Who runs low compression engines?

[satellite65]

8.5:1 or somewhere in that ballpark.

[NYrr496]

I've never run a Mopar that low, but I built a Chevy small block once that accidentally ended up with 7.8:1 and I almost just put a blower on it. Ended up swapping the heads.  Why, what are you thinking about?

[satellite65]

Doing a 440 buildup on the cheap. The engine is in very good condition, re-built about 12-15K miles ago. Wound up with the wrong pistons (long story) and as a result my CR is 8.5:1. I know the real fix for this is to swap the pistons and I'm still kicking it around but thats going to require a re-balance, all commonly available pistons are about 40-50 grams lighter than what I have (TRW L2388). The new pistons, pressing old ones off and new ones on + the re-balance will probably set me back about $600-$800. Since this is a street only car I'm considering using what I already have. I'm not too concerned about sacrificing a few hp's or ft. lbs. as long as I can make it run nice. As an added bonus, I figure I'll be able to run the cheapest crap gas out there.

Here's what Im considering:
440 + .040"
906 heads - home ported. Presently 87cc's but I can mill a little if I have to.
TRW L2388 true flat tops (no valve reliefs)
Forged crank (already balanced w/above pistons)
LY rods w/ ARP bolts
Melling HV ol pump and Milodon 7qt pan
Old Torker 440 manifold or Eddy RPM
3310 750 Vac Sec
1 3/4 pri - 3" collector el cheapo headers
Dane 60 w/3.54:1
Whatever torque converter is necessary.

[Jim_Lusk]

It still may not run well on low octane. My GTS had a stock 73 8.5 shortblock in it when we bought it in 1983. It still didn't run well on low test gas, BUT with the 10.5 shortblock in it now (well , the last time we had it on the street) it runs well on the 91 octane (the best we can get now). I don't know how the formulation changes affected this, but it is obvious that they have.

If you want more compression with the current shortblock, get a set of closed chamber heads.......

[satellite65]

Oh, I forgot- Comp solid cam XS 268 S

[NYrr496]

I guess if you polish the combustion chambers smooth to eliminate hot spots and mill them down a little to bring down the cc's and polish the piston tops, it would run ok.  The thing is, with open chambers, you get a lot of area for detonation to start.

[satellite65]

OH! This is KILLIN' ME. I'm tired of agonizing over this thing. I just want to drive the damn thing already.

Thanks guys,

Steve

[jamesdart]

what is your goal in the end? whe4n i took my pile of parts to the machinst to machine every thing fo rmy 383 he asked what i was doing. i told him stock rebuild 383, home port the heads. he called me the next day and convinced me to buy  a 440 crank he already had turned down for just a few buck more than having my crank cut. ok, the pistons were a lot more cash but i stressed over and over that i did not want any part of running race gas just to run 12s. it is a street car. i forget the exact number but it came in at 9.2 or 9.02:1. shich wa slower than i expected, i had my doubts of making the power i wanted but in the ends he was dead on with how i wanted it to perform, actually better. and i didnt even port the heads. you can make decent power without high compresion. if you got it already and its not easy to flip, just make it work. mill the shit out of the heads if you want to bump things up. it is said that closed chamber heads do not work well if your pistons are down in the hole. 

[satellite65]

Yeah, the pistons are about .120" in the hole and I just spent a considerable amount of time porting the 906's. Right now the CR is 8.5:1 but if I take .020" off the heads I can probably pick up about a half a point. How much is it really worth?

[a10secCuda]

not trying to hijack a thread but what is the lowest compression you can do on a big inch motor and still have it perform??.. i am thinking 528 with 9.5-1 and making it a street beast..

[satellite65]

9.5's not a problem............8.5's cuttin it kinda close

[cu440da]

I did my motor long ago when I didn't know any better and jut ordered the kit that said 10.5:1   Well, they were replacement pistons for the low comp motors.

