My mechanic is settling up the fresh 8BA in my 36 Ford. This is a professional shop that builds higher end cars. They do not deal with Flathead Fords on a usual basis. They are telling me that my engine has a slight spark knock under acceleration. It has two brand new Stromberg 97 carbs. After adjusting the timing I’m being told the spark knock disappears when burning 110 octane gas. They are suggesting I run this fuel all the time. I can’t believe a Flathead needs race fuel. It does have high compression offenhauser Heads. Also it has a Mallory electronic distributor. Could this be right?? Or maybe just a timing issue. Sent from my iPhone using H.A.M.B.
Flatheads will be happy with around 24* total advance. I’ve seen it mentioned that the Mallory distributors for 8BA come from the factory with a Chevy advance curve. You may have too much centrifugal advance, and when combined with the initial timing you are setting you get the knock. Get the distributor curve checked and reset (reduce the mechanical) if necessary.
That looks like a new Mallory. Anecdotal evidence says they come from Mallory with a Chevy advance curve, too much for a flathead. Send it to GMC Bubba and have it recurved for a flathead. Flatheads don't need as much advance as a Chevy. www.bubbasignition.com
I also would not trust that distributor without seeing it tested on a distributor machine. If it is in the chassis, should be easy enough to check the advance with a timing light. As noted, advance should be no more than 24 deg and a little less is better. All advance should be in by 2000 rpm. I can't see any way that it would require 110 octane. Most flatheads run just fine a regular grade gas.
I bought the engine as a rebuild never fired. I don’t know the compression ratio. Sent from my iPhone using H.A.M.B.
Ah yes, the old pig in a poke story. I, myself, would ask the previous owner of said engine if any milling or block decking was done by the machinist. Other ideas come to mind, but first things first.
I was told it was a 276 ci. The pistons are stamped .125. Here are some pics. Sent from my iPhone using H.A.M.B.
The ignition advice around 23-25 degrees TOTAL is spot on. My flatheads( I have had many big bore—big stroke) all run great on 87 or regular grade gas. Even my “race” car that ran high 14’s at 90 plus mph ran on regular 87 octane gas! Flatheads Forever!
Get out the timing light, mark off about 30 degrees in 5 degree lots on pulley (dabs of typewriter correcting fluid are a quick way) and test. I've posted quick ways to mark off a pulley accurately in my finding TDC articles. Plug in a tunep tach...very roughly you want to see timing moving up briskly right off idle and maxing out at numbers noted above. I think you are going to see it whizzing right on by those numbers and still climbing at 30 or something... Ron on Ford barn has posted a lot on timing if you want more. Your poor shop is used to seeing engines with timing curves designed for the engine...you probably have a hatful of whatever springs and weights were lying on the floor when your distributor went together. Figger out how to fix it, send it to our local distributor guru GMC, whatever or buy a converted Chevy from someone competent. While messing around, IMMEDIATELY DISCARD any Mallory points made before they went to hell (1975?) because they are going to leave you walking in about a thousand miles. Do note that timing is important (yours may actually be destructive!) and getting it right is a performance/response mod you can really feel.
I have limited experience with flatheads but I think your high end shop is just high. I will say from my extensive reading on the subject that detonation issues are the flatheads worse enemy, followed closely by those 3 cheesy main caps and a tendency to run hot. When I assembled my motor I went for a maximum 8:1 compression and a maximum 8 pounds of boost from the 471. To prevent detonation problems and maximize timing I used a MSD electronic distributor and a correlating 6-BTM box. The blower timing module box automatically retards if detonation is detected, if it misses the ping the box comes with a dash mounted control that allows for another 12 degrees of retard via manual override. 92 octane fuel, 350 HP from an 8BA. No ping Turn down your timing, monitor operating temperature, check your dwell on your points, find a different garage. 110 octane?
