[Chevrolet 38]

I hope it's not detonation ....

I have a 327 from 1963 corvette on Cams
I don't know too much about it as i got the car with it, was told forge internals. I assume high compression ratio.
I have headers going to lake pipes


Runs great, I have no issues with it
I usually don't rev past 4k .. more than enough torque on the street .. sometimes Max 5k

It's being running really good .. I added some 98 octane = 93 RON (for USA?) no ethanol

I also added some upper cylinder lube. I don't know and I doubt the heads are done with harden seats so .. dunno if it's good idea or not. Some ppl say with super high grade fuel no addititives are required anyway.

When I was driving, rev to ~ 4k i heard a loud BANG .. like pop corn but Loud .. and just once

... my first thought was, I hope it's not detonation

Then I thought, if it was, and it was that loud? Wouldn't my motor be in pieces ???

The 327 is still sounds and feels great like a champ

Then another BANG .. it was about ~ 4k rev too ...

Then I stay under 4k and it is all normal. Engine still plenty of power, didn't feel any difference.

Was it backfire ???

Does 4 month old fuel cause backfire?
Does the upper cylinder lube additive to fuel cause backfire? Did it lower octane ???

any suggestions ???

 

[BogiesAnnex1]

Detonation/preignition sounds more like a loud fast rattle generally not a one shot but rapid fire until you back off.

One loud bang is either a fuel explosion in the exhaust system or a the same thing back through the intake. Or some thing mechanical busting.

Upper cylinder lube being oil based would reduce apparent octane of the fuel. But as I said before that is usually a sudden and harsh rattle sound not a lot unlike there’s a bunch of BB’s in the cylinders.

Check the spark plug gaps, their failure at an RPM can result in after or back fires. So can failing valve springs react like that. Failing ranging from getting tired and weak unable to control a valve or valves as tge revs go up to actually being broken.

Bogie

 

[Chevrolet 38]

Detonation/preignition sounds more like a loud fast rattle generally not a one shot but rapid fire until you back off.

One loud bang is either a fuel explosion in the exhaust system or a the same thing back through the intake. Or some thing mechanical busting.

Upper cylinder lube being oil based would reduce apparent octane of the fuel. But as I said before that is usually a sudden and harsh rattle sound not a lot unlike there’s a bunch of BB’s in the cylinders.

Check the spark plug gaps, their failure at an RPM can result in after or back fires. So can failing valve springs react like that. Failing ranging from getting tired and weak unable to control a valve or valves as tge revs go up to actually being broken.

Bogie

Thanks very much.

yes, defn a one off BANG ... no harsh rattle sound like a bunch of BB shots. That is a really good description.

I am starting to think it is firing back up the intake .. the BANG was a clear pop Bang .. and I would have thought if it was out the exhaust it won't be that clear of a bang sound ??? I dunno. I was not revving it hence I am thinking it's from the intake ???

Thanks .. I will check my plugs. . etc.

 

[ExcellenceAuto]

Could it be a lean pop?

I am not well versed on this condition, but others on here can explain mo better.

 

[eric32]

What carburetor type are you using if its carbed? Does this happen if its punched straight to wide open throttle? Or does it happen in any other instances? It could just be something as a lean pump shot from the carb if running one and a momentary lean condition from the big incoming gush of air and it will do that if you don't have enough pump shot or if a Holley style carb the correct power valve for your application to open up in a fast enough time to cover the momentary lean condition while the carb is taking in the extra fuel and it needs that quick time to catch up on the power circuit.

On an Edelbrock style carb it would take a change in the color of spring being used for the vacuum opening of the metering rods being used. On a Qjet I don't know for sure how its adjusted as I have not messed with them over the years and have only read about them and there basic functions and don't know a lot about them.

Hopefully someone else can chime in on that part.

 

[Chevrolet 38]

Thanks .. yes it was full throttle. I should check the plugs.

it’s holly carb .. not too sure the cfm

what spark plugs are best for a street 327 Sbc mild cam ??? Thanks

 

[eric32]

Mild cam can mean a lot between different people. Do you mean like near stock, or an RV type of a cam? Also on your Holley carb on the front of the main body you should see some numbers on the driver side of the tower such as 3310-7 or 80457-7 for example. Underneath it you should have four numbers like 1259. The main numbers you want to post are the top numbers which would state what carb you have.

