Ok I am building a 1940 buick special with the 248 engine. I am installing a t5 behind it and an open driveline. I am trying to decide what gear ratio rear I want. So to do so what rpm range should I try to shoot for. I am talking at speeds like 70mph for cruising. This is going to be a car we drive a lot. Thanks in advance.
Tire size has a lot to do with the gear selection also. If you are using a 28" or so tire I would go with about a 3.50 axle. That 248 is not blessed with a excess of power so if you expect to make use of 5th gear don't get carried away with to low a axle ie 3.20 or less. Even with a 3.50 any hills or much of a head wind you will be in 4th.
Which T5? There are numerous gear sets, over a fair range. Ernie's article: http://www.flatheadv8.org/ernie/ernie-t5.htm
Those straight eights are very long stroke engines. Not capable of real high rpms...so no big hp numbers, but they're great on torque. They're happy cruising at a substantially lower rpm than most. Sorry I can't give you an actual number...it'd be a guess. Maybe someone else can be more specific. But figure that if a modern V8 is happy cruising in overdrive at 70mph at Xrpm, the comfortable cruising rpm for the 241 at 70mph will be somewhat less. Ask the guys over at the Team Buick website...they'll be able to give you more detailed info.
A Cadillac 346 V8 flathead has a max rpm around 3500 if it helps to give you an indication. I'm expecting to run my Cadillac flathead engine with a T700R4 o/d auto tranny and 3.4:1 diff gears and 29 inch diameter tires.
there are online calculators that can predict maximun RPM , yours comes in at 3500, factory says max HP is 3600 so dont push it too far past that. Especially if its not 100%. The special 8 is 110 HP at 3600 and 206 at 2000 so try and gear it for a bit over 2000 (near torque peak) at 70. There are calculators online for RPM at 70 including tyre sizes and ratios for you to play with. Youll have to see whats close given what ratios are available. You can use carbs off a Super and other normal mods if it struggles a bit at speed. http://thefirstgensite.com/code/maxrpm.htm
I drive my 41 buick special original on average at 50 mph , I have had it at 70 mph on interstate before . I might suggest if your going threw all this work you may want to look at a 263 straight eight 1948-1953 years .On your gear choice I would think 320's ish because the motor is a torque monster ,no worries there .I am speaking from driving mine since 2000 . Good luck
Do you have any plans for the motor itself? Mine is currently 100% original and will cruise around 55mph tops...I am looking into a complete re-build, milled head, custom grind cam, custom intake and headers to squeeze as much out of the ole 248 as I can. This is only because its an all original surviver car (3rd owner) and I really wanted to keep the motor in it. If you arent attached to the 248, id think about swapping it out... Andy
AGREE - to put that money into the older small straight eight , when you can get a 263 ci 1948 to 1953 years .That way you get rid of babbit bearings the old oiling style and I am sure some other benefits too .I will be clear tho ,I am keeping my 248 ci in my 41 special because it runs and I am not rebuilding it . The longevity benefit of a 263 ci may serve your purpose better .good luck and enjoy the build .
Correct!! I have a 1950 3spd in which I have installed a 3:34/1 gear set. Cruises all day at 70mph, 2600/2700 rpm. I don't recall exactly, but max torque is somewhere just below 2500. Ben
I would be shooting for 1600 - 1800 RPM @ 70 in overdrive. Maybe a little higher if it is to be driven in hilly country. You want your cruising speed to be in the "sweet spot" of max torque. For best smoothness economy and power.
Peak torque (208 lb ft) is developed at 2000 RPM. I would think if that gave you 65 to 70 mph, you and the engine would be happy campers. Probably prevent down shifting on hills and pretty good 50 to 75 acceleration. So with 28 inch tires, a 3.55 to 1 rear, a .73 OD, 2000 rpm gives you 64.7 MPH. A 29 inch tire would make it 66MPH A 3.22 rear give you right around 70 with either tire diameter.
107 HP at 3400 RPM is the Buick spec. It would take quite a few cubic dollars for a Buick 248 ci Str8 to make 206HP at 2000 RPM.
Not 206HP, 208 ft lbs of torque. Cruising at or near peak torque means the engine is running at its most efficient speed and you have the punch to maintain speed up hills. If you need max HP to pass a car you need to shift down.
"1600 - 1800 RPM"...that's about what I would've guessed. Maybe even as low as 1500 rpm. They sound pretty much like most other 8cyl overhead valve engines.
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