I bet the 95 mph top speed for 67 MKIVs was rev limited, not power limited.
5273 * 95/80 = 6262 rpm for stock 67 Sprite with 145s and 4.22
Isn't 6250 the redline? My car is in Nebraska for the winter so I can't
look - maybe it's lower, but how could that be - would they spec top speed
at above redline? Mine was still climbing quite rapidly when I let off and
slowed down - I just didn't have the guts to run the RPMs up any further on
a non-race-prepped motor for no good reason, since both the speedo and tach
(inaccurate tach) were off the scale.
The point is that a good street 1275 will take you well in excess of
100mph, with the right gearing. The racers run them at much higher rpms.
I will be interested in seeing what yours will do in fourth gear after the
rebuild. What RPM will you be turning at 100 mph with the 5-speed in
fourth and the 3.7 diff/145s?
ps. Anyone know the published top speed of the 68's with 3.9 gears - did
the emissions equipment slow them down considerably or are they supposed to
have a higher top end than the 67 with 4.22s?
Les Myer
At 08:39 AM 2/9/98 -0800, you wrote:
>Les,
>of course you are right, I copied down the wrong number.
>Regarding your 100mph, old test articles say thet a stock 67 Sprite when
>it was new didn't make it to 100 mph. I think they said 95 or something
>like that. Of course that was a _stock_ Sprite...
>Ulix
>
>On Sun, 8 Feb 1998, Les Myer wrote:
>
>> Ulix,
>>
>> Wouldn't my car at 80 mph with 185s be:
>>
>> 4624 * (4.22/3.7) * .947 = 4994 rpm
>>
>> Then my car at 80 mph with 145s would be:
>>
>> 4624 * (4.22/3.7) * 1 = 5273 rpm
>>
>> hmmm.....not as big a difference as I thought.
>>
>> Interesting that at 100 mph with 4.22 and 185s, rpms would be:
>>
>> 4994 * 100/80 = 6242 rpm
>>
>> Guess I was spinning it right up there.
>>
>> Thanks for the data.
>>
>> Les
>>
>> At 07:26 PM 2/8/98 -0800, you wrote:
>> >Les,
>> >a 145-13 is 566mm in diameter, the 185/70-13 is 598mm
>> >Circumference is pi*diameter, so I can just divide 566/598=0.947
>> >So you turn 95% as many rpm as with stock tires.
>> >Going from 4.22 to 3.9 is 3.9/4.22=.924 or 92.4%
>> >
>> >Now to get the rpm.
>> >
>> >17.3mph/1000rpm for a 3.7 with 145s
>> >1000/17.3 = 57.8 rpm/mph
>> >80mph * 57.8 = 4624rpm in a '79 Midget
>> >
>> >In your car:
>> >4624 * (4.22/3.7) * .924 = 4873rpm
>> >
>> >In my car: 3792rpm.
>> >
>> >I'll try my top speed after I rebuild the engine :-)
>> >I think I'll have to use 4th to get there though...
>> >
>> >Ulix
>> >
>> >
>> >On Sun, 8 Feb 1998, Les Myer wrote:
>> >
>> >> So how many RPMs am I turning with a 4.22 and 185/70R-13 tires during an
>> >> 80mph commute to work on I-94 (tach is off (reads 6500), but have
>> >> calculated MPH by the mile markers)? Really would like to know. You
gotta
>> >> do 80 here in Michigan (70mph speed limit) to keep from getting run
over by
>> >> the 18-wheelers going from Chicago to Detroit and vice versa. I cringed
>> >> every time until I put the bigger tires on and I think the bigger tires
>> >> look a lot better than the little skinnies I had before. I think they
are a
>> >> viable option to swapping diffs. Plus, it softened the ride immensely.
>> >> The only things I don't like about them is the wheel hop I get when
dumping
>> >> the clutch at RPM and I don't think it handles quite as well in the
30 mph
>> >> range.
>> >>
>> >> BTW, 80 is not the top end - have had PUDDLES briefly over 100mph with
>> >> these tires - kind of weird, the exhaust note seems to get much
quieter at
>> >> very high rpm. I try to keep the cruising speed at 70-75 mph for
>> >> long-distance driving.
>> >>
>> >> Les
>> >>
>> >> At 10:13 AM 2/8/98 -0800, you wrote:
>> >> >I swapped out the 4.22 for a 3.7 rear end in my car, which is equipped
>> >> >with the 5-speed. Wow! It has transformed the character of the car on
>> >> >the highway. WIth the ribcase and 4.22, I felt completely out of
place on
>> >> >the highway, the car felt like it was doing something it wasn't
designed
>> >> >to do. I used to drive ~60mph. The combination of 3.7 and 5-speed
lowers
>> >> >my rpm to 72% of original. At 4000 rpm, I go the same speed that
used to
>> >> >take 5500rpm, which according to me calculations should be 84mph.
74mph
>> >> >at 3500rpm. I ride the fast lane unstressed. (All numbers based on
>> >> >theory, not the inaccurate instruments of my car. rpm to mph
conversion
>> >> >based on Haynes manual that says 17.3 mph/1000rpm in 1500 with 3.7
rear).
>> >> >
>> >> >Of course there are tradeoffs. Inclines on the highway will get
>> >> >me downshifting early, since the motor has less power at 3500 than
at 4000
>> >> >which is what I used to see at cruise.
>> >> >The long rear end makes for a long first gear which makes for harder
>> >> >takeoff from a stop. First gear on the Datsun trans is shorter than
the
>> >> >one of the ribcase, which helps to offset this some. This shorter
first,
>> >> >coupled with the 4.22 resulted in a very short first gear that is
nice on
>> >> >steep hills, but is also too low to make any significant speed in the
>> >> >process on accelerating from a stoplight. I didn't notice this
until now.
>> >> >Now, after taking off, there is rush of forward motion in 1st, that
wasn't
>> >> >there. This is hard to explain, let me try again...if you take off
on a
>> >> >mountain bike in the lowest gear it has, yes - it is very easy to take
>> >> >off, but this low gear doesn't get you to any useful speed before
you have
>> >> >to upshift...
>> >> >
>> >> >I have yet to try taking off on an EXTREMELY steep hill in my
>> >> >neighborhood to test the difference in that respect.
>> >> >
>> >> >If you do a lot of highway cruising, I would recommend this combo
(maybe
>> >> >not with a 948).
>> >> >
>> >> > Ulix __/__,__
>>
>> >> >..........................................................
(_o____o_)....
>> >> > '67 Sprite
>> >> >
>> >> >
>> >> >
>> >>
>> >
>> > Ulix __/__,__
>> >.......................................................... (_o____o_)....
>> > '67 Sprite
>> >
>> >
>> >
>>
>
> Ulix __/__,__
>.......................................................... (_o____o_)....
> '67 Sprite
>
>
>
|