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Old 01-15-2021, 01:44 AM   #1
Fortunateson
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Default 266 versus 276 CI?

Just about ready to order parts for my flathead build that I had an earlier thread on. My machinist says to only go .030 overbore which would give me around 266ci I believe. A lot of the flatties I read about are 276ci. So how much more horsepower or torque would the extra ten cubic inches provide?
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Old 01-15-2021, 03:16 AM   #2
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Default Re: 266 versus 276 CI?

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Just about ready to order parts for my flathead build that I had an earlier thread on. My machinist says to only go .030 overbore which would give me around 266ci I believe. A lot of the flatties I read about are 276ci. So how much more horsepower or torque would the extra ten cubic inches provide?
You would never be able to perceive the difference in the seat of your pants! DD
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Old 01-15-2021, 06:02 AM   #3
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Default Re: 266 versus 276 CI?

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You would never be able to perceive the difference in the seat of your pants! DD
Have to agree with that. I would pay more attention to good ignition and well tuned fuel system.
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Old 01-15-2021, 06:26 AM   #4
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Default Re: 266 versus 276 CI?

With a 3.217 bore and a 4" stroke (my assumption you're using a Merc stroke crank), the CI will be 260 cubes. As others have said, it will not make that much of a difference in the end - it is more important to have the right combination of parts that fit the chassis, vehicle weight, transmission, gearing and overall application. The most important aspects of the build should be compression, piston to cylinder head clearance (quench area), cam selection, intake manifold selection, ignition and exhaust. If you need any recommendations - just PM me or ask on this stie.

The good part of just a .030 overbore is that the block will have enough meat left for many more over-bores - assuming we all live long enough! LOL.

With that said, I tend to bore most of my engines to 3.312 (3 5/16) and run a 4" or 4 1/8" stroke crank - but I tend to go for more cubic inches as a goal.

Last edited by Bored&Stroked; 01-15-2021 at 06:32 AM.
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Old 01-15-2021, 06:29 AM   #5
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Default Re: 266 versus 276 CI?

Probably less than a 5 HP difference depending on other component choices and intended use.
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Old 01-15-2021, 10:13 AM   #6
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Default Re: 266 versus 276 CI?

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I'll take all of the free horsepower I can get. Seeing that .125" pistons are usually the same price as .030", it's an extra free 16 ci or 6% increase, which I would think would be noticeable.

As to future over-bores, I gotta believe .125" still leaves room for 2 over-bores, 4 hones, and finally 8 sleeves. That oughta be enough.
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Old 01-15-2021, 10:30 AM   #7
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Default Re: 266 versus 276 CI?

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I'll take all of the free horsepower I can get. Seeing that .125" pistons are usually the same price as .030", it's an extra free 16 ci or 6% increase, which I would think would be noticeable.

As to future over-bores, I gotta believe .125" still leaves room for 2 over-bores, 4 hones, and finally 8 sleeves. That oughta be enough.
In my latest build, we went 3.375 (3 3/8") + .030. Not by choice, but necessity. It was the 5th block we went through and the only good one.

It was already out to 3 3/8" when I got it. Ronnie diligently sonic tested each bore at 12-16 points depending on the readings. According to his findings, there would still be enough meat left for one more overbore in the future. This 59L block may have been a unicorn, but it goes to prove the importance of sonic testing the block when going big.
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Old 01-15-2021, 11:16 AM   #8
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Default Re: 266 versus 276 CI?

Thanks for all the replies. I started a thread on this last spring but am only now ordering the parts. (I usually have the parts shipped to my mail box across the line and pick them up there as it saves a bit on duties and taxes but with the border being closed and not looking like it will open anytime soon I'm going to order them from my local old car parts place.)

The engine will have the .030 overbore with a Merc crank, a Max 1 style cam, Eddie Meyer heads, Edmonds intake with three 97s, and be hooked up to a '39 trans with possibly LZ gears. All that going into a '32 5W.

