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PostPosted: May 20, 2017, 10:08 pm 
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So I was learning execl the other day and decided to in put my M5ODR1 gear set and rear end ratio and tire size. I wanted to get a feel for what
kind of RPMs Id be running at different speed ie. 30mph, 55mph, 75mph and top speed ( theoreticly ) and how they related to my power band.

An example of my thinking is you go through a tight corner at 30 or so and want to exit and accelerate near you highest engine torque numbers..

for my D2.5 it starts pulling at 2700 rpm . I found that the M5ODR1 wasn't half bad for what I think would be a good street set up..
however the ford 8.8 3.73 rear end seemed put the speeds a little on the low end for a car of this size and weight. I put in a 3.08 ratio
and the numbers obviously went up and seemed to give a nice balance of speeds , hp and tq

so my question to all of you ( with four cylinder experience )is this what kind of ratios,rpms,tire sizes,and cruising speeds do you get / like??

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PostPosted: May 21, 2017, 3:14 am 
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I have a similar spreadsheet and have spent a lot of time thinking about this myself. My comments are based on assumptions of 140-150whp Duratec 2.0, a 7000-7200 rpm redline, M5OD-R1, Ford 8.8, and a 25" OD tire. If you're looking at a 23" OD tire, I believe you can pretty much just move the comments one up to one ratio taller, as the gearing would be roughly one step shorter by comparison. My thoughts on top speed and track performance are also in relation to having considerably more aerodynamic Lalo bodywork. With more 'traditional' 7 bodywork, I'd guess that the aero would dominate any 4th gear track concerns. When the tires hit the pavement, there are obviously more factors to consider than I'm looking at here. But this is what I've got to work with so far, and even with just looking at these factors if really comes down to how each individual wants to balance them to best meet their own goals.

I know from my experience even atuox'ing a 4cyl FWD econobox on modified suspension and sticky street tires, that a 60+mph top speed in second is a must for me. I'm not sure exactly how much taller I would want go, but I do know that production cars with with similar power to weight ratios to what I'm expecting seem to typically have 70-75mph 2nd gears. For the 1/4 mile I think that I'd probably want to avoid shifting too shortly before the lights. So either finish in 3rd, or shift early enough that it's able to feel like it's building some steam in 4th as well. The 3-4 rpm drop is also relatively large by comparison to more 'sporty' transmission options. I look at 4th gear as expected max track velocity, but have no idea what that would actually need to be for any given track. While I want to see some track time, competitiveness isn't really an issue. I guess I see this as max velocity I'd ever expect to take it. This is not being built for any type of LSR use, so I don't really care about maximizing the power band in 5th vs speed...Not that I haven't looked at it for entertainment purposes either. So rather than speed in 5th, I look more at rpm at various likely cruising speeds.

Ultimately, my plan would be to run it with the stock ratio for a while at first, to really get comfortable with the car and get a better feel for exactly how much of a rear end ratio change I really want when theory meets reality.

3.07/3.08: This is what comes stock in my diff. It's rather long legged, possibly too much so for me. I expect it would largely be used as a 4-speed, even at local highway speeds. I'd probably need to get outside of the city where the speeds are a little higher, and possibly on flatter land, before being really able to just hang out in 5th. Around town this would probably be a little on the dull side by comparison to the other options, and would never really be able to run out more than 2nd gear before getting into arrest-me territory. I believe 2nd would be more than tall enough for pretty much any autox course, and I'd likely be able to finish the 1/4 in 3rd. Full terminal velocity would probably still happen in 4th.

3.27/3.31: This provides a decent balance and is probably one of my main contenders. Around town it might be able to at least justify the shift to third, but not use much of it, and should have a little more usable 5th on the highway as well. I believe 2nd is in rather good autox territory, although the shift to 4th would be a bit closer than I'd like to the end of the 1/4 mile. Even with the Lalo bodywork, I still don't think I'd be able to run out 4th at any track with my expected power.

3.55: This flips the balance of the previous set, and is my other main (and current leading) contender. It think it would feel more biased towards around town driving, with a bit of 3rd becoming more accessible, but would still have a tall enough 5th for the occasional longer highway trip. It's possible 2nd will be a little short for some autox courses, but it'll get to stretch 4th a little longer in the 1/4. I'd guess 4th should still be suitable for any track I visit, as even if it 'can' run out 4th, I'd probably run out of intestinal fortitude and back of before making such a precariously timed and brief shift while barreling towards a corner at those speeds anyways.

3.73: My gut says this may be a little on the short side for me. It would definitely be able to used 3rd more around town, but would be starting to rev higher than I think would be ideal on the highway. I'm pretty confident that it would need some 2-3-2 shifting (or extensive rev limiter riding) on higher speed autox sections, although 4th would finish pretty strong in the 1/4 mile. I also wonder if 4th might be a little short for the occasional 'bucket list' track, possibly requiring some exercise in 5th that I'd rather not have to think about...I'm looking at you Road America.

