Lowering compression ratio?

Jbird · February 10, 2005, 09:35:23 PM

A good friend/customer bought a 70 Challenger, 440 six pack from the original owner. The car had been sitting in a garage for 23 years with the intake off. My friend brought the car to me to see if I could make it run. After bolting the intake back on, changing the fuel pump, starter, thermostat housing and a liberal treatment with Marvel I fired it up. It smoked out the exhaust for a bit then settled down, sounding real nice. I let it run until the thermostat cycled a couple of times then I shut it off. It dieseled horribly. I had to turn the ignition back on and pinch off the fuel line to get it to shut off.
I was a bit skeptical with the original owners claim that it was built with a 12.5-1 compression ratio. I'm no longer skeptical. I plugged a comp gauge into #2 cylinder. It jumped up to #120 on the first stroke then climbed to #180 at stroke four. I guess we're going to have to pull it down and change the pistons unless someone knows of a magic head gasket that will drop the CR down a couple of points. Any suggestions?
Jbird

Beck · February 10, 2005, 09:47:24 PM

No head gasket is going to drop near what you need. I would spend the bucks to put some of the GOOD gas in it and run it for a while. Do you have a race track in your area that is open this time of year? There are a few places in the St. Louis area to get racing fuel year round. What is the best gas can you buy at the pump these days? Make sure everything else is sound before taking it down to swap pistions. Different pistons may cause another problem. Unless you take it apart and weigh a piston and find low compression ones the same weight you will have to have it reballanced. A piston swap turns into a complete rebuild pretty easily. If you drive it a bit you may find out the tranny leaks from dried out seals also. While the motor is down you may find there are more things to repair after the long storage.
Instead of installing different pistons you may consider a pair of heads. Does anyone make aluminum heads with slightly bigger chambers? You can live with more compression with the aluminum heads.
Beck

Jbird · February 10, 2005, 10:05:09 PM

I'm probably a little premature with my assessment. I'm going to examine things a little more thoroughly tomorrow. I was just a bit amazed it sounded so healthy after sitting for almost a quarter century.
Jbird

Jokester · February 10, 2005, 10:10:10 PM

I think I'd tinker with it a bit before I tore it down. You could back up the timing a bit and slow down the idle speed to aid in shutdown. If the idle is as low as it will go, try drilling tiny holes in the primary carb throttle plates to let some more air in. That will allow you to reduce the idle speed some. You could also use an idle solenoid to hold the throttle open for idle, but then release when you turn the key off to let the throttle close all the way.

To eliminate the possibility of hot spots due to burnt spider eggs in the cylinders, you can run the engine up to about 2 grand and dribble some water down the carb throat. Not so much that you kill the engine, just enough to steam out the cylinders.

I agree that driving it to determine other bugs is a good idea. Anytime I've bought a car that sat over 2 years I had to replace every rubber part on the car. That included hoses, master cylinders, wheel cylinders, water pump, fuel pump, and on and on. Might as well get a complete list compiled and repair them all while the engine is down.

My 2 cents.

.bjb

GPster · February 10, 2005, 10:25:31 PM

Could this kind of thing be fooled with a change of cam? Maybe some aftermarket cam grinder can figure away around it. It seems to me that these blocks are just a High Deck version of the 383 (and maybe the later model 400 smog era engine) maybe a set of heads can be found that are more fogiving. He'd be able to pop the hood and brag but he'd probably have to hold back at flaunting it. GPster

enjenjo · February 11, 2005, 01:02:01 AM

I just went through this, I found a guy that would grind a cam that would fool the engine into thinking the compression was lower than it is. Delta Cams is the place that made it.

Dave · February 11, 2005, 06:24:53 PM

Quote from: "Jbird"A good friend/customer bought a 70 Challenger, 440 six pack from the original owner. The car had been sitting in a garage for 23 years with the intake off. My friend brought the car to me to see if I could make it run. After bolting the intake back on, changing the fuel pump, starter, thermostat housing and a liberal treatment with Marvel I fired it up. It smoked out the exhaust for a bit then settled down, sounding real nice. I let it run until the thermostat cycled a couple of times then I shut it off. It dieseled horribly. I had to turn the ignition back on and pinch off the fuel line to get it to shut off.
I was a bit skeptical with the original owners claim that it was built with a 12.5-1 compression ratio. I'm no longer skeptical. I plugged a comp gauge into #2 cylinder. It jumped up to #120 on the first stroke then climbed to #180 at stroke four. I guess we're going to have to pull it down and change the pistons unless someone knows of a magic head gasket that will drop the CR down a couple of points. Any suggestions?
Jbird

I had a 427 chevy with 12 1/2 to ones in er in jet boat back in the 70's we used double gaskets .. steel shim and composition. I lowered it enuff to run low lead regular. The was another post not long ago on the subject.
Use aVaition permetex and coat the gaskets fully on both sides bolt er together and touque em 2 times and it should work.
Dave

Jbird · February 11, 2005, 09:58:24 PM

I played with it again today. I drained the fuel tank again, thinking maybe there was enough old gas in it to lower the octane of the fresh gas I put in it. When I drained it the first time the old gas was really orange colored but it didn't have that rancid smell old gas usually has. I got some of ARCOs finest 91 octane, the highest octane pump gas I know of around here.

I adjusted the timing down to 9 degrees BTDC which created a nice off idle flat spot. I can shut it off with the key nowand it only detonates a couple of times. When I start it hot it pings hard once just as it starts. So far it's not leaking anything, knock on wood.

Jokester, I'm a little leary about running water through a high comp engine, don't need any cracked pistons.

Beck, I'm gonna mix a few gallons of Trick racing fuel with what's in the tank now before I give it the test drive.

enjenjo (The nekid mower), Do you have contact info for Delta Cams? And how did it run with the new cam?

My friend was hoping to run it this season and tear it down next fall.
Thanks guys, Jbird

enjenjo · February 12, 2005, 02:29:06 AM

They have a web site, I am not at home right now, so I don't have a book mark. The engine isn't finished yet, but in theory it should work good.

Phil1934 · February 12, 2005, 02:40:12 AM

Back when Chevy was selling hi comp LS 7's Crower offered a cam with a late intake closing to bleed the compression back into the intake.

Jbird · February 12, 2005, 07:25:31 PM

Well the owner came by this am to hear it run, he didn't need a stethoscope to hear the pistons rattling merrily in their bores. Kind of strange though, last night they were barely audible.

We've agreed on a solution. Tear it down and do it right. A load off my mind. That'll give me a chance to do the brakes while the engine is in the machine shop. It took five of us to push it off the trailer and into my shop. The left front caliper locked up when the brakes were applied while they were loading it on the trailer.
If I run into anything interesting during the tear down I'll snap a pic and post it for your amusement. Thanks for the advise.
Jbird