[bigblue66]

I'm trying to figure out how to fix a problem with detonation on my 66 Tempest. I've read a bunch of posts here on the topic and tried some things but still stuck. Some details:
- Stock(?) 389 w/Edelbrock 650 CFM 4bbl
- Just swapped out points for a Pertronix Ignitor - didn't really help
- Engine runs about 180-190 degrees
- Running 93 octance plus a bottle of 104 Octane booster

Main problem is pinging during even mild accelleration both from cruise and stopped. This gets a little worse as engine reaches operating temp of about 185. I just disconnected the vac advance from the distributor, plugged it and took a short drive - problem did not *seem* quite as bad.

Some Questions:
1. Why does the factory manual specify initial timing at 6 degrees BTDC? Many posts here say to start at 10 so that's about where I am now.
2. Compression check shows all cylinders at about 190psi - this seems high but I've not been able to contact previous owner to find out about cam swaps etc. Was just happy it was running at time of purchase and *had* plans to do a complete rebuild anyway.
3. Carb vacuum ports - Manifold vacuum vs. Ported vacuum. I swapped the vacuum advance and transmission vacuum hoses to test. The dist. vac advance should be on the carb's ported vacuum port right?
I checked these ports with a vacuum gauge:
- manifold port - relatively steady 18 in at idle
- ported port - 0 at idle but jumps to 18+ when revved
Bottom line is that swapping the dist vac advance to the ported vac source on the carb did not seem to help much.
4. Is the tranny vacuum line supposed to be connected to manifold vac or ported vac? When I switched it this didn't seem to matter but I never got above 40mph in my test drive.
My biggest problem is more than likely just a tired old 389 - it starts, idles and runs but I wouldn't say smoothly.

Thanks.

[Schurkey]

Trans modulator MUST be on manifold vacuum or you're going to have sky-high trans pressure at idle. Not a great plan in the winter--pressure goes so high it can crack the case.

Distributor can be on ported or manifold vacuum, whichever works best for you.

Depending on the centrifugal advance curve built-in to your distributor, 6* at idle may be plenty. What curve do you have? What is your total timing?

Exhaust heat riser valve stuck closed, or partly closed?

[form406]

maybe if that engine is running high compression 10.1 and/or over octane boost wont help much and you need to lower your compression..our gas sucks these days... jmo...

[johnsma22]

Your compression ratio is too high, and it is as simple as that. Every time you run that motor you are hammering your rod bearings into oblivion. Either satisfy the octane requirements of the combo you have now by running racing fuel, or change your combo and lower the CR, either by swapping heads or dishing the pistons, etc.

A cranking cylinder pressure of about 160-170 psi is about as high as pump gas will tolerate. It is not really a case of "bad" gas, it is more accurate to say that the modern fuel injected, electronic timing, variable valve timing autos of today do not require the higher octane fuels.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbon atoms chained together. Octo = eight.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

John

[Motor Daddy]

Awesome post John. Never heard it explained that way!

bigblue66, There are a few ways to deal with the problem.

1. Increase octane, or mix fuels.
2. Decrease compression ratio by either changing to a bigger combustion chamber, or dishing pistons.
3. Decrease the DYNAMIC Compression ratio by getting a cam that closes the intake valve later, which effectively reduces cylinder pressure.
4. Reduce spark advance until pinging stops.
5. Reduce load by either reducing mass, or increasing effective gear reduction.
6. Reduce cylinder temps by increasing the effectiveness of the cooling system.
7. Reduce intake air temps (cold air induction).
8. Reduce fuel temps.
9. Increase elevation (move to Colorado).
10. Use a combination of all the above!

What you are experiencing is Detonation. It is caused by the fuel spontaniously combusting creating another flame front after the plug is fired. With more than one flame front, cylinder pressure increases faster, and causes the peak pressure to occur MUCH sooner with respect to crank angle. When this happens, the engine is "fighting' itself and the loser is the head gasket, bearings, pistons, ring lands, resulting in SERIOUS MECHANICAL DAMAGE, AND DEATH!

[george kujanski]

One version of above mentioned death......

[mzbk2l]

Originally Posted by johnsma22

Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane).
John

Johnsma22, excellent info - I had also never heard it explained that way. Your explanation sounds logical, but how does that work above 100? (105, 108 octane, etc.) Not trying to be a smart-*ss; that question popped into my head when I read your post.

[Motor Daddy]

The visuals!

[Brad B. Hillebrand]

As a suggestion:

You might be running too lean....check out the plugs. Try to richen it up
Plugs might be too hot, move to a cooler plug...Domestic plugs = higher number is hotter plug. Overseas plugs =higher number is cooler plug....

Brad

[HWYSTR455]

Lean won't effect cranking pressures. Lower the CR, think it's obvious. You can use some of the new Cometic head gaskets to avoid a rebuild, just remember to use longer pushrods...

[Brad B. Hillebrand]

The thought is that he is checking compression with the engine running. The high pressure is caused by the detonation thats occuring. Fuel, timing and AFR and plugs could be the cause......I would think that the engine would be toast by now if CR was the problem, assuming the guy drove it before selling...just a thought....

[johnsma22]

Originally Posted by mzbk2l

Johnsma22, excellent info - I had also never heard it explained that way. Your explanation sounds logical, but how does that work above 100? (105, 108 octane, etc.) Not trying to be a smart-*ss; that question popped into my head when I read your post.

100 octane gas is gasoline that is 100-percent octane with no heptane or lead. During WWI, it was discovered that you can add a chemical called tetraethyl lead (TEL) to gasoline and significantly improve its octane rating above the octane/heptane combination. Cheaper grades of gasoline could be made usable by adding TEL.

