Max Compression

[FreeBeeTony]

Hoping to pull the engine out of my boat this w/e.
Need to start thinking about pistons.
Have Vortec heads w/ 64cc comb chambers.
Gonna have the blocked decked so the pistons are 0.015 below the deck and get a gasket w/ a comp thickness of 0.025 giving me a quench of 0.040.
What is the max compression I should go for that is safe (no detonation) and still allow reg gas?
If I use a LCQ piston w/ a 15cc dish I come up w/ a comp of ~10.3.
NOTE: I used 3 different online calculators and got 3 different numbers!!!

Is this too high?
Does the cam come into play here?

 

[Bt Doctur]

Guessing 9.0 is safe with todays fuels, but it also depends on your ign system, total advance, and rpm band where the advance occurs.
Compression ratio 9.4:1 , 9.3:1, 9.0:1

 

[Bondo]

Ayuh,...... If ya deck the block flush, with 18cc D-dish pistons, 'n a .040 composite head gasket, ya ought be right at 10:1,....

With the Vortec heads, 'n tight quench, it oughta run just fine on ethanol-free gasoline,....

The Vortecs, 'n tight quench usually don't have a detonation issue at under 10.5:1,.,...
Clean fuel, 'n filters will of course be Necessary,....

 

[FreeBeeTony]

With the Vortec heads, 'n tight quench, it oughta run just fine on ethanol-free gasoline,....

Not sure about getting ethanol free gasoline......

 

[alldodge]

In the 10: ish range might need 89 octane

 

[FreeBeeTony]

Just for reference...........
Which calculator should I use?

 

[alldodge]

I'm coming up with 9.88 compression ratio

Standard Bore 3.875 and Stroke 3.25
.030 over bore 3.905

Radius = (bore) 3.905/2= 1.9525
Cylinder Volume = Radius squared x ? x stroke
1.9525x1.9525 = 3.81x3.1415= 11.97x3.25= 38.90 cubic inch

Cylinder volume (CV) = 38.90 cu in = 637.46 CC

Head volume (HV)= 64 CC
Quench (gasket thickness + piston height) = .040
Gasket volume = Radius squared x ? x gasket thickness
1.9525x1.9525 = 3.81x3.1415= 11.97x .040= .4788 Cu in

Gasket Volume (GV)= .4788 Cu in = 7.8 CC
Total Combustion Space (TCS)= HV+GV (64+7.8)= 71.8 CC

Compression ratio = CV+TCS/TCS
637.46+71.8/71.8= 9.88 ratio

 

[Scott Danforth]

stock 350 boat motors from 1986 on had 9.4:1 compression with 64cc chambers and +3cc flattops.

when I was looking to build a 383 stroker, I was looking at +12cc LCQ style dish to get to 9.7:1 with a stock .020 deck and 0.041" gaskets

the SBC has a 3.48" stroke. a 5.7 (350) SBC from early 80's on has a 4.00" bore. AllDodge , the ole 283 and 302 and 327's had 3.25" stroke. Stroker crank is 3.75" for a 377/383 build and 3.875" stroke for a 396" build with a 4.030" bore

As Bondo stated, the LCQ style piston and vortec heads can live up to 10.5:1 without detonation

I use the summit racing calculator https://www.summitracing.com/experta...ion-calculator

 

[FreeBeeTony]

AllDodge where does the piston volume enter your equation?

 

[alldodge]

AllDodge where does the piston volume enter your equation?

