Thread: 383 build questions

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I'm picking up a block and rotating assembly this Friday. It's a 383 kit from Eagle. It will have a 10.5:1 compression ratio with a 64cc combustion chamber. I'm on a budget so I just want to put a set of stock cast iron heads on it. I was told by a buddy of mine that I won't be able to run pump gas with the iron heads and a mild cam. He said I would have to put aluminum heads on with a pretty wild cam or it will detonate. Is this true? Thanks.

You can run 10.5:1 on pump gas without issues if you plan your entire engine build.

Detonation occurs when excessive heat and/or pressure builds up in a combustion chamber causing the air/fuel mixture to “auto-ignite.” This produces multiple flame fronts within the combustion chamber instead of one hot burst. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. This is "head" independent - however; some combustion chambers (such as those in the Vortec heads) are better suited to complete combustion. Mild or occasional detonation can occur in almost any engine and usually causes no harm however; prolonged or heavy detonation can cause a lot of damage.

The octane rating of a given grade of gasoline is a measure of its detonation resistance. The higher the octane number, the better able the fuel is to resist detonation. Most engines with static compression in the 9-9.5:1 will run fine on regular grade 87 octane fuel. Engines with high compression ratios, such as your projected 10.5:1, require at least 89 (or higher) octane fuel. If switching to a higher octane fuel fails to eliminate a persistent detonation problem, it probably means something else is amiss. Anything that increases normal combustion temperatures or pressures, leans out the air/fuel mixture, or causes the engine to run hotter than normal can cause detonation.

A static compression ratio of 9:1 is typically the recommended limit for most naturally aspirated street engines. Compression ratios over 10.5:1 may create a detonation problem even with 93 octane premium gasoline. That said, BMW, Volkswagen and Toyota all sell cars with compression ratios over 10.5:1 and they run fine on most pump gas (BMW V8 in their “M’ Series is 12.5:1). I say most because there are some states that mandate high percent Ethanol during certain month (usually winter) and such reduces the octane and an octane booster is recommended – even with premium fuel.
Use the coolest plugs you can. The wrong heat range plug can cause detonation as well as pre-ignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application. Use the next range colder spark plug. This can happen real quickly – so stay on top of the plugs.

Make sure to have a big enough radiator. Lots of cooling is a good thing with higher compression ratios. Get a good water pump and lots of fan.

Richer fuel mixtures are less likely to experience detonation. Lean can be very unforgiving. Vacuum leaks at, intake manifold gaskets, or carburetor gaskets can lean out the fuel mixture. If the fuel mixture becomes too lean, misfire typically occurs – especially under load.

Others will certainly chime in - this could be a great thread.

Regards,
Glenn

Glenn has outlined the basics very well. I'll chime in with squish. Squish, also called quench, is the measurement from the piston crown to the underside of the cylinder head with the piston at TDC. It is set with the piston deck height (measurement from the piston crown to the block deck surface where the head bolts on) and the thickness of the head gasket. Most professional engine builders will shoot for a squish of 0.035" to 0.045". David Vizard stated that the motor made more power and was more detonation-resistant the tighter he set the squish. I've noticed lately that he prefers 0.035". Any tighter than that could produce problems with the piston crashing into the head at higher engine temperatures and higher engine speeds because everything grows and stretches. There will be a little flex in the crank, a little extension of length in the rods from both stretching and thermal growth and the piston will grow taller from thermal expansion. So, 0.035" between the piston crown and the flat part of the underside of the head adjacent to the chamber might be the tight limit on a small block chevy to give the mixture maximum velocity across the chamber to help make the motor detonation resistant and also to prevent metal to metal contact.

