Is there somewhere this magical number of 2500 ft / min came from? I've
heard it quoted often, but never the source. With a 3.49" stroke, 2500 ft /
min is the average speed, if the piston through it's cycle at 4310rpm. but
there are instantanious speeds much higher, the piston is going around 4000
ft / min at 90deg and 180deg ie, 3 o clock and 9 o clock. .
If I take the stroke (3.4in) * 2 (up and down) convert if to feet = .58 ft,
so the piston moves .58 ft / rev ,
so 2500 ft./min / .58 ft. / rev
= 4310 rev / min.
Blair
----- Original Message -----
From: "Ray" <spook01@comcast.net>
To: "Dave & Liz DuBois" <ddubois@sinclair.net>
Cc: "Mike Duvall" <duvallcom@sbcglobal.net>; <mg-t@autox.team.net>
Sent: Thursday, September 11, 2003 5:13 PM
Subject: Re: T cruising speed
> I think <61.18 MPH at a piston speed of 2500><This is the piston speed
> generally agreed on as
> the point above which excessive wear occurs> bears out the statement that
you
> are on the edge at 60 MPH cruise speed.
> In fact, the 2500 fpm limit can be extended with chromed bores and Teflon
> coated pistons, and modern lubricants, along with proper balancing.
> The XPAG engine really did not have as its design parameters max rpm
> continuously. Or, so as to not overstate, not even close to it. Having
said
> that, I think we can all agree that these are robust long stroke engines.
> In closing, I suspect that your TBO extension is more likely due to
improved
> lubrication with modern oils.
> Monstra mihi pecuniam!
> Ray
> ----- Original Message -----
> From: Dave & Liz DuBois
> To: Ray
> Cc: Mike Duvall ; mg-t@autox.team.net
> Sent: Thursday, September 11, 2003 3:47 PM
> Subject: Re: T cruising speed
>
>
> Ray,
>
> While it is correct that piston speed in FPM determines wear in the
> engines, the Blower's manual quotes 61.18 MPH at a piston speed of 2500
> FPM (this can all be calculated out, but it is much easier to depend on
> Mr. Blower) for the TD. This is the piston speed generally agreed on as
> the point above which excessive wear occurs. 61.18 MPH is equal to an
> engine speed of 4240 RPM with a 5.125 rear axle ratio and the correct
> sized tires of the day on it. For the TF with a 4.875 rear axle ratio
> and the correct tires, the figures are 64.3 MPH at the same engine
> speed. The engine speed that I am running at, while not quite giving me
> 60 MPH is almost right on the level for 2500 FPM of piston speed. The
> last time I had to overhaul my engine, I had put close to 70,000 miles
> on it, which I wouldn't consider excessive wear for an engine that was
> originally recommended for overhaul at 50,000 miles. I am quite sure
> that the MG factory did not put out vehicles with engines in them that
> would be destroyed by running them at speeds where the maximum HP was
> being developed (5200 RPM for the TD and 5500 RPM for the TF). While I
> would not run my TD up in that range nowadays (Primarily due to the
> expense of overhauling the engine), I do not submit to the attitude that
> "these engines cannot withstand speeds in the range of 4000 RPM". I
> still maintain that the cars can withstand the high RPM levels far
> better than the nerves of the drivers of today. We here in America,
> particularly, have spent too many years in cars with big, slow revving,
> totally inefficient V8s to be comfortable with the higher revving small
> engines of European cars (or Japanese cars for that matter).
>
> Cheers,
> Dave
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