Hey guys. I got a 69 Special Deluxe with a Chevy 250 and I've been looking around at transmissions and I'm not sure which to choose. I have a ridiculously high rev going down the highway doing like 65 or 70 (probably about 2900rpm or 3000rpm). The final gear on most of these trannys is a 1:1 so a tranny without overdrive won't make a difference. I think i already have a 2.83 in the back so I'm not sure what to do to make the revs lower. I think I've read that the redline is 4200 but is there a way increase that without endangering the engine? I just need to lower my revs on the highway is the main point but the redline just seems a little low compared to other engines.
You really shouldn't be turning 2900-300 with a 2.83-ish rear gear. I have 3.07's in the rear end of my '69 GMC, and I turn about 2300 when going 60-65.
Granted, I have 235/75 tires and a 307 sbc.
Just wondering, do you have a tach, or are you just going by your ear? These engines sound like they're wound tight when they're really not - nature of the beast I suppose.
So, to answer your ques., taller tires in the back, so you go further with each rotation so to speak.
I personally would check your tire size and confirm your rear gears. After that, check a calculator online for rpm's and rear gears. You can post them on here, and I'm sure someone will dig around and find it for you as well.
Pick up a cheap sunpro tach, and see for sure what kinda R's you're turning.
As far as increasing the redline, you're running out fo breath right now above 3500 rpms. A single barrel carb, 2 inch exhaust outlet, and damn near smooth cam? Yeah, it's trying at higher rpm's - but really, it needs some more flow in and out, along with a bumpstick to get it all in and out, ya dig? That'll raise your redline, or atleast give you power into the upper rpm range.
Others will chime in, but for the time being, you get the rantings of a deranged college student who is running on no sleep thanks to the finals count down. enjoy!
-Sam.
haha alright. Well I know it's running at ~2900rpm because I do have a sunpro tac. I might've miscalculated my rearend though. I'll try again tomorrow or something. At by increasing the redline, I mean allow me to go to a higher rpm without throwing a piston through the block. I can easily go 5000rpm if I floor it and leave it but I don't feel that's good for the engine. What is the redline for a 250? and isn't the only way that I can increase the redline (make it stronger so it doesn't fall apart above 4000rpm) is by getting stronger pistons, crank, etc.?
Snowman,
I agree, your gearing does not sound like what you think it is. Here is a gear trans calculator. It is in Excel. Simply input
tire choice
trans choice
It will show you either a table or a graph.
http://www.12bolt.com/gearing_calculatorIf you do not like the rpm's, either a bigger tire can be used or go to a OD trans.
The redline of your 250 relies on a # of things. The stock engines biggest crutch is the 40 year old valve springs. They must be able to keep upto the task, if not than bad things start to happen. As is the stock lower end should withstand 5K rpm just fine. You cam and intake/ exh (as Sam said) is the limiting factor now. Tom
Snowman in checking your rear axle ratio I think you missed the part to multiply the drive shaft rotation by 2. That would make your rear axle ration 4.56 more inline with what came in the trucks of that era.
Yes final drive in transmissions is always 1:1, but over drives are not considered the final drive. An over drive is usually 25-30% which would drop your rpm down to around 2k.
Larry
Still not sure what you're trying to do?
These are not only 2 different things, but opposites:
1. is there a way increase that without endangering the engine?
2. I just need to lower my revs on the highway
Cheap easy way to lower your RPM,s for now.
Use a 29-30" tall tire in the rear.
IIRC,,I used a 235 75 15 on my 65 Malibu SS that had 355's in the back,,made a big difference over the 26" tall tire.
Looked a little funky,but I did not care @ the time,I was driving on the freeways alot.
MBHD
I don't really know how I got to talking about increasing redline yet lowering rpms in the same thread so I'm sorry for any confusion. I guess I just started talking about it because it was on my mind. I just don't feel good about running it above 4k rpm because it's SCREAMING by then. My exhaust leak might contribute to what I'm hearing but once I put in the new cam/valve springs/lifters then and fix the leak then I'll see how it goes then I guess. But 5k on the stock engine? I heard that 4200rpm was the redline. Anyway being a teenager and the way I drive, I'm balding up my tires pretty quick. I'll get some new and bigger ones eventually and luckily, my dad being an 68-69 A-body buick aficionado, he knows the biggest tires that'll go on there so he's a good help. But what would be the best shift point? I usually shift around 3200rpm but if you're saying that the redline is upwards of 5k, then it seems like I should be shifting at around ~4k. Hoping to get a new tranny soon so I can use the powerband more efficiently because I am slowed to a crawl in 3rd gear and have absolutely NO power from about 40mph-65mph. But it doesn't look like I'm going to be driving for a while anyway. My rochester monojet finally gave out and now the choke doesn't want to work and I think the accelerator pump might be bad so it's a pain in the ass to start it. Need to rebuild it or just go ahead and get a new intake/carb... if only I had some money :-). I wish tennessee didn't have such restrictive work hours on 15-year-olds. Once I turn 16 though, then I can start getting some money for the car. Til then it's birthdays and Christmas haha. Thanks a lot guys.
Just redid my gearing calculations today. I actually either have either a 3.23 or a 3.5x
Not in any attempt to talk down to you mate, but, is your tach set for a 6 cylinder?
Just throwin' that out there.
Snowman,
Here's my advice and take for what it's worth. Quit guessing at your rearend ratio. Pull the cover and count the ring teeth and pinion teeth and divide. It probably could use a gear lube change anyway. If it doens't then save the lube and reuse it. Then do what Hank said. Try a bunch of different diameter tires. If your father knows someone who can mount tires, then go to the local tire shop and 'borrow' some bald tires of different sizes and see what get's your RPM's where you want them.
Then, when you deterine what trans your going to use (I agree that the 3 speed is really tough from 2nd to 3rd), apply a road speed calculator to determine what tires will get your RPM's in the range you want with the trans you buy.
I can't recommend a 5 speed enough. When I put the T5 in I would've sworn that I added 75 HP because of the gearing and also my mileage picked up by more than 4 mpg at 70 mph.
Good luck.
I
X2 - I'd do the 5-speed before anything else.