Page 2 of 2
Posted: Fri Apr 19, 2013 8:25 am
by 3xpendable
Dave, many thanks for that and apologies for my ambiguity, I was just thinking of trying to get a shorter starter to give as much exhaust clearance as possible. Does anyone have any problems with a sufficiently heat shielded SD1 starter here?
Yeah I might stick with the 3.9 final drive to begin with, but if budget allows I will get the 3.07 CW&P.
Posted: Fri Apr 19, 2013 9:04 am
by sidecar
A 3.07:1 diff will probably be too far the other way, your engine will need to produce a hell of a lot of power to allow it to rev out to 6K RPM. (You will then be doing 171 MPH)
Of course some people prefer to have a car that is over-geared so that they get a nice low cruising RPM and use fourth gear for max MPH. Horses for courses I guess.
I spent hours and hours going over this stuff along with trawling the web looking at gearbox ratios before deciding what to do with my car.
Posted: Fri Apr 19, 2013 12:12 pm
by DaveEFI
If it helps, the two Lucas starters fitted to the SD1 are the 3M100 (early) and the 9M90. The later unit is shorter and the motor of a smaller diameter. However, those numbers don't define the exact SD1 starter motor.
Posted: Fri Apr 19, 2013 2:48 pm
by mgbv8
sidecar wrote:mgbv8 wrote:
4th gear Pete!
I dont know about the 77 but I assumed 4th would be 1:1 and 5th might be an overdrive gear??
Hi Perry,
Yeah I know you mentioned fourth gear, my point is that it won't even cruise at 70 MPH in fifth gear at 3000 RPM which is a 0.83 overdrive (usually) so in fourth gear will be even worse!
I'm at home now so I have access to the gear calc that I used when working out my ratios, I've bunged in the LT77 ratio's along with a 3.9 diff and a 24 inch diameter wheel.....
3rd gear at 3000 RPM = 37 MPH, at 6000 RPM = 73 MPH
4th gear at 3000 RPM = 55 MPH, at 6000 RPM = 110 MPH
5th gear at 3000 RPM = 66 MPH, at 6000 RPM = 132 MPH
Just for reference I run a 4.6 stage III lump in my Cob with NOS,
I run 235-45-17 wheels and a 3.36:1 diff along with a T5 close ratio box, the figures are below:-
3rd gear at 3000 RPM = 53 MPH, at 6000 RPM = 107 MPH
4th gear at 3000 RPM = 67 MPH, at 6000 RPM = 135 MPH
5th gear at 3000 RPM = 77 MPH, at 6000 RPM = 155 MPH
My Cob is as aero dynamic as a brick!
Thats interesting Pete! I was going by my past experience with the MGB 1800 setup which use the same diff ratio. Come to think of it, it would have been 4th + o/d to do 70 at 3000 rpm. Its one of those numbers that sticks in my mind because the 1800 will do the 70 mph, but the V8 with the 3.07:1 diff does more like 98 at 3000 rpm.
I'll save those charts on my pc.
Pel
Posted: Fri Apr 19, 2013 2:50 pm
by mgbv8
Welshie wrote:Interesting, I'm not sure of my LT77 ratio's though so can't really work it out yet. I just wanted to know if I can get away with using the 3.9 ratio withough it scrreaming at 40mph for a bit.
Would a starter like this be suitable?
http://www.ebay.co.uk/itm/221009564102? ... 1423.l2649
No!
The solenoid looks like it will end up on the side. This wont fit as it will hit the chassis.
Posted: Thu Apr 25, 2013 11:30 am
by 3xpendable
All sorted now Perry, I had a starter sitting around all along!!
Thanks for your help though.
Posted: Thu Apr 25, 2013 3:14 pm
by task
Interesting topic as I am pretty much building the same sort of thing, when I've finished chasing the rust!
I was planning on fitting an LSD to the back axle though and was certainly going to use the RV8 style manifolds and possibly exhaust, depending if I can make the exhaust cheaper.
Posted: Thu Apr 25, 2013 8:45 pm
by mgbv8
Welshie wrote:All sorted now Perry, I had a starter sitting around all along!!
Thanks for your help though.
What are you like
You can now see what I mean about it hitting the chassis eh?
