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300 inch amc class motor

Posted: Tue Nov 07, 2017 10:32 pm
by AMXstocker1
[*]we run a local series and currently run a 427 inch amc in a 3100lb car but after looking at rules they say combinations under 320 inchs have no minimum weight. The class limits us to factory block and crank but we can offset grind a crank and by taking a .030 to .040 over 360 block and offset grinding a 290 crank to 3.00 stroke we end up with a 3.00 x 4.12ish bore 317-319inch motor my questions is this to far over square or is there such a thing and i was considering the use of some 6.2 by 1.85 journal used nascar rods assuming they should live in Drag combination that will probably make less than 550 hp. But in a 2400-2500 lb car should run upper 9s i would think. This also puts the rod stroke ratio over the 2.0 mark which im not sure if thats anything to worry about or not. Does anyone have a guess on how high a motor like this would have to spin to make 550 i was thinking a compreasion ratio around 14.1 and a solid roller cam, i have no clue on cam specs but im sure mike jones could help with that. We are limited to a 950 cfm carb but that should be plenty on this combo. We also have sitting in the corner a 3.08 low jerico that would be fun behind a high reving small motor and probably easier on parts than behind something that makes some torque.

Re: 300 inch amc class motor

Posted: Tue Nov 07, 2017 10:45 pm
by Walter R. Malik
AMXstocker1 wrote:
Tue Nov 07, 2017 10:32 pm
[*]we run a local series and currently run a 427 inch amc in a 3100lb car but after looking at rules they say combinations under 320 inchs have no minimum weight. The class limits us to factory block and crank but we can offset grind a crank and by taking a .030 to .040 over 360 block and offset grinding a 290 crank to 3.00 stroke we end up with a 3.00 x 4.12ish bore 317-319inch motor my questions is this to far over square or is there such a thing and i was considering the use of some 6.2 by 1.85 journal used nascar rods assuming they should live in Drag combination that will probably make less than 550 hp. But in a 2400-2500 lb car should run upper 9s i would think. This also puts the rod stroke ratio over the 2.0 mark which im not sure if thats anything to worry about or not. Does anyone have a guess on how high a motor like this would have to spin to make 550 i was thinking a compreasion ratio around 14.1 and a solid roller cam, i have no clue on cam specs but im sure mike jones could help with that. We are limited to a 950 cfm carb but that should be plenty on this combo. We also have sitting in the corner a 3.08 low jerico that would be fun behind a high reving small motor and probably easier on parts than behind something that makes some torque.
Whatever the cylinder head capabilities are ... will govern this engine.

I built many Chevrolet, Trans-Am 310" engines in the early 90's which used 6.300" rods and a 3.00" stroke.
Don 't give the rod ratio a second thought.

Re: 300 inch amc class motor

Posted: Tue Nov 07, 2017 10:56 pm
by AMXstocker1
planning on using the heads were taking off our 427 they flow(with out going and getting the flow sheets) around 250 cfm at .400 and 300cfm at .700 don't have flow#s above that and the exhaust side is about 250@.700 2.08 and 1.72 valves went 10.30s on our 13.1 427 at 3260lbs.

Re: 300 inch amc class motor

Posted: Tue Nov 07, 2017 11:43 pm
by KnightEngines
Not a lot to add other than it sounds like a fun project.

+1 about don't worry about rod ratio.

Big bore short stroke = fun, don't stress about the oversquare - I'm doing some road race stuff at the moment with 3.25" stroke & 4.125" bore.

You'll need to turn peak power rpm around 8000 or so & it'll want to hang together out to 8800 or a tad more. Think lightweight valve train.
The heads will need to flow around 280cfm at the lift to be used (you could maybe get by with 270).

I can't see anything standing in your way other than $$ for light parts........

