Thread: Detonated Dreams

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I've been reading posts on here for a long time without being a member. A lot of knowledge in this place. I am hoping to reap the rewards of all of your hard work now HA HA.

I have a 383 in a 63 Nova SS with a built 700r4 and 3.73 posi. The engine has a detonation problem that i haven't been able to succesfully get rid of so i now have the engine apart.....again. I am completely prepared to do what it takes so i can stop "babying" the throttle. This thing needs to be hammered on. The engine is as follows:

-4.030" bore
-3.75" stroke
-5.7" rod
-2 valve relief, flat top, hypereutectic pistons
-cast chevy heads, 441. They've been surfaced so they measure 73cc's now
-Crane Cam, installed "strait up", grind #H-272-2, .454/.480 lift, 216/228 dur (w 1.5 rockers, but i'm running 1.6 roller tips. Problem?)
-When my piston is @TDC my distance from the deck to the piston measures
.012"(is that the right measurement for deck clearance?)
-My static compression checks out to 9.83:1
-Dynamic Compression Calculates to 8.686
-Intake is a first gen Edelbrock ProFlo EFI, 4 valve, w what is essentially a performer rpm intake i believe.

Under cool temperatures, before the engine compartment gets toasty, i can run pretty hard. As soon as the intake temp gets up anywhere above "brisk" it knocks like a little baby. I've got the spark dialed back ALL THE WAY and the fuel turned WAYYYY up for the Load/RPM range that is killing me(less than 6"/2500-4000rpm). I can not ditch the knock.

I always run 91 octane or higher, if available. I've mixed a tank with 2 gallons of 103 and made some headway. In no way is this engine fast enough to warrant the need for race gas. I've heard that some things can happen to make your engine "see cylinder pressure" that rivals that of 11:1 even though you are only at 9.5:1. Not sure i understand how that would happen or what to do about it but i am VERY open minded at this point.

Any suggestions?

Welcome to CHR – I hope you find the answer to you denotation problem soon. There’s nothing more frustrating – especially when you’ve built an engine that really needs to be romped really hard!

Isolating the cause of detonation in a modified engine requires you to determine if the problem is being caused by a change you made to the engine or if it’s is one that the engine has always suffered from and is now coming to light.

It sounds like you’ve already looked at the fuel/octane possibilities. I think that your compression ratio should be fine with premium pump gas. In most instances, good high octane pump gas can go up 11:1 so you’re well below that. Octane simply determines the fuel's ability to resist auto-ignition, or literally “wait for a spark to initiate a flame.”

Somewhat common but often overlooked problem areas are sharp edges of metal either on the piston or in the combustion chamber. You mention that your pistons have a valve relief cut into them. This typically leaves a sharp edge. This sharp edge can be susceptible to “super heating” and will actually retain enough heat that it will start to glow. If fuel hits this glowing edge either prior to the firing of the spark plug or even after ignition, it is very likely that another flame will ignite. If this unintended ignition occurs very early in the compression stroke, the piston will be forced up against the increased pressure of the burning fuel and will result in a condition referred to as” pre-ignition”. Excess carbon deposits in the combustion chamber or on the piston top cause a similar effect. The carbon deposits also decrease cylinder volume at TDC, which also raises the compression ratio and actually exacerbates the problem.

If pulling timing out doesn't suppress or eliminate the detonation, then your problem may be carbon deposits or a hot spot on the piston or combustion chamber. You can try the chemical carbon remover and see if that makes a difference.

It may just be one or a couple cylinders that are detonating – look at your plugs and see if you’ve got some oil fouling.

There are some who advocate thermal barrier coated pistons and cylinder heads eliminate detonation and make more power – others on this site are far more knowledgeable than I am on the coatings.

Try to get as much cold air as possible to the engine. Heat is a killer and it sounds like you’re experiencing your problem once the engine warms up.

Others will chime. I usually know enough to be dangerous but Pat McCarthy, the Tech Inspector and others will pour a wealth of knowledge your way.

