401 pistons

82Waggy

Depends on how much compression you want to end up with. Opening the chambers alone while using flat top pistons may still be a bit high. Unshrouding the valves and polishing the chambers usually brings the 58cc chamber out to around 65cc's. In my case, this in conjunction with the 28cc KB D-cup dish piston yielded around 9.1:1 static CR with just less than .045 quench. Using the 17.3cc flat top KB would yield just over 10:1 static CR in my engine. Not sure what you may have to pay to have your chambers opened up. I have right at 1100.00 in my pair of heads total, including parts and labor. I would highly recommend you have the chamber work done by an experienced person as it is possible to do more harm than good. As discussed numerous times it is possible to reduce combustion pressure in a high static CR engine by running a long duration cam with a late intake closing point, but you may not like the low rpm drivability characteristics of such a set up. I too have found donwags dyno sheet and build useful as a baseline reference and would be very satisfied in achieving similar results. I do hope to achieve slightly higher peak torque at a lower rpm with a wider curve by using MPEFI and a cam with slightly less duration, more aggressive lobe ramps, a little wider LSA, and a valve lift optimized to the flow characteristics of my heads - probably splitting hairs. I don't really expect to achieve a whole lot more than he did and probably won't spend the money to dyno and verify my engines output.

82Waggy

One other point to consider in regards to compression ratio - as a general rule of thumb a jump in compression by 1.0:1 is usually worth about a 4% increase in total output, assuming the engine can take advantage of it and detonation does not result. For a 400HP engine this is only a 16hp gain.

Holeshot

The way I understand it, "open-chambering" the head should consist of (at least):

*unshrouding the valves

*draw a circle around your combustion chamber using your head gasket as a template. Then follow the existing angle of the chamber and extend it out to the circle you have drawn, not sure what the step up to the flat of the head is on the edge.

The fresh 401 I recently bought (assembled) has open-chambered heads & the builder said that they are 72cc, as I recall, which he said lower the rated 13.5₁ pistons (w/51cc heads) to around 11₁ (I guess I'm too lazy to do the math myself, so I expect to hear from someone with corrections!!!).

This '72 401 is now in a '71 AMX (soon to have Richmond 5-speed) which will be used for Saturday night & Sunday sniping, not a daily driver. I have an R4B w/1050 3bbl, a CrossRam w/2-660s, an UltraRam w/2-660s, or a Torker with a Carburetor Shop 850++ Holley. Between the fully ported heads, dumping plenty of fuel in it (a lil extra to cool the chamber helps), & using a fairly large cam, & the .045" quench, I think I will be able to keep it from pinging. The block is already O-ringed & studded, so I could use a thicker copper gasket if needed. I am not opposed to mixing some 104 into the tank, it actually helps the mileage as well as running better. The outcome of that will determine whether I lower the CR a bit, which I really would rather mill the valve reliefs deeper than open up the quench. Then I would be free to go with more cam later, if desired.

My '69 may ultimately get the CrossRam for nostalgia sakes...

...but an UltraRam with a Bug Catcher scoop looks pretty cool...

Gremlin4ever

What do ya think? Got some .020 Ross pistons in the 10:1 ratio using 52cc (291c) heads and oh yeah, gapless second rings from total seal. I hope to build a "million mile motor",you know, Lowe's run, pull the car to the track sort of truck. Am I on the wrong path?

1980_Cj7

Gremlin4ever wrote
What do ya think?

Got some .020 Ross pistons in the 10:1 ratio using 52cc (291c) heads and oh yeah, gapless second rings from total seal.

I hope to build a "million mile motor",you know, Lowe's run, pull the car to the track sort of truck.

Am I on the wrong path?

Do you want it to run on pump gas?

82Waggy

Regarding opening up the chambers, most builders don't hog out to the bore edge much anymore, opting instead to maintain as much quench surface as possible. You can unshroud the valves a bit though, without hurting the quench surface too much. Polishing the chamber surface is helpful as well for detonation resistance. Doing these two things should get you close to 65cc's.

Gremlin4ever

1980_Cj7 wrote
Gremlin4ever wrote
What do ya think?

Got some .020 Ross pistons in the 10:1 ratio using 52cc (291c) heads and oh yeah, gapless second rings from total seal.

I hope to build a "million mile motor",you know, Lowe's run, pull the car to the track sort of truck.

Am I on the wrong path?

Do you want it to run on pump gas?

I thought that part explained it? If not then YES! :razz:

Or are you telling me an AMC can't run "pump gas" @ 10:1? I would think 93 octane would be enough :-|

82Waggy

Depends on the cam. Intake valve closing point after bottom dead center determines actual running cylinder pressure (Dynamic Compression). General rule of thumb is no more than 8:1 dynamic compression ratio for pump gas, less if you are towing. 10:1 static compression is generally considered high for any iron head motor on pump gas - especially if towing a load.

1980_Cj7

Gremlin4ever wrote
1980_Cj7 wrote
Gremlin4ever wrote
What do ya think?

Got some .020 Ross pistons in the 10:1 ratio using 52cc (291c) heads and oh yeah, gapless second rings from total seal.

I hope to build a "million mile motor",you know, Lowe's run, pull the car to the track sort of truck.

Am I on the wrong path?

Do you want it to run on pump gas?

I thought that part explained it? If not then YES! :razz:

Or are you telling me an AMC can't run "pump gas" @ 10:1? I would think 93 octane would be enough :-|

Sorry, I wasn't trying to sound "smart alecky" with that reply, but maybe it came off that way. We just finished a 401 with 9.8:1 ross forged pistons and had a pinging problem even on 94 octane pump gas. Turns out the cam was installed incorrectly though, so it's hard to say whether the pinging was caused by that, or the piston-cam combination we picked.

We ended up having bigger problems than the pinging though, and are now re-re-building the engine after only 300 miles. Fortunately we found the machine shop/engine builder screwed up on some things and so he says he'll make things right. Read about the whole fiasco in the "Collapsed Piston" thread here.

In the process of trying to figure out what caused our problems, we learned quite a lot from the guys here and other places. Read in that other thread about static and dynamic compression ratios. Like 82Waggy said, you gotta get the cam and pistons and everything working together. It's not as simple as we originally thought.

Gremlin4ever

No problem CJ7, I haven't figured out the cam I wish to run yet and can always use the "open chambered" heads as well to lower the CR. I've chosen the piston/head combo hoping to get into the 400 hp range on pump gas. Not to mention the 291c heads have the pushrod guides built in. The factory guides have mostly all broken, and the Iskys that are on the running motor have worn out as well. Have some of the guides from AMStyle that I haven't tried yet should I need to run the open chambered heads. Thanx for everyone's imput as I will read that other thread as well :lo1l:

Holeshot

the 291c heads have the pushrod guides built in

WTF are you talking about!!!

291c's have to have flats machined onto them to use pushrod guides!!!
The older style PR guides anyway, I guess the new style like MadDog sells don't need machining(?).

82Waggy

He's talking about the flat sided penetrations cast into the heads where the pushrods come through. Later heads had round holes.

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