The Ford Barn

Brian · 01-12-2018, 04:16 PM

I've calculated I'm running 9.7:1 compression approximately. 255cu in flattop pistons, 60cc chambers. 21 stud. running 95 octane [RON] champion D14 plugs [down two heat ranges from stock D16. Two [new English ]97's .043" main jets, power valves down two sizes from stock too, I think. Crab distributor, fully retarded, brake screwed a fair way down. Getting detonation. Any ideas from the gurus? Thank you, Brian

Brian · 01-12-2018, 05:05 PM

Isky 88 camshaft. Basically, I've been running a 21 stud bored .060" with the exact same components for a number of years. Has been a great performing combo. So, have built a fresh engine with bigger bore and stroke. Now, bad detonation. Maybe richen up all circuits?

supereal · 01-12-2018, 05:31 PM

Detonation arises from excessive advance. The vacuum brake snubs the edge of the point plate to reduce the advance provided by the flyweights under the plate. The flat springs under the plate determine how much and how fast the weights can move. Advance can be affected by the springs. Before you try to make changes, I would suggest the distributor be checked on a machine. The initial advance is fixed when the distributor is mounted to the timing gear cover without provision for adjustment, so all adjustment is done by the spring tension. In my stock car racing days we ran av gas and had to adjust this way.

Talkwrench · 01-12-2018, 07:14 PM

Brian try 98 for starters. Geez that's a lot of compression for a flathead �� back the timing off and yeah you might need 45's in there too, ahh you'll know what to do, just keep an eye on the plugs.

flatjack9 · 01-12-2018, 08:43 PM

What does the combustion chamber look like? I really think you're going to have to find a way to decrease the compression ratio or use high octane gas.

Bored&Stroked · 01-12-2018, 09:02 PM

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Check the plugs - though it is hard with today's gas to really understand what they're telling you. I'd richen it up a bit and see how it behaves. Question: WHEN do you see detonation? Partial throttle as you're putting load on it . . . full throttle . . . ??? If it is under partial throttle, then richen your main jets - if it is under full throttle, then change the power valve.

Keep in mind that the power-valve richness is BACKWARDS from the way you think it should work.

40 Deluxe · 01-12-2018, 11:30 PM

Did you measure the piston-to-head clearance? This dimension is critical when running higher compression. The ideal situation is to get the piston as close to the head as possible without hitting the head (.040" is a safe figure). This way, as the piston nears top dead center, it "squishes" the fuel mixture into the spark plug area of the chamber, with the resulting turbulence thoroughly mixing the fuel and air. This gives better power and fuel mileage, while reducing detonation.
If for some reason you're running true flattop pistons instead of the factory dome ones, your "squish" clearance is probably way too much.

Brian · 01-12-2018, 11:51 PM

Yeah man, custom made Ross flattop pistons with .012" popup above deck with steel/asbestos gaskets [compressed thickness .055"] which gives the ideal squish with the Eddie Meyer heads set up for flattop pistons [true flathead] lol.
Reading original Ford literature, Ford went to domed pistons to 'improve' squish...Now I know these old flattop pistoned pump in head motors pretty well, and I certainly acknowledge they are 'rattlier' than the later dome top piston versions.
I'm thinking the additional [excessive?] compression I'm running accentuates this rattling.
I've eliminated most of the knocking by fully retarding the ignition, have now fitted fatter main jets. Yet to roadtest. But, I want to run a bit more advance, where it's at is underpowered; smooth but hasn't got that that instant response it should have.

40 Deluxe · 01-13-2018, 12:08 AM

Quote:

Originally Posted by Brian

Yeah man, custom made Ross flattop pistons with .012" popup above deck with steel/asbestos gaskets [compressed thickness .055"] which gives the ideal squish with the Eddie Meyer heads set up for flattop pistons [true flathead] lol.
Reading original Ford literature, Ford went to domed pistons to 'improve' squish...Now I know these old flattop pistoned pump in head motors pretty well, and I certainly acknowledge they are 'rattlier' than the later dome top piston versions.
I'm thinking the additional [excessive?] compression I'm running accentuates this rattling.
I've eliminated most of the knocking by fully retarding the ignition, have now fitted fatter main jets. Yet to roadtest. But, I want to run a bit more advance, where it's at is underpowered; smooth but hasn't got that that instant response it should have.

Thanks for the additional data. Obviously you've already taken care of the obvious!

Talkwrench · 01-13-2018, 04:24 AM

Underpowered? Give it a chance to run in Brian ! : )

jrvariel48 · 01-13-2018, 06:00 AM

Do you have timing marks on the crank pulley? You can see where your timing is and check the advance with a good timing light for a starting point.

john in illinois · 01-13-2018, 08:03 AM

I have a 276 moderate compression engine, 2 new 97's. I experienced detonation at cruise with .043 jets.
I changed power jets and distributors and timing and this did not help. I put a A/F meter on it and it was lean at higher speeds.

