What is the limiting factor on mopar flathead rpm?

[Ajgkirkwood]

Just curious to know why our flatheads have low rpm (below 4000) is it connecting rod stroke? The rod moves to fast because of stroke length? Valve springs being too weak causing valve float? Motor can not breath at any higher rpm?

[martybose]

All of the above, and probably more.  I suspect crankshaft and block strength would keep you from going too far with supercharging them.

 

Marty

[pflaming]

Torque! Vehicles had to climb hills which were knee deep in snow or ankle deep in mud. Very little of rural America had graveled roads let alone paved ones. It all boiled down to power at slow speeds. Few drove more than 40 miles in one direction. Because steam engines required water and coal every six or eight miles, few lived more than 15 miles from the grocery store, gas station or Doctor. I grew up on the edge of the great sand hills of Nebraska. We were at the end of the road. East and south of us were thousands of acres of grass land. I never knew a neighbor to the east or south.

 

So the answer is torque!

[Plymouthy Adams]

mainly it is because folks afraid to rev them up cause they sound too busy,...

[pflaming]

1 minute ago, Plymouthy Adams said:

mainly it is because folks afraid to rev them up cause they sound too busy,...

 

True, we have become accustomed to quiet cabins, the reality of driving and shifting per the sound of the engine is a lost reality. 

[Adam H P15 D30]

They will run all day at 3600 rpm. I cruise mine at 2800 which is over 70 mph. With the old gears I cruised at 3200 and never had any issues. Long stroke = high piston speeds  limiting rpm

 

Adam

Edited September 26, 2018 by Adam H P15 D30

[maok]

Question is, why would you need to or want to? 

 

[Ajgkirkwood]

I don’t really want to go past it’s limit. Bringing up what Adam said higher piston speeds is what I thought. Wonder if I should get a block with a shorter stroke. Just with driving on the highway I was afraid I was running at its limit going 65mph. However it seems I can go to 75mph without issues (I go to 75 then I back off) I once went to 85!  So the crank and block can’t handle much past it?

 

as for why I want to go past. Well I want to build a hot flathead 6. I need to rebuild the block so why not aim for 5k rpm?

[P15-D24]

I have never seen any crank or block issues with these engines. Failures were usually loss of oil pressure, then anything can break. 

[laynrubber]

When my engine failed it was piston ring lands let go, I don’t know if that is an old age thing or if was a revs thing. I do a lot of hwy driving and now with a tach I see it runs 3500ish. They do sound very busy at hwy speeds.

[sser2]

High reciprocating mass (pistons and upper parts of connecting rods) combined with long stroke. These conditions create high inertial forces that put strain on rod bearings. Forces at rod bearings are proportionate to square of rotational speed, so at 4,000 rpm the load on rod bearings is 4 times that at 2,000 rpm. Excessive force squeezes out oil film, leading to bearing failure.

 

Modern engines can run at higher speeds because they use lighter pistons and rods, and because they are generally short stroke.

[Silverdome]

17 hours ago, Ajgkirkwood said:

 

as for why I want to go past. Well I want to build a hot flathead 6. I need to rebuild the block so why not aim for 5k rpm?

 

Have you tried to contact Tim Kingsbury or George Asche?

[greg g]

Why build a race engine for the street??? Factory peak hp was pulled at 3600, it falls off after that.  My machine shop guy suggested 10% over that was safe and a bit meaningful, over that was just more noise.  Also suggested 80/85 % of 3600 was a safe cruise all day speed assuming good engine condition and proper oil pressure. So rather than build for a peak rpm, why not build to usable street able range.  These are torque engines not hp engines.  Torque gets you going, hp keeps you going...  My slightly modified 230, accelerates fairly briskly, has plenty of power to pass and climb hills, and cruises nicely at 65 with overdrive and gets 20+ mpg in the bargain. Personally there isn't much more I require of a 72 year old car.  

[Ajgkirkwood]

well i figured since i have to rebuild it, make it fun. 5k is out of the question, i dont plan to race it but i want to have fun getting to the speed limit. no one around here really sees a flat 6, and never one thats hopped up.

 

thoughts on machining down the skirt on the pistons to lighten them? i know i will need to get it all balanced

[Plymouthy Adams]

3 minutes ago, Ajgkirkwood said:

well i figured since i have to rebuild it, make it fun. 5k is out of the question, i dont plan to race it but i want to have fun getting to the speed limit. no one around here really sees a flat 6, and never one thats hopped up.

 

thoughts on machining down the skirt on the pistons to lighten them? i know i will need to get it all balanced

go with modern design pistons...its only money....right!

[Ajgkirkwood]

i have no clue where to find them

[Plymouthy Adams]

that can make it difficult for certain....

[greg g]

Contact Egge Machine in California, they make pistons.

[Adam H P15 D30]

While you’re at it, use the longest rods possible and measure for zero deck. 

 

Adam

[Ajgkirkwood]

22 hours ago, Adam H P15 D30 said:

While you’re at it, use the longest rods possible and measure for zero deck. 

 

Adam

longest rods possible? like pick my longest connection rod or get the longest set of connecting rods? i have a 265 block, i dont think they make new ones?

[Adam H P15 D30]

I’m not as familiar with the 25” blocks as I am with the 23” blocks. That said if I were going to have custom pistons made, I would use a 230 crank and 218 rods, pistons cut to 0 deck with a higher pin height because of the longer 218 rods.  Then find a 201 or late 50’s 230 head to top it off. 

 

Adam

Edited September 28, 2018 by Adam H P15 D30

[Ajgkirkwood]

The 265 is the biggest production flathead 6 that I know of. These connecting rods are the longest made? When I took my head off to get milled down, I think my pistons were already sticking out past the deck height. Is this bad or good or just ok? With it past deck it should be more compression, if anything it be more gained by lowering deck 0.010” rather then the head right? The full surface of the piston is  reduced and is in gasket area

[HotRodTractor]

Going off of memory... of the 25" blocks, the 265 is the largest displacement, but the shortest rods. The longest rods would be in the 237. (Same block, shorter stroke = longer rod). I know I was going to get a set of 237 rods and test with a 265 crank, but I fear that the rods might get too close to the cam/block to be useful.... its pretty tight in there on the 265.

[James_Douglas]

Special rod bolts on the 265 to clear the block.  I talked with ARP and they said they can make them.  I have not asked how much yet.  I will be building a 265 over the next year or so.

 

James.

[Dartgame]

These motors are long stroke already - shortest rod piston combo is going to be more durable...oddly enough 230’s use shorter rods than 218’s because the pistons on both are the same the cranks are different stroke.. Eventually I will be building a modded 230, but for now the 218 runs really well.