Thread: Changing compression ona built motor

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Hey all,,, I recently acquired a 383 stroker to go in my Hot rod and it is supposed to have 11.5:1 ratio compression,,,, from what I understand it's not a good idea to run pump gas with the compression that high on steel heads,,, it is a running motor and I don't want to tear it down unless I have to but I'm okay with doing that,,,
I'd like to lower the compression to 10:1 and run normal pump gas without octane booster.
So any input on how to build a 383 with the existing parts I have now and what would need to be changed is greatly appreciated,

Here's what I can tell you about the motor,,,, if you need more let me know and I'll do my best to get it.

Good ole 10/20 block,

Scat crank with I believe 3.75 stroke,
5.7 Scat rods,
Dart Iron Eagle 72 CC heads,
Roller tip 1.6 rocker arms,,, steel stamped
12-292 cam,,, 501 lift I think and 110 lobe separation,,,, need to look up more on this one.

Domed piston but I could not get the dome CC,

Holley 750 double pump vacuum secondary carb,
Not sure of the intake but it's definitely not a street intake,,, single plain high rise,,, probably Edelbrock,

Oaky that's all I can think of at the moment.

Originally Posted by HotRodN

Hey all,,, I recently acquired a 383 stroker to go in my Hot rod and it is supposed to have 11.5:1 ratio compression,,,, from what I understand it's not a good idea to run pump gas with the compression that high on steel heads,,, it is a running motor and I don't want to tear it down unless I have to but I'm okay with doing that,,,
I'd like to lower the compression to 10:1 and run normal pump gas without octane booster.
So any input on how to build a 383 with the existing parts I have now and what would need to be changed is greatly appreciated,

Here's what I can tell you about the motor,,,, if you need more let me know and I'll do my best to get it.

Good ole 10/20 block,

Scat crank with I believe 3.75 stroke,
5.7 Scat rods,
Dart Iron Eagle 72 CC heads,
Roller tip 1.6 rocker arms,,, steel stamped
12-292 cam,,, 501 lift I think and 110 lobe separation,,,, need to look up more on this one.

Domed piston but I could not get the dome CC,

Holley 750 double pump vacuum secondary carb,
Not sure of the intake but it's definitely not a street intake,,, single plain high rise,,, probably Edelbrock,

Oaky that's all I can think of at the moment.

Since it's running, warm it to operating temperature and do a compression test on all cylinders.
Remove the air filter element.
Remove all spark plugs to help the starter turn the crank.
Disable the ignition system to prevent a fire.
Wire the primary throttle blades wide open so the motor can breathe.
Let the motor have at least 5 "chuffs" on the gauge for each cylinder. Max the needle and write it down.
https://www.youtube.com/watch?v=X_tbksFYhl4

Crane Cams engineers state that an internal combustion engine can safely be operated on pump gas up to about 165 psi cylinder pressure, although I have witnessed many motors that have more cylinder pressure than that and operate OK on pump gas. I suspect that it has to do with squish/quench.

I know you didn't ask for a lesson, but here's one anyway.
As the piston ascends in the bore and approaches top dead center, the area of the piston opposite the chamber gets friendly with the flat area of the cylinder head opposite the chamber and squishes out the air/fuel mixture that is between the piston and the head. This sudden close proximity jets the mixture across the chamber toward the spark plug and the turbulence created homogenizes the mixture so that all of it will burn, instead of having rich and lean pockets that may or may not burn.

The quench part of squish/quench occurs with the piston in very close proximity to the underside of the cylinder head, where heat is released from the piston crown into the relatively cooler cylinder head, cooling the piston. The tighter the squish/quench, the more power the motor will make, up to the point where you have a collision between the piston and the head. On a small block, David Vizard has gone as tight as, I believe 0.027" before he encountered a kiss. A kiss might be defined as seeing the piston part number imprinted on the underside of the head at the next tear-down.

.

Tech,,, measured the compression across all cylinders and it ranges from 212 to 218.
now what lol

Originally Posted by HotRodN

Tech,,, measured the compression across all cylinders and it ranges from 212 to 218.
now what lol

Holy crap!
Can you hear the motor detonating? What fuel did the original owner use in the motor? What fuel have you used in the motor?
It is possible that it is not detonating if the builder engineered in a very tight squish when he built it. (0.035").
If the motor is detonating on premium pump gas, the cheapest way to lower the static compression ratio is by changing pistons. Are you up for that?
A 12cc dished piston would put the static compression ratio at about 9.5:1 SCR, perfect for iron heads on pump gas, as long as you use a tight squish.

.

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??

I've run high octane with 108 boost mixed in (a lot of it, like two cups to 2 gallons) and only ran it a few minutes total in 1 minute intervals. I couldn't hear any detonation going on best I could tell. 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.

