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Old 04-27-2005, 07:52 PM
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detonation issues and help with Mallory HEI

Hi guys! I hope I can figure out my detonation issues with my 383 once and for all: CR 10.1:1, 3.73 rear end, TH350, 750 Edel., idles at 750-800 with 14-15in Hg., gas 91 octane + 107 (8 gallons 91 + 3 gallons 107).

Here is what happening: in either 1st or 2nd gear the engine pings at about 2000rpm at acceleration. I can play with the throttle, get rid of it and accelerate just fine past that dreaded 2000rpm mark. The timing is set at 10* initial, vacuum advance was plugged. I am using the heaviest springs in my GM HEI rebuilt distributor.

Initial -10*
1500rpm - 14*
1800 - 20*
2000 - 22*
2500 - 26*
3000 - 28*
3500 - 28*

So my distributor adds 18* to the initial, giving me 28* total which is not enough. After digging on this site the most recommended setting is 34-36*. I bought a Crane recurve kit and went through different springs combos with the same result: pinging under load. I went from 14* initial to 6*. After I added 107 gas the rattling noise is not as loud as it used to be.

After messing with this distributor, I bought the Mallory HEI unit with the spring kit included. In the instruction sheet for advance curve springs it says that Silver (default) combination will give me 600 (curve start) and 2800 (total). In addition the distributor has a 24* advance (per the paperwork), not 18* like my other one.

- Should I stay with this setting? Or try the black springs (curve start at 600, total 4000)? Or black + silver together?
- Confusing part: at the bottom of that sheet is says: "With GM Advance Kits there is a sleeve which must be positioned on the advance stop pin located under the advance mechanism." I apologize, but I can't find the darn pin under the ad. mechanism! Does this sheet mean the vacuum advance 90* bent arm coming from the canister that pulls and advances timing? I'm sorry English is not my first language. The arm itself doesn't have any sleeve.

My car is at the shop with other problems being fixed. On Friday before installing this new distributor I'd like to be better prepared for that rattling/pinging noise.

I also realize that I should have done better job measuring my CR and quench that finally bit me in my a r s e.

Sorry about the long post.

Thanks in advance for your help,
Denis
'67 Camaro RS

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Old 04-27-2005, 08:13 PM
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what cam do you have? I have 10.3 on a 327,iron heads,comp 274 cam,20 degrees intial 36 total and no detenation just courious.What is your motor combo.It could also be jetting sounds crazy but i have had similar problems.
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Old 04-27-2005, 08:20 PM
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Comp Cams 12-242-2 XE268H-10
.477/.480
@ 0.050 224/230
LS110

Vortec heads with upgraded springs, valve lift allowed up to .525.
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Old 04-27-2005, 08:29 PM
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Is your tdc mark actually correct? not sure why you would ping with 10 degrees initial. Have you pulled a plug to see if your running lean.
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Old 04-27-2005, 08:35 PM
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Yes, the plugs are Rapidfires, colder range, brown in color, no black oily deposits. No spell of fuel either. I finally calibrated my 750 carb with right fuel bowls height and jets. I am running Stage 1 rich +4% secondaries (jet #1432), primaries 1433 (.113), metering rods 1447 (068x047), position No. 9. No bogginess at acceleration/de. No flat spots.

Oh, I installed 160* t-stat, the engine runs 160-170*, fan shroud, alum. radiator.

Yes, the har. balancer mark is on the spot, I am using an adjustable pointer. Sprocket and crank were aligned at 0* mark.

Thanks!
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Old 04-27-2005, 09:48 PM
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Get the Mallory advance curve kit and use the shims to get 24o of advance. See if the pinging is still there.

Buy or borrow an advance analyzing timing light and verify your timing curve. At least double check that.

I have an iron headed 400 at 11:1, running on 91 octane gas only. I run 16o initial, 34o total with no pinging. Even on hot days driving uphill.
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Old 04-27-2005, 10:20 PM
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You probably got your quench within 0.035-0.050", right?

I do have a timing light with an advance, I will get another one to double check.

Illuciano, if it pinged with my measely 24* total, logically, wouldn't 34-36* be even worse? Or my old distributor is messed up?

Should I start with two silver springs (2800 total) for now?

Thanks!
Denis
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Old 04-27-2005, 10:28 PM
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I am at .044"

I smoothed out my combustion chambers so that there are no sharp spots. Sharp spots are hot spots. I also sanded the valve reliefs down on my KB hyper. pistons.

You can get a pinging type sound from being too retarded.

The springs aren't as important right now as the amount of mechanical advance. Once you do verify your timing, It would be a good idea to get the timing in as quick as possible. Just make sure you don't use springs that will have your timing start coming in before idle. In that situation, some of your mechanical advance would be used up before your idle RPM. You would end up with less total timing kind of like you are having right now.
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Old 04-27-2005, 10:30 PM
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Thanks, it clears it out a bit. All Mallory HEI springs start too early for me: 600 rpm. My idle speed is about 800rpm. How can I deter the initial starting curve and make it, say, 1000rpm? Any tricks?
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Old 05-08-2005, 01:22 PM
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Update

Here is an update:
I installed the Mallory HEI distributor, initial set 10*, total 34* by 2800 rpms.