I think it should be good enough for fun driving...
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Old 01-15-2021, 11:35 AM   #9
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Default Re: 266 versus 276 CI?

Unless you make other "performance" modifications to your engine you will not notice any difference. Here's the quickest, easiest and cheapest way to give your engine big cubes and bragging rights. (but, always tell the truth, if asked.)
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Old 01-15-2021, 11:45 AM   #10
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Default Re: 266 versus 276 CI?

How many HP do you reckon the "Speed-Bleeders" add?
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Old 01-15-2021, 11:50 AM   #11
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Default Re: 266 versus 276 CI?

I like everything but the Max-1 cam. Look for something a little more agressive.
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Old 01-15-2021, 11:50 AM   #12
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Default Re: 266 versus 276 CI?

Perhaps your machinist did a sonic test and determined there was enough core shift to make the recommendation he did. If it were me, I would listen to him.
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Old 01-15-2021, 12:20 PM   #13
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Default Re: 266 versus 276 CI?

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Perhaps your machinist did a sonic test and determined there was enough core shift to make the recommendation he did. If it were me, I would listen to him.
The key word here is "perhaps". It has been my experience that, unless experienced with flatheads, most of these guys do the great majority of their work on modern engines with thin wall castings, and are most comfortable with the smallest overbore possible. IF the guy sonic tested it and IF he found significant core shift, listen to him. Otherwise, go for the increased displacement; as I said, it is usually free. I would at least check it out as it would be valuable knowledge you should have.

These days, guys talk "superchargers and nitrous"; back in the day, it was "bored and stroked".
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Old 01-16-2021, 11:27 AM   #14
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Default Re: 266 versus 276 CI?

Back in the day, 3 3/8 was a standard overbore for a hotrod. Along with the 4 1/8 crank, you had a 3/8 x 3/8 engine or 286ci
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Old 01-16-2021, 08:24 PM   #15
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Default Re: 266 versus 276 CI?

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Originally Posted by Fortunateson View Post
Just about ready to order parts for my flathead build that I had an earlier thread on. My machinist says to only go .030 overbore which would give me around 266ci I believe. A lot of the flatties I read about are 276ci. So how much more horsepower or torque would the extra ten cubic inches provide?
There is no substitute for cubic inches. I went to 286, and love the torque.
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Old 01-16-2021, 09:06 PM   #16
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Default Re: 266 versus 276 CI?

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There is no substitute for cubic inches. I went to 286, and love the torque.
Isn't it amazing that some of todays vehicles with 1/2 the displacement of a flathead V8 will, in most cases, out perform and probably blow the doors off most flathead V8 Fords.

Last edited by 34PKUP; 01-16-2021 at 09:12 PM.
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Old 01-16-2021, 10:44 PM   #17
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Default Re: 266 versus 276 CI?

To be clear, there have been a couple of errors in calculating total displacement in this thread. The engine the O/P is describing (3 3/16"+.030" bore with a 4" stroke) is 260 ci, not 266 ci. The classic hot rod "3/8 X 3/8" flathead (3 3/8" bore and a 4 1/8" stroke) is 296 ci, not 286 ci.
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Old 01-16-2021, 11:20 PM   #18
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Default Re: 266 versus 276 CI?

Guess the old memory is getting forgetful. The engone I built for Ron Horell was that 3/8 x 3/8 with an L-100 cam 9:1Cr and it is impressive in a 35 PU.
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Old 01-17-2021, 02:11 AM   #19
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Default Re: 266 versus 276 CI?

BUT, a lot of the dirt track guys went with 3 7/16 x 4 1/8 for 306.

That was the engine to beat at Ascot, in the LA area.
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Old 01-17-2021, 09:07 AM   #20
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Default Re: 266 versus 276 CI?

And just think about it. They all had 91/21a Rods in them and Floating bearings.
Gramps
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