I'll give you a for-instance on a build. You have a block that has never been decked and measures 9.025" block deck height (measurement from the centerline of the main bearing bore to the block deck where the head bolts on). Your stack of parts measures 1.875" (crank radius for a 3.750" crank), 5.700" rod length and a piston compression height (measurement from the centerline of the wrist pin to the crown) of 1.425" for a total stack height of 9.000". This means that when you assemble the motor, the piston will be down in the bore 0.025" (piston deck height) with the piston at TDC. To reach the target 0.035" to 0.045" squish, a very thin head gasket would have to be used, such as a 0.015" steel shim gasket. To use a shim gasket, the block decks and cylinder head mating surfaces must be flat and smooth, but it can and has been done. Chevrolet used to use steel shim gaskets in the factory motors they assembled. Another way to reach the target squish would be to cut the block decks 0.010", leaving a piston deck height of 0.015", then use a slightly thicker composite gasket such as a Victor Reinz #5746 gasket that compresses to 0.025". This would result in a squish of 0.040". If for some reason, a builder wanted to use an even thicker gasket in this build, he might cut the block decks 0.025" so that the piston was exactly even with the block decks at TDC. This would produce a piston deck height of 0.000" and is known as zero decking the motor. You could then use a gasket that compresses to 0.040" and have your 0.040" squish.

While you are learning this stuff, you might also want to google Singh grooves. These are channels that are cut into either the head or the piston crown and allow routes for the squished mixture to be jetted toward the spark plug.

All of what I'm writing about here is aimed at the elimination of detonation on pump gas with a moderate static compression ratio. There are many instances of iron head motors being run successfully with 11.0:1 static compression ratios if using the correct squish. Of course, a builder must also pay attention to dynamic compression ratio. This is the factor that uses rod length and camshaft intake closing point to build to a certain result.

Let's say for instance that a builder puts together a 9.0:1 motor and chooses a very long cam that closes the intake valve at around 70 degrees (@ 0.050" tappet lift) after bottom dead center. The motor is fairly low compression to begin with. Closing the intake valve so late allows the intake mixture that was just drawn into the cylinder on the intake stroke to be blown back up the intake tract. The intake valve is still open a little while the piston is travelling back up the bore on its compression stroke, so the piston pushes the mixture back up toward the carburetor, reducing the amount of mixture that is trapped in the cylinder when the intake valve finally closes and making a weak explosion when the spark plug fires. This is what happens to poser newbies who use a long cam in an otherwise stock motor. The motor won't make enough power to pull the hat off your head, but boy it sure sounds great to the geeks down at the drive-in with its rumpety-rump.

The flip side is building a higher static compression ratio motor and using a cam that is too short for the scr. Its a high-compression motor to begin with and let's say a cam with an intake closing point of 30 degrees
ABDC is used. The valve closes early enough so that most all of the mixture is trapped and you get a cylinder pressure that is too high for the available fuel. This results in pre-ignition and/or detonation and/or cracked pistons and/or bent rods and/or a broken crank and/or a failed head gasket or two.

So, as you can see, all these things have to be taken into consideration before you can finalize your choice of parts that go into a build. I wince every time a newbie comes on and knows nothing about the scr, squish, piston deck height, block deck height, chamber size, piston crown configuration, piston compression height or practically anything else about the motor, but says he has chosen such and such cam. I usually just click to another thread and smile.

There is another scenario that is possible, although no builder in his right mind would do because it leaves no material on the block decks to take another cut on the decks if you have to clean them up in a later build. That is cutting the block decks to pop the piston out of the block. Let's say that you have a piston deck height (piston crown to block deck) of 0.010" and you, for some unknown reason want to use a 0.060" copper gasket. Maybe you're thinking of adding a blower later, so a copper gasket would be a good choice. To arrive at your preffered squish of 0.040", you would have to cut the block decks 0.030" to do it. This would pop the piston out of its bore by 0.020". This can and has been done, but it would not be the avenue you would want to take on an unblown application.

The other things you want to be cautious of is the configuration of the piston crown and the piston compression height when you are choosing the parts for a build. Rebuilder pistons, although cheap, are normally manufactured with a reduced compression height (centerline of wrist pin to crown) of about 0.020". They do this figuring that the machine shop is going to deck the block 0.020" to clean it up and the piston will be at the same piston deck height (piston crown to deck) as the old original piston was. This is fine for a low-buck crate motor, but for a street rod motor builder, it is unacceptable. Always note the compression height of the pistons you are considering buying and add up your stack. If it is less than 9.000", you're going to have to do some considerable decking on the block to get the piston deck height (piston crown to deck) where you want it to set the squish. If you're doing the building and not paying attention to the piston compression height, you could end up with a squish (or non-squish dimension of 0.065" to 0.070" and then wonder why the motor will not operate on pump gas of any kind without detonating.

