Grant MacPherson Posted October 27, 2021 Share Posted October 27, 2021 Changed rear gear from 2:56 open to 3:42 yukon posi. Now looking for rev limiter. It is like a different car. 6 Quote Link to comment Share on other sites More sharing options...
Dtret Posted October 28, 2021 Share Posted October 28, 2021 You have a rev limiter built into your foot. Don’t cost a thing. Lol. 1 Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 28, 2021 Author Share Posted October 28, 2021 the older I get the slower the signal to the brain gets. thought a rev limiter would be cheaper than putting a 454 back together. Self control? Quote Link to comment Share on other sites More sharing options...
Canuck Posted October 28, 2021 Share Posted October 28, 2021 That would be a significant difference. You will need to visit the gas station more often. I have 3.73 in mine and wish it was more like the stock 3.31 or 3.42. Quote Link to comment Share on other sites More sharing options...
bob 71 Posted October 28, 2021 Share Posted October 28, 2021 msd 6al ignition box. you would have to get creative to hide it if you are trying to look stock. has some different diagrams for different distributors Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted October 28, 2021 Share Posted October 28, 2021 In the glove box, on top of the glove box, inside the passenger fender on the fender well alongside the battery. No one sees it there on Big Red. Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted October 28, 2021 Share Posted October 28, 2021 Nice ! Great feeling when you make a change and it makes such a difference. 1 Quote Link to comment Share on other sites More sharing options...
Glen Posted October 28, 2021 Share Posted October 28, 2021 The guy who rebuilt my trans said he’d be happy to do the exact same swap to my 70 SS - 3:42 Posi unit for the stock open 2:56. I’m aware of the potential… just need some persuasion. Is the swap all its cracked up to be? Quote Link to comment Share on other sites More sharing options...
sbarnette Posted October 28, 2021 Share Posted October 28, 2021 3 hours ago, Glen said: The guy who rebuilt my trans said he’d be happy to do the exact same swap to my 70 SS - 3:42 Posi unit for the stock open 2:56. I’m aware of the potential… just need some persuasion. Is the swap all its cracked up to be? I guess it just depends on what you’re looking for. My 71 has 3:31 gears and I’m at 3500 RPM’s at 70 MPH. I would like to run lower RPM’s on the highway. I guess an overdrive transmission may be in my future. 1 Quote Link to comment Share on other sites More sharing options...
Canuck Posted October 28, 2021 Share Posted October 28, 2021 3.42 gears with a 27” tire should be about 3,000 rpm at 70 mph. With the 2.56 you would have been at 2,300 rpm at 70 mph. My 3.73 rear end puts me at 3,300 rpm at 70 mph. Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted October 28, 2021 Share Posted October 28, 2021 Taller tires. 275/60 r15 1 Quote Link to comment Share on other sites More sharing options...
MCfan Posted October 28, 2021 Share Posted October 28, 2021 Going to a 28" tall tire (i.e. 275/60-15 as Tom suggested) will change your effective gear ratio from 3.42 to 3.28 which is very close to the 3.31 many of us run in our BBC Montes. RPMs @ 70 mph with a 28" tall tire and your current 3.42 rear gear would be around 2875 - not great but possibly livable. Of course, running 28" tall tires with a 3.31 rear drops 70 mph rpms a bit further to 2785 with an effective ratio of 3.17. Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 28, 2021 Author Share Posted October 28, 2021 does anyone know the correct spedo gears to go with the 3:42 gears? 28" tire. Quote Link to comment Share on other sites More sharing options...
1970mcss Posted October 28, 2021 Share Posted October 28, 2021 I don't see a 3:42 gear in the parts book. What year and trans is in your car??? Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted October 28, 2021 Share Posted October 28, 2021 71 small block elcaminos came with 3.42 12 bolts I believe. Quote Link to comment Share on other sites More sharing options...
1970mcss Posted October 28, 2021 Share Posted October 28, 2021 I checked the chart in my book and it doesn't show a 342 in the chart. Maybe someone else has some info Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 28, 2021 Author Share Posted October 28, 2021 I don't think GM offered a 3:42. I used Richmond gears. What did the chart show? 3:08 - 3:55? might have to guess at some where in between. Quote Link to comment Share on other sites More sharing options...
1970mcss Posted October 29, 2021 Share Posted October 29, 2021 Yes, I will take a look tomorrow. 400 transmission? Quote Link to comment Share on other sites More sharing options...
Leghome Posted October 29, 2021 Share Posted October 29, 2021 My 700r4 with 3:73 and the motor turns at 2700 at 70 Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 29, 2021 Author Share Posted October 29, 2021 yes, turbo 400 Quote Link to comment Share on other sites More sharing options...
1970mcss Posted October 29, 2021 Share Posted October 29, 2021 9775187 might be the part number. It shows it has 45 teeth. Quote Link to comment Share on other sites More sharing options...
rotinrob Posted October 29, 2021 Share Posted October 29, 2021 A quick way to come close is most speedometers register 60 mph at 1000 rpm cable speed (easily checked). Do you know what color gears are in your TH400? On my MC I had to change out the speedo gear housing, gear and add a box for my 4.56 gears. I don't think I changed the gear on the output shaft. The box came from the GM dealer I used to work for. The Olds parts book actually had a chart listing the gear and box required for 4.56 gears in a Toranado (trans basically a TH400). Must have been some serious tow package's back then (or maybe gas was just starting to creep to $1.00/gallon). As for a revlimiter I have been using the Flamethrower 3 ignition with built in revlimiter on most of my builds now, works great and looks stock. Need to run 12v to it for optimum spark. Haven't replaced the HEI in the MC yet (on the list of things to do) so my foot is the rev limiter. Actually now it is the used up valve springs as it will start floating at about 6200/6500 instead of the usual 7500 needed to get the full benefit of the 4.56 gears (also on the list, springs on the shelf, self on the couch). rotin Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 29, 2021 Author Share Posted October 29, 2021 thanks everyone Quote Link to comment Share on other sites More sharing options...
DragCat Posted October 29, 2021 Share Posted October 29, 2021 On 10/28/2021 at 3:09 AM, Glen said: The guy who rebuilt my trans said he’d be happy to do the exact same swap to my 70 SS - 3:42 Posi unit for the stock open 2:56. I’m aware of the potential… just need some persuasion. Is the swap all its cracked up to be? Lots of good suggestions and info in the above posts from members 👍 I couldn't add anything specific for your needs so just watched on. Mine isn't a highway cruiser. I swapped out my old series 2 posi in my 12 bolt to a series 3 with 355 gears (my buddy had it in stock 🤣) and yes its a different monster! I never changed the speedo gear (I've already got to much on the to do list or I'm just being lazy) but the local solar powered speed limit checker has me 15mph off my speedometer so good enough 🤣 I also have 275/60/R15 drag radials on the rear. Also a rebuilt TH400 and new converter. On my build, the original posi and gears without the drag radials ( just BFG's) was a pig, I hated it, but with the new was fantastic. Again, I wasn't interested in hwy cruising. My buddy offered me his old ( he upgraded to the 7) msd 6al and I'm contemplating the change. I have an HEI dizzy so I either have to buy the adapter parts or replace it with a standard dizzy. Some people like the msd's some don't. Just my .02 Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted October 29, 2021 Author Share Posted October 29, 2021 I have an early pertronics mod in the distributer and was told that they have one that has a rev limiter in it. Quote Link to comment Share on other sites More sharing options...
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