Can someone help me calculate Dynamic Compression Ratio for my proposed build? The following is what I have:

Bore 4.290
Stroke 4.25
Rod Length 6.385
Head Volume 118cc
Piston Volume -17 or -20
Head gasket .039
Deck Height .020
Cam adv duration 276 / 284
intake closing 45.5* at .050
LSA 110
Lobe centerline 104
Intake seat closing as calculated by a tool I found is 62 (I cant find seat closure on the cam card)

I have found a few calculators on the internet, but I get different results and don't know what I can trust.

I would like to run pump gas and the research here tells me that I should be around 8.25:1 DCR in order to run 93 octane without problems.

Thanks for your assistance here!

 

 

Something like 7.7 static compression ratio!!! I stopped there.

491 cubes is what I figure. Looks like a big lazy pig. I must be doing something wrong or this build is off.

-Mark.

 

stingr69,
here is what i get:
assuming a piston clearance of 0.005 in and distance down to first ring is 0.213 in,
displacement = 491.455 ci (looks like a 4.250 bore 0.040 over)
could call it a 492 i guess

for piston volume -17, SCR = 9.76:1
for piston volume -20, SCR = 9.99:1

for Intake seat closing 62 deg ABDC, and
for piston volume -17, DCR = 8.01:1
for piston volume -20, DCR = 8.20:1

you can contact cam mfg for better data, or maybe a better cam depending on rest of components, but this will run fine on 93.

here's a link to calcs you may have already found:
http://forums.corvetteforum.com/show....php?t=1903517

if you PM me with you email, i'll send pdf of calcs

hope this helps . . .

 

I had to make a few assumptions, like what is the pistons compression height? I assumed it to be 1.29", which going by your numbers leaves you with a quench height of .059". This is a little big in my book. Below is a screen shot from my excel file. I have highlighted a few numbers for you that you should work on. First is the compression height, what is it? Second, is the rod to stroke ratio. Its at 1.5:1, you'll have a better luck warding off detonation and ping if you increase this closer to 1.7:1. Do this by getting even longer connecting rods. Next is the quench height, the closer you can get to .035~.040 the better. Your compression numbers are at the bottom going from my assumptions. Your in a pretty safe range as is but I would definitely reduce the quench height to make sure your are really safe. For comparison I have my combination to the right. Its a smaller cube Big Block but as you can see my compression is quite a bit higher and I still run 93 octane with ~10% ethonal (thanks Missouri). Computer controlled timing makes it all possible.

 

the only thing is that the 6.385 in rod length is about all you can fit in a std height block; it's already into the oil ring. race pistons might be different???
i agree on the quench; get it down to at least 0.040 in

 

agreed ... ~7.7:1 scr ... 491 ... gonna be a slug if those piston volumes are truly negative as in dish/reverse dome.
------------------------------------------------------------
Suggest a piston w/ small to mid-dome ... around +18cc ... that'd produce about 9.6:1 scr ... such a piece is forged probe p/n 13729-030 (no +40; only +30 & +60)

4.280" d
1.270" ch
+18cc dv

it's for use with aftermarket 6.385" rod.
--------------------------------------------------
There's a hyper mid-dome that should work w/ stock 6.135" rod ... Sealed Power p/n H603CP30 (no +40; only +30 & +60)

4.280" d
1.525" ch
+23cc dv

tighter quench & ~ 1/2 pt more comp at about 10.1:1 scr
------------------------------------------------------------
just guessing ... perhaps s489 scr calcs errant because he's mixed positive dome & negative dish volumes. I've seen a calculator that does that ... may've been KB? Yea, I know ... a pos dome results in less(neg) comb chamber volume ... but industry std is + for dome & - for everything else.
-------------------------------------------------------------
lr172 ... if ... IF your "-17" and "-20" piston volumes are actually mispoken and they're actually domed with a positive value, you'll probably be fine on scr. I don't have much faith in meanifulness of common dcr calculators' results so someone else can handle that ... but you must have an accurate scr before you can begin to get any sense of dcr. Strongly suggest you shy away from KB / UEM dcr calc ... and their hyper pistons. SP makes a great hyper. Note both pistons I reference are +30 ... neither cat showed +40.

here's a decent scr calculator from sealed power/federal mogul:
http://www.21cgt.com/FMWebCatalog/frmConversion4.aspx

 

With a 10.200 tall deck block, you could spec out a 6.700-6.800 rod and get a ratio between 1.576-1.600:1 without requiring drastic compression height alterations, but the 6.385 is the commonly accepted rod for 4.25 stroke 9.800 deck builds.

