Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   Sounds of detonation when exhaust is too loud (

Argess 08-08-2010 05:01 PM

Sounds of detonation when exhaust is too loud
I've had some ignition troubles since last year (with my 428 Ford), due to a barely adequate ignition system that wouldn't do the job after I did a rebuild and the compression got bumped up a bit. Finally got that sorted out and now that I have no misfires, I have a question on timing....specifially timing advance.

When I was playing with it last year, I found the engine fighting me at 28 intial and 38 max, and wound up with 20/38 which seemed good. I seem to remember 24/38 was also OK, but kept it down to 20/38 for safety in the low end. Took away all of my idle and low rpm acceleration problems.

After my igntion changes to obtain a stronger spark, I'm not so sure I can live with 20/38. At 20/38 I have a very snotty sounding engine that really barks...almost a crackle when you give it a bit of throttle ( I like it!). And this is coming out of the exhaust. The engine sounds fine otherwise. I don't think I can ever hear a "ping" or a "knock" from the engine with sidepipes.

So I tried bumping the ignition back. Didn't fiddle with the max this time, but just set it back from 20/38 to 16/ 34. Power feels the think, but no crackle or bark. Idle is OK, but there's a small bog sometimes when I accelerate. No doubt 18/36 would be a compromise.

I'm trying to figure out if the snotty, crackle/bark is a good thing or a bad thing....i.e. might be light detonation. If you watch this you-tube video, you can hear some of the cars have this crackle, and some don't.....hard to listen and ignore the tire squealing though....LOL

Next step is to try to capture an audio for both settings and post it...wait....before that, I need to try lugging it in 4th or 5th at around 1500 rpm and listen for ping/knock.

I`m really paranoid about detonation as I broke a piston before due to my balancer outer ring slipping and inadvertently setting the timing wrong. New balancer now and I double checked it for accuracy.

Info: Cam is a Crane Fireball 294 and CR is suposed ot be 9.5:1, but after the block deck and cyl head re-surfacing, it may be a bit over 10:1. I know cranking pressure went up about 7%. High test around here is 91 octane.

LATECH 08-08-2010 06:21 PM

Would it be safe to blend some race gas in to see if the engine sound improves.How about an alcohol spray? You could put a piezeo sensor on and monitor it for a sensor. You know a knock sensor. Knock sensor would be the way to detect a problem more so than the fuel additives I would think.seems I remember you had access to a DSO so that might be the way to go. Just a thought.Maybe someone else has a better Idea.

mashori 08-08-2010 09:08 PM

could you maybe inspect the spark plugs to see if there is any speckling on the porcelain?

cobalt327 08-09-2010 02:58 AM

It would seem to me that the "crackle/bark" you're getting isn't any indication of detonation- at idle/unloaded you won't get detonation (unless the ignition timing was WAY too far advanced) anyway.

Is there a way to reduce the header/sidepipe noise volume to a lower level? Like w/temporary baffles or an additional muffler system to use long enough to see what (if anything) you can hear?

Another idea would be to do some acceleration testing to see if there was a reduction in speed w/the timing you'd like to run (w/the gas you are going to use). If there was no slowing from the timing you "like", then that's a good indication there's no detonation- detonation kills the power, so you'd expect to see a reduced speed/increased ET if it was detonating.

Argess 08-09-2010 09:26 AM

Thanks guys. I just might try that electronic listening idea, because even if I'm not having a problem, I could play around until I did, and would then know where my limit is.

Sparkplugs look pretty good. When I did have detonation with the engine prior to re-build, I noticed the plug porcelain was sparkling white, although I didn't see any aluminum or other specs on them. I should pull a few again. Due to the engine design and coolant flow, my #1 cylinder gets the hottest and is more prone, so I'll at least check that plug.

I havn't been able to think anything up for exhaust baffles, but I'll keep thinking.

I have to get some more test driving in and pay more attention...maybe get a video so I can post it for all to listen to.

One other interesting note, is that even with my timing set at 20/38, I didn't have that bark/crackle until I re-set my valves. Silly me had them set with too much lash as I went with the Ford spec vs the Crane spec. Memory doesn't always serve me well. With the lash set to Crane specs the cam overlap comes more into play (which could be responsible for the change in sound) and the engine felt a bit more snappy down low as decreasing the lash advances the valve opeing times a bit. The bark/crackle went away again in hot weather when the weak ignition wasn't doing its job, but came back after I made changes to improve the spark quality.

cobalt327 08-09-2010 09:40 AM


Originally Posted by Argess
I havn't been able to think anything up for exhaust baffles, but I'll keep thinking.

Even if you did come up w/something, the back pressure would be working against you- you'd have a kind of EGR deal going on if the back pressure was excessive and that could mask the detonation. Not one of my better ideas. :P

The plugs will begin to "pepper" if/when the aluminum of the piston becomes molten- you really do not want to see that, EVER!

BTW, what heads are you running?

Argess 08-09-2010 10:10 AM

Cast iron 390/428 heads with hardened valve seats and bronze guide inserts. 72cc cardioid chambers, but likely with 10 thou removed during resurfacing.

Back during testing, I could tell the engine was fighting me at 28 intial, but not at 24, so I sort of thought 20 was safe. I'm actually searching for the lowest initial timing that still gives me a good idle and low rpm operation.

Never had any problems cranking engine. With idle set at 20 degrees, the dist springs are already working a bit. Cranking is more likely about 15 degrees.

It did seem to me that the crackle went away about 1/2 way through rpm range. Sort of ends when the advance stops advancing. Could have been my imagination, so that's another thing to check next test drive.

V8 Super Beetle 08-09-2010 11:05 AM

I wouldn't want to be one of those people after the starting point of the car. :D Looks like fun though.

Argess 08-10-2010 06:32 AM

Well.....this is embarassing. I tried to make a couple of videos with the different timing, but when I dropped the timing from 20-38 to 16-36, it still sounded the same. I have no idea why. It was a drastic difference last time.

Now it could be I don't have the timing quite as far back as it should be as my mech advance springs are working at idle and changing the timing affects idle speed, maybe it's not in the same place....or maybe it's a mechanical problem that's getting worse since my last test.

But as I made the videos, I thought i better post them. The engine doesn't sound good at all in the audio....i.e. different than when you're there. However you can still hear the exhaust as a seperate sound from the engine sound. The exhaust note is just muffled due to the recording process with a digital camera, so it doesn't sound like the "bark" I have been referring to.

The idea was to hear the engine at light acceleratin from idle to 3000 rpm. I didn't do very well.

20-38 Timing

16-34 Timing

Guess I'll just have to do some more trial runs.

All times are GMT -6. The time now is 04:16 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.