The piston crown configuration is another place to pay attention. If the piston does not have a large, generous area on the crown to mate with the underside of the cylinder head, you will not generate enough squish to make the motor detonation-resistant. I'm talking about if you have to use a dish piston to make your target static compression ratio. Some pistons have the dish machined out over the entire top of the crown and leave only a thin ring of actual crown material maybe 3/8" wide around the perimeter of the crown. This is unacceptable. You want to choose a piston with a D-cup configuration. These will have a generous pad of material to mate with the head and will generate the proper squish to quell detonation. Here's an example. Note the generous squish pad on the right side of the photo....
http://kb-silvolite.com/performance....etails&P_id=92

Okay - you've just been given your first two lessons. Want to stay for the rest of the "class" and learn from some real pros? If so, let us know what you plan on putting this motor in. What kind of driving will this build see? Do you want lot of noise or do you want good solid power that sounds as stock as the 1969 Nova 6 cylinder your grandmother drove but can pull a 13 second quarter mile time? Do you want to go with an automatic or stick? Do you want to bruise the boulevard or cruise on the highway? (Makes a huge difference in the rear end department!)

If we haven't scared you off with the intro - we'll look forward to seeing what ya got and helpin' where we can!

Glenn

Thanks for the reply guys. Ok, here we go. It's going in a 92 1500 series truck. It's single cab, short wheel base two wheel drive truck. I plan to drive it every day and go to my local 1/8th mile dragway every Sunday afternoon. I will run nitrous on it if I have to. I'm just looking for it to run low low 8s or high 7s. It has a 700R4 tranny which I would like to keep. The pistons are flat top with -5cc worth of valve reliefs cut in them. They have a lot of piston crown. I completely understand what you mean by "squish" so it won't be a problem getting the 0.040" gap your saying I need. The kit does have I-beam rods so I don't want to over power those. I plan to run NGK TR6 plugs and running it a little on the rich side doesn't bother me either. This will be the first chevy engine as well as carbed engine I've ever built so I'm lost as far as induction goes. What cam, intake and carb should I consider? I'm planning to run long tube headers with glass packs dumped for exhaust. I'm planning on running hydrolic flat tappet cam as well. Is this ok? Any suggestions and advise? Thanks again. You guys are a lot of help.

I think I'm going to assemble the bottom end this weekend so I need to know a few things right off the bat for that. What should I torque the main and rod bolts to? It has ARP main studs. Which assembly lube would be best for a motor that will sit up for a while? Should I spray the block down with a light coat of oil since it'll be sitting up for a while? Should I plastigauge the main and rod bearings? Should I check crankshaft end play and if so what are the limits?

I don't know who figured your static compression ratio or how it was figured, but it's wrong. A 383 with 5cc valve reliefs and 64cc heads will be 11.14:1 static compression ratio. You'll have to run a ton of cam even with the tight squish to keep the motor out of detonation. A ton of cam ain't gonna be much fun in a street motor. I suspect you'll need around 245 degrees @ 0.050 which will give you an operating range of 3,500 to 7,000. You'll need a converter that stalls at 3,500 minimum and a rear gear somewhere in the 4's.

Also, stock iron heads are a bad plan.

Ouch - 5cc relief and 64cc heads looks like you'll have closer to 11:1 compression ratio. I think cam wise you'll be looking at 242-245 degrees at.050 which will make this thing a dog on the street unless you put some really tall gears (like 4.11 or 4.56) and plan on a stall converter in excess of 3,000 - 3,500 may be better..

Sorry - Techinspector - I was thinkin' and typing while you had already responded - but I think we're on the same page. As described by Contentsunderpr this will not be a lot of fun on the street (unless he lives in an area where school zones are 70MPH!). I'm sure your figures are more precise..

Regards,
Glenn

Hmmm. That sounds bad. Should I go for some heads with the 72cc combustion chambers or get new pistons?

Let's go back to square one.

First off, I have heard some bad juju from professional engine builders concerning the Eagle kits. If I were doing this build, I'd start with a Scat cast steel crank kit with Scat I-beam 5.700" rods.

