haacer

iTrader: (1)

For those who aren't aware, the highest available octane in California is 91. I'm putting together a 383 stroker and wondering how high the compression can go. My builder has recommended the -5cc flat-top pistons that only have valve reliefs. I think this puts compression anywhere between 11.5:1 and 12:1 depending on deck height.

The other option is -12cc dished pistons which should give around 10.5:1. This seems like a safe choice but I'd like to go with the higher compression if it's safe on California gas.

ss.slp.ls1

iTrader: (36)

You need to talk to calculate your dynamic compression ratio, talk to your builder or calculate it yourself. That will determine your octane requirements more than your static compression ratio.
I'm in OC btw.

SS MPSTR

iTrader: (1)

Do not run -16cc pistons in your N/A build. The -5cc will be perfect - mill the heads to the achieve the desired DCR (if necessary) using your cam timing events to determine what that will be. I'm running 12.4:1 on mine with 91 octane. I do run a couple gallons of VP110 to prevent detonation on hot days though - the higher compression will make you run hotter.

bombebomb

I would think someone in California with only 91 could run the same compression as someone in ohio with 93. The reason why there is different octanes is because of elevation. The higher you are, the thinner the air is so the cylinder takes in less air hence the lower octane needed. The lower you are the more dense the air is causing you to need higher octane (93).

So I would think you could run the same as anyone else would.

96capricemgr

iTrader: (1)

The LT4 was 10.8 bone stock.
As said dynamic compression is something you have to consider too.

haacer

iTrader: (1)

Thanks for the input guys.

I've been talking to Lloyd Elliot and the only way to overcome that kind of compression with 91 octane is to run a big cam. I'm more interested in a DD than a race car right now so I'm going with the dished pistons. I may live to regret it but as 96capricemgr among others has pointed out, this is basically a budget build. I'm fairly sure that I can pull the engine out again and put something beefier in it in the future.

From LE:

when using the 212/218 cams with a 62 degree IVC, you end up with a DCR of 8.93

Static compression ratio of 10.94:1.
Effective stroke is 2.99 inches.
Your dynamic compression ratio is 8.93:1 .
Your dynamic cranking pressure is 183.20 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.93 :1.
V/P (Volume to Pressure Index) is 168

_________________________________
These calculations assume zero deck. I've been considering whether or not this is really a good idea though... I'm only boring the engine 30 over to be sure that I have the option of going further in the future. It seems like one should leave some meat on the block for future reworks... any thoughts?

ss.slp.ls1

iTrader: (36)

1) Get a bigger cam! Seriously. I DD my CC503 (224/230 .502"/.510" 112) and I love it.
2) Don't get the dished pistons!

haacer

iTrader: (1)

Whoops...those were the wrong numbers. Those were with -16cc dished pistons. These are with -12cc pistons. I want to be sure that I can drive this thing in the summer without race gas.

Using a 280 advertised duration cam (226 @ .050 duration), you end up with a 68 degree IVC and a 8.99 DCR that is still doable on pump gas with good tuning.

Static compression ratio of 11.47:1.
Effective stroke is 2.83 inches.
Your dynamic compression ratio is 8.90:1 .
Your dynamic cranking pressure is 182.41 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.90 :1.
V/P (Volume to Pressure Index) is 158

ss.slp.ls1

iTrader: (36)

Wait, so you're saying that with 12cc dished pistons you have a DCR of 8.90:1 and a SCR of 11.47:1?

haacer

iTrader: (1)

Those are the numbers LE ran by me. He's assuming zero deck and 54.5 cc head volume with a .040 gasket.

I've seen a variety of numbers for head volume but I have to think Lloyd knows what he's doing.

ss.slp.ls1

iTrader: (36)

Calculate your numbers using flat top pistons w/ 5cc valve reliefs and see what you get.

There are guys that run a 9.1:1/12.x:1 DCR/SCR that run on pump gas. You could always change your chamber volume and gasket thickness to reach your ideal compression ratio.

haacer

iTrader: (1)

From LE:

12.0 static compression ratio (SCR) and 91 octane is doable with a 8.7-8.8 dynamic compression ratio (DCR),. This will need a 73 degree IVC (typical mid 230 duration cam) to see this kind of DCR. [...] Most consider 8.8-8.9 to be the max for a mail order tune, mechanical water pump, etc. An electric WP, hands on tuning, etc can allow 9.1-9.2 DCR but most are using 92-93 octane fuel when doing this. I would say 8.7-8.8 is the max for a mail order tune and 91 octane.

haacer

iTrader: (1)

All the calculations from LE were assuming zero-deck. While squaring the block seems like a good idea..what about zero-deck? This seems to leave no room for future modifications or repairs. Any thoughts?

ss.slp.ls1

iTrader: (36)

Is seems like you can use the 5cc relief pistons with a larger cam. A mid 230's duration is very streetable in a 383 with a good tune.
Next thing you will need to concentrate on is getting a good pair of heads to feed your stroker.