QuickStyle

what octane gas should i run for my 383? (engine details in signature)

dont have the heads yet, but im debating between afr 195's 68cc and 74cc

with 68cc my compression ratio is going to be 9.55:1
with 74cc my compression ratio is going to be 9.02:1

what compression ratio would be better to shoot for?
i want to run the lowest octane possible (87) but if i have to run higher octane then i guess i have to. the car is for street use only.

TraviZ

iTrader: (2)

9:1

Sonix

iTrader: (1)

AL heads and 219/219 cam? I'd say 9.5:1. You won't have detonation if you have your quench right, and you'll optimize the setup for that.
9:1 will be a bit lazy on the low end with that cam.

I think the recommended CR for that cam is in the range of 9:1-10:1 (assuming iron heads), so you'd be right at the bottom of it.

Damon

A 383 with flattops? You'll be wanting the 74cc heads. I think you might have miscalculated those compression numbers.

A 383 with flattops (-6cc of valve reliefs) sitting a full .025" in the hole at TDC (stock deck height) and a massive .039" Fel Pro composite head gasket (9cc actual volume- not an estimate) you'll still be at 9.32:1 with 74cc heads. Same same but with 68cc heads you're at 9.88:1

Go to a slightly thinner .028" GMPP composite heads gasket, everything else the same, and you're at 9.55:1 with 74cc or 10.14:1 with 68cc.

It's easy to build compression when you have more cubic inches.

QuickStyle

im pretty sure i calculated it right, here are the numbers i used:
bore - 4.0310
stroke - 3.75
head gasket bore - 4.166
head gasket compressed thick - 0.041
combustion chambers - 68 or 74
piston volume - flat tops with valve reliefs [-12cc]
deck - 0.0125
these are the correct numbers 100% sure
i probably should have added that the compression ratios i listed were static.
here are the dynamic compression ratios with the lpe-74129 cam

68cc - 8.77 dynamic CR
74cc - 8.29 dynamic CR

Quench Distance is 0.0535

let me add another question to this, do you think i would be able to get away with using 87 octance with the 68cc heads at 9.5:1 static and 8.77:1 dynamic

Sonix

iTrader: (1)

if you're sure that the flat tops are 12cc, i'll trust ya. They're usually 5-6cc's though, that's why I asked.

You decked the block then? .0125 seems like an oddball #.
If you use a thinner gasket (if you still have that choice, using damons suggestion), shoot for perfect quench, and use the 74cc's. I think that'd be your best bet. That'd be 9.27 static, and whatever in dynamic. That's .028 gasket, with .012" deck height = el perfecto .040" quench.

9.5:1 static, 8.77:1 dynamic won't be happy on 87 octane.

rgarcia63

74cc if your going to use 87 octane rated fuel.

QuickStyle

i just got info that my heads are in and ready to ship. found out that afr's new eliminator heads come as 75cc or 65cc. im going to call afr on monday to confirm this.

65cc- 9.84:1 CR static - 9.02:1 dynamic
75cc- 8.94:1 CR static - 8.22:1 dynamic

i was pretty set on using felpro-1003 head gaskets at 0.041 thickness seening as that is was afr says needs to be used with their heads. could you explain how 0.040" quench is perfect, i dont really understand the quench.

as for what octane im going to run, i was just wondering if i could get away with 87 octane. if i have to run higher octane, then that is fine.

also my measurements after deck were all equal.....

1 - .0125
2 - .0120
3 - .0125
4 - .0115
5 - .0125
6 - .0120
7 - .0145
8 - .0145

Sonix

iTrader: (1)

.040" quench gives the best flame travel. Smaller quench is best, however below .040" you're getting into risky territory, if you have some piston rock, they can slap the heads. Higher end race motors will run tighter, like .025".
If it's a higher quench #, you're throwing the quench effect away. Better quench means you can get away with higher compression. ie, 9.5:1 with .070" quench, will detonate more than 10:1 with .040" quench will. Either way the final say is how well you can tune it. I'm just saying that if you get your quench right on, you'll have the best chance of keeping away detonation, and making the most power.

here's some quench reading for you
KB Performance Pistons-Engine Building with KB Pistons
Compression
just stumbled on these, there was one really good site that I can't find now...

