rithsleeper

As I'm waiting on parts to try listening for knock and trying some ideas you guys put before me, I started thinking maybe I did my math wrong and I have too high of a compression ratio... Then when I went to figure dynamic and static I'm getting numbers that don't make all the sense. Real thing is, my cam card isn't giving me actual ABDC intake closes. It only gives me @ .050.

Could someone do some quick math for me and see if they get a number that looks good for 93 octane gas?

383 stroker, 64cc AFR heads, 6" rods, 3.75 stroke, head gasket is .039 I believe with 4.166 bore. did not measure deck when the head was off it. -5cc dished piston

Here is my cam card

383tpimachine

Pistons? Dished or dome and home many cc

bjankuski

This is a solid roller cam so you have to use the .02 numbers which are 280 degrees and the 106 installed centerline numbers, we also need to know what your piston volume is in cc. (domed/dish/flat with valve reliefs)? So your abdc number will be (106 + 280/2) - 180 = 66 ABDC use the 66 abdc to figure your dynamic compression after you find your piston volume information. assuming a static compression of 11:1 which would be a 11cc dish volume with a 4.20 diameter head gasket gives you 8.7 dynamic compression which is pretty high for pump gas, but without knowing what you have I am only guessing.

rithsleeper

Sorry, yes, -5cc dished Pistons.

bjankuski

using a 5 cc dish (I assume a flat top piston with valve relief's) and 0 deck height that gives you a static compression of 11.75 and a dynamic of 9.25. Do you know if your deck height is 0? If you are running 9.25 dynamic compression that is probably too high for street 93 gasoline. Maybe you can make it work but you will have to keep the engine cool, like 160 degrees and even then it may not work. ( I am also assuming conventional SBC, not an LT1)

Find out what your deck height is.

Churchkey

Has the block been decked to 9" ?

If the block has not been decked the slugs are probably .025 in the hole @ TDC.

Dish pistons are + CC

anesthes

I'm getting 10.67:1 static, 7.19 dynamic

It's got 64cc chambers, plus the 5cc dish.

4.030" bore, 3.75" stroke, felpro 1010 gasket.

If the block is zero decked, 11.08:1 static, 7.40 dynamic


-- Joe

bjankuski

I never even caught the 383 bore comment vs the bore size that he listed of 4.165. If you use the bore size of 4.03 all else being the same I get 10.92 static compression and 8.65 dynamic with a 280 degree cam. The is pretty fast acting so it will close the valve and start building compression at 66 degrees ABDC. It is better but still on the edge for 93 octane pump gas.

anesthes

Yeah he was talking about his head gasket bore, which is probably a felpro 1010.

Since we have no idea what the deck height is, or the compression distance in relation to the cross pin, who knows.

-- Joe

Kubs

Just to compare to others, I used a calculator I wrote and using the following:

4.030 bore, 3.75 stroke, 6" rods, 64cc heads, 0.039/4.166 gaskets, and zero decked I get

11.09:1 Static
8.77:1 dynamic

This is close to my 355ci in my race car which is 11.32:1 static and 9.00:1 dynamic with an 0.035 quench. It runs great on 93 pump gas with no pinging or spark retard.

GREGGPENN

I have 20cc pistons under 56cc AFR195s in a 383. I chose the deeper dish to avoid potential problems. That said, a year after installation, I couldn't remember if the chain set was installed straight up or 4-deg advanced. The problem with 4-deg advanced is my cam was built with a 108 ICL.

If I installed it 4-deg advanced, my DCR is 8.6. If I installed it straight up, it's 8.35. I run Q-Trip 91 octane w/o issue. But, I also have a zero deck and .040 quench.

I think the combination of the AFR chamber and dish pistons makes for a very efficient chamber. So much so, that my full advance at WOT is only 30-deg.

If the OP can find a small dish (10-12cc), it might be safer and allow more flexibility with octane selection (i.e., 91 which is more prevalent in some areas). OTOH, I think he's likely to be OK with 8.7 DCR as well. That's because I don't consider any of my timing table "conservative". In fact, there was very little KS activity with the stock 89 BIN -- which is off-the-chain aggressive.

FWIW, the change in HP between 4cc and 20cc piston is pretty small. So small, it might come down to which piston type you feel is more efficient. I've seen BOTH flat-top and dish arguments on this topic.

Obviously, good heads and chamber design can tolerate more DCR.

bjankuski

Greg, what are your can specs? The reason I ask is you need to use seat to seat duration to figure dynamic compression not .050 numbers, your combination gives you around 10.1 static compression so I think your dynamic compression will be lower then what you posted.

cardo0

What is your compression pressure? For pump gas it needs to be <220psi on a warm block (with the throttle plates fixed open of course).

GREGGPENN

It's a SMALL cam...custom ordered before I decided to build a stroker. I also angle-milled the heads originally intending to HONE my 50k mile 350. Later, I decided to stroke it. *** = purchased before decision to build 383.

269/269 (adv duration) 214/214 108ICL 112LSA ***
AFR195's that are milled to 56cc ***

383 stroker (.030" overbore)
.040 gasket (Cometic -- IIRC the #5404 with a 4.030 bore)
zero decked
20cc dish
5.7" rods

As noted, there's a possibility the cam is installed 4-deg advanced (in addition to 108 ICL)

I ran the numbers again. I get 8.4 DCR if installed straight up or 8.65 DCR if installed 4 deg advanced.

bjankuski

Looks good, sorry for the confusion

cv67

Youll be ok on 93 ran my 350 on it with 64cc .015 in the hole .028 gasket with that cam
Should work good on a 383
Dbl ck your tune. You can always toss a can of Torco in never hurts
If you dont have cats Wild Bills octane supreme is real good stuff and cheap. Its toxic as hell, has lead but I can tell you first hand is not the crap youd get from the auto pts store. Use a good pushrod on that cam.

383tpimachine

My DCR is 8.77 and then a crap load of boost on top.

Gibbles

There is an excel file posted on a few related threads at grumpys performance garage forum.

​​​​​​​Super easy to use, just enter details into the sheet and it auto calculates.