stewdecky Posted May 13, 2011 Report Share Posted May 13, 2011 Just wondered what RPM most of you fell comfortable with you flatheads cruising on the highway? I am thinking of putting a tach on my '46 ut really don't know what to look for in the RPM range. Quote Link to comment Share on other sites More sharing options...
41/53dodges Posted May 14, 2011 Report Share Posted May 14, 2011 (edited) i think the stock redline is about 3600, they sound like their coming apart at only about 1800, but 3000 is the "safe line" as i would call it. i got a tachometer out of a p51-mustang on my truck! all electronic, run off the points once you make some circuitry. it's called a weston AC-42 tach. top speed:3500 redline:2800-2900. search "tachometer" in the search bar here, and you may find some helpful postings Edited May 14, 2011 by 41/53dodges Quote Link to comment Share on other sites More sharing options...
Alshere59 Posted May 16, 2011 Report Share Posted May 16, 2011 i think the stock redline is about 3600, they sound like their coming apart at only about 1800, but 3000 is the "safe line" as i would call it. i got a tachometer out of a p51-mustang on my truck! all electronic, run off the points once you make some circuitry. it's called a weston AC-42 tach. top speed:3500 redline:2800-2900. search "tachometer" in the search bar here, and you may find some helpful postings Do you have a schematic for the tach by chance? Quote Link to comment Share on other sites More sharing options...
41/53dodges Posted May 16, 2011 Report Share Posted May 16, 2011 for the tach or the conversion circuit? i made one for the circuit, but dont have it now. i have no idea on the tach itself, i just built around it Quote Link to comment Share on other sites More sharing options...
ggdad1951 Posted May 16, 2011 Report Share Posted May 16, 2011 Do you have a schematic for the tach by chance? I believe for a the tach you'd hook up to the coil and battery for it's circut? Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted May 16, 2011 Report Share Posted May 16, 2011 Hand held tach-dwell meters are dist side of coil and ground. Dash, steering collum (spelling?), hood mounted will have specific directions according to manufacturer. Usually 3 or 4 wire, black to ground, red to bat side of coil, green to dist side of coil and fourth wire for a light is so equipped. Pos ground and 6V may not be compatible with the tach itself. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted May 16, 2011 Report Share Posted May 16, 2011 (edited) As there are multiple threads on this question I am not sure what thread I should post this to. But here is how I hooked up my 12V negative ground 6G tachometer. Edited May 18, 2011 by Don Coatney Quote Link to comment Share on other sites More sharing options...
greg g Posted May 18, 2011 Report Share Posted May 18, 2011 the HP rpm is 3600, any thing north of that is basically wasted due to piston speed losses, any thing from 2800 to 3300 is all day cruising. Factory pulled random engines for bench testing, running them for 50 hours at 3600 rpm. So if you figure 3600 is 70 mph, thats 3500 miles worth of testing. Wonder if after the test they went back to the assy line??? Quote Link to comment Share on other sites More sharing options...
Young Ed Posted May 18, 2011 Report Share Posted May 18, 2011 Greg maybe that explains the engines that got a * in the serial number and came factory oversized? Quote Link to comment Share on other sites More sharing options...
TodFitch Posted May 18, 2011 Report Share Posted May 18, 2011 the HP rpm is 3600, any thing north of that is basically wasted due to piston speed losses, any thing from 2800 to 3300 is all day cruising.Factory pulled random engines for bench testing, running them for 50 hours at 3600 rpm. So if you figure 3600 is 70 mph, thats 3500 miles worth of testing. Wonder if after the test they went back to the assy line??? Actually it was a little tougher than that: The 50 hour test was at max BHP, so it was at wide open throttle with enough load on the dynometer to keep it at 3600 RPM. So that was 3500 miles of running uphill at 70 MPH... Quote Link to comment Share on other sites More sharing options...
ggdad1951 Posted May 18, 2011 Report Share Posted May 18, 2011 Greg maybe that explains the engines that got a * in the serial number and came factory oversized? serial number? Quote Link to comment Share on other sites More sharing options...
Young Ed Posted May 18, 2011 Report Share Posted May 18, 2011 serial number? You'll just have to drive it enough to wear out the one you put in and then you can rebuild a proper block Quote Link to comment Share on other sites More sharing options...
ggdad1951 Posted May 18, 2011 Report Share Posted May 18, 2011 You'll just have to drive it enough to wear out the one you put in and then you can rebuild a proper block that block already cost $1000 with its repairs! I think a stamp or engraved or not worrying about it is a more propper response! Quote Link to comment Share on other sites More sharing options...
Young Ed Posted May 18, 2011 Report Share Posted May 18, 2011 That too. You know where to find an excellent sample. My garage! Quote Link to comment Share on other sites More sharing options...
greg g Posted May 18, 2011 Report Share Posted May 18, 2011 Ed, do you have documentation that the Star inclusion in the engine number is an indicatio of a factory overbore??? and if so what was it size wise??? I believe my current enign from a p28 has two stars in the number. Quote Link to comment Share on other sites More sharing options...
Young Ed Posted May 18, 2011 Report Share Posted May 18, 2011 I believe I read it in a plymouth bulletin way back. I think it wasn't always an overbore too some of them had standard bores but undersized crank or rod bearings. It makes sense they would do that rather than tossing perfectly useable parts. Quote Link to comment Share on other sites More sharing options...
Reg Evans Posted May 18, 2011 Report Share Posted May 18, 2011 Just wondered what RPM most of you fell comfortable with you flatheads cruising on the highway? I am thinking of putting a tach on my '46 ut really don't know what to look for in the RPM range. Two of my trucks were set up with 25" engine's and a 3.0 rear end with P235 75R15 tires(26.5"diam.). I could cruise all day long at 65 mph while according to the rpm calculators turning~ 2500 rpm. I have installed new tires (28.5" diam.) on my present truck with this drive train and can now travel down the road at 65 mph while the engine is only turning ~2300. I'm not sure how good my engine feels at this rpm but I know I feel good. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted May 18, 2011 Report Share Posted May 18, 2011 My original P-15 engine had two stars. I do not know it it was an overbore. Quote Link to comment Share on other sites More sharing options...
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