Who runs low compression engines?

[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.

[sportfury70]

I have a 72 .040 over 440 w/ TRW 2388 pistons, a Mopar .557 cam, non milled  452 heads w/2.14/1.81 valves, BG 750 double pump carb on a Holley street domunator intake and it runs really strong.  I'm sure it does run to it's potential but strong non the less.  I had it in a 70 dart w/ 3.23 then went to 4.10 and it just used more gas.  Now it is in a SWB truck. 

[satellite65]

Sportfury70, those are the same pistons I presently have. How far down were they @ TDC? What was the compression ratio?

[sportfury70]

I did not calculate the compression ratio.  I ordered 10-1 pistons in 2000.   5 years later when I built the engine, I noticed there were no valve reliefs and the pistons were down like .150  I did some research and found they were basically stock replacement forged pistons. 

[satellite65]

Yeah. They're stock replacement forged pistons for low compression smog motors from the mid '70s. Mine are .150" in the hole too. Are 452 heads open or closed chamber?

[Steve DeTar]

A blower/ roots type with 6lbs of boost will wake it up.  Steve

[satellite65]

A blower/ roots type with 6lbs of boost will wake it up.  Steve

I'll bet it would. Trying to keep it simple. Besides, I cringe at the thought of cutting a hole in the hood of my car, this particular one anyway. Next one? Maybe.

Thanks

[dusterdarryl]

Yeah. They're stock replacement forged pistons for low compression smog motors from the mid '70s. Mine are .150" in the hole too. Are 452 heads open or closed chamber?

open chamber mate,   not a bad head as far as iron goes

[sportfury70]

I just looked at the cam specs on comp cams.com.  That is more mild than the Mopar cam I have.  I say go for it the way it is.  I don't think you will be disappointed.  I know I wasn't when I put the .557 cam in mine.  If you need a recommendation for converter, I run a Turbo Action 3800 and I love it.  Shoot me a pm if you have any questions on my engine build and it's performance.

Lewis

[satellite65]

Thanks, you have a PM

[Chris_Holliday]

I run a very similar type setup with no problem.  My stock bottom end has the flattops .14 in the hole and the compression is like 7.8.  I run a .509 and yes the idle quality is poor.  The stall converter makes up for it and when it hits 2800 rpm it starts to scream. 

[satellite65]

Well, it's a street car and idle quality is sort of important to me. A slight chop is no problem but if the things going to be thrashing around on the motor mounts, pulling 6" of vacuum and stinkin' up the neighbor hood while getting 3 miles to the gallon thats not going to cut it. On a race car I wouldn't care how it idled as long as it didn't foul the plugs waiting to stage. I would like to get the CR up to about 9.5:1 and unfortunately, new pistons seem to be the only (best?) way. Thats going to require a re-balance. I don't think I could cut my 906's enough to get it. Thanks for the reply anyway. If anyone else has more ideas I'm open to suggestions.

Steve

[ksdartguy]

I had a roller that was close to the 268 comp, you talked about.
It idled 11in. vac. in an 8.2cr. 383. It was my procharger motor,
but it ran 12.70s @ 109 without the boost and at 3600lb. with me in it.
The 9.8 pistons will make a diff for you.

Rick

[mxlplx71]

anyone want to guess what amount of hp you get per point of compression?
mines around 8:1 and ive run these before, still got my 70 rr sideways

[satellite65]

anyone want to guess what amount of hp you get per point of compression?
mines around 8:1 and ive run these before, still got my 70 rr sideways

About 30 HP / 20 ft lbs if I'm guessing correctly. It's not just about horsepower and torque numbers. Like I said before, it's a street motor. I'm looking for a fair idle, good street manners and good overall efficiency. I want an engine that pulls strong up to 5800 RPM and I would like to avoid a very high stall converter if possible. I know that big cams and low compression are generally a bad mix so I'm trying to avoid this (again).

I'm seeing that several of us that built these motors years ago seem to be in the same boat. When I was going over what I wanted with my machinist 20 years ago I told him that I was looking for about 10:1 with steel shim gaskets. If pinging was a problem I could knock off a half point with FelPro blue gaskets. He thought it was a good plan and ordered the pistons that are in the motor now (2388's). Is it possible that there was a misprint in the TRW catalog back then? That would explain it.

Some of you seem to have gotten some pretty respectable ET's with these low comp engines. It's really just a question of streetability. I'm still undecided as to which way to go and I'm not ordering/doing anything until I'm sure.

Thanks for the help/input/replys

Steve

[Chris_Holliday]

I will say my car is very streetable and does not load up at idle in town but the .509 cam gives very low idle vacuum.  I would go with less duration and the same lift with a Hughes or Comp extreme cam next time I build to improve the idle.  If I remember correctly I think Mopar Muscle or HotRod did a feature article where they kept adding head gaskets on a 440 to decrease compression and show the Hp and tq loss along with the idle quality.  30 HP per compression point seems right.  I figure my low compression motor is down 90HP from the ET I see other people run with more compression. 

[Dave_n_68_383S]

My guess.. that far down and that compression..not good.. Quench is going to be for crap. cut the deck, mill the heads.. get into 9.x on zero deck if its going to be a natural motor.

[jyrki]

We run a couple of 440's in the 8.6:1 - 8.8:1 range (one with stock '74 short block and .060" milled ehads, and one with TRW L2266's and unmilled heads). They worked pretty well, but didn't respond to big cams. The best results were 16 mpg on the highway, 12.94 with 3.23 gears and regular 205 wide tires and 12.45 with 3.91 gears and M&H DOT tires. The car weighed approximately 3500. The reciepes in both were as follows, 3310 holley, M1 or Torker2 intake (run the same), electronic ignition with quick and short curve, homeported open chamber heads (both engines had different number casting heads on each side from the factory! And were left that way), MP 280/.474" cam, 2" primary headers and 2.5" dual exhaust with H-pipe. We tried different cams, valvetrain parts etc., but I think that of those, the 280/.474 with factory rockers worked best and was most cost effective. 

[signet]

 jyrki, do you recall the trap speed on those runs?

thanks

[forphorty]

.030 over 440 trw 2266, about 8.5, pocket ported 906s, stock valves and rockers 292 .501 comp cam, torker, 750dp, ger 10 inch convertor, stalled about 3800 i recall, 3.23 8.75 sure grip 28x9 et drags, 68 satellite with 6pak hood, about 3500#  best  12.35 at 106 1.69 60ft.