Engine Detonation 350 Oldsmobile with Quadrajet

[rhinosoft007]

Hi All,

Not sure if I've posted here about my issue but maybe someone could assist on what I could look at next?  Not strictly speaking a carburetor issue...but maybe it is?

PROBLEM
Engine starts and idles great, zero noise or vibration. When I move off sedately, car changes gears smoothly, no bangs or flaring.

What I have been experiencing is detonation when I give the engine a lot more gas in first gear or when under heavy load and changing gear from 2-3. The valve train (I guess it is?) starts rattling really badly so a bit worried about that - need to back off right away. At cruise, perfect, I cannot hear any noise and power delivery is fine but possibly is pinging just can't hear it.

I removed the engine then pulled the camshaft and heads to inspect for damage - none found. Well, I did throw away the CompCam valve train - one rocker arm was jamming on the CompCam stud, back to standard valve train now. I did not check pistons or bearings, there was no metal in the oil filter. I wonder if one valve has been 'tweeked' from the CompCam jamming, causing the noise? Hmmm.

If you read below, I have changed distributors and carburetors (one item at a time and tested) and for the life of me I cannot get rid of that noise. Advanced/Retarded timing every test and used a lot of fuel to get nowhere.

Engine builder replaced rotators with standard springs and retainers - he is backing his choices. 73262K2 - Lunati KIT 1.495". Not sure if the noise may be valve float due to incorrect springs?

DETAILS

350 Oldsmobile had an engine rebuild, pretty mild  :

- block decked, align honed, .30th over etc....
- CompCams camshaft CL42-227-4 H252 .433 intake/exhaust valve lift, flat tappet
- standard valve train
- heads improved (needed it) - #8 heads with larger 2.000 intake valves
- 17057253  and 17059241 Quadrajet carbs. Both updated using recipe 1. 17059241 running long blue power valve spring.
- 9:10 compression
- running dual exhaust through Thorntons cast manifolds
- recurved distributor (original set 10 to degrees) and mechanical set to the same 10 degrees.
- automatic 350
- running high octane fuel (in Australia ULP 98) plus added some octane boost but I hate that stuff so no more.

I ordered the specific parts for both carbs a while back and they all look to be in good order.

Someone told me core shift may have come into play after the rebore? That could cause overheating and damage the pistons - sounded plausible.

Thank you for reading this post and if anyone has any ideas, please post back - I would appreciate the help and thank you very much.

Graeme

[quadrajam]

Have you verified the timing mark on the balancer is located proper?
What is the total centrifical + initial timing?
Was the cam degreed in?
What is the cranking compression?
A 9:1 engine with a good quench distance should run fine on 87 octane.
Proper spark plug heat range?

.......  Could it be an exhaust leak at the manifold donut?
Are the motor mounts good?

QJ

[Pav8427]

Just a thought. Did you use a timing chain set to make up for align hone?

[rhinosoft007]

Thank you guys.

QJ - setup the timing mark with balancer using TDC with micrometer as per standard steps. Cam was degreed in as per cam card. New motor mounts,. Timing 34 deg total.

I will have to try a compression test next, will advise. Spark plugs look good, just standard ACDelco.
Exhaust leak maybe? I will jack it up and run engine to verify. 

Pav - used a double row timing set yes. 

May be something simple, I really hope so. Exhaust leak may be the next check - shop did whack a lot of orange sealant around the flange.

Will report back, thank you. 

[quadrajam]

Sometimes it takes some engine load, AKA cylinder pressure to reveal a smaller
exhaust leak, so it might not show up reved up in the driveway.

I dont think you have valvetrain issues, that noise would be heard reving up at any load.



QJ

[Cliff Ruggles]

Right to start with you do NOT have enough camshaft in the engine so it's going to be octane sensitive.  252 seat timing is WAY too small for a 350 CID engine over 9 to 1 compression.  The tight 110LSA just makes it worse closing the intake valve WAY too soon which narrows up the power curve and spikes dynamic compression pretty high early in the RPM range.  Olds engines also use dished pistons (most of them) and crappy combustion chamber shapes.  Between the bathtub chambers and too much quench area they tend to run hotter and ping easier than other designs.  A LOT of quench area is NOT your friend with these things BTW.

I'd yank the cam and tink it off the nearest dumpster and install a cam with about 20-30 degrees more seat timing and wider LSA.  Summit sells a 214/224/112 cam that will work much better than the tiny little Comp Cam you are currently using.

I've ran into EXACTLY the same issues you are several times with engines in the 9 to 9.5 to 1 range pinging with any heavy load on them and I couldn't pull out enough timing to make them stop.  Every single time they had a short seat timing Comp Cam in them on a 110LSA......FWIW.

As far as Comp Cams valve train parts, specifically their "roller tip" rockers.....absolute and complete JUNK!  I have yet to remove a set from on engine that weren't blue or even black where the poor fitting grooved rocker balls were wearing heavily into the bodies.  The grooved rocker balls and poor fit to the bodies are also a design flaw.  Instead of the oil "pooling" and running out on the tips of the rocker arms to get atomized for cooling and lubrication it runs straight down the studs and back to the sump....duh?

I'm also NOT a fan of their heavy retainers and they recommend WAY too much spring pressure for their tiny little flat camshafts which is really hard on everything in the valve train and a big contributor to all the wiped out lobes we read about every time we log on to a Forum related to this hobby......FWIW......

[rhinosoft007]

Thanks for the updates guys, sorry I missed the replies on this post, may have forgotten to tick the correct notification button.

Cliff, thank you for your comments, will go down that track.

Check my latest post, now its the carb, related maybe.?

Graeme

[Cliff Ruggles]

Once you get the right cam in that engine your detonation issues will go away.  Carbs and distributors aren't going to get you there.....IMHO....

[Jeff K]

 I didn't see weather your running a vacuum advance. I know Cliff is pro vacuum advance but I've had  trouble with them.
 
 In 1974, I blew up a real nice GTO 389 with dual quads because I connected the vacuum advance in the points distributor to the ported vacuum in the lower left corner next to the air  fuel screws on the front AFB. This vacuum outlet was for the Roto Hydra-matic (Slim Jim) equipped Catalinas and Grand Prix equipped motors.
This was in a real strong 1964 2+2.. It had a 1966 389 with 092 (10.5) closed chamber heads.

[Cliff Ruggles]

Vacuum advance is a load sensing device.  It only adds timing at very light engine load.  Any heavy or full throttle it's not adding any timing.

Doesn't matter if you hook it up to manifold or ported vacuum.  The only difference between manifold and ported vacuum is that the VA is applied at idle and coasting with a manifold vacuum source. 

Using ported vacuum it's only applied at light part throttle or when you are driving very gently.

Neither scenario will contribute to detonation at heavy engine load.  If you experience any pinging at light engine load it's telling you that the engine doesn't need as much timing as that VA is adding, but it will not destroy the engine. 

If you run in to that scenario simply modify the VA to add less timing or find one that does.  You can measure the travel in the VA or use a timing light to find out how much timing it adds.  A drill bit makes a good measuring device for VA units.  They are also easy to modify.  Here I use a MIG welder and round file to fine tune them to add the amount of timing desired.......