355, vortec heads, rpm air gap intake, longtubes, 212/218@.050" cam, Edelbrock 650CFM carb, 5000 miles on the motor. 3.08 gears and 700r4.

I have not been able to get rid of detonation with this motor. It pings every time I accelerate heavily. Cruising is fine but any time I try to have some fun I get spark knock. The strangest thing is that the spark knock does not respond to retarding timing.

Things I have tried:
-Running 93 octane (I always do)
-Running 110 octane
-Retarding timing to 5* initial advance (26* total)
-Reading the plugs after a WOT run
-Unhooking vacuum advance
-Verifying my timing marks and timing light are correct
-Compression test gives 150psi=dynamic compression ratio not too high/cam is correctly timed
-Installing wideband AFR gauge to make sure im not lean
-All sorts of carb enrichening including pump shot, secondaries and primaries
-Trying two stages cooler plugs (RS9YC)
-Motor only has 5000 miles on it so carbon build up is unlikely
-160* thermostat so I run at a consistent 170*
-Checking for oil getting in chambers=all clean

There is one last thing I can think of--I used sealed power H345DCP pistons which have a compression distance of 1.540" which gives me 0.084" quench because I didn't deck the block. I wouldn't have used these pistons had I known at the time the importance of quench. HOWEVER, I don't know if that would really cause the trouble I'm having.

If I haven't figured something out by this summer I'll be tearing it down and replacing the pistons. Any other ideas?

 

have you tried adjusting your cam timing?

you may have to go to a "bigger" cam?

http://www.e30m3project.com/e30m3per...omp_ratio2.htm

 

If your @ .084,you have no quench/squish.Anything much over .050 has no detonation resistance to speak of.

 

I let the quench slide on a build before.I think my cam was 230@.050. Only disapionted with the power. Never pinged on junk gas. So what type of carb are you using? I had a 650 Holly Double pumper.

 

Carb is an Edelbrock 650CFM thunder AVS. Makes me feel better hearing about your experience with quench.

 

I have zero experience with that carb.Holly's slop the fuel in as they are a racing carb. {the one I was using }Your carb is spoon feeding I think if you can run it richer even beyond what all other indicator say you should be at, your ping will go away. True you'll waste more fuel but you won't be pinging. best fix is the one you don't want to hear... Fix your quench. Anyone else think this is good advice?

 

Sounds to me rather like, there are sharp-edged things in the combustion chambers, glowing red hot.

Typical sources of this problem are decking the heads and not busting the little burr off from around the chambers, and leaving all the sharp corners on the valves and seats after grinding them. All that stuff should have the corners gently rounded over.

First thing I would do though, is VERIFY the accuracy of the timing mark by MEASURING - not just "I think it's right" - and then verify the ignition timing using a tape or similar MECHANICAL method, NOT a "dial-back" light. Those heads are ordinarily pretty detonation-resistant, so there's definitely SOMETHING wrong; and it isn't the carb.

Yes your quench is terrible; but at the same time, your static CR is quite low and entirely reasonable. Lower than stock motors that use those heads and have no problems. While it's not "helping" you any, it's most likely not "The" "Problem" either.

Also wouldn't hurt to make sure your vac advance is actually working... that is, that the thing isn't stuck on the dist shaft in the fully-advanced position, regardless of the presence or absence of vacuum.

If none of that helps it, I'd pop the heads off and go to work with some very mild abrasive, to make sure everything is smooth and no sharp corners in the chambers.

 

Doing some quick calcs

you have about 9:1 Static CR

4.030 bore
3.45 stroke
8 cyl
355 cu

I made these assumptions to calculate SCR:
head gasket bore: 4.060
head gasket + deck thickness: .124 (.040 gasket plus your .084 in the hole.
head CC: 64
piston dome cc: 0 ( I ASSUMED FLAT TOP PISTONS )

the additional .084 in the hole from the deck lowers the compression quite a bit, if you had "0" SCR would jump to 11.03:1

Are you sure about this piston depth in the hole? because that is a huge difference... 11:1 is too high for a carb and 93.. you would need to run 100 unleaded,blend 93 with 100, or switch to E85..


edit.. I found your piston specs... it says the valve reliefs are 5CC so if I include that in the calculation
your SCR drops to 8.5:1 ?! that is still assuming you are .084 in the hole..

ARE YOU SURE ABOUT THAT #?! because if you have "0" quench the SCR is still 10.24:1

 

I re-read your 1st post are your telling us your pistons are in the hole .084 at TDC ?!

seems like too much and this would actually lower the SCR...

how did you come up with this value?