I'd also be starting with a roller tappet, one-piece seal block. It just makes no sense in this day and age to begin your build with a flat tappet block and have to worry about wiping the cam out due to lack of extreme pressure lubricants in the oils that are available today. There are tons of these blocks and even complete builder motors in the boneyards. Chevy began using rollers in 1987 in cars and I think about 1992 in trucks. If you could find a complete L31 5700 Vortec builder motor from a '96 to '98 Chevy truck, you would also get the L31 Vortec heads on it. These are the best production heads that Chevy ever produced and will outflow many aftermarket heads.

A Scat stroker kit in this L31 motor would make a dandy street/strip motor, using an RPM Vortec manifold and 750 vacuum secondaries carb, along with 1 3/4" long-tube, equal-length headers and a cam to match the SCR.

The Vortec heads are lift-limited to about 0.450" as produced stock because of retainer/seal interference. You can run more cam by either cutting the height of the valve guide boss and dropping the seal down, or you can upgrade the valve springs/retainers to CompCam beehives that will clear with more lift. I would opt for the spring/retainer change, because that way, you can run the springs recommended by the cam grinder for the cam you choose. Nothing more need to be done to the heads to produce good power on pump gas. If looking for the heads in a boneyard, look for a sawtooth design on the ends of the heads, then pull a valve cover and look for casting number 12558062. There's nothing wrong with using the rail rockers that come on these heads either. They'll work fine with moderate spring pressures without pinning the studs.

Using these pistons....
http://kb-silvolite.com/performance....etails&P_id=94
with the 64cc Vortec heads will produce 10.3:1 static compression ratio. Using this cam...
CompCams HYDRAULIC ROLLER-Great for street machines. Best with headers and aluminum intake. Use 2500+ stall converter. Rough idle. 2500 to 6000 08-450-8 286HR 286 286 230 230 .560 .560 110°
will produce these numbers.....
RPM HP TQ
2000 145 382
2500 184 386
3000 236 414
3500 300 450
4000 359 472
4500 406 475
5000 444 467
5500 450 431
6000 427 373

All I'm saying here is that by starting with a production motor and using selected parts to build it, you can have a motor that produces 450 horsepower and 475 ft/lbs of torque without expensive aftermarket heads. I might expect to pay $400 to $500 for a complete L31 builder motor, but not having to spend money on heads makes this the deal of the century.

I've already got the Eagle kit. What is the bad news you've heard about them? Should I check anything on it? I need to put something together with what I have. I haven't bought any heads, intake, carb or cam yet. I just need to know what I need to get to make the bottom end I have work. It would be nice to be able to start all over but I don't have the means to do that.

The only thing you need to change out are your heads. I have an Eagle rotating assembly with SRP forged pistons (-5cc) and I-beam rods in my 383. If you go to aluminum heads with 68-72 cc chambers (mine are 68cc AFR 190's) and a cam like a CompCams 280 Magnum which has an intake closing point at 66 degrees or a cam with a higher intake closing point, you should have no problem with using 91 octane like I do. A lower intake closing point will raise your DCR. For example, I considered CompCams XE-274 which has an intake closing point of 63 degrees, but that cam would have made my DCR 8.64 which is problematic (Anything over 8.5 with aluminum heads risks detonation.) My DCR figures out to be 8.469, and I have yet to experience any detonation. But I would not recommend my combination for iron heads. You can normally run .5 more DCR with aluminum heads. I would not worry about your Eagle rotating assembly. The only difference I have heard between SCAT and Eagle is that SCAT has a little nicer finish. But I have also heard that some builders prefer Eagle connecting rods to SCAT for some reason. Personally, I think it is a matter of comparing six to one-half dozen. My short block builder sells Eagle, and they are a highly respected machine shop in this part of the Midwest.

Ok. That's the heads the guy told me he was planning on putting on the motor. I guess I'll just save up for a set instead of chancing it with cast iron heads. Which cam did you go with exactly?

Originally Posted by contentsunderpr

Ok. That's the heads the guy told me he was planning on putting on the motor. I guess I'll just save up for a set instead of chancing it with cast iron heads. Which cam did you go with exactly?

CompCams 280 Magnum. Great cam with good street manners. Run a 2400 stall converter with it. My engine dynoed at 450 hp and 468 ft. lbs. of torque.