coppercutlass

So Im still hearing this noise out of something . I hear it mainly under the trans tunnel. I have never really experienced detonation issues so I don't want to hurt anything. But at which point would detonation show up. I have taken 1st gear well into the 5k + rpm range then I take it up into second gear and I can hear it and then as soon as I hit 3rd it's there. Now I know detonation happened under load but will 3rd gear show more load than 1st and 2nd. That's what was told to me but I want a little more insight into it. I figure if I take 1st gear up that high I would hear detonation. Im planning on buying some 110 and seeing how it goes hopefully we get some clear weather here. Any input would be appreciated. The noise im hearing sounds more like a chirping sound almost like that of a really far gone water pump thats dry or even bad alt. than a metal popping sound. I know it's not the alt unless as the rpms go up the load on it changes , I thought it could be the converter but it would do it all the time. My only other thought is the trans and that some thing is starting to go as I hear it directly under the trans tunnel but I also know noise travels. I am getting ready to install my th350 but I wanna rule it all out before I do the trans swap.

MDchanic

The noise you describe and its location are not typical of detonation, but I am not going to tell you you don't have any.

Your friend is right that the way to provoke detonation and hear it best is low RPM / high load (lugging), but if it occurs at high RPM, you generally can't hear it.

- Eric

cutlassefi

X2. That's why it's called the silent killer. You won't always hear it.

coppercutlass

So what I'm hearing is not detonation if it's happening at high rpms. I also don't think it would be detonation. My total timing is around 32 total and I'm running 93 pump gas with right around 10 to 1 high 9's ever since I even gave it a thought about it being detonation its been bugginf the crap out of me.

oldcutlass

It will also show up under medium throttle load if your vacuum advance is not set up correctly.

MDchanic

To clarify, if there were no other sounds, you'd probably be able to hear high-RPM detonation, but the sound is masked by mechanical sounds from the engine and by wind noise.

When I was setting up my car, I thought I had it right and was out on the highway for a test drive. Traffic was such that I was passing someone at 65mph in the left lane (I'm almost always as far right as I can go on the highway), the car was in third gear, it was a slight uphill grade, and I was giving it just a little bit of gas (thus not activating the vacuum advance to retard the timing, so I was at full advance), and I heard just the tiniest bit of rattling bouncing off the concrete barrier a couple of feet away from my ear.
I would never have heard the detonation otherwise, and would have gotten it a large portion of the time while driving on the highway. I took a couple more degrees out of the vacuum advance and all was fine.

Just a practical illustration of what look for and how it can affect you.

- Eric

coppercutlass

No vac. Adv. I heard it off the barrier as we'll on the highway but I can hear it when I'm on it regardless of the barrier there to bounce off it. I think it's my trans. I remember hearing the same sound a few times when I first fired it up and put it in drive. Maybe pump idk.

coppercutlass

I might add it only happens under wot. As I'm running through the gears. If I'm in 3rd and I mash it to pass it won't do it like say I'm doing 65 and mash it there is no noise. And since it won't down sift on it's own unless I do it manually . I think something is up with my trans . I'm gonna get 5 gallons of 110 . I think for the amount of tming I have it at and the compression if it's still there with 110 it's my trans

MDchanic

Sounds like the trans to me.

Full throttle detonation in one of these cars sounds a lot like there's a tin can full of marbles rattling inside the engine.

- Eric

droldsmorland

Unusual to hear spark knock from trans tunnel area, plausible but not common. Likely somethings ready to come apart in that area? Have you pulled the insp cover to have a look at the flywheel for cracks, crank & converter bolts etc...Check the converter for obvious ballooning. If you see bulges its likely the noise source as the outer vanes could be floating around, though youd here it most of the time not just high rpms? If you see a questionable converter dont drive it until its replaced or you can lose a leg/foot.
If all checks out above then it could be ignition related? You said your all in at 32* Thats not crazy. What is the mechanical advance doing? Where is your initial set? Whats your build compression ratio? Retard it and road test. Keep retarding it until it goes away. If it doesn't go away you have something else going on like the above or a vac leak, lean condition or carbon in comb chamber? Wrong plugs can cause knock. What have you recently changed? Bad gas can cause this too.

coppercutlass

The plugs don't look lean they look nice and tan. I usually leave it a bit rich not a lot. Flexplate is new, all that's fine. My compression is in the 10 to 1 range high 9's. I run it on 93. I was thinking possibly plugs too but I got the std. Replacement autolites if it is detonation I would assume my plug is too hot.