 

How much initial is in the distributor do you know how to change the curve?
With all things right should be able to go 30-32 total. Curious to see what you find.

Bob Jennings Dyno shop in North hills ca can do a # on your dist if you tell himwhat you have stil uses the old sun machines.

 

Just a thought, when I had my timing that far retarded I had similar issues with my vortec heads. Bringing it to 34 total really helped.

 

I belie e he is saying is quench is .084 .. tbsts including the factory .025 in the hole plus the head gasket plus the pistons short compression height so I believe the pistons are .045 in the hole with a .039 head gasket ?

91' firebird .. 305 TBI .030 over , balanced bottom end , speed pro hyper flattops , 10.6:1 CR , all ARP rod bolts and main studs , melling HV oil pump , trick flow super 23 175cc heads, Howards HR 214/218@.050 .488/.495 lift on 114 LSA , full roller 1.6 RR = .520/.528 lift , summit dual plane intake port matched , shorty headers into 2.5" Y w full 3" flow master system and 3" cutout in front of axle , msd billet dizzy, 620cfm TBI , custom CAI , tubular adjustabke LCA pan hard , umi torque arm wonder bar , BMR trans cross member , COMP engineering SFC's, disc rear w 4.10's, drilled an slotted rotors, EBL flash , VRFPR, WBO2,

 

if this is true then OPs SCR is more like 9.22:1 which should be OK with 93 octane fuel.

A/F readings at WOT need to be at least .8 with a wide or narrow band O2 sensor (lambda scale) or 12.5 - 13.0:1 max. on the Gas scale. Just remember a narrow band is just that.. narrow. .8 can be 12.5 or 13.0 so if your A/f is using narrow band shoot for .8

if A/F is too lean at WOT it will knock. DO NOT TUNE WOT A/F for Stoich.
(14.7:1) it can' be done without knocking the motor.. best performance is always with a slightly rich tune at WOT not too mention a rich mixture will never hurt the motor.. but go too lean.. and bang you break parts.

with a carb the main jets set the fuel curve for WOT operation.. for mild street car the edelbrock (q jet clone) is OK ,, but I have not played with a q-jet in over 20 years. the principle is the same but you also have to deal with metering rods..
it sound like you have a lean fuel curve at WOT. I would ditch that q-jet copy for a 4150 holley that is easier to tune and more people will be able to help you. Otherwise you need to just play with & find the right jet /metering rod combo to richen the mixture check out you-tube or contact edelbrock for help.

total timing mechanical timing 28-34.. initial timing.. 15-18 degrees.
you need to see what the car likes..

 

3 minute mark and on

OP, do you have owner's manual and calibrations kit for your carb?

 

^Yes this is correct

@FRMULA88 Yes I've got the owners manual and the calibration kit. I've tried 12.5:1 at WOT all the way down to 10:1.

@sofakingdom I think you're probably right about hot spots. Basically the only thing left to check. I didn't deck the block but I did grind the valves and seats and didn't do any smoothing out.

Well... if the heads come off I'll probably fix my quench while I'm at it

 

I calculate your SCR to be just under 9.4:1.(9.394:1 to be exact).This would normally be fine with the Vortecs.Tho they are less resistant to detonation,they rely on fastburn chamber design which uses quench to fight the detonation.It is a quench designed chamber.When you remove the quench,you basically disable the most important part of how & why the head works.If playing with the timing doesn't stop it,it's time for new pistons or decking the block.You need to get the quench below .055 to have any effect so the heads can do their job.Preferably .045 to .040.Tho the L31 Vortec engine uses a 9.4:1 CR,it does so with a .053 quench(.025 deck hgt w/ a .028 gasket).It also has a knock sensor that picks up detonation & signals the PCM to pull timing.

 

Stroke is 3.480 for a 350. So a tad more compression....

 

Also,keep in mind,if you get your quench down to .045" & keep a 5cc flat top,your SCR is gonna be approx 10.2:1.Your current cam will put your DCR @ approx 8.6:1.You'll still be on the ragged edge of detonation.Using a D cup piston such as KB-193 that has a 12cc dish & a 1.561 pin hgt would put you rite in the sweet spot of 9.5:1 SCR & an 8:1 DCR w/ a .045 quench.

 

Or maybe its also time for a bigger cam

 

Using a bigger cam would only be another attempt @ putting a band aid on to correct a mismatch of parts.The correct thing to do would be switching to a better set of aluminum heads that would be better suited to run the higher CR.