Good luck and once again, welcome
Glenn

>012 piston to head isn't enough clearance---when everything gets warm and expands they are probably hitting the head and will also pound out the rod bearings

yep could be jerry . i think he is trying to say he is in the hole 012 from the deck ? so if a 038 gasket would be 050 or so . sounds like the intake air is getting heated up added engine temp adds to this hot oil up on the under side of intake = hot intake .. the hyper pistons hold abit more heat some use a bigger oil pump and loose rods in hopes of spraying more oil on the bottom s of the pistons to get them running cooler. timing could need to be tune in along with reworking the timing and engine water temp . like glen said hot spots in the head chamber and pistons and the head gaskets fire rings not hanging in the bores, not sure if i care for cam ?

Yeah--

I was just commenting on things as he stated----But he hasn't been back on with any more info

At first, I though NAAAA, this fellow has figured the SCR and DCR incorrectly. I refigured it and he's right on the money, so the cam and cylinder pressure match up, the squish looks ok at 0.051"/0.052" depending on whether he's using a 0.039" or 0.040" gasket and the motor absolutely should not be detonating on 91 octane fuel. I have a Crane catalog right in front of me that has the Cam Facts for this cam....."Good low end and mid range torque and horsepower. Daily useage and off-road, towing, performance and fuel efficiency. 8.75 to 10.75 static compression ratio advised."

OP, I'm gonna take you at your word when you say better fuel makes it a little better and go out on a limb here. I'm betting the inertia ring on the damper has slipped circumferentially in relation to the hub and you are getting incorrect ignition timing figures when you put a light on the motor. Here's a tutorial I wrote for the Crankshaft Coalition Wiki a couple of years ago. It explains how to correct the condition.
http://www.crankshaftcoalition.com/w...op_dead_center

The other long shot I was thinking about is that the noise is not spark related at all and is a mechanical interference noise. What made me think of that is the 1.6 rockers. Did you check for piston to valve clearance, spring coil bind clearance, retainer to guide/seal clearance and rocker slot to stud clearance at full lift? I know with the other things you said, it is unlikely, but just asking if you checked this stuff.

I run a lot of DynoSim combinations and the usual improvement I have seen going from 1.5 to 1.6 rockers is a single digit improvement in hp and torque. When you consider the extreme additional pressure that is generated at the lifter/lobe interface with these higher lift rockers, it hardly seems worth it. You want more lift, use more cam that is designed for more lift from the gitgo. That's just my opinion.

Glenn offered a reasonable explanation also, with hot spots in the chamber/piston crown, so clean all that up and round off everything while you have the motor apart. As Pat said, use a head gasket with a large enough I.D. so that it doesn't overhang the bore and glow red hot. The bore of the gasket should just exactly match the counterbore at the top of the cylinder. Using a bore that is too large is just as bad as one that is too small in my opinion. A very large bore gasket leaves a crevice where unburned gases can hide.

OP posted:
"In no way is this engine fast enough to warrant the need for race gas."
If the ignition timing is advanced as far as I think it is, that would explain the general lack of performance. Peak cylinder pressure could be occurring at the wrong piston position.

OP posted:
"I've heard that some things can happen to make your engine "see cylinder pressure" that rivals that of 11:1 even though you are only at 9.5:1. Not sure i understand how that would happen or what to do about it but i am VERY open minded at this point."
I think what you're alluding to is the DCR, which you already understand, apparently.

Thanks to everyone for all the great posts/ideas.

Glenn S-
I am cleaning up my chambers to rid the hot spots and am also looking at different valves to eliminate those edges, but i never thought of the piston. Makes sense. Is there anything i can do about those w/out pulling them from the motor to get them rebalanced/machined?

Cold air intake is def in the works.

I do need to do a better job of reading the plugs consistently. They are usually tan/brown w a little black spotting.

Jerry C-
.012 is the distance from the top of my piston to the top of my deck @ TDC. Is this to little?