Rocket Richard

When you look under your car is there anything loose or suspiciously close to the floor pan or anything? One thing that comes to mind is an exhaust pipe just barely tapping on the underbody when the engine is under load and flexing a little on the mounts.

coppercutlass

No I know that sound and my exhaust isn't very close. It's not a metalic pinging sound more of a chirp like somthing is running dry possibly in the trans.

oldcutlass

What about header or collector leaks?

coppercutlass

I would expect a leak even when reving it up then wouldn't ? I heard a similar sound when we first fired up this engine but as son as put it in drive it chirped went away and never did it again at idle. Maybe bad trans pump not enough oil psi at high rpms ? I know my exhaust is not perfect and it does have a leak here or there but nothing audible really. Unless someone has experienced a chirping sound from bad exhaust connections. But I would expect a ticking at best at idle and drownd out at high rpms.

coppercutlass

I'm thinking either the trans is foaming up or not enough psi for the load maybe runing dry at those rpms. I don't have a book for the th400 that shows what's engaged in 2nd and 3rd as far as clutches or gears go.

MDchanic

I'm NOT a transmission expert, but all of that information, including full color foldouts, is in the Chassis Service Manual.

- Eric

don71

I'm with Richard here, any chance something is loose? Trans mount, flywheel, exhaust leak, how about your drive shaft loop, any thing like that.

coppercutlass

Took the loop off still , headers done seem to leak . trans mount is good , crossmember is tight I was just down there within the last 200 miles and I checked everything. I did nothice there is a lot of movement enough for my driveshaft to bhit the loop bolts. But the sound was still there when I took it off. Fly wheel I just installed pretty much right after break in as I found out it was junk and I put in a nice new prw flexplate.

coppercutlass

If it was any of that it would do it in 1st gear I would assume but the sound is like instinataneous when you hit 3rd gear I wil let off and gradually bring in the rpms and it seems to lessened. Almost like something is slipping or grinding inside if I bring in the power all at once . Btw guys thanks for the input. I have looked and thought about many of your suggestions whic makes me think I'm on the right path. The last time I had detonation I just switch to 93 and problem solved. The reason I asked about the load in 3rd gear is because this problem has come up but not similar symptoms to my issue and I'm a bit perplexed . Like I said earlier I can take 1st gear to 5000 rpm plus and no noises its only mid way through the rpm at wot. In 3rd. Possibly second but 3rd is the worst.

don71

Ok, a lot of movement? whats the rear end doing? Pinion angle? Check there. Everything.

coppercutlass

Enough movement for the drive shaft to move over 1.5 inches to scrape the driveshaft loop bolts. Nothing is adjustable in the rear so pinion angel is where it should be. But if it was that wouldn't I hear it when I launched it which I have done plenty of times ., from my understanding the most movent would occur during launching so if I hear it in 3rd and it's something moveing I would assume it would be heard well before 3rd gear

cutlassefi

Just an fyi, I've been standing right next to a running motor during a pull and never heard it detonate. However it lost almost 15h from a previous pull AFTER advancing the timing. I'd say it was detonating.

coppercutlass

I mentioned the whole lean thing since it was brought up I know there are sevral factors that go into what causes detonation. I was just asnwering for whom ever brought up the subject. Thanks for clearing that up mark. What's your take on the situation then.

coppercutlass

Ok there is a sliver of a chance it's my shifter cable. I thought I had it clamped away from the tail shat housing. Well I guess some thing went missing and the shifter cable has been riding on the yoke for some time and cut through the rubber and is just starting to hit the metal that surrounds the inner cable. It has ridden enough on the yoke to create a small very very small grove. This could be it possibly . Im re doing some trans lines which is how I found out about the shifter cable ., I also moved my alt. to pass side , Im picking up 5 gallons of sonoco 110 so if the noise is still there I can rule out detonation I think with the 110. Then if it's none of those it has to be the trans ! . This has gotten to the point this has really gotten under my skin . Im close to narrowing the issue down. I just hate it has taken me so long but that's life .

MDchanic

I'd move the shifter cable first, and blow big bucks on the 110 second.

- Eric

coppercutlass

5 gallons is 43 bucks not too bad compared to a 5 gallon from a preformance shot at 75 bucks.

coppercutlass

WOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOH let me rephrase that WOOOOOOOOOOH F*O^&ER*&I^O&T_(GBKVYI yeah. Im positive it's my shifter cable. it made no noise then I popped the zip tie and it faintly came back. I really got on it and I mean got on it and man this little sbo boogies. Now the only issue I have is tossing belts thank god I got a gree light and no cars at the major intersection by my house wooooo avoided disaster. Im gonna fix that and then hopefully some track time. I may add I think this noise im hearing was the same noise I was hearing with my last 350 that I "thought I hurt " So I took an engine and took all the good parts off and scrapped it and sole the good parts because of this noise. Although that little 350 with the cast flat tops I built did show some pre mature wear it was still running fine .

coppercutlass

I tend to over evaluate things . Im overly cautious and don't like to risk it. people may think im a bit of hack because I do things on a shoe string budget but the hack ends there when I assemble stuff or try to diagnose something I look at all the major stuff but overlook the stupid things. JESUS so much frustration over something so stupid. Again guys thanks for the help and input on suggestions. no time to find a way to keep my shifter cable in place with out popping the zip tie.

Rocket Richard

Great news. Sometimes the simple fixes are the most rewarding.