Help me figure compression so I can tell Lloyd what to mill them to!

I need to let Nightrain66 know what to mill my heads to. I am wanting to get 11.4:1 compression. I have a 355, TRW L2256F Pistons, Fel-Pro .039 Head Gaskets and the pistons are about .030 in the hole. I cant find out anything about these pistons online, they are flattops with four valve reliefs like the stock ones. If somebody knows something about these pistons and how to figure compression, then please let me know how much they need to be milled.

If you mean a modern day LT1 car, the valve reliefs on the pistons are 7cc (so -7cc from combustion chamber if your not using all variables on a spreadsheet or something).

To get that compression with those gaskets, its going to be a bit of meat to remove from the head. Any chance on going Imp .026 gaskets?

Using this online calculator:

http://www.gafba.com/calculators/cmpratio.asp?mode=ntr

I get these numbers for the L2256 piston:


Bore Size: 4.03"
Piston Size: 4.028"
Stroke: 3.48"
Deck Height: 0.01"
Piston Ring Land: 0.3"
Piston Dome/Dish: 3.55 cc
Head Gasket Bore: 4.125"
Head Gasket Height: 0.039"
Cylinder Head Volume: 54 cc
Cylinder Volume: 727.391 cc
Gasket Volume: 8.541 cc
Deck Volume: 2.09 cc
Total Volume: 795.634 cc
Engine Displacement: 355.11 cid
Compression Ratio: 11.66:1

This would mean you wouldn't want to mill the heads any except to clean them up. I would recommend you measure the combustion chamber volume on the heads to verify they are indeed 54cc instead of relying on this information though.

Brian

Originally posted by Speeds8erM-1
I need to let Nightrain66 know what to mill my heads to. I am wanting to get 11.4:1 compression. I have a 355, TRW L2256F Pistons, Fel-Pro .039 Head Gaskets and the pistons are about .030 in the hole. I cant find out anything about these pistons online, they are flattops with four valve reliefs like the stock ones. If somebody knows something about these pistons and how to figure compression, then please let me know how much they need to be milled.

Bore Size: 4.03"
Piston Size: 4.028"
Stroke: 3.48"
>Deck Height: 0.03"
Piston Ring Land: 0.3"
>Piston Dome/Dish: 6.0 cc (TRW L2256)
>Head Gasket Bore: 4.166" (this is for typical Fel-pro gasket)
Head Gasket Height: 0.039"
Cylinder Head Volume: 54 cc (need to cc for accuracy!)
Cylinder Volume: 727.391 cc
Gasket Volume: 8.9 cc (per Fel-pro)
>Deck Volume: 6.27 cc (.030 down the bore)
Total Volume: 795.634 cc
Engine Displacement: 355.11 cid
Compression Ratio: 10.68:1

You need to lose ~5 ccs to get to 11.4:1.

Easy enough... use the Impala gasket Doc mentioned.... I don't know the actual volume of that gasket but all things considered equal (4.166 bore) that should reduce your volume ~2 ccs. Then you need another 3 ccs which should be nothing more than a cleanup on the heads... probably ~.012.

-Mindgame

58cc 10.27
64cc 9.60
68cc 9.21

5.7" Rod 350

looks like roughly 48cc chambers
Steven

Thanks everybody!

I already have Fel-Pro's and want to use them anyway because of the nitrous I will be using. So .015 needs to be taken off with the Impalla gaskets or the Fel-Pro's?

Originally posted by Dr.Mudge
Any chance on going Imp .026 gaskets?

My reference shows the Impy head gasket as .029", the Felpro FP1074 as .039" and the stock F-body head gasket as .049".

Have you thought of using Cometic (not sure on spelling) head gaskets? You can get them in almost any thickness needed.

Steven

No because I had Fel-Pro's for a regular SBC and just traded them in for these. :)

sorry missed that one.
Steven

Curt,
Now that I think about it... you're right, they are .029. So we're talking (.026 vs .029)... a ~.2 cc difference. Small, but still significant.

