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Megasquirt 258 install
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Mr_Roboto View Drop Down
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Joined: Jun/12/2022
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Points: 221 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/12/2022 at 8:50pm


Finding this forum had a post with a stock timing curve and an MSD dist, it is a huge win, it means I won't have to guess on the initial timing table but that the work is reduced to interpolation and data entry. VERY good. The only thing I may have to end up doing is pulling timing in the cruise ranges since I don't have EGR.
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farna View Drop Down
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Points: 19672 		Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jul/31/2022 at 3:10pm
On the bottom end, somewhere around 6000 rpm (5800-6200 range) you get a harmonic vibration on the AMC 232/4.0L (242)/258. All sixes have it somewhere in that range, though some of the smallish short stroke foreign jobs may bet closer to 6200 and possibly a bit more. Running THROUGH that isn't an issue. Back when AMC cars were running in IMSA (late 70s/early 80s) the teams would run up to 6500 rpm. The trick is you don't want to run AT that harmonic speed. There will be enough vibration to beat out the bearings. But once that speed is determined (depends on stroke and reciprocating weight -- so varies) you just go THROUGH it and gear the car accordingly. If that mark is 6000 on your engine, you can run it at 5800-5900 or 6100-6200.... just not 5950-6050 rpm (just to be safe...). Once over the harmonic speed it's good to keep on going for a while! Most builds don't go much over 5000 rpm. A little blip up into the harmonic won't hurt, just staying there will.
Frank Swygert
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Mr_Roboto View Drop Down
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Points: 221 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Aug/02/2022 at 11:05am
Originally posted by farna farna wrote:

On the bottom end, somewhere around 6000 rpm (5800-6200 range) you get a harmonic vibration on the AMC 232/4.0L (242)/258. All sixes have it somewhere in that range, though some of the smallish short stroke foreign jobs may bet closer to 6200 and possibly a bit more. Running THROUGH that isn't an issue. Back when AMC cars were running in IMSA (late 70s/early 80s) the teams would run up to 6500 rpm. The trick is you don't want to run AT that harmonic speed. There will be enough vibration to beat out the bearings. But once that speed is determined (depends on stroke and reciprocating weight -- so varies) you just go THROUGH it and gear the car accordingly. If that mark is 6000 on your engine, you can run it at 5800-5900 or 6100-6200.... just not 5950-6050 rpm (just to be safe...). Once over the harmonic speed it's good to keep on going for a while! Most builds don't go much over 5000 rpm. A little blip up into the harmonic won't hurt, just staying there will.


Honestly I'm more concerned that I'm getting borderline on the piston speed. The Jeep Strokers guys quote about 4K SFPM being the limit.

That said I should in reality probably be yet more concerned about valve train honestly. These have long pushrods and to get the stuff in order to turn that high will likely require some investment. Pushrods are one of those odd things that tend to be worth a few extra bucks to both measure and to buy good of. Then again that's a lot of valve train parts now that I've said it.
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farna View Drop Down
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Points: 19672 		Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Aug/02/2022 at 5:41pm
I don't know... stock pushrods have been used up to about 6500 rpm. I know the Team Highball guys would hit up to 7K for very short bursts, but typically ran under 6K.
Frank Swygert
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Mr_Roboto View Drop Down
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Points: 221 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Aug/02/2022 at 8:16pm
Originally posted by farna farna wrote:

I don't know... stock pushrods have been used up to about 6500 rpm. I know the Team Highball guys would hit up to 7K for very short bursts, but typically ran under 6K.

Any info on their builds out there? I'm presuming that may well be lost to the ages on a quick search. I'd be curious to see what they did to make things work together well and what the issues they had were.
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