I run a .509 cam in mine, (more specs in sig block) and recently went to the track and ran 12.88 @109. If I were able to get traction, the mph number indicates its capable of about a 12 flat in the quarter. Low 12's are more realistic, but it just goes to show that low comp motors aren't the boat anchors alot of people make them out to be.

[satellite65]

Well, Im goin for it. We'll see what happens.

[JohnnyfromChicago]

I bet it will run fine. Ever driven an 8.5:1 318. Runs like a sewing machine. I'd use a 650 carb or even a 575 and stay away from the single plane intake.

[Capt Jack]

yeah, this is sort of the plan for the 360 I have.  run low compression ala the original motor design (a '77 truck 360) and just use the car as a cruiser.

all I really want is

it should idle like glass
drink any gas I can find
have enough torgue to move the 3500+ lb car from stoplight to a stoplight with little effort.
sound nice with headers
be relatively gas stingy
look nice when the hood is open.

[ksdartguy]

Well, Im goin for it. We'll see what happens.

Go for it, I built a 383 for my nephew that has almost .100 down pistons and we ran 452s shaved a bit with steel shim gaskets.
Its under 8.8CR and ran 12.30s at 113mph in his 73 Dart.

Rick

[dusterdarryl]

I wouldnt be afraid of running a single plane, maybe not a tall race victor but I have a similar sounding 440 with 915 heads, 528 purle stick, pistons down the hole, no valve reliefs etc & it loves the ol torker II

[satellite65]

Thanks for the encouraging words. I was going to do it anyway, just wanted some other opinions. Thing is, this motor actually ran pretty good when I first built it 20 years ago. Then I listened to a Chevy friend and tried to make some "improvements" which basically castrated the poor engine. I haven't driven the car in 12 years and now it's time.

I'll keep Y'all posted.

Steve

[grudd440]

i estimate my 440 at around 8.5:1 or lower. i realized what i was working with and tried to keep the engine combo good.  comp xe 274, performer rpm intake, rpm heads, hooker headers. it sounds good. i think it will do good considering the low compression. it showed approx 430 horsepower and 460 lbs of torque on my desktop dyno.

[GON_RACIN]

The current motor in my Dart is an 8:1 motor.  Like yours, my pistons are around .120 in the hole at TDC.  Even with the low compression and some missmatched parts and the car still runs highs 7's in the 1/8 on motor, low 7's with a small shot of gas and probably some high 6's after I iron out all the bugs.  Personally I wouldnt go cutting on the heads for only a half a point of compression, its not worth the trouble.

[Thin White Duke]

Low compression will actually burn MORE fuel than a high compression motor. 

[268darts]

Sounds like a good turbo motor. The guy's at Mopar Performance said they would mill as much as .125 off the 906's with no problems. That ought to bump your compression a bit. 915's or 516's would be an easy swap. Or just put it together and run the snot outta it. If you don't know what adding a point of compression feels like, how would you know the difference? 

[grudd440]

gon racin, what gears and converter r u runnin?

[satellite65]

i estimate my 440 at around 8.5:1 or lower. i realized what i was working with and tried to keep the engine combo good.  comp xe 274, performer rpm intake, rpm heads, hooker headers. it sounds good. i think it will do good considering the low compression. it showed approx 430 horsepower and 460 lbs of torque on my desktop dyno.

Yeah, Desktop Dyno is a great tool but I kind of take the actual numbers with a grain of salt. I would love to be able to build an engine and run it on a real dyno to compare the two. My D.D. says 410 horsepower and 488 ft. lbs. of torque for my combo. I'll be REAL happy if I get those numbers.

The current motor in my Dart is an 8:1 motor.  Like yours, my pistons are around .120 in the hole at TDC.  Even with the low compression and some missmatched parts and the car still runs highs 7's in the 1/8 on motor, low 7's with a small shot of gas and probably some high 6's after I iron out all the bugs.  Personally I wouldnt go cutting on the heads for only a half a point of compression, its not worth the trouble.