Originally posted by Speeds8erM-1
Thanks everybody!

I already have Fel-Pro's and want to use them anyway because of the nitrous I will be using. So .015 needs to be taken off with the Impalla gaskets or the Fel-Pro's?

Speeds,
Are you not machining the block in this build or is this just a head swap?
I'd want to get that quench tighter..... machining for a 9.0-9.005" deck would be a good idea.

If that's not an option then you are gonna have to do it with the head. You need ~5 ccs, which means a cut of ~.030 to get there. Nothing for an aftermarket head with thicker decks but I honestly don't know about the LT1 head. I don't think it would be a problem but it may very well be necessary to angle mill, which means milling the intake too. Hopefully someone can clarify some of this for us.
Either way about it, you need to pay close attention to your valvetrain geometry upon assembly as you will likely need a custom length pushrod.

-Mindgame

I cant have the block decked, the shortblock is already together. :) You think .030 to get there? I might just go with the Impalla gaskets and mill them .015. I would rather keep the Fel-Pro's.

An old cylinder head rebuild book I own shows .006 per cc. So yep, you're looking at about 15-18 thousandths when using the Impy gasket. That's pretty much standard cut for any production head except maybe the thin smog heads from some years ago.

That'd be the direction to go in my opinion. Best of luck.

-Mindgame

I had him go ahead and have them milled .030. They are waiting to go on the engine now, I just need to figure out what gaskets I need for sure. I found out some info on the TRW's, the compression height is 1.563 and the four valve reliefs are -6.1. What gasket do I need now? The chambers on the heads had a little work done too, dont know how much that effected it, I just want to be in the ballpark for 11.4:1 compression or so. I have the Fel-Pro's right now, waiting to go on, if they will get me the compression I want, then they are going on, if not, I guess the Impalla Gaskets will be what I will have to get because I can probally trade these for them.

Good info. That helps in your calcs.:)

So you're deck height is ~.022, piston volume is 6.1 ccs, and with a .030 cut on the heads.... you're probably close to 50 ccs. If you check out the calculator mentioned at the top, there are a few discrepencies.
One being that late model engines don't have that much (".3") crevace volume (top of piston ring from crown of piston). They're pretty tight these days.... even tighter on the LS1, probably to keep down on hydrocarbon output. I used .200 instead but it may be even tighter than this... not that that is gonna matter too much.;)
Anywho, from what I'm punching in, you're right there at ~11.4:1. Play with it and see what you get.
If anything substantial was taken from the combustion chamber, which I suspect there wasn't, that number will drop.... remember we're assuming 50 ccs now so....

Good luck man!

-Mindgame

Thanks so much for your help! My head is about to explode over this d@mn car. :) I played with that calculator a little bit and came up with different numbers but I didnt mess with it that much.

How did you figure deck height? I am not sure how to figure it?

No problemo. Just don't have an aneurism!:)

Let us know how everything turns out. If you want to know the exact numbers I plugged in let me know. I checked em 3 times just to make sure though so you should be set.;)


edit: Ok the deck height is the distance from the deck of the block (which is 9.025" from the centerline of the crank) to the top of the piston at top dead center.
You take the stroke and divide it by 2 (3.48/2) + the rod length + the pisto's compression height. You should come up with 9.003.
9.025 - 9.003 = .022. So the piston is down the bore .022 at TDC and .022 is your deck height.

HTH

-Mindgame

We checked it again tonight, this time with the right tool to get an exact number, the pistons are .029 in the hole. So, whats the compression looking like now?

If my pistons are .029 in the hole, is that my deck height?

Ok, .029..... using the same calculator I get 11.26:1.

-Mindgame

D@mn it! :) What would the Impalla Gaskets put it at? Sorry for all the questions, I am a compression idiot. :)

I get 11.59:1.

It's ok to be ignorant of something but give yourself maybe 5 minutes of study and you'll have the whole concept down. Or you could just plug the numbers in.:)

-Mindgame