Airplanes and off-road race vehicles are still allowed to use leaded gasoline, and octane ratings of 100 or more are commonly used in high-performance piston airplane engines and racing engines. In the case of AvGas, or racing fuels, ratings above 100 is the gasoline's performance rating, not the percentage of actual octane in the gas. The addition of TEL boosts the compression level of the gasoline -- it doesn't add more octane.

[74Grandville]

you didn't happen to just watch modern marvels with Gas being the topic.... I just saw that information on TV yesturday during lunch.

[Lloyd-TX]

Originally Posted by johnsma22

A cranking cylinder pressure of about 160-170 psi is about as high as pump gas will tolerate. It is not really a case of "bad" gas, it is more accurate to say that the modern fuel injected, electronic timing, variable valve timing autos of today do not require the higher octane fuels.

John

Have to disagree with you on your cranking compression limits, John.

As an example, my 467 CID 455HO in my blue Formula has regularly checked at 210 - 220 PSI cranking PSI, with IRON 7F6 cylinder heads, and runs fine on 93 octane pump fuel ONLY.

I've never experienced ANY audible detonation, spark knock, or "ping". Nor have I ever seen any evidence of detonation when examining my used spark plugs, etc.

Judiciously chosen engine components, zero decking the block, proper attention to details in the machining operations, and especially cam selection all mean MUCH more than the cranking compression numbers. BTW, my engine makes best torque and power at 32 degrees total ignition lead, all in by 2600 RPM. I also run a light spring vacuum advance, limited to 12 degrees of additional lead. This results in a total ignition lead of 44 degrees at cruise speeds and light throttle.

Obviously, my engine was machined and assembled with pump fuel quality in mind.

[johnsma22]

Originally Posted by Lloyd-TX

Have to disagree with you on your cranking compression limits, John.

As an example, my 467 CID 455HO in my blue Formula has regularly checked at 210 - 220 PSI cranking PSI, with IRON 7F6 cylinder heads, and runs fine on 93 octane pump fuel ONLY.

I've never experienced ANY audible detonation, spark knock, or "ping". Nor have I ever seen any evidence of detonation when examining my used spark plugs, etc.

Judiciously chosen engine components, zero decking the block, proper attention to details in the machining operations, and especially cam selection all mean MUCH more than the cranking compression numbers. BTW, my engine makes best torque and power at 32 degrees total ignition lead, all in by 2600 RPM. I also run a light spring vacuum advance, limited to 12 degrees of additional lead. This results in a total ignition lead of 44 degrees at cruise speeds and light throttle.

Obviously, my engine was machined and assembled with pump fuel quality in mind.

I agree with you 100% Lloyd. You obviously have the knowledge to pick the right components and keep your engine in a state of tune that will alow you to run with the numbers you have stated. However, I think that you would agree that the "average" hobbiest may not have the knowledge and resources available to them to run on the ragged edge. It is much safer for them to just be in the "comfort zone" and not have to worry about it. Wouldn't you agree?

John

[johnsma22]

Originally Posted by 74Grandville

you didn't happen to just watch modern marvels with Gas being the topic.... I just saw that information on TV yesturday during lunch.

No, I wish I had seen it. You can never have enough knowledge as it pertains to this hobby, and especially this subject.

I was so angry at the results of my ignorance on the subject of compression ratios, combustion chamber volume, and octane requirements when I had my original motor built about 10 years ago, that led directly to its demise from detonation, that I did a lot of research to get up to speed.

John

["Puddles the Pontiac"]

i have had the same issue on a past engine. i ran lead substitue for a while (don't know if it was really working) i changed to a colder plug and pulled alot of timing out. also used 93. though the best thing i ever did to fix my detonation, REBUILD THE MOTOR.

[Lloyd-TX]

Originally Posted by johnsma22

I agree with you 100% Lloyd. You obviously have the knowledge to pick the right components and keep your engine in a state of tune that will alow you to run with the numbers you have stated. However, I think that you would agree that the "average" hobbiest may not have the knowledge and resources available to them to run on the ragged edge. It is much safer for them to just be in the "comfort zone" and not have to worry about it. Wouldn't you agree?

John

I agree with you to some extent, John. However, if people will LISTEN and follow recommendations from the more knowledgable folks here, they too can assemble quite a nice engine package without too much chance of engine damage.

Forgot to mention that my engine has a calculated compression ratio of 9.98:1. We had JE custom make a set of pistons with a small dome to achieve that number with the large chamber 7F6 heads. For MOST people, might want to limit the static compression ratio to 9.5:1 with iron heads, or 10.25:1 for aluminum heads to provide some "cushion".

BTW, you give me WAY too much credit. Most of the credit for my engine must be given to fellow PY members, and to my cylinder head specialist and machinist, Dan Barton of Westside Performance here in Houston.

[HWYSTR455]

I believe it can be done, with 210-220 cranking pressure, and have done it myself. But I have to admit, it is the ragged edge. I think round ports are more forgiving too, and the Ds at the same CR suffer. Audiable detonation is one thing, but the inaudiable is what kills you in the long run (hammers rod bearings for one). Cam selection is critical once your there, along with weight and gear ratio. These days I tend to play it safe with CR, you don't loose enough power to justify high CRs. You narrow your cam selections when you drop CR, but it's acceptable, and when your in EBF, you can still get gas. Oh, and not sure who checks cylinder pressure with the engine running, haven't heard of doing that personally...

[HWYSTR455]

By the way, Lloyd-TX, my current D port engine is at 9.98 as well, run it on the street, it's my daily driver....