Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: Milwaukee Midget on November 07, 2007, 01:22:43 AM
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Newbie here, with a gearing question.
I'm working on an MG Midget to compete in the I/GT class. My vintage road race buddies think I'll be able to get peak power out of my engine at about 7000 RPM, and it should wind to 7800 without throwing a shoe.
With a 1 litre 5-port 4, should I be building the engine for a wider torque band and going with numerically lower gears, or taking advantage of the high RPM potential and gearing the car for peak power at the traps? My calculations, with 25 inch tires, at 7000 RPM with 3:90 gears, gets me about 130 mph. 5400 RPM with 3.08 gears will get me the same speed, but with those gears, I suspect I'm losing some much-needed torque multiplication from the rear-end. The Midget is small, but it is a bit of a brick and there's a lot of wind resistance to overcome. And despite years of racing development of the Austin "A" block, 105 horse is about the most anybody I know has been able to get out of them.
Any thoughts?
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What gears are you running in the trans? Do you plan on running a overdrive in your trans? All this will have a lot to do with optimizing your final ratio and your engine potential. The best way is to buy the gear split software from Land Racing and play with it. You can see the graph and the effects it has from different gear combos before you settle on transmission gears or the rear gears. Good luck on your project.
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RPM baby.!....cant do without it...
kent
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When i ran my Lotus 907 in a real brick, '29 Ford roadster, I was pulling about 8400 rpm in the mile. I sold the car to a guy who thought that if I got a record using 4.60 gears he would be really fast with 3.20s It refused to go even 120 mph and finially blew.
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MM
The record holder in that class before 1980 was an AH Sprite (about 97 MPH). Since that time it has been held by 2 different Abarths. The last guy to get that record did it by less than 1 MPH...
Good luck with the Midget, I would plan on peak HP to be at 122 and keep the gear spread close if possible
Have fun, that is a fun class
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...................The last guy to get that record did it by less than 1 MPH...
Good luck with the Midget, I would plan on peak HP to be at 122 and keep the gear spread close if possible........................
I agree, don't get greedy, go for the record and then you will have data to see what is realistic and can make appropriate changes.
Good luck,
Sum
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Horse power is what makes you go fast, build it for max hp and then gear it for your max speed at close to the max hp rpm. Torque makes it easy to pull away from a stop light but doesn't give you mph.
Rex
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If you only need to make about 105 mph (?) to be competitive, you need either a much smaller tire or a much taller final drive ratio.
Using the 25" tire, 7000 RPM and 3.90:1 = 133.5 mph.
Required ratio for 105 mph: 133.5 ÷ 105 × 3.90 = 4.96:1.
I don't know of any factory axle ratio above 4.22, which will be far short, and you don't have much ground clearance to throw away on smaller tires.
If you use the 4.22, the engine will only turn 5,957 RPM. The speed of the car will be limited to the power it can make at that RPM.
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What gears are you running in the trans? Do you plan on running a overdrive in your trans? All this will have a lot to do with optimizing your final ratio and your engine potential.
I was planning on using the stock gearbox, 3.200:1, 1.916:1, 1.357:1,1:1. I suspect an overdrive might actually cost me some top end, given the added friction and the small displacement of the engine.
Last I checked, 121.779 is the current record, so 122.000 is the magic number. In fact, if I go with Panic's suggestion, the 4:22's, I'm at 123.37 at 7,000 rpm (on paper, anyway), which is right where I want to be.
I'm grateful for all the posts. RichFox's comment intuitively rings true, if I'm reading him right - with a small engine, wind it tight and let the torque multiplication of a numerically higher rear end provide the extra leverage to push it to speed.
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RPM baby..!....ring its neck and pick up the pieces....
kent
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The torque multiplication isn't a variable - the engine must be turning very close to its power peak, and you calculate the gear ratio backwards from that. What exact ratio is, not really important.
An OD, in this case, would be no use since it slows the engine down even more. The exception might be if there were an even higher numerical set available (4.50, etc.), and the OD would allow an extra gear - but only a real advantage if the 4-OD shift is closer than your existing 3-4 (which is terrible).
When you shift the engine falls off to 5,158 RPM. It's important that any development work you do doesn't create a major hole in the torque curve above this speed, even if it reduces peak power slightly. This shift will occur (4.22:1 ratio) at 91 mph, and if you don't have enough power at 5,158 to pull up above 91 the car won't go any faster. I don't think it's likely but moving the torque curve around with resonance etc. sometimes creates some strange effects - don't trip over one. You don't need a dyno to detect this - just accel in 2 or 3 from 5,000 and see if it picks up a lot suddenly.