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 12:06 am
by AMXstocker1
the more i think about it aluminum rods are seeming like the way to go instead of used nascar stuff.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 12:08 am
by Diodedog
Use a 343, it has the 360 bore and the 290 crank.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 12:14 am
by AMXstocker1
we have 343 the crank has to be destroked either way the 343 has a little shorter deck height witch might help make the piston a little liter, the only problem is redoing the block for 1/2 inch head bolt instead of 7/16s

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 9:20 am
by Frankshaft
In theory, with all the same parts from your 427, your 320 inch deal should make the same peak horsepower, but do it 20+% higher in rpm. With the steep 1st gear jerico, and an appropriate gear spread and rear end gear, even at the same weight, it WILL be faster. See 600hp at 6200 vs 600hp at 7100 rpm thread, lol. With the lighter weight, it will be WAY faster. Rpm wins again. Sounds like a way cool deal.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 9:37 am
by Abbottracingheads
Put as much compression as you can get in it. You can't get too much in that little engine.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 12:14 pm
by Jeff Lee
Garrett Ghezzi’s NHRA SS/MA 290 AMX runs 10.50 - 10.70 ET’s with his 290. NHRA spec engine has approximately 10.5:1 compression, the little valves (1.78” intake???), roller cam, 625 CFM Carter and factory iron 304 heads. Somewhere around 3100#’s with driver.

As long as you can get your car to the weight you described, this is definitely a worthwhile project. I used to race a ’70 AMX in S/ST and it had fiberglass hood & trunk lid, gutted interior & lexan windows with 451” Mopar big-block (which is heavier than an AMC). It weighed 2850 w/ me in it so it probably takes some work to get around 2500#.

I would encourage you to get a AMC Victor EFI and convert it to 4150 mounting. Made +10 HP across the board on Ghezzi’s 290 and that was not even ported.

FYI - you can de-stroke a forged 390 crank to 290 specs. Expect to pay $350-$500 for machine work. 290 cranks were cast. They made a forged Group-19 290 crank but very rare.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 1:18 pm
by AMXstocker1
The amx is staying at its current configuration were having adkins port some sr heads for it and changing the cam we have a 70 hornet hatch back that already has a cage that were going to put a bunch of fiberglass and lexen in and see if it cant get light.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 2:28 pm
by PackardV8
In theory, with all the same parts from your 427, your 320 inch deal should make the same peak horsepower, but do it 20+% higher in rpm.
X2 - I did a recent computer simulation on the three different Studebaker V8s with the same bore but differing strokes, they all made the same horsepower, but the 289" did it at 6,500 RPMs, the 259" at 7,500 and the 224" at 8,500.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 10:03 pm
by Jeff Lee
PackardV8 wrote:
Wed Nov 08, 2017 2:28 pm
In theory, with all the same parts from your 427, your 320 inch deal should make the same peak horsepower, but do it 20+% higher in rpm.
X2 - I did a recent computer simulation on the three different Studebaker V8s with the same bore but differing strokes, they all made the same horsepower, but the 289" did it at 6,500 RPMs, the 259" at 7,500 and the 224" at 8,500.
Don’t those Studebaker engines have unusually thick cylinder walls? I know the AMC 290 does. If so, it would be interesting to juggle the bores and strokes around on your simulation but keep the same CID. Maybe there’s something in under-square, square, over-square? Not hijacking the thread here, it may prove relevant to the OP. Because if you have to grind a crank, nows the time to know.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 10:25 pm
by PackardV8
Jeff Lee wrote:
Wed Nov 08, 2017 10:03 pm
If so, it would be interesting to juggle the bores and strokes around on your simulation but keep the same CID. Maybe there’s something in under-square, square, over-square? Not hijacking the thread here, it may prove relevant to the OP. Because if you have to grind a crank, nows the time to know.
Tried that, but the computer programs rely on CFM. That has to be measured on a flow bench with the appropriate bore diameter fixture. Only if the change in bore diameter changes the CFM does the horsepower/RPM move much.

Re: 300 inch amc class motor

Posted: Wed Nov 08, 2017 11:24 pm
by Jeff Lee
Got it.