When I did the compression test the carb was wide open front and back as it's mechanical secondary's not vacuum. Let it turn until it stabilized on pressure which was 5 or 6 chuffs. Of course I was shocked at the amount of compression on the first cylinder when I saw it. I thought,,, man no way,,, I've never seen that much compression on any motor,,, but' I've only assembled low end rebuilds.

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.

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

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.

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.

Of course if it all gets to be way too much then I can always keep the Vortec head motor with 3 Rochester carbs in the car. But that one wouldn't give me the fun factor I don't think. My concern with that setup is the motor would choke out if I hit the carbs wide open from dumping too much fuel. 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.

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

Later guy's,
HotRodN

With 72cc heads, about the only way to get to 11.5-1 compression is with domed pistons. It you want to get by really cheap, pull the motor apart, find someone with a milling machine, and lop the domes off the pistons. Just make sure you measure the thickness of the dome area to make sure you're gonna have some left when you're done.

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.

.

TDK, being a machinist by trade and having access to a manual mill I've thought about fly cutting the piston tops to lower compression. These are BTW domed pistons,,,, just don't have the info on what the CC of the domes are. I believe we have an electronic thickness age at work and would verify the thickness before cutting them. Having read Tech's post it too is a option I'm considering. Having the input on components makes building the engine type I'm after a breeze. This may seem wishy washy but I'm still pondering just what engine I want in the car. Mild to wild,,,, at lest I have those options to choose from LOL.

Hey Tech,
Thanks for getting back on here and posting.
I'm familiar with the idea of a gap being in the valve train valley,,,, just wish there was a way to get either in there to test for leaks. One time I did squirt some in the valve cover to see if it made a difference but nothing happened At least nothing blew up in my face LOL.

Now as for changing out the heads I'm not opposed to doing that either. I could sell the Dart heads for something and recoup a little of the cost. I'll be looking tomorrow for a number on them to see can I identify the intake runner.

I once had a Patriot head and a Pro-Comp (I think it was) flow tested and the dude was surprised at how well the Patriot head flowed and it did better than the other all across the board.

As for the air gap,,, being in the southern part of Texas (near Houston) we don't see a lot of real cold down here but your advise will be taken. I do have a Performer RPM as well from another motor I could pull and use.

As for the love thing,,, it may be looking up LOL,,, met a lady who really likes NASCAR and probably knows more than I do. But it's all in the first date stages so I'll have to see how that goes. At least I was invited to attend the race with her and friends next weekend so I must have passed some of the "He's an okay guy test" over the weekend ))

So I'll keep pondering all this and come up with a decision soon so I can move forward with the build. There is still plenty of other thing's to do but prefer to get one thing completed before moving on to another section.

Tech,
One thing I haven't mentioned is what car the motor will go in,,, I'm building a 32 three window coupe that' a fiberglass body and estimate the total up weight to be 2400 to 2600 depending. Just FYI,,,

I never heard of a double pump vacume secondary Holley carb

For an engine to be reliably driven on the street-I would sell this one and build another with a block set up for hydraulic roller lifters and alum heads-

Yea been thinking along those lines. But was just going to sell off what I wouldn't use and get what was needed. Again what I'd like to have is 400 crank HP. Sooo,,, if it can be done that way pass on how you'd change this one up,,,, Later Cuz,
HotRodN

The reason for keeping what could be used is because getting a block,,, having the machine work done to convert to 383 and all the other parts. I would have way more in it than what I paid for this engine. Just think it's wise to use what you can from this build and scrap the rest.

Having a block that is set up for OEM type hyd roller lifters is a valuble part of the scheme to just sell what you have and build whats best-by the time you get different pistons/rings/pins, balalance them to crank, etc-buy heads and valve train, you can sell what you have as an assembly and build a much more street suitable set up for todays gas

HotRonN, if your 383 is an older (pre-'87) block you can convert it to hydraulic roller lifters instead of selling it and starting over. Here's one of several articles that walk through the process - Converting a Hydraulic-Roller Camshaft - Tech Article - Chevy High Performance Magazine Being able to do your own machine work is a huge plus, so IMO the approach is to go through the engine you have and map out a detailed plan for everything you'll need to do to pull down your compression, improve your volumetric air flow, and ensure reliability with a roller cam. Once you have the plan, big parts down to nuts, bolts, gaskets and all of the machining you can put numbers against your shopping list and make an informed decision. Tech's one of the most down to earth, no BS sources for questions that come up about pieces & parts that you'll need, IMO. If the numbers don't pan out after you have the plan, then sell the whole shebang and start out with a later model, OEM roller cam block like Jerry is suggesting, or shop Chevy Performance and buy one from GM that's ready to roll, and comes with warranty.