There are close-ratio internal sets for this box that move 3 much closer to 4, which is what you want. Typically, they compromise 1st quite a bit but that's not much harm on a long course. If you haven't chosen a cam or tuned intake/exhaust lengths yet I suggest you explore your power spread vs. gear ratios before making a final choice - closer ratios means the power peak can be narrower.
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Now, if you were interested in making the car go faster (as opposed to working in class limits), for much less money than would be spent on porting, cam, rockers, Webers, etc. a used Eaton M45 could be belt-driven off your crank pulley and pressurize your existing carbs. It's safer at Wendover than at home because of the low pressure, and your pistons may work well with moderate boost.
More on small blowers: http://victorylibrary.com/supercharger/super.htm
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Thanks, Panic. I was oblivious to the tranny gear spacing issue, and what you're saying makes a lot of sense.
Regarding the supercharger, I've got one on my MGB summer driver. Moss Motors makes a simple little kit using an Eaton and a single SU carb, and while my wife's Impreza will still blow it into the weeds, it's certainly the cheapest way to increase horsepower (and torque) on what is essentially a tractor motor. Much less finicky than the twin SU's, and my highway MPG went from 24 to 28. More power, better gas mileage, and less maintenance. The trifecta.
But I'm building the Midget for the class. It's going to be tough, but I like the discipline that the rules bring to the effort – it’ll keep me focused.
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Thanks, Panic. I was oblivious to the tranny gear spacing issue, and what you're saying makes a lot of sense.......................
If you need them I have spreadsheets on my site that will show the rpm drop between gears for a shift point at any rpm and for any gear change:
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
c ya,
Sum
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An overdrive tranny might not be a bad idea. Here is why. My buddy has a overdrive MGB tranny in his MGA. It is electric. I think one of the kind that replaces the tail section of the tranny. It has a switch on the knob. Being that you have a B I am sure you are familiar with this. Anyway his overdrve can be used in any gear, which effectively makes it a gearsplitter, and give him 8 forward gears. How much loss it would add to the driveline I have no idea.
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Doh!! Memory failure - thanks.
Yes, the external Laycock de Normanville OD can be used in intermediate gears, there is a chance that 3rd + OD is mid-way (or at least useful) between 3rd and 4th to reduce your 3-4 shift RPM drop (4th to OD will slow the engine down too much). If it's less than the 26% loss you have now it will help, but not much, the most common is .80, so you'd only get another 400 RPM. It may also help between 2nd & 3rd.
It's an ambitious project in itself, since it involves shortening the driveshaft, re-angling the pinion stub and/or engine height, new brackets to support the OD box?, floor mods. however: it will make a taller (4.22:1) gear streetable again.
An Eaton of the correct size can be had and installed for less than $500...
Rough sizing: Eaton turns about 2 × engine speed for best efficiency, so Eaton nominal size can be as small as 50% of engine size: a 1500cc engine can use an M45 (= 45", 737cc) at 2 × speed to get roughly 1 bar.
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" . . . but I like the discipline that the rules bring to the effort . . ."
Boy -- we don't hear that very often here.
Stan
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True, but now we're talking different trannys altogether, with different ratios. Yeah, the OD MGB tranny might be made to fit with a custom 948 bellhousing. There's probably somebody out there making 'em.
The OD is 0.82:1.
The MGB tranny has different ratios, so with the OD operating in each gear (bypassing the stock 1-2 electric lockout on the od), my ratios at the yoke effectively become:
1= 3.44:1
1od= 2.8208:1
2= 2.167:1
2od= 1.77694:1
3= 1.382:1
3od= 1.13324:1
4= 1:1
4od= .82:1 (probably useless)
Compared to
1 = 3.200:1
2 = 1.916:1
3 = 1.357:1
4 = 1:1.
That's a bit more elegant - you don't fall off the table on the 3-4 shift, and if my math is close, I'd be looking at 5700 at the start of 3rd, 6200 at the start of 3rd OD, and 6100 going into 4th.
Another advantage is that I've got that tranny out in the garage, which would let me do the Rivergate 5-speed swap into my MGB.
I can hear her now - "Honey, why are both the MG's up on jackstands?"
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Seems like a supercharger could add enough torque to the upper mid power band to build momentum right at the shift into overdrive. If you can find a cam with a peak torque curve about 5500RPM or so and a peak horsepower curve around 6800RPM (maybe a 292/312 split diff or so) the overdrive could probably give you an edge.