Snkebait95

iTrader: (16)

Hey guys, im in the middle of assembling my engine and just wanted some other peoples last thoughts on the combination. Also was wondering how one can find out what the best compression ratio is for their combo?... Going in a 4th gen with a t56. Car has adj rear suspension with 4.10s and a 10 bolt. An online calculator said my compression will be at 10.85:1 if I stay with the combination below. My goal is mid 11's n/a, and whatever I can get on a 200 shot.

The engine:

Iron 370 by TMS
Stock crank
eagle h beam rods
-4cc forged and coated mahle pistons
862 casting heads never milled- ported with ls1 intake valves, flows 300 cfm
Ls7 lifters
Comp cam- 238/240 .605/.609 112 lsa
Comp 26918 valve springs
42lb bosch injectors
Comp hardened pushrods
Vengeance ported FAST 102/102

Does anyone see anything out of place? I saw an old thread Vengeance made 500rwhp with a similar combo in a vette. Thanks for helping to anyone.

batboy

iTrader: (6)

I'm calculating about 11 CR with GM gaskets. You could use Cometic .040" gaskets to bump up CR a bit more.

Snkebait95

iTrader: (16)

Forgot to mention i already bought gm gaskets, also using arp head studs. Thanks for the tip batboy, i probably would have done that if i didnt already buy the gm ls3 gaskets.

mark21742

You might look at getting about .020" shaved off the heads to give you a little more bump. I ran a 238/242 in my 04 GTO with a 4,000 stall and loved it! A bit peaky in my 346, and a hair small in my 408, fun daily driver.

Your bigger cubes will really eat up the large duration and it should act like a mild can in a stock cube ls1....once I jumped up to the 408 I pushed my static compression up to 12.5:1, then dropped a 255/271 cam in which put me at a pump gas friendly 8.22:1 dynamic compression

Snkebait95

iTrader: (16)

Cool thanks for the share sir! I was thinking about milling the heads, and i wouldnt mind having to buy different length pushrods, but is the bump really worth it? How bad is 10.85:1 (at least that what i think it is) opposed to 11-11.5:1?

Also, i assume there is really know way to find what ratio is best, just pick what youre shooting for and build accordingly?

batboy

iTrader: (6)

I double checked and again I'm coming up with about 11:1 CR for you. I bet you are not considering the fact that most LS engines have the piston coming out of the hole a little (average .005" to .006"). How high of CR to use depends on many factors like gasoline octane, cam specs (dynamic compression ratio), etc. If all you can get is 91 octane like me, then 11 CR is probably a good compromise. If you can get better gas, then you can probably go up to 11.5 CR by milling the heads. With your build, a half of a CR point is worth maybe 5 hp.

mark21742

As a rule of thumb going a full point (10:1-11:1) will give you 3-4% bump in torque and HP across the entire motor's rpm range. I shoot for the highest ratio possible for the fuel I'm running + the possible bad tank of gas.....92/93 octane you can push as high as 8.5:1 dynamic compression, and I've heard of dome people running higher, but that is really pushing it, which is why I picked a safe 8.22:1. If you mill too much off the heads you can get to the point where your valves become shrouded and performance starts to drop off, that is where welding the chambers comes in if you want more. I've got a lot of work into my 241's and have them shaved down to 57cc chambers.

Then you can look at getting the perfect quench lol

mark21742

Used to be able to get 94 around here, but now its getting harder to find 93

Snkebait95

iTrader: (16)

Oh ok thanks guys. We get 93 here in south west Ohio. 11:1 doesnt sound bad then. Thanks for answering the questions! I would say 5-10 is not worth it in my case where milling the heads and new pushrods would be the cost.

What is the desired dynamic compression ratio of a lsx engine from a street/ strip performance stand point like my combo? If this has already been answered multiple times on this forum can i be directed somewhere that can tell me what a desired DCR is and how to calculate that? Thanks again everyone.

batboy

iTrader: (6)

With that cam and 11 CR, your dynamic CR will probably be about 7.8, so you could bump up the static CR a little. The rule of thumb for pump gas is about 8.0 for safety (about 11.25 static) and not to exceed 8.5 dynamic CR. You'll always find someone claiming to have used more than that with no problem, but I'd rather settle into the lower range and have some flexibility with bumping up timing.

Snkebait95

iTrader: (16)

Thanks for the info sir

mark21742

Your dcr is calculated from where your intake valve closes at .005", not at the advertised .050" lift. I like wallaceracingcalculators.com

batboy

iTrader: (6)

I used the Wallace Racing calculator and I used the Comp Cams specs to determine 74 degrees ABDC (valve timing @ 0.006 lift as per the cam card).

Snkebait95

iTrader: (16)

Thanks for the help, I think im sitting pretty decent, could be better, but not bad i assume

I was messing around with those calculators and heres some food for thought....

Using advertised duration at .006- 287 int. 289 exh. 112 lsa 110 degree intake centerline

IVO is 33.5 ° BTDC ( - indicates ATDC)
IVC is 73.5 ° ABDC
EVO is 78.5 ° ATDC ( - indicates BTDC)
EVC is 30.5 ° BBDC
Overlap is 64 °

- What can be determined by this info?

chrisfrost

iTrader: (8)

Go to "Wallace Racing" and look on Their site for "engine Calculators" and They have maybe 150 different calc formulas from static comp dynamic comp to camshaft calc to 1/4 and 1/8th mile times calc gear change calcs ,,,,,,,,You name it n They have it all for free also horse power gain from compression change calculator

Snkebait95

iTrader: (16)

Yeah thats what I used. Cool stuff to mess with