I then read JWL's section on tuning 2 97 on a stock engine. He found .046 jets worked, and discussed the lean at high speeds with Strombergs. I tried this and no detonation and pretty good A/F with .046 jets. I still have to fine tune it after winter.

John

rotorwrench · 01-13-2018, 11:01 AM

To get more than 9.1:1 with the old style aftermarket or 81A heads, the engine usually has to have a stroked crank. Has this thing got more than 4-inch stroke? Those old 21 studs don't have much room in there for more than a 4-inch stroke. Kind of limited on bore too.

BUBBAS IGNITION · 01-13-2018, 12:12 PM

After testing hundreds of crabs on a test machine they have the best advance action Ford ever offered. They may get nasty and slow down a bit due to stiff grease etc but mostly will give ya 11 degrees at 2500 (24 engine) everytime.

With your combo of parts the compression is way too high and the air fuel is too lean.....

Put some fuel in that engine !!!!

Brian · 01-13-2018, 02:11 PM

Thanks boys, have fitted 46 jets and advanced dizzy back up, much better- almost there. Will get a few miles on motor today [only done 120 so far], then get into final tweeking. Thanks again.

Pete F · 01-14-2018, 10:36 AM

Quote:

Originally Posted by Brian

Isky 88 camshaft. Basically, I've been running a 21 stud bored .060" with the exact same components for a number of years. Has been a great performing combo. So, have built a fresh engine with bigger bore and stroke. Now, bad detonation. Maybe richen up all circuits?

Brian
When you put the cam in did you degree it.
The reason I’m asking is when I built my engine and degreed the cam it was 15 degrees off and it was a Cloyes gear set.
If the cam is advanced that would cause the compression to go even higher .

Sent from my iPhone using Tapatalk

Brian · 01-14-2018, 11:12 AM

Pete F, yes, I degreed cam in

Bored&Stroked · 01-14-2018, 12:45 PM

Hey Brian: Where did you see the detonation? On part throttle when you started to put some gradual load on it? Or, under full throttle? Have you checked the power valve numbers/settings as well?

drolston · 01-14-2018, 01:16 PM

How did you even get to 9.7 compression in a 255 cube motor? Conventional wisdom is that compression higher than 8.5 is diminishing returns on power, and prone to detonation.

Champion D17 plugs have .5" thread reach. Your plug threads are probably protruding into the combustion chamber. The sharp edges can get too hot and contribute to pre-ignition.

Maybe try doubling the head gaskets to lower compression ration.

01-12-2018, 04:16 PM	#1
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	detonation I've calculated I'm running 9.7:1 compression approximately. 255cu in flattop pistons, 60cc chambers. 21 stud. running 95 octane [RON] champion D14 plugs [down two heat ranges from stock D16. Two [new English ]97's .043" main jets, power valves down two sizes from stock too, I think. Crab distributor, fully retarded, brake screwed a fair way down. Getting detonation. Any ideas from the gurus? Thank you, Brian __________________ Unfortunately, two half wits don't make a whole wit!

01-12-2018, 05:05 PM	#2
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: detonation Isky 88 camshaft. Basically, I've been running a 21 stud bored .060" with the exact same components for a number of years. Has been a great performing combo. So, have built a fresh engine with bigger bore and stroke. Now, bad detonation. Maybe richen up all circuits? __________________ Unfortunately, two half wits don't make a whole wit!

01-12-2018, 05:31 PM	#3
supereal Senior Member Join Date: Jul 2014 Location: Iowa Posts: 1,634	Re: detonation Detonation arises from excessive advance. The vacuum brake snubs the edge of the point plate to reduce the advance provided by the flyweights under the plate. The flat springs under the plate determine how much and how fast the weights can move. Advance can be affected by the springs. Before you try to make changes, I would suggest the distributor be checked on a machine. The initial advance is fixed when the distributor is mounted to the timing gear cover without provision for adjustment, so all adjustment is done by the spring tension. In my stock car racing days we ran av gas and had to adjust this way.

01-12-2018, 07:14 PM	#4
Talkwrench Senior Member Join Date: Aug 2012 Location: Perth, Western Australia Posts: 2,687	Re: detonation Brian try 98 for starters. Geez that's a lot of compression for a flathead �� back the timing off and yeah you might need 45's in there too, ahh you'll know what to do, just keep an eye on the plugs. __________________ *"Came too close to dying to stop living now!"*

01-12-2018, 08:43 PM	#5
flatjack9 Senior Member Join Date: May 2010 Location: Oshkosh, Wi Posts: 4,527	Re: detonation What does the combustion chamber look like? I really think you're going to have to find a way to decrease the compression ratio or use high octane gas.

01-12-2018, 09:02 PM	#6
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,068	Re: detonation Sponsored Links (Register now to hide all advertisements) Check the plugs - though it is hard with today's gas to really understand what they're telling you. I'd richen it up a bit and see how it behaves. Question: WHEN do you see detonation? Partial throttle as you're putting load on it . . . full throttle . . . ??? If it is under partial throttle, then richen your main jets - if it is under full throttle, then change the power valve. Keep in mind that the power-valve richness is BACKWARDS from the way you think it should work.

