Need Help I think my engine builder F'd up.. Compression
 

I have a 408 short block built by Liverois Motorsports...
4.03 bore
4"stroke
6.125" rods
Pitsons are wiseco 6464LX3 -3cc dome with a 1.10 compression hight i belive and almost .3 top ring land
LS3 heads milled .030
.051 LS9 gaskets

Im no engine builder, ive done some myself but i let our Modified Engine builder help me with the top end... He had my heads milled .030 and told me id be at 11.4:1 (they were new L92's)

Now im either really loosing my mind or am i really at 12.4:1 ??? And if so... Wtf do i do now? Its a street car

Cam is 240/248 .629/.629 113+2 lsa
111° intake center 18°overlap and intake closes at 51°
.... Im hoping my math is wrong

Thanks in advance

The guy that suggested milling the heads is not an employee at Livernois, correct?

Assuming a 64cc chamber and 0 deck, I get 12.6:1 with a 3cc dome. With 3cc valve reliefs I get 11.7:1.

Using this calculator.

No he didnt work at livernois.. The pistons are listed as -3. They are basicly a small dome with reliefs

https://cimg8.ibsrv.net/gimg/ls1tech...5ae99b78e1.jpg

I know its hard to say looking at a picture.. But thos are my pistons

Hello, lets see if we can help you out here.
The title of this thread is a drop misleading.
If we can help in any way we are glad to, we don't know what your build is without more info
if you are trying to calculate compression we need some #'s to gather the correct end #

# of cylinders 8
finished bore of engine 4.030"
stroke 4"
conn rod length 6.125"
head gasket bore 4.080"
head gskt compressed thickness .055"
center of mains to top of deck ?
how far down is the top ring ?
cylinder head chamber cc's ?
dome or dish volume -3cc
how far down or how far above is the deck ?

I don't know where your deck height is at or where the piston is.
You mention LS3 heads that have been milled.
Have they been cc'd?
A LS3 "70cc" +/- 2cc oe

if you gather this info we can gather the correct #
What are your goals with this build?
It looks like you already planned on running more than 93 octane already so a little more compression is not going to change that fact.
old rule(may vary some based off of camshaft profile and induction efficiency)
iron block and iron heads 10.0:1 on 93 octane
iron block and aluminum heads 10.5:1 on 93 octane
aluminum block aluminum heads 11.0:1 on 93 octane

I planed for 93 octane, talked it over with a few people including some guys over with you and everyone said id be ok in the mid 11's.. Your guy told me 11.4:1 to be specific and thats what i was aiming for, you guys did nothing wrong at all... I am overy happy with my shortblock and suggest you to anyone trying to build there own.

The Engine Builder that builds my cousins tour type modified engines is who may have screwed up here

The heads have only been milled. Honestly im not entirely sure on what you mean by cc'd.. Measured?

As for the deck hight im not sure, i hava all my paperwork and order #s and who built it ect.. I can het thos for you
I dont believe there was refrence as to how far out or in the pistons were either, tilt the piston towrds the valley or away and it would be level.. Idk i think it was basicly 0
Gaskets are .051 compressed, they are ls9 gm mls gaskets so i belive 4.06 bore?

Top ring down would be top ring land thicknes? If so you have it stated at .2950
Final bore All-4.030
Piston diameters All-4.0265

The 2 big things here are what the heads were to begine with... Seems everyone is saying 68.5 if this is true id be around 63.5cc

2nd is the pistons are listed as -3cc dome... I have to talk with greg again (other engine builder) but i think he may have did the math at +3cc

So that puts me sky high...

My cam specs are 240/248 .629/.629" 113lsa+2
111 intake centerline
115 exhaust centerline
Intake opens 9° BTDC
Intake closes 51°ABDC
exhaust opens 59°BBDC
exhaust closes 9° ATDC
18°overlap


I dont know. This is probably more my ignorance then anything. Greg knows his stuff

"cc's" measured amount of fluid the chamber will hold
deck height is the # we need, we already have stroke, c.d. and rod length
most piston calculators use a - for a dome and a + for dish
the heads would be on the low side of GM +/- of 2cc@70 if your saying they are 68.5 to start with.
Don't guess measure
gather that deck height from your build data, get the chamber cc's and we can get you a #

The engines together.
I told him what the heads were when he asked.. I whent by what gm said

I got worried when i kept seeing posts about 68cc stock over and over again.. Almost all the posts i found were people saying they were 68 to start..

