V8e36

I have a lm7 iron block 5.3, its going into a bmw 3 series (approx 3200lb car) have some questions about the engine setup. so far i rebuilt the lower end with new bearings seals etc, gapped the rings to accept a small shot of nitrous down the road. installed a bigger cam, 226/230 112 +4, .545/.545 Lift. the 706 heads came back as being cracked when i had them pressure tested. im sourcing a set of 853 heads locally and will be putting my heavier valve springs in and trunion upgrade assuming they check out ok. now these heads have a larger combustion chamber so the compression will be lower. according to an online formula this setup would yield a 8.7:1 cr vs stock which is 9.5:1. The car will be driven mostly N/A and only occasionally on spray. here lies my questions;

How much power would i lose keeping the lower compression ?

How much will it be felt?

If it would be better to bring the compression back up near stock whats the most cost effective way to do so?

Mill heads down and deal with possible PTV issues shorter pushrods and intake fitment?
or get different pistons?
Other option that i havent thought of?

I have found a lot of good info on here so far, i wanted to provide as much info as possible so im not wasting time with more info replies. thank you in advance

00pooterSS

iTrader: (40)

With that size cam and that static compression it's going to be a dog.

That's a good size cam for a 5.3, I would want at least 11:1.... hell I'd want it to be near 11:1 even if it was a stock cam.

You should be aiming for well over 10, so being at 8.7 you'll be down 40 or so. But that isn't the worst part, the worst part is how it will feel in comparison. It'll feel real lazy in comparison to running up near 11 where you should be aiming.

I'd be looking for a good set of 706's, and milling them, and thinner head gaskets, cometic .040 or ws6 store .045 gaskets

00pooterSS

iTrader: (40)

If you do flat top 4.8/5.3 pistons

.045 head gaskets

706 heads milled down 18-20 thousandths should give you right around 59cc

That would be 11.04:1

--------

If you leave your dish pistons in and do the same as above youd be at 9.96:1
Do all the same as above but dish pistons, milled 706s, and a .040 gasket and you're at 10.07

V8e36

this is kinda what i was expecting, one of the heads is good that i have, ill look out for another good 706. ill also check out those thinner gaskets.

milling 18 to 20 thousandths off with the thinner gaskets do you think id have PTV clearance issues? or intake fitment issues? also same question but with the flat tops too.

thanks for the reply

HioSSilver

iTrader: (8)

Compression is power. Does that answer your question? Run all of it you can.

00pooterSS

iTrader: (40)

Intake should fit fine, that I'm aware of problems come once you go over 30 thousandths. But each situation can vary, you'll very likely be just fine though.

Piston to valve.. with that cam you should have quite a bit of extra valve clearance already. So bringing the head in .030 total SHOULD be okay. You'll have to check though to confirm.

Flat tops won't change anything except possibly the valve clearance. I can't say for sure because I'm not positive if you had valve contact if it would contact down in the dish of the piston or not. If it did then you'll have a little less clearance with the flat tops.. But lets approach this from a different angle. The block you're running and crank have the same specs as a LS1 and you could run that cam in a LS1 no problem that has flat tops in it.

Again you'll have to measure your current setup to see, but I'm willing to bet you have a mile of available valve clearance right now.

Take your good head and a old head gasket and put it on the motor and lightly tighten some head bolts in it. Before putting the head on oil the piston and the valves to keep them from sticking to the clay and clay the piston. Then turn the motor over a few revolutions and see what your clearance currently is. Then you can see how much you can take away before running into problems

00pooterSS

iTrader: (40)

in case you asked for more info I went ahead and looked for a video

Do this. see what you got. Go from there

V8e36

That's good to hear , sounds like a nice weekend job ill see what my clearances are now and go from there as far as how much to mill and to go with flat tops or not . Again thanks for the replies and all the great info

chrisfrost

iTrader: (8)

Hello You should have lots of valve to piston clearance but should clay check just to be sure . Also don't forget about quench which is the distance between piston at tdc and the flat portion of the head with gasket and milling,,,,,,,,,,,etc. The best quench range is .030" to .040" and quench alone cools the fuel/air charge by forcing it into the combustion chamber from between the top of piston and flat portion of head next to comb chamber w3ith some velocity mostly into comb chamb. So with a good quench the ignition can advance a little bit more without pre-ignition .Anyway sounds like a fun project good luck

chrisfrost

iTrader: (8)

Hello again seems like not a whole lot of people know about Wallace Racings website. They have every engine calculation and car , weights, gear changes, 1/8 mile , 1/4 mile cam , compression change hp gains, ……………..etc literally EVERY calculator You can possibly think of and many many more. Check it out sometime

low2001gmc

iTrader: (148)

706 are prone to crack sooner or later so getting a replacement would be a bandaid.

I would get a pair of 862s, same heads different casting to at least maintain compression. Milling to increase compression would greatly help as some have said here.

G Atsma

NOT all 706's, only the Castech ones. Not all 706 heads are Castech. There have been no issues with non-Castech 706 heads

HioSSilver

iTrader: (8)

How do you tell the difference?

Che70velle

iTrader: (6)

This? I’ve heard people say that Castech has the battery symbol in the head. Every GM LS head I’ve ever had in my shop has the battery symbol, from 706’s to LS7’s.

G Atsma

The Castech ones have a battery-shaped raised logo on top of the intake port, under the rocker arm support rail, and in the spring deck cavity portion of the head.

99 Black Bird T/A

iTrader: (4)

FWIW - Ran a set of 706 5.3's TEA ported for 15 years and 130,000+ miles. I don't think the cracking issue applies to all 706's. Pulled a set of 706 5.3's two weeks ago off a 220,000 mile 5.3 that didn't have any issues. 706 heads are are dirt cheap and its fairly easily to find a decent pair.

A set of stock 243/799's are also pretty inexpensive too.

Regarding the compression, I'd go for as much as possible and still have good P to V clearance. You can probably do 11.5 to 1 on 91 octane gas.

HioSSilver

iTrader: (8)

I had to replace a set of 706 on my uncles 5.3. Just kept using coolant. Put 243s on it to avoid the cracking issue. Milled the 243s to not lose compression to. Worked out well.

V8e36

its been a little while since i asked this question and have read some great info , since last posting i have sourced a used 706 and having it checked by my machine shop tomorrow. i checked my piston to valve clearance and have more than enough room to mill the heads, use flat top pistons and thinner head gasket. as long as the head is good they will be milled down and i will be sourcing some new pistons and headgaskets. again thanks for all the great info

00pooterSS

iTrader: (40)

**** yeah