Assuming a 4.37 x .039 gasket, if your piston volumes are in fact negative (dish), your static CR is indeed 7.61-7.74:1. If domes, then you have 9.73-9.96:1 static CR. The latter sounds a lot more like it, eh? If not, Id rush back to the drawing board.

IMHO, it would be best to deck the block or order custom comp ht pistons to get that quench in order with the gasket you choose. Also, 4.290 bore? Might as well go on to 4.310 for a more commonplace 496 cid. If your block can't take that, I'd be looking to sleeve it or get another one, anyway.

BTW, why so much cam advance? I'd be looking at ~108-110 ILCA, and strongly consider a 112 LSA with those durations and this bore/stroke combo. Just my $.02.

 

Thanks for the responses here. I really appreciate the help. They are dome pistons. I apologize for representing them incorrectly. I am new at this. The 17cc are Hyper's and the 20cc are forged. I am leaning the forged way, but wanted to make sure my DCR wasn't going to get too high.

I guess there is no reason not to go to 4.31. I just thought I would convserve the bore to maintain it's value, given the bores are in good shape now.

I know the quench should be smaller. I had considered a .029 head gasket if I can find one. That would get me to .050 and I didnt' think it would kill my DCR. Any problems with that or should I spend the $ on decking?

The block has not been decked yet and it is a 9.80 block. I will look into a longer rod, but I am doing this on a budget and would like to stay with off the shelf pistons if I can. Will I get in much trouble wth the 6.385 rods?

Not sure on the cam advance. I just pulled that off the cam card. I will research this more before I do that. Thanks for catching this. I will also look at 112 LSA. That was the original recommendation that I got.

Thanks again guys.

 

A better rod angle would be desirable, but 6.385's (that's what I've got) are fine w/4.25 stroke in a std deck ht block for our purposes. You'll find a fair selection of relatively inexpensive off-the-shelf pistons (SRP's for example) for that combo and a 4.310 bore which should work well with cams on the order of what you're looking at. Stay below 6300 rpm, unless you use better-premium quality parts in the bottom end, and you should never have any short-block issues.

IMHO, unless you're saving a stamp I'd deck the block and use the premium quality Fel-Pro .039 x 4.37 gasket. The thing to do here is make sure your cylinder walls will tolerate (sonic test) and that the bores will clean up sufficiently at a .060" overbore. Then order the pistons, finish honing the cylinders and decking the block after you have them in hand.

As I mentioned, you can always sleeve any problem cylinders. When doing big inch ultra-high compression BB Buicks back in the day, we used to routinely sleeve all of them to ensure/increase integrity (those blocks were notorious for critically thin walls which compounded any core shift issues).

Yeah, that 104* is pretty early. And, with the ~10.2 you're likely to have with "stock" pistons you might look at that 112 LSA, too. It's not like you're going to be lacking for peak torque, so you might as well flatten the curve while improving manners for streetability.

 

OK here is a combo I slapped together for example - If I assume you go with the forged 20cc domed pistons and all else as posted you are just a hair under 10:1. As we do not have reliable cam data, I went and grabbed a Crane H-286-2 hydraulic cam. This one has 112° lobe separation angle and should have a basic RPM of 2500 - 5500 RPM with a redline of 6200 RPM and valve float at 6800 RPM. Very streetable. It would have your DCR at 8.2 or so. Crane recomends 9.5 to 11.5 CR so you should be able to run pump gas with no problems. Not real sexy but a VERY nice 500ft-lb of torque across the board driver.

-Mark

P.S. - the intake closing angle used in the above was 40°ABDC so shoot for a cam that has a SEAT TO SEAT intake closing point close to that.

 

i assumed piston was domed; should have stated that. am using self made mathcad worksheet to try to eliminate black box errors - apparently didn't fix unstated or bad assumptions by user . . .
i'm running 112 also with roller cam on street and auto - working fine

 

Thanks for running this. I am not running into many piston options. It is either 20cc or 37cc. It looks like I will roll with this, as 11:1+ seems too high. Once I get all those details in order, I will come back with a few different cam choices. Hopefully you guys will be able to help me sort through them.