01-12-2018, 11:30 PM	#7
40 Deluxe Senior Member Join Date: May 2010 Location: now Kuna, Idaho Posts: 3,779	Re: detonation Did you measure the piston-to-head clearance? This dimension is critical when running higher compression. The ideal situation is to get the piston as close to the head as possible without hitting the head (.040" is a safe figure). This way, as the piston nears top dead center, it "squishes" the fuel mixture into the spark plug area of the chamber, with the resulting turbulence thoroughly mixing the fuel and air. This gives better power and fuel mileage, while reducing detonation. If for some reason you're running true flattop pistons instead of the factory dome ones, your "squish" clearance is probably way too much.

01-12-2018, 11:51 PM	#8
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: detonation Yeah man, custom made Ross flattop pistons with .012" popup above deck with steel/asbestos gaskets [compressed thickness .055"] which gives the ideal squish with the Eddie Meyer heads set up for flattop pistons [true flathead] lol. Reading original Ford literature, Ford went to domed pistons to 'improve' squish...Now I know these old flattop pistoned pump in head motors pretty well, and I certainly acknowledge they are 'rattlier' than the later dome top piston versions. I'm thinking the additional [excessive?] compression I'm running accentuates this rattling. I've eliminated most of the knocking by fully retarding the ignition, have now fitted fatter main jets. Yet to roadtest. But, I want to run a bit more advance, where it's at is underpowered; smooth but hasn't got that that instant response it should have. __________________ Unfortunately, two half wits don't make a whole wit!

01-13-2018, 04:24 AM	#10
Talkwrench Senior Member Join Date: Aug 2012 Location: Perth, Western Australia Posts: 2,687	Re: detonation Underpowered? Give it a chance to run in Brian ! : ) __________________ *"Came too close to dying to stop living now!"*

01-13-2018, 06:00 AM	#11
jrvariel48 Senior Member Join Date: Jan 2012 Location: New Jersey Posts: 1,426	Re: detonation Do you have timing marks on the crank pulley? You can see where your timing is and check the advance with a good timing light for a starting point.

01-13-2018, 08:03 AM	#12
john in illinois Senior Member Join Date: May 2010 Location: Illinois Posts: 2,184	Re: detonation I have a 276 moderate compression engine, 2 new 97's. I experienced detonation at cruise with .043 jets. I changed power jets and distributors and timing and this did not help. I put a A/F meter on it and it was lean at higher speeds. I then read JWL's section on tuning 2 97 on a stock engine. He found .046 jets worked, and discussed the lean at high speeds with Strombergs. I tried this and no detonation and pretty good A/F with .046 jets. I still have to fine tune it after winter. John Last edited by john in illinois; 01-13-2018 at 08:08 AM.

01-13-2018, 11:01 AM	#13
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	Re: detonation To get more than 9.1:1 with the old style aftermarket or 81A heads, the engine usually has to have a stroked crank. Has this thing got more than 4-inch stroke? Those old 21 studs don't have much room in there for more than a 4-inch stroke. Kind of limited on bore too.

01-13-2018, 12:12 PM	#14
BUBBAS IGNITION Senior Member Join Date: May 2010 Location: SPEEDWAY INDIANA Posts: 4,148	Re: detonation After testing hundreds of crabs on a test machine they have the best advance action Ford ever offered. They may get nasty and slow down a bit due to stiff grease etc but mostly will give ya 11 degrees at 2500 (24 engine) everytime. With your combo of parts the compression is way too high and the air fuel is too lean..... Put some fuel in that engine !!!! __________________ If it Makes Spark, we do it !!!! www.bubbasignition.com [email protected]

01-13-2018, 02:11 PM	#15
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: detonation Thanks boys, have fitted 46 jets and advanced dizzy back up, much better- almost there. Will get a few miles on motor today [only done 120 so far], then get into final tweeking. Thanks again. __________________ Unfortunately, two half wits don't make a whole wit!

01-14-2018, 11:12 AM	#17
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 3,834	Re: detonation Pete F, yes, I degreed cam in __________________ Unfortunately, two half wits don't make a whole wit!

01-14-2018, 12:45 PM	#18
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,068	Re: detonation Hey Brian: Where did you see the detonation? On part throttle when you started to put some gradual load on it? Or, under full throttle? Have you checked the power valve numbers/settings as well?

01-14-2018, 01:16 PM	#19
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Re: detonation How did you even get to 9.7 compression in a 255 cube motor? Conventional wisdom is that compression higher than 8.5 is diminishing returns on power, and prone to detonation. Champion D17 plugs have .5" thread reach. Your plug threads are probably protruding into the combustion chamber. The sharp edges can get too hot and contribute to pre-ignition. Maybe try doubling the head gaskets to lower compression ration.