After some investigation it turns out the heads on the engine are Offenhauser 400 polished type high compression heads. My research shows that combustion chamber would be about 54cc. By comparison I am reading that a stock cast Iron head has about 76cc I guess this explains the spark knock. With these heads the timing must be set back to 14 degrees to stop the detonation or spark knock under hard acceleration. Not good. The block does not appear to have been altered or radically decked. Check out the pics. I’m thinking the only solution would be a new set of edelbrock 74cc heads from Speedway. Please know that the numbers as far as cc are just a guess What do you guys think ???? Sent from my iPhone using H.A.M.B.
I'm in the camp of those who think the problem is in the distributor and timing curve. If the pistons are not kissing the heads, I'll buy 'em (at a reasonable price, of course) if you insist on replacing them.
How do you know that you're at a total of 14 degrees? I can't imagine having to run that low of timing - as I've ran quite a bit more than that with a blower and small chambers (6 lbs boost). My guess is that your advance curve is incorrect (too much) and/or that it is incorrectly jetted and running lean. What can you tell us about how you're measuring initial static timing and "all in" timing? Also, noticed something in the picture of the cylinders - has me a bit curious. Hopefully I'm just imagining things . . . but check it out a bit and let us know. Notice on one cylinder how the upper ring-wear pattern stops more than 1/2" from the top of the cylinder - I've never see this . . . unless there is something wrong with the rings on this piston, or the cylinder was not bored/honed correctly. You should see the ring wear/polish pattern all the way up to about 5/16" from the top of the bore (like on the other cylinder). How are the rest of the cylinders wearing? How long has this engine ran since it was rebuilt?
Take it to someone who does not build high end cars. Get your timing and jetting right and that car will run on panther piss.
The engine only has a few hours on it but was built by a local reputable builder. I will look into the other cylinders for wear. Sent from my iPhone using H.A.M.B.
The 400's I have checked are generally around 60 cc's as cast. Should give you a compression ratio of around 9.2:1
That sounds about right in line with what they calculated. The shop is suggesting 8:1 max. Edelbrock has a 74 cc head that should come out to be below the max that will solve detonation issue and run pump gas. I never saw this problem coming , they checked the TDC to ensure accurate timing. It is within 2 degrees. Super confused about this. Thanks so much for all the info!! Sent from my iPhone using H.A.M.B.
I have a 276 with Edelbrock 74cc heads. I will still knock if too lean or if timing is off. I agree put on a correct distributor and try stock .045 Stromberg jets before you spend money on new heads. John
there is a little more to building a flathead that won't ping than jetting and timing. remember those horrible smog engines from the 70's that always pinged?? that was because of horrible combustion chamber design. if you don't make sure your piston squish is set at about .050 you are enlarging the combustion chamber flame front too much and adding to your problems. keeping this distance low will allow your motor to resist detonation. bruce and bored&stroked have built and worked on more flatheads than most guys on this forum. listen to what they have to say. Hell, I've bugged Dale a few times when I put together my mystery rebuilt motor and I'm certainly glad I did! take your time and do the things that they say are important. I setup my older mallory with a curve that bruce suggested years back and it's run great! my piston clearances are set to Dale's recommendations. get that distributor on a sun machine and dial it in or you will be just parts swapping. set the carbs up correctly. do you know what's in them now?? idle jet settings?? are they rebuilt or just new gaskets?? sync'd correctly?? oh! one more thing, the timing mark on a late flathead is two degrees advanced not top dead center, hence your "within two degreees" comment.
Wow, their stuff is elaborate. Can one of these knock sensors just be bolted to the engine and wired to an idiot light? My hearing is toast and I have 'imaginary detonation syndrome'.
No, the knock sensor sends a very low, very short voltage pulse for each knock it detects to the computer controlling the ignition, nowhere near compatible with an idiot light. The knock sensor has a piezoelectric element within it that is "tuned" to the frequency of the engine knock.
i'm not familiar with the sensors, but my understanding is that engine knock in general is a standard frequency?? basically when standard combustion happens it is at "x" frequency but spark knock happens at a much higher frequency which is basically standard across gas engines?? Am I correct in my understanding?? I like to verify these things as I just bought a M112 eaton supercharger that i'm planning to bolt on to a flathead at some point. it will be a mild boost but I want the ignition to help make sure nothing bad happens....
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