Aaron-74

I got my engine kit today. The piston specs are :Chevrolet: 350, .030 in. oversize, 4.030 in. bore, 3.48 in. stroke, 5.7 in rod length, .125 in. dome, hypereutectic piston. Size point 2.18 from top of the head. Skirt size 4.0287 +/- 0.00025 inches. 10.72:1 with 64cc heads


I don't know the head gasket thickness but the part number on the gasket it 7733PT2.

I have 74cc heads (thy've been milled). I think it was Vettdrms that said I'd have a 9.8:1

Can that be verifyed by the given information??...

Thanks,
Aaron

Aaron-74

Here's a picture of the dome.

boredsoIstrokedit

That head gasket is .039" compressed. That's about all I can tell you for sure. Compression sounds about right though at around 10:1 with a .125" dome.

Edit, should have said about right at 9.5 to one, but you need to know your deck height for sure, and yes, the cc of the dome.


[Modified by boredsoIstrokedit, 4:49 PM 3/11/2003]

gerry72

68cc chambers with that dome? You are probably going to be closer to 11.0:1. There are plenty of static ratio calculators, like at smokeemup.com that offer up the ratio based upon input information. You're going to need to know the dome displacement cc, not the height, and the gasket compressed volume to get an accurate result.

Aaron-74

It's a 74cc chamber.

77Dream

:confused: that makes the car go right?

:D :jester :lol: :lol:

Aaron-74

Umm..I think it does. I'm not real sure.

Twin_Turbo

that compression, for what deck height is that (I am too lazy to calculate it) ? Do you have a stock deck? if so, it's around 9,025 and w/ the generals tolerances the pistons wil be down the hole further than 9.00"" at TDC. So, if you add a 0.039 gasket you will almost certainly have more than 0.060"quench height, which is about borderline for proper quench action. With that much compression you better make sure you ahve the quench around a safe 0.040 or so otherwise it's detonation for you.

Marck

Aaron-74

And that means I need to get a zero deck??

I'm pretty sure it's the stock deck...The pisonts in there now sit down a little. It's not flat.


Why does having a .060 quench cause detonation?? It's a slightly domed piston. So, that could help the quench right?? Besides, it's rated at 10.72:1 for a 64cc head. I have 10cc more than that.


[Modified by Aaron-74, 4:59 PM 3/11/2003]

Twin_Turbo

well, not necessarily a 9.00 deck height. Just make sure that the amount the piston is down the bore (measure! ken has a degree wheel) to the total deck height + compressed gasket thickness is withing the proper range for quench height.

The quench area is where the flat spot on the piston meets the flat surface on the head. Close at TDC the mixture is forced out there violently and this wirls up the mixture which significantly rduces the tendency to detonate and it improves the burning. It will get even more important with your dome so that no mixture is trapped in the quencha rea and as a result won't burn w/the bulk. The dome is not a part of the quench area, only the flat surface. Your heads area also open chamber design and have only 1 quench area whereas the lower cc chambers of closed design have 2 (one under the spark plug).

Marck

Aaron-74

I can figure all that in school. I'd just like to know what my compression will be.

Twin_Turbo

yeah, okay. Well, figure it out for .040 or so quench height. There's a nice calculator in DD2K and ken has DD2K.

Aaron-74

I don't think I really under stand the quench thing. The pistons needs to be atleast .040 under the deck at TDC??

Wrecked82

I love it when you guys talk like that. Totally greek, or whatever.

Aaron-74

I guess you could call it geek-like i guess. A part of owning and working on cars is learning and becoming geek...but all well. :cheers:

427Hotrod

Quench height is how far the piston is from smacking the head at TDC. Most stock Chevy's have the piston "down" in the hole ( as you described yours) approximately .017-.020. Many replacement pistons are even further down to allow for block cutting during rebuilds, but most Hypereutectics are a little better in that regard. The only way to know is to measure. Generalities and shortcuts get you a motor with a lot of trick parts that doesn't perform up to your expectations or the car with the milder but well scienced out motor in the other lane.

So on top of the .020 deck height, you add the compressed gasket thickness and you arrive at total quench height. Tighter is generally better. .030-.040 is a pretty good area to be in for a steel rod street toy. You are at .059 or so. Will it run? Sure. Could it run much better and be less detonation prone with a tighter quench.

The point is to not allow any places in the chamber where fuel can puddle and not burn. By squeezing the flat surface between piston and head flat surface as tight as you can without it hitting, you are forcing the mixture over into the combustion chamber. The OEM's and race engine builders work hard to control these areas. Placing the rings higher on the piston eliminates dead air between cylinder wall, top ring and and head surface. They even go so far as to NOT stamp the part numbers on the dome to cut out THAT little dead area in the letters!

Remember..it's the little things that make it all work!


JIM
You're going to have to find out the volume of the piston dome and valve reliefs to accurately determine compression ratio. It will make a huge difference and it is useless to even try to calculate it even in the ballpark until you have those numbers, It's all total shot in the dark speculation 'till then.


JIM


[Modified by 427Hotrod, 9:41 PM 3/11/2003]

Aaron-74

alrighty then..thanks for the info.

cardo0

I like this question Aaron - it's definitely a not an easy one. :banghead:
You need to call (E@mail) the piston manufature for dome volume. If not possible there is couple ways to estimate. My choice would be to soften enough modeling clay (Walmart crafts) and make an impression with entire top of piston - on a level surface of course. Now using a calibrated cylinder or even an eyedropper or burret if you can afford one measure amount of oil to fill just impression. Do this with different pistons until you get consistant numbers you trust. Remenber 1ml = 1cc.
Next measure cylinder head chambers with again oil until consistant volumes are found.
Now you need to assemble the short block (piston, rings, rods, bearings) to measure any piston at top dead center (TDC). Measure distance from deck to piston top (ensure at TDC) flat area - not top of dome. Multiply this distance times 4.03"(bore dia) times 3.14"(pi) and divide by 2(dia to radius). Convert this to cc (multiply by 16.387).
Now add chamber volume and gasket volume (~3cc) and piston to deck volume and subtract the piston dome volume. This is compressed volume.
Fun? Yes? :crazy: Not done yet.
You have to divide the result of the above madness into the result of the following: 4.03"(bore) times 3.14"(pi) times 3.48"(stroke) divided by 2(dia to radius) and multiply by 16.387(in to cc) and then add the compressed volume back in, yes again.
This now is a static compression ratio neglecting piston ring crevase volume (<1cc).
Static compression is only a guide Aaron. Larger overlap cams can handle more without preignition. Another long subject. Good night and good luck. :seeya

Aaron-74

I think i understand what you're telling me to do. We have a CC measuring thing here at home. I cant get any TDC measures until I get to school...next week to pull the motor. But, I would like to know what compression I'm iuning. So, I'll be crunching some numbers. Summit said the ratios are figured on an .030 over motor. So, I'll see what I can figure. Maybe instead f decking the blocl if the quench is over .040, maybe I could get higher compression height pistons???????

jackson

Seems you may've considered only 0.038 or so gaskets so far? After you've measured well and verified the block's decks are straight and do not need cutting... and quench is still too high ... take a look at the thin steel shim gaskets ... depending on brand & pn, they range between 0.015 & 0.025 thick. With proper gasket choice, chances are you can get the quench down low enough to avoid decking block or tall CD pistons. You should find out from piston mfg just what is your CD & dome volume. If they are either TRW, Speed-Pro or Sealed Power ... I can tell by the part number ... just email/post their part number.