I'm having a issue with my car detonation at WOT. If you ease into the second carb it fine, it's only when you stomp on it. I have a 428pi with bk/bj carbs and the cam below. Dual point mechanical advance only dist. 35* total dwell. 6* intial timing/32* all in timing. C7AE factory ford cast iron heads. Less than 5% leak down, 230-240- psi compression numbers. Idels currently at 17" of vacuum at 850 rpms. 3.50 rear gear if important. Currently running 92 octane pump gas. Didn't know if the fuel is the issue and should be running 106 or a carb issue or something else. Any suggestion would be appreciated. Compression is about 10:5.
Thanks
Rich Herr
cam specs..jpg
You forgot the important part. How much compression are you running?
Roy
I would like to know what octane fuel you are currently using besides the compression ratio previously asked for.
The compression is about 10:5 and running 92 octane from the pump.
Although I have stated previously that compression testing numbers really are not a good determinant of the actual "measured" or "mechanical" compression ratios, for a number of reasons, but if your instrumentation and testing process was at least somewhat of the norm, those values of "230-240 psi" seem somewhat high for what one might think is a "10.5" example, at least "dynamic" wise, or being compatible with "pump gas". :-\
Scott.
Quote from: pbf777 on April 18, 2025, 05:38:19 PMAlthough I have stated previously that compression testing numbers really are not a good determinant of the actual "measured" or "mechanical" compression ratios, for a number of reasons, but if your instrumentation and testing process was at least somewhat of the norm, those values of "230-240 psi" seem somewhat high for what one might think is a "10.5" example, at least "dynamic" wise, or being compatible with "pump gas". :-\
Scott.
I was thinking the same thing Scott. I was thinking he had more like 12 to 1 compression ratio.
Roy
You can doctor your gas with some race fuel to see if it makes a difference. If it makes a difference you can either keep race fuel on hand or use what I do and instead use a octane booster to doctor my gas on the cars that need it.
http://www.wildbillscorvette.com/OctaneSupreme01.htm
Thanks for that link, Bob! I used REAL LEAD product years ago with great results. Lost track of it, but now found.
Use caution when handling this leaded product.
Rich,
One important thing to do is read the sparkplugs. Might also suggest a Dyno run to watch it under load to monitor air fuel and timing. Hope this helps.
R.R.
This is the octane booster that I use:
https://www.summitracing.com/parts/MEF-100132
Unless your head has hardened seats, then get some lead substitute to put in also.
Roy
I raced a lot of FE´s and also I have driven them on normal roads and in my opinion all FE´s need 95 Octane due to the head design + piston clearance between the heads at top dead center. Thats also the reason why spark plug reading is not really effective as they usually should be grey when the engines ran perfect. I measured a lot of FE´s on the Road and adjusted them and when I even went to safe rich settings the spark plugs were white-grey which has to do with the combustion chamber setup from the factory.
For your problem it would be good to know if the popping comes when you are at normal throttle and then hit the pedal hard to the floor which would tell me your second carb is reacting to fast. Maybe it's also just fuel as I said I never would go under 95 with these engines. Also please take a close look to your ignition is there enough voltage at the Coil+Distributor and at the end check your spark plug wires and more important the spark plugs and gaps. Sometimes just spark plugs can cause this problem. I only use NGK since 25 years.
Kent, do you know how ninety five octane in Germany compares to ninety two octane, here in the united states?
Roy
Thank you to all. I will test it out and see if it solves the issue.
Rich
Quote from: Rich Herr on April 18, 2025, 05:12:50 PMThe compression is about 10:5 and running 92 octane from the pump.
We usually get our fuel at two sites bear our home
Noticed similar issues at one if the sites that we get what we thought was 93 octane 100% of the time
On a late Friday night coming back from an event a tanker was dropping a fuel delivery.
I noticed he had two hoses leading from the truck to the ground
Got friendly with the driver how you doing tonight, how long in the job, you like it, what dont you like.
As he was dropping 9,000 gallons off
Asked him what was he dropping, he pulls out the invoice and says looks like all regular 87 octane
problem was that one the tanks was the 93 super unleaded I was getting
Called the state and reported them, find out they have had many incidents of misfueling at this site.
Lower octane that posted on the pump
Guess were I don't purchase fuel anymore
The new cars with the computer controlling everything are more forgiving our older cars are not , they ping or diesel on with low octane fuels.