With what you've got, i'd still use a 75cc head with a .028"ish head gasket. But again, AFR is THE expert on AFR heads. I'd call 'em up and tell them your dilemma, show them your choices, and ask them to recommend one. If they think you can use a .028" gasket on there heads, then I think they'll list off what I mentioned. (actually, the 65cc heads with the .028" gasket would be absolutely ideal, but would most likely need premium fuel, unless you were using a sophisticaed FI system, rather than a carb).
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.
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just found you are using FI. I'm not familiar with FI much, but if that has a knock sensor, and computer adjusted timing, fuel curves, etc etc - I'd use the 65cc head and the .028" gasket. You can tune that up to run just fine - and most importantly, power level would be highest with that combo. Perhaps some of the custom EFI tuners could comment on this.

QuickStyle

i just found that exact web site right after my post.. i put in "quench height" on google search and there you go.

i understand the quech idea now. .040" = perfect the .028 head gasket would make it.

the only 0.28 head gasket i could find[Mr.Gasket mrg-1134] comes in a bore of 4.130 when the recommended gasket bore is 4.166

the only difference between the mrg-1134 and the recommended felpro 1003 is the compressed thickness and bore is off by 0.036 smaller both are composition type. i will ask afr on monday when i call them about my situation.

compression would go up to 9.2:1 static CR with the 0.028" gasket with the 75cc heads
and up to 10.1:1 static CR with the 0.028" gasket with the 65cc heads

10.1:1 seems a little high to me for a street car. are there any cons to running this much higher compression, obviously more chance of detonation, but would this make it harder to pass smog [California] drivability problems ect.ect. if im going to have to run 91 octane with the 75cc heads, i might as well get the 65cc heads to benifit from the extra power levels.

Sonix

iTrader: (1)

yea, the bore size of the gasket doesn't really matter. As long as it's over 4.060" it's a non issue. You can ask AFR, because some gaskets work good with AL heads, and some do not. (I'm not sure if this is an urban myth or not). But AFR can tell you beyond the shadow of a doubt. Aside from the MRG gasket, there's a GM made one (the one damon mentioned). I found the part # for it here on the forums (do a search), it's like $14 per, so a pretty good price too. Just another option.

Yea, 10:1 is about the limit for a street car, with a street cam **and iron heads**. That's the key. With AL heads you can/need to use an extra 1pt. So 11:1 is the limit with AL heads. That's kinda pushing it, but you get the idea. AL loses it's heat faster, so you *can* run more CR, but it also loses it's heat faster (), so you *have to* use more CR to keep the same power level.
heat = HP eh?

I run 10:1 on iron heads and full timing on my car, FWIW. and a carb.
FI has better fuel atomization, and a better adjustable timing curve, with spark knock detection to pull timing, so you can get away with a lot more CR. What, stock 4th gens had like 10:1 didn't they?

Personally, i'd bite the bullet and use the 91 octane with the 10:1. You might be able to use the 87 if you've got a solid tune down, or say if you're taking a road trip in the cold or something you can dump in a tank and not worry... Drag strip or heavy driving in the summer, i'd keep 91 in it to be on the safe side.

I don't think it'll make any difference for smog levels either way.

ZimGTA

On my FI 383 I am running TF 23 degree heads, Comp Cams 306 and my c/r is 11.25 and I have no knock counts. I run 92 octane. You seem set on running 87 octane, with all you are putting into this combo, is less than a dollar a tankfull for 89 that important? Alright 2 dollars to run 92.

QuickStyle

im not exactly set on using 87 octane, even though it would be nice to be able to. i was just wondering if i would be able to get away with using it, but it seems like im just going to have to use 91 octane [max available here]. this isn't going to be a daily driver, just a weekend cruizer and it will probably see one or two tanks of gas per month, that averages out to $2-$4 more for gas a month.

i think i will take your advise and go with the 65cc with a 0.028 head gasket but will still have to contact afr about this, just to see what they think about it.