Thread: Changing compression ona built motor

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Originally Posted by HotRodN

Hey Tech,
It was built just for drag racing and I have no idea what the squish or deck height was set at. Will certainly find out when I tear it down. To make sure I understand what you call squish,,, I'm thinking the gap between the top of the piston and the chamber in the head. Is that correct??

Yes, exactly. It is the sum of the piston deck height and the gasket thickness. For instance, a piston deck height of 0.022" added to a compressed gasket thickness of 0.020" would yield a squish/quench of 0.042".

Originally Posted by HotRodN

One other thing I noticed when I pulled the plugs was they had oil on them which may have contributed to the high compression. No doubt this thing is a beast with the 750 carb compared to the 670 I first tested it with.

You could be right about the oil sealing the rings better. That's the difference between a dry test and a wet test.
As far as where did the oil come from, 2 possibilities come to mind right away, valve seals or the intake gaskets not sealing. When a fellow builds a motor, he should have the machine shop cut the block decks at 90 degrees square from each other. If they are not square, then the heads will not sit squarely on the block and the intake manifold will not sit squarely on the heads. If the intake gaskets do not seal, that leaves a pathway for oily vapors from the crankcase to be pulled into the cylinders when the intake valve is open. You know how you find a vacuum leak with a flammable substance, for instance spraying ether around the motor, well that won't work if the gap is on the bottom side of the gasket. I hope I'm explaining this so that you can understand it. The top side of the gasket could be tight and you wouldn't find a breach with a flammable substance, but the bottom side of the gasket being open would allow the motor to draw in oily vapors from the crankcase. This breach could be caused by someone in the past whittling on the block or heads or even the intake manifold and not making sure that everything was cut so that the whole mess was sealed up. Your machinist will have formulas for cutting different parts to make them parallel and make them fit.

Originally Posted by HotRodN

I've never seen that much compression on any motor,,,

Neither have I, not on any of the street motors I ever put together. Of course, I always built for pump gas.

Originally Posted by HotRodN

I'm definitely up to changing out the pistons and feel it's the best thing to do at the moment. I have no desire to have a 500+ HP motor in my street rod.
I've read one of your other posts on squish and setting up deck height etc. Great info. When I tear it down those measurements will be taken and posted for input.

Well see, here's the problem. I don't put a lot of confidence in Dart Iron Eagle heads and feel that even if you got the SCR down to a sustainable number, the motor wouldn't make the power that it could with a good set of heads. These heads were cast in 165cc intake runners, 180's, 200's, 215's and 230's, none of which impress me much. Most of them will make less or about the same flow as a set of cast iron production L31 Vortec Chevy heads, which isn't really much to write home about. You will not make 400 hp with a set of L31 heads on a 383. If you have to keep the Iron Eagle heads, then you have to keep them, but don't expect all that much from them.

Originally Posted by HotRodN

Will also get one of the books you mention by David.

Buy used to save a few shekels....
Here's a very elementary offering by David....
http://www.amazon.com/gp/offer-listi...s=david+vizard
And a more advanced offering....
Amazon.com: Buying Choices: David Vizard's How to Build Max Performance Chevy Small Blocks on a Budget (Performance How-To)
He has published scores of books that you may want to read eventually.

Originally Posted by HotRodN

If you could,,, give me a component list for what combination would get me the 400HP I'm wanting to end up with on this engine. Of course I'd like to keep as much of these original parts as I can (budget on less work hours LOL). The rod's, cam, heads etc.

Well, see, there we are again with those heads that I don't like very much. We need to settle that problem before I suggest a complete build for you, because I'm pretty sure you would be disappointed with the build if you have to use the Iron Eagle heads. I would like to see you do the build with a set of Pro-Filer 195's, but don't know if you can pop for the $1100 price tag. These heads will support 500 hp and 500 ft/lbs of torque on a 383 and it would be a walk in the park to put a 400 hp example together, although I'm pretty sure you'd like 500 hp better.
SBC 23 Degree Cylinder Heads

Originally Posted by HotRodN

Had already planned on changing the intake to a Endurashine Air Gap and running the 670 carb (or 750 with vacuum secondary's if I had to) for performance.

Don't get suckered in by the marketing hype of the Air Gap intake. Some fellows who live in colder climes tell me that they have driveability problems with the manifold due to cold air working its way around the runners and that they wish they had used the standard Performer RPM. You can use any carburetor you want to, but the 750 will make the most power. Use vacuum secondaries if the converter stalls at less than 3000 rpm's or a double pumper if the converter stalls at 3000 or higher.

Originally Posted by HotRodN

My concern with that setup is the motor would choke out if I hit the carbs wide open from dumping too much fuel.

Never happen.

Originally Posted by HotRodN

I've also had problems getting it to idle or run without the choke partly closed. It's starving for fuel and I'm almost convinced the idle mixture holes need to be opened up a tad more. Three deuces look cool but are a pain to get tuned just right IMHO.

You just need to find a good reference book and study the carbs. Learn exactly how they operate and what you have to do to make them work properly.

Originally Posted by HotRodN

Hey this is all fun stuff for me and I'm having a ball doing it,,,, can't keep a girl friend though ha ha ha. They all say it's not just a hobby but an obsession,,, they may be right

You're lookin' for love in all the wrong places. Go to the dragstrip and hang out.

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