Pat M-
Water temps are steady at 185-190 in any weather. I have a great radiator. The thermostat is a 185. I could go lower.

I am shopping cams as we speak. I would love more power as long as it stays between 1000-5000 rpm. Any suggestions???

TechI-
The damper was new when i built the motor. I will definitely look into that scenario.

The heads were machined to be used with 1.6 rockers, but no i didnt check the other stuff you mentioned. Would those things be load/heat sensitive? Either way those 1.6's are gone. I already have a set of 1.5 rollers to eliminate the question mark.

You're right about the power being limited by my retarded(in every sense of the word) timing. Unfortunately predetonation doesnt make power either HA.

How do you guys feel about my DCR? Is 8.6 a good number?
How much HP/Torq do you think i can get out of this combo with the current cam?
Could i get more usable power(1000-5000rpm) from a bigger cam w/out using a hi stall converter?

Louey-
Engine temps are steady at 185-190
Vaccum idles at 12"+
I never actually checked the timing with my light. I just took the control modules word for it (and i don't even have those numbers written down for you), that's how you all know I'm an inexperienced hack.

have you had the dist. setup by a shop???.check the advance curve.since you have efi do you have a dist???

The distributor is the Mallory unit they(Edelbrock) suggest for the EFI. It is locked down at 10 degrees and advance is controlled by the computer based on load(vaccum) and engine speed(RPM).

Am suspicious if the timing you are reading is actually what is real. If timing turns out to be accurate, is heat range of plugs correct, are you sure engine is not lean, and is the temperature gauge accurate. Is there a vaccuum leak somewhere that is possible getting worse as the engine reaches operating temperatures? For sure it is detonation and not mechanical noise?

I am curious about the timing now as well but i can't check that til the engine is back together. I will get the coldest plugs possible if that is a possible fix, or even just a step in the right direction. I've got the injection dumping lots of fuel in the areas where i am having the knock as well as pulling all kinds of advance out. It just makes it run terrible and never gets rid of the knock. The distributor is not a vaccum advance so that shouldnt be an issue in this scenario, correct? If the manifold saw "less/leaked" vaccum it would retard the timing more than it needed to accross the board which would hurt power but help knock yeah? As far as the mechanical noise, what could make that noise, and be sensitive to engine speed, engine load, engine temperature, and ambient temperature? I will check anything at this point.

Any possibility that your engine control electronics are getting whacked out and are really putting a very bad or inconsistent timing advance curve to the engine. Bad temperature sender,bad ambient temperature sender,bad R.P.M. information being sent to control module. With your electronics on board, could be sensors or the wiring, or the control unit itself, but when you get engine back together, check the basics, especially if what your are reading about your timing is accurate.
As for as mechanical noise, the first thing that came to my mind was something in the valve train. Second thing that come to mind would be proper piston clearance and ring gap if using hypo pistons. Is cam truly installed straight up and not advanced?
I would do a compression test, would be interested in what it pumps!

Planning on a compression test when we get the heads back on. Then check the timing. People have mentioned valve train noise but that would be a more consistent noise, not sensitive to outside variables wouldn't it? Cam is def strait up, unless there is 4 degrees ground into the cam.

Valve train noise can be affected with engine temperature and load. Too much lash will grow even bigger with engine temp up to normal, as compared to a cold reading. Also too much lash on the exhaust will sometimes tick louder under load as it has to open against increased cylinder pressure, but most of the time who can hear that when the engine is screaming unless you have a stock and quiet exhaust system, which i assume you do not. The valve lash can grow more with aluminum heads as the expansion rate of aluminum is so much greater than cast iron. Good luck on your engine and know that when you find out what was wrong you will be that much more educated and satified with your car and you will have information that you will be able to pass alone to the next guy! I would like to know what eventually fixed this problem!
that cast iron.

I was under the impression, that if you have hot spots inside the firing chamber, it is pre-ignition. The motor won't last long with this condition.
Might check for the proper reach of the spark plugs.