck" is sticking above the deck. You can use the generic chamber volume numbers for vortec heads but you should really measure the actual volume for an accurate number.

Bextreme04 · Feb 10, 2021

hirschdalechevy said:
Quench is what you are after, what pistons say on the box for compression means nothing. If you bolt vortecs on a stock small block with stock heads, (as long as the stock heads are 72cc) they will bring compression up and flow better and have a way better combustion chamber than old stocker heads which means more power.

Since you are building a new motor all that changes. If you are after as much power as you can get, buy as much head as you can afford. Afr 180cc or 190 cc will blow vortec's out of the water everywhere.

Quench,
You need to go to a compression calculator online and put your spec's in to figure out where you want to be, With good quench and the right cam I shoot for 9.5 with an iron head and 10 for an aluminum head for 91 pump gas.

On a stock 350 the pistons are down in the block deck .020 to .025 , then you add your head gasket thickness , (anywhere from .015 to .060) and that's your quench area. You really want to shoot for a quench of (.039 to .042). Then the head cc and type of piston comes into play for the compression that you are shooting for.

You will need to know your piston's compression height, your blocks deck height, your head gasket thickness and bore, then head cc to get a close compression number.

Example,
A 350 chevy has a 9.020 deck height and a 5.7 inch rod and a 3.48 stroke, and lets say your piston has a 1.560 compression height. So you cut the crank stroke in half, (1.74) add you rod lenght ,(5.7), then add your piston compression height, (1.560) and that equals 9.0 , so your piston is down (.020) in the block and if you run a common , (.039) mls head gasket your quench would be ,(.059), no good, you want to be around, (.040)

So what you can do is deck the block to bring the pistons up close to 0 deck and run the .039 heads gasket and there you have it, (.039 quench)

Then you take the head cc, (64cc) and you take the inverted dome pistons, (16cc, 22cc, 25cc etc.) to bring your compression where you want it at.

Then the cam comes into play, (cylinder pressure) , but thats a longer story.

The better the quench the more the motor will handle compression on pump gas.

Hope this helps you out a bit.

Remember , if you want to go all out buy as much head as you can afford. Stock Vortec heads can only handle so much valve lift as well so that limits your cam choice.

What he said... This is all the information you should need to get where you want to be.

hirschdalechevy · Feb 10, 2021

Preston Tucker said:
Ok thank you kindly. With a .090 head gasket and a .025 deck clearance my compression ends up around 9.9:1 for a ratio, is that too much for 87 octane?

That would not be good at all, .025 + .090 = .115 quench.

Preston Tucker · Feb 10, 2021

Bextreme04 said:
A .090 headgasket is going to kill your quench area and probably make the change to Vortecs counterproductive. Do you already have an assembled shortblock? 9.9:1 with Iron heads is premium territory, especially with a huge quench like that. Make sure you are entering the correct numbers in the calculator... a dome piston will be entered as a - number and dished pistons as a positive. Deck clearance will be the same... - number if piston "deck" is sticking above the deck. You can use the generic chamber volume numbers for vortec heads but you should really measure the actual volume for an accurate number.

No i dont have an assembles shortblock, I have a 4 bolt block out of a 74 that was blown up, only using the block, had it checked out and its all good. Thanks for the info, its not going to be a **** motor, its going in a lifted k20.

I want to stay out of the premium if i can but if i cant avoid it I will settle. Does this look right? I need advice from knowlegable people.

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Bextreme04 · Feb 10, 2021

What pistons are you basing those numbers from? You will most likely want a piston like this. https://www.summitracing.com/parts/slp-8-h124cl30

You'll most likely end up with a flat top piston with only the +5cc dome. Almost 0 deck clearance, and closer to a .037 compressed gasket thickness. That will bring you down closer to 9.5:1 CR(you can then run a bit thicker gasket to bring it down more if you want while still keeping good quench). I wouldn't be any higher than 9.5 if you want to run pump gas in a 3/4 ton truck that will be towing or hauling anything. Unless you are running aluminum heads, then you can go a bit higher.

SquareRoot · Feb 10, 2021

I have vortecs om my 350. If I had to do it again I'd run AFR 190 cc heads every day of the week.

Preston Tucker · Feb 10, 2021

Bextreme04 said:
What pistons are you basing those numbers from? You will most likely want a piston like this. https://www.summitracing.com/parts/slp-8-h124cl30

You'll most likely end up with a flat top piston with only the +5cc dome. Almost 0 deck clearance, and closer to a .037 compressed gasket thickness. That will bring you down closer to 9.5:1 CR(you can then run a bit thicker gasket to bring it down more if you want while still keeping good quench). I wouldn't be any higher than 9.5 if you want to run pump gas in a 3/4 ton truck that will be towing or hauling anything. Unless you are running aluminum heads, then you can go a bit higher.

Ok I was looking at this style dished piston. https://www.jegs.com/i/United-Engine-Machine/648/IC9926.030/10002/-1
I dont have the budget for aluminum unfortunately, its either this or the stock 64cc heads that came with it and those are more of a dog for power and i have the same compression issue.

Preston Tucker · Feb 10, 2021

SquareRoot said:
I have vortecs om my 350. If I had to do it again I'd run AFR 190 cc heads every day of the week.

Unfortunately I cant afford that, i have about 4k to build this engine.

Preston Tucker · Feb 10, 2021

Bextreme04 said:
What pistons are you basing those numbers from? You will most likely want a piston like this. https://www.summitracing.com/parts/slp-8-h124cl30

You'll most likely end up with a flat top piston with only the +5cc dome. Almost 0 deck clearance, and closer to a .037 compressed gasket thickness. That will bring you down closer to 9.5:1 CR(you can then run a bit thicker gasket to bring it down more if you want while still keeping good quench). I wouldn't be any higher than 9.5 if you want to run pump gas in a 3/4 ton truck that will be towing or hauling anything. Unless you are running aluminum heads, then you can go a bit higher.

Does this look reasonable?

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hirschdalechevy · Feb 10, 2021

I would not run that set up.

400 crank has a 3.75 stroke , divided by 2 = 1.875
5,7 inch rod
your piston you picked , 1.433 compression height

That equals 9.008 and a stock 350 deck is 9.020, that equals piston down .012

That equals .050 quench on your chart of parts, which is not great for higher compression.

You could deck the block .012 and run a .039 or .041 head gasket and be there with a d-dish piston with a larger cc invert and get good quench and your 9 to 1 or so.

If you want 9 to 9.5 to 1 at close to 0 deck you need a d-dish piston, not a flat top or a large round dish

SquareRoot · Feb 10, 2021

Preston Tucker said:
Unfortunately I cant afford that, i have about 4k to build this engine.

Vortec costs add up. You'll need a vortec intake. Carbs suck on vortec intakes.

383 vortec high compression

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