Pel
Posted: Fri Apr 26, 2013 4:52 am
by minorv8
I have a Moggy with RV8, LT77 and 3,08 diff ratio. The car weight is about 1000 kg with full tank less driver so it is relatively light. I would say that the 3,08 diff is pretty much spot on. I´d say with 3,9 diff you basically have a 4 speed box where 1st gear is useless and you probably get bored with highish revs on highways.
Acceleration is not a problem even with the Moggy body. If a Cobra is a brick I really don´t know how a Minor should be described, pile of bricks ?
Posted: Fri Apr 26, 2013 8:10 am
by sidecar
minorv8 wrote:
Acceleration is not a problem even with the Moggy body. If a Cobra is a brick I really don´t know how a Minor should be described, pile of bricks ?
You be surprised, I bet your Moggie is more aerodynamic than my Cob, the Cob is an open top car and that's what causes the issue, you get a huge amount of turbulence about 6 feet behind the winscreen.
Your 3.08:1 diff is quite tall but if the rolling cirumference of your wheels differs to say my car for example then they will offset this somewhat.
The online gear calulators are very useful because they take the wheel size, diff ratio, and gearbox ratios into account
Posted: Fri Apr 26, 2013 8:26 am
by DEVONMAN
sidecar wrote:minorv8 wrote:
Acceleration is not a problem even with the Moggy body. If a Cobra is a brick I really don´t know how a Minor should be described, pile of bricks ?
You be surprised, I bet your Moggie is more aerodynamic than my Cob, the Cob is an open top car and that's what causes the issue, you get a huge amount of turbulence about 6 feet behind the winscreen.
Your 3.08:1 diff is quite tall but if the rolling cirumference of your wheels differs to say my car for example then it is no good just looking at the diff and gearbox ratio's in isolation. This is why the online gear calulators are very useful.
My Devon is pretty much like a moggy (probably worse due to flat screen) but it's amazing how a 1300kg pile of bricks gets as light as a feather and wants to fly at 135MPH
BTW I run a 2.88 diff with a 6 speed T56 gearbox and that is a little too high as it cruises at 1900 rpm at 70 mph. Soon to change to a 3.08 just to get a better pedal feel at motorway cruising speed although I don't dislike it as it is as there's good torque from my engine.
It really depends what your main use for the car is.
I set out to get through the line at the pod in 3rd to minimise gearchanges but I also like stress free motorway cruising.
But 0-60 mph is also appealing at 4.0 secs
Regards Denis
Posted: Fri Apr 26, 2013 9:34 am
by minorv8
I am running 195/50x15 tyres, previous ones were 185/60x14.
Devonman, your Devon is fast ! I used a calculator for checking the 0-60 (based on 1/4 figures) and only got 4,06 sec. So your car is definitely faster than mine
I have ran the car occasionally at strip but have not enough traction from the line. Best 60 ft time is a lousy 2,01 sec, trap speed is usually a consistent 170 km/h. I would like to have the 1st gear ratio around 2,9 - 3:1, it might help with the traction. But anyway, I am satisfied with 12,99 sec time card, best so far is 13,039 with less power.
Posted: Fri Apr 26, 2013 10:23 am
by mgbv8
sidecar wrote:minorv8 wrote:
Acceleration is not a problem even with the Moggy body. If a Cobra is a brick I really don´t know how a Minor should be described, pile of bricks ?
You be surprised, I bet your Moggie is more aerodynamic than my Cob, the Cob is an open top car and that's what causes the issue, you get a huge amount of turbulence about 6 feet behind the winscreen.
Your 3.08:1 diff is quite tall but if the rolling cirumference of your wheels differs to say my car for example then they will offset this somewhat.
The online gear calulators are very useful because they take the wheel size, diff ratio, and gearbox ratios into account
I run a 3.08:1 diff in mine. Its about 1100kg and I run large 28" tall rear tyres. Its quick as fook. Stick a 3500 in a stock bodied MGB with the 3.9:1 diff and its not worth using 1st gear. And you are well short of gearing for motorway use even with overdrive. Been there, seen it done it a few times for folk over the years with conversions in their MGB's. The 3.08:1 diff is not a problem in an MGB V8 conversion.
Sorry for the swift reply, just about to dash out to work
Perry