I'm not  going to bother milling the heads. I think you're right, it's probably not worth the trouble for a half a point. I just finished prepping the heads last night and they're ready to be cleaned and re-assembled. Sometime this week I'll pull the shortblock out of the car and start freshening that up. I called Dynamic yesterday and inquired about a converter. They recommended a 9.5" "tight" converter for my car. Not cheap but probably worth the money spent.

[NYrr496]

Desktop Dyno is actually pretty good.  My brother and I used to take known combinations and dial them in.  Came pretty close.  If I remember right, there was a later version that required a lot more information. That one was the one that was accurate.

[satellite65]

Mine is the 2000 (Y2K) version, pretty old. I figure it can't be too far off but not totally accurate. It makes a lot of assumptions: fuel mixture is "perfect", no detonation (you can plug in 15:1 CR and the software doesn't care) etc. etc. Very valuable tool though and a lot of fun to play around with. I "blew up" about $200K worth of turbo motors this week alone .

[GON_RACIN]

gon racin, what gears and converter r u runnin?

4.30 gears w/ a 28 inch tall tire.  3500 stall.

[speedymopars]

Compression isn't everything. Sure, if you are competing regularly you want to optimize the cam and compression to get the most cylinder pressure (read:hp) without detonating.

If you aren't competitively racing every weekend, you aren't going to notice the (very) few HP you lose.

If you are really worried about it, put in a small duration cam to optimize the engine (of course that means it will be a low end engine only), or a shot of NOS/turbo/blower to take advantage of the lower compression.

[FastmOp]

I use to run stock shortblocks at the  track all the time, to me yours is a step up over stock cause of the pistons. there really tough pieces and if you wanted to spray some nitrous they would not have any trouble holding as much as you can get in a plate.
I miss'd the cam numbers but I use to take a stock 440, stick in a hugh's cam that was the fast rate 250/256 560 lift, thats with stock 906's just worked enough to clear and handle the springs. A M1 with an 850 DP, and super comp hookers and run LOW 7's (1/8 mile) with a 4000 stall.and 4.30 gears. Thats stock short blocks, blow one up get another from the shed and go back to the race track and run the same number till that one blew

[satellite65]

'I miss'd the cam numbers but '........

Cam specs for XS268S:

Duration (seat to seat) IN-268  EX-274
            (   at .050"   ) IN-230  EX-236

Lift                           IN-.488" EX-.501"

Lobe Centerline (LSA?)     110*
Intake Centerline             106*

[FastmOp]

You should have a fun car with about a 3000 stall and 3.91's that would make a great platform for a hot rod, if the shortblock and heads are apart then i'd deck the block and heads as long as you know the PtV clearance and you can have all the compresion you want. That would help the part throttle torque and make the motor more efficent. but I covered the lazy torque up with converter. if you upped the compresion you could get away with something like a 2500 stall.

[signet]

i have a 1976 440 bottom end with the smallest cam from Summit and a 650 cfm Holley with the Torker manifold.
452 bowl worked heads and steel shim gaskets,fenderwell headers hooked to a Midas hacked full length exhuast.
advance curve in stock distributer comes in real fast.

3.23 gears and a low stall factory converter in a lightened Valiant.

when it was on the road it was great,fueling up on Regular 87 15 miles to the Imp gallon.

it was fun to drive except for the rear end wanting to be first around the corner.

[dusterdarryl]

it was fun to drive except for the rear end wanting to be first around the corner.


 

  oh,    thats the truth

[grudd440]

im planning on running a 50 shot of nitrous. shouldn't hurt anything. i just hope the 9 inch tires will hold.

[satellite65]

Got the engine on the stand and partially diassembled. Measured the deck height @ .150" in the hole. CR works out to 8.13:1 (w/steel head gaskets). I think I'm going to suck it up and go for a set of slugs. Probably KB237's (w/5cc valve reliefs). Should work out to .010" down. With a .039" thick head gasket the CR works out to 9.8:1. That sounds more like it.