Markp
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| posted on 7/7/03 at 10:48 PM |
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0-60
I need a bit of advice.
I have had my car on the road for about a month now and generally i'm happy with the performance.
I have a 1600 Xflow and a four speed box (all straight for the MK2 Escort)
The only trouble I have is I expected it to go quicker, I don't feel the pull I expected.
I have a new clutch and the engine tuned but still none the better.
Apart from changing the engine and box is there ang thing else I can do??
Will changing the rear axle ratio help?
I need it to be quicker but I don't want the hassel of changing the prop, speedo, engine mounts, exhaust, etc...
Any help would be great.
Mark
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stephen_gusterson
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| posted on 7/7/03 at 10:56 PM |
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how much does a 1600 xflow make in power? about 80hp?
if so, and your car weighs about 600 kilos, thats around 135 bhp / tonne.
My mazda 323zxi had about 122 bhp tonne and that did about 8 seconds.
So, I would suspect that your car lies somewhere between 7 - 8 secs.
7 secs should feel quite brisk!
have you timed it?
a std 2 litre sierra efi makes 115hp. Thats 190 hp / tonne.
atb
steve
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ned
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| posted on 8/7/03 at 08:47 AM |
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Depending on which diff you're running, changing that to a higher ratio would increase acceleration but reduce top speed.
Ned.
beware, I've got yellow skin
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stephen_gusterson
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| posted on 8/7/03 at 09:04 AM |
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it would be interesting to know if changing diff ratios really does that.
I dunno either way, but I saw a lot of info on a jag xj-s site once that said the diff ratio made very little difference.
I have no opinions either way - it would be interesting to see if anyone can confirm or deny from trying both....
I recon it has more to do with engine torque - more torque allows a lower ratio.
atb
steve
[Edited on 8/7/03 by stephen_gusterson]
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ned
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| posted on 8/7/03 at 12:34 PM |
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well, why don't we ask jasper in a month or two. he's running a 4.1 i think he said in his car and hes always changing gear. see if he
puts in a 3.9 or 3.6 and ask him what difference it makes. I think it will be noticable.
Ned.
beware, I've got yellow skin
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stephen_gusterson
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| posted on 8/7/03 at 01:22 PM |
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The acceleration of an object is due to the power applied (force) and what you are trying to move (mass).
So, acelleration will depend on power applied at any given time.
The amount of revs being pulled at that given time is irrelevent in terms of pure ratios - it matters more where the power is being delivered in the
rev range.
What matters is available power, and the spread of torque from the engine.
Gearing on road cars tends to drop the revs into the power band after a change - my 318i drops to 4,500 from a 6,200 change. If the axle ratio drops
the engine into a more preferable area of the power band, it will accellerate faster.
A bike engine is likely to generate power further up the rev range than a car engine, so it needs to keep the revs higher in order to apply peak power
- which would tend to point to faster acelleration with a higher (4.1) ratio axle.
With a car engine developing peak power and torque half to 3/4 through the rev range, it may accellerate faster with a lower ratio (3.5) axle.
The XJ-S 5.3 had a 2.88 ratio. The 3.6 litre version had something nearer 3.5 IIRC. Same car, same approx weight, but different power / torque curves.
thats my theory anyways.
atb
steve
[Edited on 8/7/03 by stephen_gusterson]
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Jon Ison
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| posted on 8/7/03 at 07:48 PM |
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well all this maths is way over my head but one thing i know for sure, higher in the diff ratio from the one used at donnington for the cadwell race
gave quicker lap times, we aim to be on the rev limiter in top at the same time as we want to brake at the end of the fastest straight.......we allso
had better acceleration out of the hairpin with the higher diff too.
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steve26
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| posted on 8/7/03 at 10:58 PM |
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quote: "Gearing on road cars tends to drop the revs into the power band after a change - my 318i drops to 4,500 from a 6,200 change. If the
axle ratio drops the engine into a more preferable area of the power band, it will accellerate faster."
Hi Stephen, you are right in saying that the gearbox ratios are designed to keep the engine within its power band. However by altering the axle/final
drive ratio you dont actually affect the use of the engine's power band, all you do is vary the torque driving the rear wheels i.e. taking the
torque output from the engine / gearbox and multiplying it by a factor which will either increase or decrease acceleration. In other words with a
higher diff ratio but the same gearbox the engine revs will drop to the same values when you change gear, but the engine will accelerate faster
through its rev range.
Markp - as Ned said I would check the ratio of your diff as there are a number of standard Ford ratios available - ranging from a 3.54:1 (less
acceleration) up to a 4.4:1 (more acceleration / less top speed). The 3.54 was commonly fitted to the RS2000 Escorts (around 105bhp) and if this is
what u r running, would certainly cause your 1600 Crossflow to feel sluggish. As a stab in the dark (and from playing about with different ratios in
my 1300 race locost) I would reccomend somewhere around a 3.9:1 (fairly commonly available) or a 3.77:1 (fitted to 1.6 Capris, 1.3 GT / Mexico
Escorts, and others) is likely to be suitable for general road use. In addition, if you can get hold a MK 1 or 2 Escort GT/Sport Gearbox this has
better gear ratios to keep the engine more within the power band than the standard 1100/1300 box. It is also identical housing + mountings etc to the
11/1300 box so would be a straight swap. However to be honest both diff and gearbox swaps are only likely to be moderately subtle improvements to
performance and the problem may lie elsewhere, for example engine not running as it should etc. Have you had the car rolling roaded - i.e. do you have
any idea what power output the engine has. As Stephen said, a 0-60 time would be useful to know as it gives an idea of whether everything is working
as it should + whether ratios are about right, or whether it is your own 'need for more speed' that is resulting in u'r
dissapointment in performance.