So when you think your getting 93 octane sometimes you are not
Best of luck
Certainly it could be one of many reasons as mentioned.
It might help if you could determine when you are detonating.
Many engines detonate at peak tq rpm because they are advanced curved for total advance at peak horsepower rpm. Maximum cylinder pressure is at max tq rpm. Max HP is later and it requires more spark advance.
For example, using generic numbers, a stepped curve, set to say 35 degrees at peak tq rpm and then advancing at a rate similar to 1 degree/1000 rpm to peak hp rpm, to say 38 degrees, is how some tailor around peak tq rpm detonation.
This used to be considered exotic/expensive tuning, but advances in things such as a programmable MSD (cleverly hidden) have brought this into the realm of the 'small guy'.
I'd suggest running the timing events of that camshaft through the "dynamic compression" calculation.
My initial impression is that you need to go to more camshaft to bleed off cylinder pressure. In other words, your cam is too mild and creating static compression that the fuel octance cannot handle.
exactly Shelbydoug thats what I also tried to explain in my school English. Regards to the 95 octane question I mean 95 octane US not EU in EU it would be similar to 98 I think.
First, although "Octane-Booster" is convenient and quite popular in the effort to better the anti-knock quality of a lesser than needed fuel, I (and many others) really don't recommend its' use beyond 'emergency' situations were for example you find yourself a long way from "good" fuel and it can be conveniently stored in the vehicle for such instances. Better, is to utilize 'actual' fuel that presents the necessary octane value, even as mixed with the poorer fuel from the pump. :)
As far as for what it might be as pumped into your vehicle, regardless of that which you might have selected at the pump, this is not a new issue! As I have worked at a gas station at one time and have known both others of the same, owners of establishments, and even the tanker delivery drivers, and I can assure you that "93" ain't always just that! ;)
Scott.
As far as for the camshaft timing, yes, that profile as outlined is definitely on the "conservative" side, and although may not necessarily be 'the' culprit alone, it certainly is intended to create peak cylinder pressures at the lower revolutions, in the application presented. :)
Now as for whether it's the "wrong" cam or not, this has more to do with what one might consider as acceptable drivability characteristics within the intended engine revolution range; and it could perhaps be just the right camshaft, just not coupled to the surrounding engineering that permits such with said fuel. :-\
Scott.
Quote from: pbf777 on April 21, 2025, 11:38:19 AMFirst, although "Octane-Booster" is convenient and quite popular in the effort to better the anti-knock quality of a lesser than needed fuel, I (and many others) really don't recommend its' use beyond 'emergency' situations were for example you find yourself a long way from "good" fuel and it can be conveniently stored in the vehicle for such instances. Better, is to utilize 'actual' fuel that presents the necessary octane value, even as mixed with the poorer fuel from the pump. :)
As far as for what it might be as pumped into your vehicle, regardless of that which you might have selected at the pump, this is not a new issue! As I have worked at a gas station at one time and have known both others of the same, owners of establishments, and even the tanker delivery drivers, and I can assure you that "93" ain't always just that! ;)
Scott.
So you are saying is given the owner doesn't want to rebuild his engine with lower compression different cam change timing etc. and it is determined that the detonation is the result of too low of octane that keeping a 55 gallon drum of race fuel to doctor your tank is ideal. The octane booster would be the next best thing. If this is not what you meant then please clarify.
Quote from: pbf777 on April 21, 2025, 11:51:54 AMAs far as for the camshaft timing, yes, that profile as outlined is definitely on the "conservative" side, and although may not necessarily be 'the' culprit alone, it certainly is intended to create peak cylinder pressures at the lower revolutions, in the application presented. :)
Now as for whether it's the "wrong" cam or not, this has more to do with what one might consider as acceptable drivability characteristics within the intended engine revolution range; and it could perhaps be just the right camshaft, just not coupled to the surrounding engineering that permits such with said fuel. :-\
Scott.
You can make it as complicated as you want to. For modern gas, you need a modern cam ground for it. That one is ancient history for a station wagon.
Thank you for all the information. After reading comment and going back to engine builder, the compression was not measured but determined it was way more than 10:5 as many commented. I found 100 octane fuel near me and that fix the issue. I appreciate all the advice.