My friend and I verified the TDC mark with the piston stop. Right on the money. We also checked my quench my putting a soldering rod in the piston area and letting it squash it. My quench turns out to be at 0.078in. I am screwed. The car pings at around 2000 rpm under load, HOWEVER, it happens if my TH350 shifts too early and I am taking off in the 2nd or even 3rd going 15 mph. We ran the car in the 1st gear all way up to 4000rpms with no detonation noises. I also jetted up my 750 carb (position 13 on the chart, secondaries - Stage 2 rich with purple springs). The spark plugs look good (greyish, no fuel). I also hooked up my ported vacuum to the distributor, the car behaves the same way, runs cooler and seems to have more power.

I realize that the modifying my shift rpms in the tranny is almost like a bandaide fix, but besides using a thinner gasket, or removing the heads, are there any other solutions? Zero decking the block? What if I installed the 72 or 76cc heads, will this cause the same problems since the quench is all messed up? Going with aluminum heads?

Re: shift rpms - (1st to 2nd gear) 3000 rpm will be a good strating point?

Thanks for reading this and helping me out,
Denis
'67 Camaro RS
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Old 05-08-2005, 02:14 PM
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Are you absolutely sure you're at 10.1 to 1 C/R? Did the quench area mathmetician also figure swept volume? Not being a smart *** here just trying to save additional frustration and cost.
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Old 05-08-2005, 02:59 PM
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Well, Doing some quick calculations, I find out that you're having a dynamic compression ratio of 8.7.
That's really pushing it with a set of iron heads. 8.5 is normally the aim if you got Alum. heads....8 if you got iron heads, and a quench of 0.035-0.040"

So you're 10% above the borderline with that cam.

To make things worse your quench is way above the recommended 0.035-0.040" to maximize power and keep detonation away..

Two bad things combined to give you trouble.

The fact that you can remove the pinging by easing the throttle, indicates to much dynamic compression at 2000 rpm.

I bet that you can get the pinging away by retarding the ignition further.
I know it will run like C*** at idle, but if you can remove it, it's quite clear that you got too much timing at 2000 rpm.
And since a lot of other people can run with the same amount of timing at 2000 also with a 383, maybe also with 10:1, something like the cam or quench is pulling a leg on you.....

One last thing to check.
I have previously experienced pinging problems with engines burning a lot of oil. Oil in the combustion chamber reduces the overall octane level.....

If you got a crank case vent hooked up, try and remove it... just for testing it....


The fix (sorry to say):

1: Change the quench (Watch static compression ratio!!!)
New thinner head gasket, enlarge combustion chambers, change heads.
You will probably have to lower the quench to get the most out of the engine.

2: Change the cam to lower dynamic compression
You keep the heads, add a thinner head gasket and lower the DCR with a new cam with for instance XE274 or XE284.
They're getting a bit wild, and you're gonna get into trouble with that Edelbrock carb I guess....

3: Retard ignition enough to remove the pinging, and add manifold vacuum to the vacuum canister, to help you idling.
Lower power, but drivable until you're ready to dig deeper into the engine...

Well just my 2c......

Comments welcome!

/ 59Vette
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Old 05-08-2005, 04:57 PM
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Thanks, guys!

Here are a few numbers (killerformula helped me here):
You Entered:
Bore 4.06 (inches)
Stroke 3.75 (inches)
Rod Length 5.7 (inches)
Cyl. Head Vol. 64 (cc)
Deck Height 0.025 (inches)
Head Gasket Bore 4.166 (inches)
Head Gasket Thickness 0.026 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -12 (cc)

Compression Ratio:
10.128 : 1

Bore / Stroke Ratio:
1.083 : 1

Rod / Stroke Ratio:
1.52 : 1

Total volume:
882.881 cc's

Compressed volume:
87.176 cc's


The tricky numbers are: deck height (unknown) and top ring land height (unknown). It was my first engine build, and I should have measured everything right the first time.


The timing was retarded to almost 2*. It ran like crap, of course, and the headers got so hot that it melted my TV cable. My little starter survived :-) It still pinged under hard accleration even then.

- 59vette, by going with the 274XE cam, the Comp cams suggests using a 2200+ torque converter. Will I be okay with my 1900-2000 rpm one?

- Why will I get into trouble with my 750cfm carb and the above cam? Do I need to get a bigger model?

- Will the tranny shifting RPM operation be okay for now? I can install weaker governrr springs.

- If I change the heads, should I go with aluminum ones? Any preferences? The problem with mine is that I had to invest into the Vortec style intake etc.

- Any other ways of figuring out quench without removing the heads?

Machine shop, Nairb and other engine gurus: how much meat off the Vortec head can I port to increase the comb. chamber volume? I've heard that these heads are not really port-friendly due to their somewhat weak design?

Thanks, I really appreciate your input.
Denis
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Old 05-08-2005, 09:11 PM
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Probably simple question: I played with numbers, 76cc heads will bring my CR to 9.4:1 CR. Is quench as important in this application as with running high CRs and street gas? Or will my engine detonate the same as it is now?

Thanks!
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