Detonation at WOT?

[Mike F.]

My car seems to detonate at wide open throttle (or 10/10ths) on the freeway in the 3rd or 4th gear. What am I missing?

I have a mostly stock(rebuilt) 70 L-46, 10.4:1 c/r, M21, 3.70. The carb, plugs, wires, cap, rotor, vacuum can, and Breakerless ignition are two years old or less, and have no more than 5k miles on any one item. TCS is hooked up and functioning properly (as is everything else, as far as I can tell).
I'm using the silver(medium) springs on the distributor weights and a B26 vacuum can. Initial timing (no vacuum) is 8*@800rpm, 36*@3000rpm, 4000rpm, 5000rpm, 6000rpm. I'm using 93 oct. gas. I can lug the car around in 2nd, 3rd, or 4th gear all day long (partial throttle) without any sounds of detonation. The car runs great until I get to 10/10ths.

The only time I notice detonation is when I'm traveling on the freeway in 3rd or 4th gear (3000rpm or above) and I floor the accelerator (10/10ths). Once I floor it, the vacuum can shouldn't be in play (no/low vacuum) and the mechanical advance should already be at 36*. If I back off the throttle to 9/10ths the problem goes away. What is happening in that last 1/10th that causing detonation?

If I back the timing off enough to stop the 10/10ths detonation, my initial timing is 3*or 4*(which seems low). Am I missing the obvious?

TIA,
Mike

[Joe R.]

Re: Detonation at WOT?

Hi Mike:

First, let me say that there are others on this board that have a LOT more experience with this topic than I do. Hopefully they will comment soon. In the meantime, I will offer just a few thoughts. I agree that the stated conditions for your engine make it surprising that you are getting detonation.

That being said, the conditions for detonation are a combination of advance, engine speed, and load. The most vulnerable engine speed is around peak torque, probably in the 3000-4000 RPM range that you report, and the most vulnerable load condition is full throttle. Since your advance is all-in at 3000 RPM, you may just be at the hairy edge of triggering detonation when you go to full throttle at this RPM. It would be interesting to try using the next-stiffer mechanical advance springs to see if you can delay full mechanical advance until after the peak torque RPM.

[Duke W.]

Re: Detonation at WOT?

Check the B26 VAC with a Mity-Vac to make sure it meets spec and backs off immediately when you release vacuum. What you report may be due to a binding condition of the breaker plate or VAC.

With the TCS fully functional I don't think the VAC sees engine vacuum unless you are in third or fourth gear, which explains why a binding condition would not affect detonation in first or second.

With an aggressive centrifugal curve, detonation usually occurs at lower RPM, such as applying half or more throttle at 1500-2000, which will cause manifold vacuum to drop to near zero. Detonation in the mid/upper rev range only at WOT would indicate too much total WOT advance, which could be due to the VAC not backing off immediately when manifold vacuum drops to near zero or a missing or defective limit bushing that allows over advance at high revs, however you indicated that total WOT advance does no exceed 36 at 6000, so I assume to tested to that speed.

If you floor the throttle in fourth gear is the detonation sustained or just transient?

Also, when you say "mostly stock" what do you exactly mean? What modifications were made, and how did you determine the compression ratio you reported? Does it have a real L-46 camshaft that is installed at the as-designed inlet POML indexing of 114 deg. ATC?

It can also be instructive to use a long piece of hose to place a vacuum gage in the cockpit and observe manifold vacuum behavior in normal driving.

Duke

[Mike F.]

Re: Detonation at WOT?

I did check the B-26 with a Mity-Vac, started to pull in around 6"-8", all in by 13" (My vacuum at idle is 15".). I'll try running a vacuum gage into the cockpit and/or disconnecting the vacuum line to the B-26 while making some runs.

Mostly stock = ported heads, Comp Cam XE262h-10, 10.4CR per engine shop paper work.

[Duke W.]

Re: Detonation at WOT?

The problem is the camshaft. The inlet valve of that XE262 closes several degrees earlier than the L-46 due to shorter duration and earlier indexing, so the DCR is higher. Your choices boil down to install the correct cam, lower the CR with a thicker head gasket, or live with less than ideal total WOT spark advance.

You can't beat the L-46/82 cam for a high compression high performance road engine, but guys keep wanting to believe that some aftermarket design is better than the original Chevrolet design. NOT TRUE!!!

Some time ago I provided simulation results of the XE 262 vs. the L-46 cam (everything else identical) and IIRC the L-46 made better low end torque (due to lower overlap) and greater top end power (due to the later closing inlet valve) The the XE262 may have made better peak torque, but high VE in the mid range can cause detonation. There's a reason why Chevrolet designed the L-46 cam with rather late phasing for the era, but modern Corvette engines have relatively short duration cams that are phased even later, and the McCagh Special cam I designed for base engine rebuilds with massaged heads has the same design philosophy.

About a month ago someone was looking for an "improvement" of the L-46 cam, but I believe he decided to stick with the L-46 based on my arguments and agreement from Joe Lucia.

The only "better" camshaft than the L-46 is the LT-1 cam. All other things the same both will make about the same top end power but the LT-1 cam offers and extended peak power rev range due to its mechanical lifter design.

Duke

[Joe C.]

Re: Detonation at WOT?

There's nothing wrong with the camshaft, but it is designed to be used with lower static compression. Actually, it is much more efficient than the GM cams because it has faster ramps: i.e.: faster valve actuation which builds higher cylinder pressure...........thus, your detonation.

The spec sheet for your cam shows the test engine had 9.25:1 SCR. Since your engine does not detonate severely, I would retard the cam by 3-4 degrees, which will delay the IVC and lower cylinder pressure. Next best thing would be to retard the timing. I absolutely would not use a thicker head gasket because this effectively detunes the engine across the entire rev range, but, even worse, it exacerbates an already bad situation by increasing the already excessive quench area in the chambers. Increased quench, while lowering SCR will also make the engine LESS detonation resistant.

If you'd rather change the cam, then replace it with the next size Comp Extreme Energy cam with longer duration, which should be the XE 268H.

[Mike F.]

Re: Detonation at WOT?

Thanks for all of the help and explanations.

Some days it's worse than others (may be dependent on gas brand). It's never crazy detonation, more of a mild or transient problem.

Would increasing the octane a point or two fix the problem as well? It's easy enough to mix a gallon or two of race/aviation fuel in with the 93 oct..

[Duke W.]

Re: Detonation at WOT?

That will probably do the trick - probably no more than 20 percent avgas or equivalent.

Duke

[Domenic T.]

Re: Detonation at WOT?

Mike,
The mixture of av gas does it for me. I mix up to 50/50 but did not really measure. The 20 will most likely do the trick.
Years ago as they were reducing the compression ratios I had the same problem you describe. The car was a 280Z that I converted to a high compression 327 with a super T-10. I installed water injection, I believe it was Eselbrock, and it stopped the pinging. I usually filled the water tank up every time I filled up and could tell when the water tank was empty as the engine started pinging instantly.
I have considered this on my restored cars using another air cleaner lid and mounting the system on exhausting bolts for long trips when I am not able to mix with avgas. I used to mark my distributor and grease the gasket so I could retard the timing when I could only get lawer grade fuel, but that took a big hit on performance, so the water did a good job, but not as good as the mix. The CA smog laws ended that car years ago. It was grandfathered in as the mod was done in the late 70's, but a few inspectors that couldn't spell automobile shut it down. They were treating it as a stock 280 Z - 6 cylinder, and didn't have a clue that it was a Chevy conversion done years before.

Dom