Correct, left that out. The bore stroke didn't look right but I was more concentrated ion the math. Your question was which calculator should you use, so may attempt was to do the math and which ever one came closer you would use. I like doing things which work the brain. So I recalculated and came up with 9.32 ratio with 15CC piston. Also show flat top assuming no valve reliefs at 11.05, and 15CC dome at 13.68

So if someone can show where my calculations are in error, this would help, or point to a better on line calculator

Standard Bore 4.0 and Stroke 3.48
.030 over bore 4.030

Radius = (bore) 4.030/2= 2.015
Cylinder Volume = Radius squared xPi x stroke
2.015x2.015 = 4.06x3.1415= 12.75x3.48= 44.38 cubic inch

Cylinder volume (CV) = 44.38 cu in = 727.25 CC

Head volume (HV)= 64 CC
Quench (gasket thickness + piston height) = .040
Piston Volume (PV)= 15CC
Gasket volume = Radius squared x Pi x Quench
2.015x2.015 = 4.06x3.1415= 12.75 x.040= .51 Cu in

Gasket Volume (GV)= .51 Cu in = 8.35 CC
Total Combustion Space (TCS)= HV+GV+PV (64+8.35+15)= 87.35 CC

Compression ratio = CV+TCS/TCS
15CC dish = 727.25+87.35/87.35= 9.32 ratio
Flat top (TCS-15CC) = 727.25+72.35/72.35= 11.05 ratio
15CC dome (TCS-30CC) = 727.25+57.35/57.35= 13.68 ratio

 

[FreeBeeTony]

AllDodge

I like math too............that's how I noticed the missing PV.
Your math looks good!

Any recommendations on sourcing the LCQ pistons?
Found a set of KB pistons..........~$900!!!!

 

[Bondo]

Ayuh,.... You don't need forged pistons,..... hypereutectic is what ya want,....

Waayyy Better than cast pistons at 1/2 the cost of forged,...

These, would allow ya to do a clean-up bore, 'n hone, for brand new cylinders, without the pitfalls of doin' a .060 over-bore,.....

I'm not advocatin' any supplier, nor manufacturer,..... the above was just the 1st I found,....
This page has 1000s of options,....

 

[FreeBeeTony]

OK............I was looking for a piston with the reverse vortec dome.
The D Dish is good enough?

 

[Bondo]

Ayuh,.... LCQ, D-dish, Reverse-dome,..??

Vortec heads shine when the Quench is tight, .043", to .045",....
The pistons want to be flat, where the head is flat,...
Those pistons linked oughta get ya 10:1 at zero deck,.... they look vortecish in the pictures,...

 

[alldodge]

Those pistons linked oughta get ya 10:1 at zero deck,....

Guess my calculations must be off, because the ones in the first link have 18CC dish, and using mine above (15CC) it would come out less then 9.3 ratio. What am I missing?

 

[Bondo]

Guess my calculations

Ayuh,.... I Guessed, didn't calculate,.....

Is that zero decked, with a fel-pro composite gasket, .040, I think, maybe .043,..??

 

[alldodge]

He has +.015 deck and .025 gasket which would give him .040 Quench. I like the +15 deck just cause it leaves more metal for threads (not much) and also deck can be resurfaced again if needed (hope not) some day.

I think the big issues is his heads has 64CC chambers, that's a tad big I'm thinking/guessing

 

[alldodge]

BTW Tony, need to chose the pistons before getting the block decked, they have different compression heights

 

[Scott Danforth]

I think the big issues is his heads has 64CC chambers, that's a tad big I'm thinking/guessing

64cc is perfect for a 5.7 boat motor

smog heads are 76cc (early to late 70's) and they kind of suck, however were standard on the large 4.125 bore (400 cube)
Some marine motors up to 1986 came with heads 14011083 which are 64cc
the standard heads on most marine motors (non-vortec) from 1987-1995 are casting number 3901068 and are 64cc
vortec heads are 64cc (casting numbers10239906, 12558062)

the 5.0 has smaller valves and a smaller combustion chamber (53cc) you cant use them on a 350 unless you keep the RPM down because the small valves peter out on airflow.

conversely, you cant use the larger chamber on a small bore 5.0 because you shroud the larger valves

 

[alldodge]

64cc is perfect for a 5.7 boat motor

Sounds good, have any thoughts on setup pistons, deck height, gasket?