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EFV-8 Club Forum / General Ford Discussion / cruising rpm

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Posted By Discussion Topic: cruising rpm     stephen miller 09-25-2014 @ 6:51 PM reply profile send p.m. Member Posts: 28 Joined: Feb 2013 what is a good safe cruising rpm on stock 33, original 21 stud engine ken ct. 09-25-2014 @ 10:49 PM reply profile send p.m. Senior Posts: 1513 Joined: Jan 2010 With probably a 4.11 rear I would say approx. 55 mph , any more than that the engine would be screaming. I have a 36 that had 4.11 and changed the rear for a 37 which had 3.78 in it. Easy swap using the whole rear 3rd member. Use the 36 backing plates and had to grind a clearance issue with the rear brake levers. ken ct omo. 1934 Ford 09-26-2014 @ 6:27 AM reply profile send p.m. Senior Posts: 568 Joined: Oct 2009 If your 33 still has a single throat carburetor, it may not be the same as my 34 Fords, but here goes. My 34's have 2 barrel carburetors and 4:11 rear ends and the ideal running highway speed is 53 MPH. They'll go faster but the RPM's go up dramatically above 53. They will drive across the country at 53 without overworking. We've done 2 Great American Races and dozens of Glidden type tours and the 53 MPH is the number to make them last like ours have for 42 years. 1934 Ford's since 1972 JT Ford 09-26-2014 @ 7:18 AM reply profile send p.m. New Member Posts: 112 Joined: Oct 2009 When you are talking about speed, are you talking the speedometer reading or gps speed? The speedometers on these old fords is usually not clocked speed. My '49 is 10 miles off. 1934 Ford 09-26-2014 @ 2:36 PM reply profile send p.m. Senior Posts: 568 Joined: Oct 2009 My speed is in MPH and is 99.65% accurate. Great American Race tested and timed at all speeds. Correction factor is only 2.1 seconds for each 10 minutes at 50 MPH. These little flatheads can click off the miles at 50 MPH. I like 53 MPH for Intersate driving when it's hour after hour. I like it even better on secondary roads. 1934 Ford's since 1972 len47merc 09-26-2014 @ 4:27 PM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 Just a thought to keep in mind - these speedos are only as accurate as one may have your car 'rubbered' to be as close to original as possible. Specifically, regarding tire size the closer one gets to the original tire diameter the more accurate the speedo. When running 670s @ 27.75" diameter and 32 psi my speedo is 3-3.5 mph slow against a GPS at 55 mph (i.e., speedo reading 55, GPS actual 51.5-52. Changing to 710s @ 28.8" diameter and 32 psi the speedo is 4-4.5 mph slow against the GPS and so on. When first bringing the car back to life with the more-than-somewhat dry-rotting original 650s on it (original diameter unknown but visually less than both the 670s and 710s) the speedo was basically on the GPS mark at 50 which was as fast as my nerves and stupidity would allow the car to go on the old rubber. After dropping the pressure on the 710s to the designed 26 psi for a '47 (Merc, 28 for a Ford) the gap improved somewhat as the tire diameter was lessened by the lower tire pressure. When taking the occasional full tank trip I always use the speedo with known offset for controlling speed and a Samsung phone with GPS app for mileage. Regarding cruising speed, for me with a 3.78 rear I prefer 55 mph max on the speedo (with the 710s @ 26 psi) which equates to roughly 58 mph GPS actual and ~2,600 rpm. Don't like pushing the motor any harder (especially for long runs) and engine compartment noise starts becoming bothersome/prohibitive for long periods above 2,750 rpm or so. Applying this to a 4.11 rear I'd have to agree with '1934 Ford' that 53 mph max (GPS reading) for you Stephen is real good input. Steve carguy 09-27-2014 @ 4:48 AM reply profile send p.m. New Member Posts: 167 Joined: Oct 2009 Please note that he is asking for RPM not MPH Bill Brown '34 Cabriolet len47merc 09-27-2014 @ 7:18 AM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 You are right carguy in that the dialogue veered slightly off subject and somewhat indirectly addressed his question - my apologies for not catching that versus focusing specifically on rpm. The intent of most of the replies appears to be to provide thoughts, considerations and data to Stephen that might support his decision on the rpm number best for his original 21 stud with an assumed 4.11 rear. Hopefully the data provided will give him adequate info to make the judgement that is right and safe for the current set-up and condition of his engine/car. Steve supereal 09-27-2014 @ 11:31 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The "sweet spot" for best power, according to the Ford test specs, is 2,500 rpm. This translates to about 50 mph with a 3:78 rear gearing. Older models with mechanical brakes, loose steering, or other signs of age, have no business running over 50 mph or on Interstate hiways. stephen miller 09-27-2014 @ 2:41 PM reply profile send p.m. Member Posts: 28 Joined: Feb 2013 thank you for you information. i will follow the advice. stephen

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