My car is a 1936 Ford Coupe with a fresh 8BA FLATHEAD. I am using Offenhauser 400 heads. I bought the engine rebuilt and do not know some of the part details used in the build. The pistons are Egge and stamped .125 bore. I do not know the stroke. The engine runs good but has detonation under load. The spark knock is bad. The clearance from the top of piston to the combustion chamber in the head is .22 thousandth per the plastic gauge. I have been on a wait list with Edelbrock for almost three months for a set of 1116 high lift 74cc heads. Apparently the company was sold recently and parts are way behind. I am considering using a pair of taller head gaskets from a company named Cometic. They have Fathead head gaskets up to 1.40 thousandths. My question is this. If the gap above the piston is a good reference to compression??Also if this is a good indication on compression ratio. What is a good gap to work towards to run the car on pump fuel?? I have seen a few post of .50. - .60 thousandth as the idea gap. Any help or info would be great Thanks! Sent from my iPhone using H.A.M.B.
Sounds like you MAY have answered your own question. Thicker gasket will lower the CR. Not sure about the old flatties but.....OHV engines need a certain amount of "squish"... and just jack'in up the head with a thicker gasket sometimes causes other problems as far as performance is concerned. I want to think(since the valves are NOT in the head) a flathead will perform different. I would go for it ...IF.....have you played with the advance in the dizzy? What about the thermostats? (cooler engines tend to spark knock less) High test gas make a difference? You lugg'in the engine? Lotsa stuff you could try before jack'in up the heads. Jus say'in 6sally6
Yes 50-70 Does you head gasket compress down to 70 ,copper best are 90 not compressed . I used clay balls or playdoughy to check head clearance with no gasket assuming the Best copper head gasket will compress down to 70 I had my iron 59ab milled down and held 70 clearance and the motor has a lot of pull for a stock 59ab with 8ab crank and cam .
Have you checked your timing curve? You may be wasting money on heads. I have the same heads with Ross domed .125 over Pistons and 4-1/8” stroke with no detonation. Flatheads do not like much timing, 22-26 degrees total. Very finicky about that and how quickly it comes on! Also check to see if you are running lean, maybe you are not getting enough from your power valve. I would be more suspicious of timing or fuel mixture than compression ratio. What carb and distributor are you running?
The detonation stops when running 110 octane. I think I might try the taller .140 head gaskets. We used a fuel mix meter to adjust the carbs. I have two new not rebuilt Real-deal Stromberg 97’s on an Offenhauser 2 x2 intake. The timing will not adjust out the problem. We have tried for days. ... it must be compression I guess... Sent from my iPhone using H.A.M.B.
I’m running 2 BIG 97’s (250 cfm each) on an Offy intake. Using an HEI distributor. .022” sounds tight, should be .050”. Are you checking the whole timing curve or just initial timing?
I do not think it is compression. I milled my heads on my 8ba for 20thousands piston clearacnce and it ran fine on regular gas and no knock.
I can't speak for the comp ratio you'llget but in order to maintain the "quench"needed your piston to head clearance should be in the .040 range. If that leaves you with too much compression then either relieve the blok or head between the valve pocket and the cylinder for improved airflow.
Checking the entire timing curve. I’m using a Mallory HEI distributor as well. It’s a real mystery. Trying the tall gaskets are the least expensive option to try next. I have access to 93 octane ethanol free gas locally. If it would run well on that I would be cool. Sent from my iPhone using H.A.M.B.
I have built two engines (a 255" Merc and a 258" Ford) over the last couple of years. I was careful to obtain a "squish" of .050 on both. Both run fine and are very crisp and have 160 lbs of compression. Perhaps you can use this for a comparison.
Thanks guys. The block is not relieved. It also does not appear to have been decked in the past. Compression is about 9:1. Maybe a little more. Sent from my iPhone using H.A.M.B.
Yes, that would be way high, but you're guessing. Again, what is the cranking compression? As long as cranking compression is not too high, less piston-to-head is better. As mentioned, squish is a good thing. The other factor is cam timing. Longer cams can help tolerate higher compression. What cam are you running? What total advance are you running and at what RPM is it reached? jack vines
Yeah, I don’t think it’s your compression ratio or Heads. You should be able to solve with tuning. According to the book “The 335hp Flathead” my combination is close to 10/1
Thanks Jack. I don’t know the cam. I know it is an Isky but not the model. The engine was bought assembled and rebuilt. I will plan to check the real cranking pressure and post it for more advice. Sent from my iPhone using H.A.M.B.
.140 head gasket thickness is way too much. The Offenhauser 400 heads need a lot of carving to get a good combustion chamber. Lay a head gasket on them and you will see what I mean. You probably have over 9:1 comprsssion.
I run a 304ci engine with 0.3 thousand and it doesnt detonate.....(yeah , tight... .4-.5 would be perfekt) Proper timing .... I bet ignition and maybe fuel also . while the heads are off, check the stroke and TDC mark on the pulley. 4 degrees at idle , 22 at 2000rpm Egge pistons are known for a different crown than the head shape. Did you measure just the center or also on the sides of the chamber. If the piston has a sharp point in the center and is ok on the sides, I would just grind the center down , carefully
Try NOT to cut the deck on a flathead. There in not much iron to cut and if you try to cut in excess, you might invite more problems! Flatheads Forever! John in s. Texas
I have a 276 with 2 English Stromburgs that had detonation problems. I tried every thing I could think of for timing ,and spark plug heat range. The only thing that stopped it was bigger jets. According to the A/F meter it is rich, but it runs good and plugs are good. John
Almost all the engines I build need a skim cut off the deck. Most never more than .006. It's impo rtant to have a straight deck to get good sealing.
Thanks so much guys for the great advice and input. Will keep trying to get it right. Sent from my iPhone using H.A.M.B.