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Last edited by Driven5 on May 21, 2017, 4:22 pm, edited 5 times in total.

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PostPosted: May 21, 2017, 6:12 am 
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Location: Connersville, Indiana
I think the most important thing is too decide what your car is going to be. My car is a Sunbeam Alpine, built for long distance cruising. I'd guess the typical road weight would be about 2750 pounds. Tranny is the stock Ranger M5, axle ratio 3.80, tires 24". This gives me 3000 rpms @ 70 and I love it, as I'm at the bottom of the power curve at cruise and we can go up from there until I run out of bravery. I might add the big four with balance shafts seems to be perfectly content at those speeds. For a while I had a 3.55 rear, but it seemed the engine was always playing catchup and I did not like it. It gave me 60 mph in second, which I liked, but I don't spend very much time at 60 in second. I spend hours on the road at 70 in 5th. Highway mileage, low 30's. In a lighter car, I think I would be pretty happy with a 3.55. I would really hesitate to go lower.

The D2.5 has such a torque band I can drive around town in ANY gear. Accidental third gear starts are no big deal. The engine can handle it. Fourth gear starts are not particularly traumatic. Driving around town in 5th is pretty common. Never tried starting in 5th.

You night consider stuffing the 4.0 gearset into your M5. Gives you a sort of close ratio setup:
2.3 gearbox 3.72, 2.20, 1.50, 1.00, 0.79
4.0 gearbox 3.40, 2.05, 1.31, 1.00. 0.79

I think that if push came to shove, I'd rather go with the 4.0 ratios than swap rear ends.

Bill


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PostPosted: May 22, 2017, 11:22 pm 
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Quote:
My comments are based on assumptions of 140-150whp Duratec 2.0, a 7000-7200 rpm redline, M5OD-R1, Ford 8.8, and a 25" OD tire.


yup basicly the same set up the D25 is 175 hp @ 6000 and 175 tq @ 2700 withe the 2.3L M5ODR1 , 3.73 8.8 and a 25.7" tire
ill have to admit I was a little depressed seeing the numbers with the 3.73 it gave me a top speed of around 98 mph..

Quote:
Ultimately, my plan would be to run it with the stock ratio for a while at first, to really get comfortable with the car and get a better feel for exactly how much of a rear end ratio change I really want when theory meets reality.

3.07/3.08: This is what comes stock in my diff.


right now Im thinking on running my 3.73 lsd for a while also to see if i like it as well ... we'll have to compare notes after logging a few miles ...however I dont think my car will see much track use or auto x ( maybe 10%) ? mostly DD in the spring summer and fall as long as I can get away with it..

Quote:
I think the most important thing is too decide what your car is going to be. My car is a Sunbeam Alpine, built for long distance cruising. I'd guess the typical road weight would be about 2750 pounds. Tranny is the stock Ranger M5, axle ratio 3.80, tires 24". This gives me 3000 rpms @ 70 and I love it, as I'm at the bottom of the power curve at cruise and we can go up from there until I run out of bravery. I might add the big four with balance shafts seems to be perfectly content at those speeds. For a while I had a 3.55 rear, but it seemed the engine was always playing catchup and I did not like it. It gave me 60 mph in second, which I liked, but I don't spend very much time at 60 in second. I spend hours on the road at 70 in 5th. Highway mileage, low 30's. In a lighter car, I think I would be pretty happy with a 3.55. I would really hesitate to go lower.


That was one of my concerns with the 3.73 is having the 70mph cursing speed to far up the RPM range.. but it sounds like you've found a sweet spot at the 3k mark for 70 MPH
Ill have to play with my numbers some more ..

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PostPosted: May 22, 2017, 11:45 pm 
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Soo... I must have a problem with my basic math or my new found use of excel ...I found this site to double check my numbers on because I dint feel they were acurate
and low and behold they weren't according to this calculator http://www.grimmjeeper.com/gears.html looks like Ill be learning more about execl and basic math ..
untill then I think ill stick to a good calculator ...

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PostPosted: May 23, 2017, 11:25 am 
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Yeah, your comments about a 98mph top speed had me wondering about your math...If you want any help figuring out where it went wrong, let me know.

Your 25.7" tire puts your 3.73 similar to my 3.55. But even my D20 ends up with similar peak hp to a stock D25, yours will still have a considerably more low end grunt to pull that gearing. The lower torque on mine is why I'm also by no means counting out the 3.73 for my build either. Bill's 24" tires would be similar to tossing a 4.10 in yours, but you'll have a lot less weight to lug around than Bill as well. So considering all of that, your 3.73 sounds like it might be just about the perfect starting point anyways :cheers: .

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PostPosted: May 23, 2017, 12:50 pm 
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One thing to consider with gearing is if you ever decide to do autoX in it. You want a car that will be able to hold a gear (typically 2nd) up to 100kph or a little higher while still not dropping out of the torque-band at low speed. Otherwise you're on the rev-limiter or having to shift. Neither are good for autoX (especially the latter).

Cheers.

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