The number after the - does not really matter as with time the number will change like 3310-7 to 3310-8, and what that means is Holley made some change to it and its most of the time just a minor tweek or a change on the casting design etc to where they up the number.

The carb model number will help and of its a double pumper or a vacuum secondary will change things slightly on how you would go about to determine on what is causing the backfire when hitting wide open throttle.

 

[Chevrolet 38]

Thanks a lot … it’s got vac secondary.
The cam is ok on first crank start, but have to pump the gas to keep it alive. After it warms it can idle … but u can heard it’s not stock.

I think I will change out all plugs anyway. Is there such think as high performance and cheap but short life plugs?

 

[eric32]

Well do you know by chance on your cam specs? That right there can help a lot with your carb tune itself. Depending on what your carburetor exact model is, it might be to lean where its at. The rest of your engine specs if you know them would hep with things as well. Also do you know your timing curve and how much are you running? Timing is very critical on things as well and if your timing curve is not good then that can make your carb tune way off and never be right.

Just some things to start off with

 

[curtis73]

Backfires out the exhaust are usually caused by a rich condition (which could be caused by ignition timing allowing too much unburned fuel out the back). Lean pops backfire through the carb. Detonation sounds like 8 leprechauns under the hood tapping on the engine with little hammers really fast.

 

[Chevrolet 38]

unfortunately I don't know the cam specs . . as I got nothing with the car.
Most likely it's Isky Cams, on mallory 9000 (no vac advance) on MSD 6AL

I actually think it might be rich backfire out exhaust as the car do smell pretty rich

Since it was a very clear and loud BANG .. and it didn't feel like coming from the hood

I assume the backfire out exhaust is not too bad vs out intake

 

[AutoGear]

You've asked for sparkplug recco's. \We need to know what cyl heads are you working with. Vortec? Traditional 327-era iron heads? Chinese aluminum castings? Something else

If theyre GM heads, best would be a casting number under a valvecover, but if you dont have that pull an existing plug and we can see if its reasonably correct, and then cross it over to different brands if needs be.
Are you running headers (tubular extractors) or GM iron exhaust manifolds?
Manual choke or electric?

 

[Chevrolet 38]

Sorry, I should know these things ... nah, the heads I got for the 327 are iron ... I do need to do valve cover gaskets so thanks for letting me know to get the casting numbers. The info I have are the motor from 63 corvette .. the rotation assembly are forged. (someone has done them) Seems to me the motor is doing good .. it really wants to rev up after 5k .. but that's the limit for me atm .....
Intake is a little risen edelbrock single plane. The one kinda looks like a squashed spider - no air gap.
Yep, headers .. out to lake pipe exhaust.
Manual choke .. I don't think there is one, I use the throttle so I guess manual. Defn no electric.

 

[cerial]

How do the exhaust gaskets look? Any darkspots or can you feel any leakage(being careful about rotating parts of course)

Next up is a compression test to see if you have a valve/valve spring issue.

Then its time for you to force feed that intake some smoke.

All cheap and easy checks you can do with the thing in the car.

 

[lmsport]

Will it do it sitting still and just opening the throttle?
If so it might be a weak or broken valve spring.

 

[realcarsrob]

Don't you love this forum ? Clear advise from those with some experience. Other sites would call this guy a dumb arse and abuse him. Mate I hope you find the answer and get sorted. I have learned something from this post myself. Awesome site.

 

[Phillip Nickles]

Chevrolet 38, your description of what you experienced sounds like you had a post combustion chamber explosion in the exhaust system due to a weak or a problem in the ignition system. The fact this happened above 4K rpm leads me to believe this may be true.
The most recent experience with this was on a 66 Pontiac Catalina with a 455 and electronic ignition. It would happen most often with headlights on but even without headlights.
The car was usually being driven sedately also.
It would backfire badly enough to blow out the baffles in the muffler.
The problem was traced to a poor connection at the bulkhead connector at the firewall. The circuit in question powered the headlights and ignition. Correcting this connection solved his backfire problem. The ignition system would stop working for an instant and load the exhaust And combust the unburned fuel in the exhaust when the ignition came back up.
I know a lot of people that read this forum have turned off their ignition, coast and turn on the ignition for a bang. Hahahaha