Or in fact it could be a combination of a number of factors that are reducing performance. What size wheels + tyre r u running? 13 or 14" are
about right. Anything much bigger would take acceleration away which would need to be compensated by a higher diff ratio (more like 4.1 or 4.4)
...etc....etc.
p.s. to check your diff ratio there might be a metal tab attached to one of the diff housing bolts with the ratio stamped on. If not you have to put
the car in neutral and jack up one of the rear wheels. Put a mark on this tyre and turn the diff / propshaft until the tyre rotates fully one turn,
counting the turns of the diff. This will give you the ratio.
Hope this is of some help.
Best Regards, Dan S
Locost 69
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craig1410
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| posted on 11/7/03 at 10:47 PM |
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Hi,
Just my tuppence worth:
Changing the diff will only make a significant difference to acceleration if one of these statements is true:
1. With the original higher ratio diff you were unable to apply enough wheel torque to spin the wheels right through first gear (tyres are limiting
factor therefore lower diff ratio would not help if you can spin them already)
2. Having the lower diff ratio will allow you to accelerate through the desired terminal velocity (eg 60MPH for 0-60) more efficiently than if you had
the higher diff ratio. (eg. My Vitesse can hit 60MPH in 2nd gear so if I lowered my diff ratio then I would need another gearchange and that would
actually make my 0-60 worse!. But if I was doing 0-80 timings then lowering my diff ratio would help because I could make better use of third gear and
would be approaching the peak power RPM at 80MPH with the lower ratio. With the standard ratio I would not be developing as much power at 80MPH due to
lower RPM as I can exceed 90MPH in third at present)
This second effect is why some cars are on paper faster (0-60 MPH) but in practice they are not so fast as other cars which do not have such optimised
gear ratios. This is common for hot-hatches because it sells cars to boy racers...
One general rule to bear in mind, it is not peak power or peak torque that is important for acceleration, it is average power over the RPM range used
whilst accelerating through the gears which is important. If you put your car on a rolling road dyno you will get a nice power and torque (at the
wheels) curve which you can use in conjunction with the gear ratios to calculate the optimum gearchange point for each gear. Each gear will most
likely have a different "optimum" change point due to the different steps between each gear ratio. I did this for my Fiat Uno Turbo a few
years back and it allowed me to calculate the actual "average" power output between 0 Mph and 60Mph. This allowed me to very accurately
predict my best theoretical 0-60Mph time which I could almost achieve if I got a perfect start and good gearchanges (powershifts)
So, my advice would be, if you have enough torque already to spin the wheels comfortably in first gear and you are not a "paper racer" and
don't care about specific timings such as 0-60 then just keep the gear ratios that you have. Lowering them too much will just make 1st gear a
handful to the point where you won't use it any more and make the car buzzy at cruising speeds.
All IMHO of course!
Craig.
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stephen_gusterson
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| posted on 11/7/03 at 10:54 PM |
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I can see the logic in both craig and steves posts.
The stuff I read was based on an auto box from a car that already produces huge torque (xj-s v12) so ratio changing would have lesser effect. The car
could chirp wheels from a start quite well even with 2.88 diff. chronic in wet.
Apparently a torque converter serves as a multiplier, and can increase torque (im told) by a factor of three. On a car with only 3 gears thats
needed!
On jaspers car, I can imagine a lower ratio would make the car less thrashy at constant speeds. My metro turbo sounded like it was gonna explode over
100, but was able to go well past the red line at 6k and 114 mph. Four speed metro / mini box wasnt intended to go from 34 to 93 hp when designed in
50's!
atb
steve
[Edited on 11/7/03 by stephen_gusterson]
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MikeRJ
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| posted on 12/7/03 at 04:32 PM |
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quote:
Originally posted by stephen_gusterson
how much does a 1600 xflow make in power? about 80hp?
a std 2 litre sierra efi makes 115hp. Thats 190 hp / tonne.
You didn't take into account that the Pinto weighs about twice as much as the crossflow
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stephen_gusterson
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| posted on 12/7/03 at 06:54 PM |
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and that the pinto is taller!
power to weight takes whole vehicle into account, not just engine.
A tuned xflow seems like a good engine for a low bonnet line.
Im hoping Dave Ashurst will convince me of that with a demo one day!
atb
steve
[Edited on 12/7/03 by stephen_gusterson]
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kingr
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| posted on 13/7/03 at 12:24 AM |
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I can bear witness that 140Bhp out of a 1.7 Xflow does make it shift very nicely indeed.  
Kingr
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