Turns out he did measure them. They were 71, even so the pistons are like i said. Listed as -3cc and they are a dome with reliefs

He said that the pistons had more valve relief than they did dome.

I assumed -3cc meant less volume since thats what all the calculators baicly say.. Mabey there listed wrong idk

You didnt give me the deck hight measurement in your paperwork.. Im pretty confident the pistons arnt out .. They were flush


So i guess it was my bad.. 71cc would put me at 66cc? If you go by 6...

I still dont understand the piston tho, why is it labeled- 3cc? All the calculators take that as less volume and it sends the compression up...but he said mine were basicly 6cc valve reliefs and 3cc dome..

If it's all together your going to have to see what you get.
Your dyno tuner will be able to tell you if it's happy with the octane you are trying to run.

Piston companies and calculators use a total volume # to make it easy to do your math.
I would think milling your heads will only net you a couple cc's, the trouble is you don't know where you started nor did you mention someone valve jobbing the heads sinking the valve a hair further. All variables
These are all factors in attaining the correct final # 's and why we insist on checking everything.
I don't have a way to see what your build was without any info.
If you pm us you contact info we can look up in the system and see if we have data on your short block build.

If they were 71... then .030" is 6cc *typically* which means they are 65cc. And they are reverse dome - it's how most shelf Wisecos are sold. So it's a dome with huge reliefs. It ends up being 3cc *dish* for calculating purposes.

If we assume a zero deck height the compression is 11.58:1 and if it's .010" out of the hole, it's 11.87:1... either would be fine.

Thank you, I was confused with the way the pistons were listed.. So 71cc milled .03 would actually be 66cc tho right? I guess my pistons are out .0025

And to Livernois the whole reason i was concerned about this was because i didnt think he measured the heads... So when i started seeing everyone say therea wound up 68 i freaked... Also the way wiesco lists there pistons messed me up. ..

But he did measure them they were 71 to start... No valve job... Stock new L92 heads all we did was lap the valves

If i know how far out the pistons are then i shouldn't need to know deck hight correct? The 4" stroke never changes. ..

So
4.03 bore
4" stroke
6.125" rod
66cc heads
Ls9 gasket so .051 and 4.1" bore
.2950 too top ring
Compression distance 1.10

You're right... it's .006" per cc. So it's only a 5cc mil. I transposed the numbers. So it would be 66cc. You just need to know how far out the pistons come from the block. If you know that, you'll know compression for sure. But that means it's likely around 11.5:1 give or take a little depending on where the piston sets in the head.

N/A86 seems confused about the definition of deck height.
1. how far out the pistons come from the block
2. dimension from main bearing centerline to deck of block

#1 is used in most compression ratio calculators

Looks like 11.5:1-11.7:1 depending on where the head ends up.
You may be able to run on 93 with reverse cooling and a conservative timing curve but that is up to your tuner to tell you when it's on the dyno. If you want to run an aggressive timing curve I would recommend 95 96 octane at a min.

I understand what it is.. But if i know how far out i am and what my stroke is then does it really matter?

Thanks for the info guys

9.240" +or- o.e. tolerance
looking at your build your deck height is@9.235" your pistons are out of the hole .005"-007"
with the head gaskets you have chosen you still have enough piston to cylinder head clearance. Min. .040"
the deck height and where the piston is relative to it is a major factor into figuring compression accurately.
If your deck height was extra tall and the piston was in the hole the compression would be lower than one that was above deck.

We can make a custom cam that will work fine with the higher compression. Give me a call. Kip

Looking at your cam in Kips cam motion dynamic compression ration calculator I would probably give him a call and get a cam that gets you down a bit on the dynamic side if i didn't fat finger the specs you put out your at ~10.15 dynamic.

Agreed. Call Kip

Definitely will do. I originally had the cam speced by pat for 11.4-1....