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MG MGA - RPMs to shift at

Hi all,

Over the years I have found this forum invaluable and made great friends.

In 1966 I owned a 60 MGA 1600 which I kept for many years. In 2001 bought another 60 1600 with 3.9 MGB differential... great fun!

I'm wondering at what RPMs people typically top out at?

For me it is:
1st-14mph
2nd -28mph
3rd - 45mph
4th - 65mph

While I'm comfortable with this, I wonder if I should be topping out at 4500 or 5000?

Thanks in advance, Paul
Picture will follow!
Paul Hinchcliffe

Paul
I don't think it really matters what revs you change up at, the best answer I can give you is, "It depends".

It depends on how you feel on the day, if you are enjoying a very leisurely cruise, then changing up at 2000 rpm will give you a very comfortable days drive.

But if you are out there to push the car to its limits and get the very best performance out of the engine, then you need to rev it to at least 5500 rpm in the gears.

Probably somewhere in between those ranges will suit you better.

I don't normally rev my engine much above 5000 rpm but the other day I was closely following a modern car which had just overtaken me in a rather dismissive manner.

So, my old racing driver instincts came out of hibernation and I changed down a gear and soon came up behind him.
I think the thought of having a 60 year old car on his bumper upset him slightly and he put his foot down.

My MGA easily matched his speed and he was obviously a little aggrieved by this and accelerated even more. But then I realised that I was doing 6000 rpm in 4th gear and so I decided that maybe I should back off before I did some damage.

It was fun though but I think it would be more sensible to stick to my more leisurely 2000 rpm gear changes in future ( that is, unless someone overtakes me :^) )

Cheers
Colyn











Colyn Firth

Paul,
I think you would need to put your car on a suitable and good tuner's rolling road to find the power band of your car to see where it picks up best and tops out in the rev range.

Obviously you don't put it on the rollers until it is fully serviced, maintained and repaired otherwise it won't get to its available optimum.

For road use you're not looking to bust a gut with it but fully set up for road performance will also give a combination or compromise of best pick up, mpg, reliability, starting, ect..

The peak power of your particular engine and set up with adjustment tuning might be lower in the revs than you think or perhaps higher. There's usually little point going much above peak as it's just noise and wear.
Nigel Atkins

meant to include all at 3500 rpm
Paul Hinchcliffe

As Colyn says.

In the pre classic and pre 5 gear days I used to cruise all day at 4500 and that seemed fine. I don’t think I would cruise at much higher revs. But in the end it’s down to you.

Paul
Paul Dean

Ah, the beauty of the 5-speed. I run the 3.9 with my T9. I would be banned from driving if I sat in top gear at 4500 RPM as 4250 gives me 100 MPH.

Like others have suggested I change gear by the seat of my pants feel and what the road and traffic suggests at the time. Sometimes it is the blood red in the eyes as Colyn experienced. Other times it is just an effortless cruise (wife in passenger seat).

Steve
Steve Gyles

As a paper exercise, at 3,500rpm from your figures you get (in round figures) -

. 1st - 4mph per 1,000rpm
. 2nd - 8mph per 1,000rpm
. 3rd - 12.8mph per 1,000rpm
. 4th - 18.5mph per 1,000rpm

If you want change points for acceleration then say the standard engine has peak torque at 3,800rpm (and peak bhp at 5,600rpm) but you don't really want to go above 5,000rpm too often then you get -

(mph per 1,000rpm times 3.8 and 5 for range)

. 1st- 15mph - 20mph
. 2nd- 30mph - 40mph
. 3rd- 48mph - 64mph
. 4th- 70mph - 93mph

These are paper figures and you can change below these for more leisurely driving.

Different cars and different drivers will have many variables so everything is in general.

Nigel Atkins

It's no different from any other car ever made. The harder you are on the throttle, the higher into the power range you go before shifting. You can shift at 2.5K RPM if you're driving along on city streets but try that when you're on full throttle and you'll ping, and go nowhere fast.
Steve Simmons

I rarely go over 3k RPM unless cruising on the highway and drive quite fast on the roads where I won't get a big fine for doing so. There's nothing to be gained by flogging the engine at high revs, you just have to find the balance of where the revs and the power balance out with an easy mode to the next gear which leaves you in the power band for the chosen gear, up or down. Shifting at 4500 seems a little late to me, I think you will find your fuel consumption and noise will go down a LOT by shifting a lot earlier. Don't labour the engine by changing at 1200 RPM, but try shifting at around 2500 max and you will find very little loss in performance and a much mire relaxing drive

1950cc and 5 speed - cruise is comfortable at 2800 rpm, power is always available!
Dominic Clancy

Ah!, but the problem comes when climbing steep mountain roads in 95F degrees and the temp gauge starts climbing. I took a drive in my MGA Mk II stock a month ago up to Banner Elk and (I'm a conservative touring driver) maintaining 2800 to 3400 RPM saw the temp rising at an alarming pace topping out (per the gauge) at about 205. Last week I drove the stock TD up to Lake Lure (admittedly not as steep but still pretty hilly and a much hotter day) and avoided the dreaded +200 by using third (and even second) a lot more and kept the RPMs up over 3200 and the temp never busted 200F. On the uphill stretches I'd stay in third at 3800 to 4K for extended periods.

I think it all depends on what the ambient temp is, where you're driving and how you feel about your car.

My seat of the pants limit is about 4700 RPM, my friend in his '52 TD limits himself to slightly +5500 RPM. Reading some of the old literature, these MGA and TD engines, in good tune, can be coaxed to +6 or 7 grand without damage although few of would ever stay in that range for more than a few seconds.

I tend to lug the poor engine and am trying to break myself of that trait. Lugging is probably worse for the engine that brief trips up in the 5K RPM range is.

Just my seat of the pants feelings.

Jud
J. K. Chapin

The engine won't so much as blink an eye at 4500 RPM, but you can break it putting the pedal to the floor at 1500. Lugging is the worst thing you can do! I pretty much never let the RPMs get below 2K in any driving situation above 1st gear, and that's only when cruising along sedately. When accelerating, I never give it more than a little gas below 2500. They just don't like it. Comfortable highway cruise is 3200-3800. High speed highway cruise is 4-4500. My normal shift point when passing on the highway is about 4500 because that's where the cam begins to drop off. But every engine is a bit different on torque and HP curves so your shift points may not match the next guy.
Steve Simmons

The aim of most gear changing, whether on the race track or the road, is to be at the desired RPM in the next gear you are choosing.
For this I have developed a program but it needs to be viewed with Microsoft Excel on a Wndows computer. It does not work on IOS software on iPads etc.
The factory figure for a pushrod MGA with standard gear and rear axle ratios and original tyres was 17.2 MPH at 1000 RPM in top gear.
However these days most vehicles have tyres of various sizes. To accurately use the program you need to determine your own MPH per 1000 RPM not with speedometer but with GPS or road markers such as the markers every 5 Km on main Australian roads.
Once you have that you change the wheel size on the spreadsheet until you get your measured MPH per 1000 RPM. After that all calculations will be correct.
You can change the gear ratios to close ratio etc.... and the rear axle to 3.9 etc ...
There are separate spreadsheets for KPH and five speed gearboxes.

http://www.angelfire.com/amiga/mga/index.html

Mick
M F Anderson

Mick,
great stuff, I'll use that if I may for others?

Did you already allow about 3% off the tyre size for rolling circumference and standard tyre pressure and standard weight load of car (or is that being too fastidious of me)?
Nigel Atkins

The point about the use of lower revs highlights the the need/usefulness of a (reliable, not ego) rolling road print out to see where the torque curve is at different rev points, plus this take out the variables on the set up and wear of different engine that varied anyway when they left the factory leet alone 60 years later.
Nigel Atkins

Nigel,
Feel free to use the spreadsheets however you wish.
There are so many different opinions on how to measure the effective size of a tyre that I do not do that.
There is the unloaded diameter, the loaded diameter, twice the loaded radius etc ....
I need a high level of accuracy when developing software for traction control and launch control.
I measure the time taken at a constant RPM in top gear over a measured distance (several miles) and then calculate the speed at 1000 RPM.
I then set the spreadsheet RPM at 1000 and keep adjusting the wheel size until the speed shown in the spreadsheet matches the earlier measured speed.
Once the wheel size has been set all the other values can be changed as required and the results are then given.

Mick
M F Anderson

Sorry Mick I meant to put a smiley at the end of the of that question.

Car Bibles, before he went all commercial, on his Tyre Bible section had a great explanation and calculator for rolling circumference but that's all gone now or backroom archived so no longer available.

After this the few other calculators I found on the web seemed to miss some factors or round up or down figures to give final figures I couldn't fully relate to.

I find the 3% off the stated tyre diameter to be about good enough given that reading to 1 mph on the speedo is perhaps taking my concentration off driving for too long and too much.
Nigel Atkins

I want to thank everyone for your contributions!

While I was looking at top end RPM's to shift at, I had not been considering the low end limit, below which lugging prevents high risk to damage engine!

Good knowledge and discussions all.

thank you, Paul


Paul Hinchcliffe

sorry, %$)"? spell check, should have been

' below which lugging preSents high risk'

Presents, not prevents

yeeesssh!

paul
Paul Hinchcliffe

This thread was discussed between 02/07/2019 and 05/08/2019

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