Date: Mon, 16 Dec 2002 18:36:57 EST
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Engine Performance vs Tire Size/Comments Redux
Content-Type: text/plain; charset="US-ASCII"
In a message dated 12/14/02 4:00:16 PM, lchase@ATTGLOBAL.NET writes:
<< A while back my stock engine was rebuilt (stock 2.1L to a Eurocars 2.2L)
After the rebuild my "Seat of the Pants" conclusion was that I had
gained 10-15% in Engine performance (Acceleration & Top End MPH).
Now to the topic ..... Engine Performance vs Tire Size
Since then I've had the opportunity to try two different tire sizes on
the same vehicle. Since I was planning on going to a large than stock
tire, I anticipated sacrificing a little power / performance.
But it seems to be more than a little.
Tire Details:
Size / Width / Diameter / Rev per Mile
Original Owner Tires
27X8.5R14 / 8.6" / 26.5" / 795 ... Dunlop Rover - 10-15% Engine Power
Increase (as noted above)
First Replacement Tire Set
215/65R15 / 8.7" / 26.1" / 818 ... Michelin - No Change Engine Power
Increase
Second Replacement Tire Set ... Current Tires
215/75R15 / 8.5" / 27.7" / 755 ... Michelin LTX 0% Engine Power
Increase
Does it make sense that I'd loose 10-15% in performance by increasing
the tire diameter 1.2"?
>>
Larry and cognitave listees,
I am moved to make a comment here. In the past, see archives, I have offered
a detailed discussion about the issues of perceived performance gains,
transmission and gearing issues, powerplant choices and the impact of tire
size changes. In those ramblings, I indicated that the key issues for vanagon
performance included the torque delivered to the tire contact patch, the mass
of the vehicle, the drag (function of airspeed and model) and the work
insisted upon by the driver/operator (includes the effort to accelerate from
one velocity to another and/or the effort required to scale a grade). I
quoted numbers for the amount of reserve thrust remaining at any given
flatland velocity (up to 65 mph) for combinations of powerplants (2.0L, 1.9L
and 2.1L boxers, 1.8L, 2.0l inline 4's and 1.6L diesels and turbo diesels),
manual transmissions (including the key final drive ratios) and tires
(including stock 185SRx14, 27-8.50x14 and 215/75x15). The powerplant choices
were dictated by the analytical performance curves at my disposal.
In my warning about performance losses going from the DZ diesel gearing to
the AC or later WBXer transmissions with inline 4 transplanted engines, I
indicated that tire size changes had the same effect as final drive
regearing. The change in total thrust available is linearly related to the
effective diameter. A change in tire size going to a tire with an 8% larger
diameter would give an 8% change in radius and therefore an 8% reduction in
thrust at the tire contact patch. Depending on the amount of excess thrust
the powertrain is capable of at any given rpm, this change could range from
inconsequential to catastrophic. For the 1.6L diesel, this could drop the
flatland top speed from 62.136 mph to 54! Other impacts remain for the
interested peruser of the archives.
Note however that your numbers for tire diameter refer to the unloaded
inflated body diameter. This number is only of use for the tire dealer in
determining how many can be stacked end to end before striking the ceiling.
The real number is the measured rotations per mile. This gives the loaded
radius at the indicated inflation pressure and vehicle loading. By those
numbers, the 818 rev/mile tires are 2.8% smaller than the 795 units and 8.3%
smaller than the 755 rev/mile units. The real rolling diameter of the 818
rev/mile tire is 24.66 inches while that of the 755 rev/mile tire is 26.71
inches for a delta of 2.05 inches in diameter or 1.025 inches in radius.
Finally, on the issue of seat-of-the-pants power increases I have strongly
suggested that they are virtually meaningless. Just as meaningless as the oft
quoted Lilley (SP?) power enhancements. Measurements, even relative
measurements probe reality and the answers (as noted by Boston Bob, myself
and others) can often trigger fiscal remorse. I have also put into the
archives multiple and inexpensive ways of measuring power increases, even
with the vanagon as a test mule. Many of these inexpensive methods are more
accurate than dyno time. Nonetheless, I have given a quick way to measure the
impact of a roughly 8 HP change in the engine power output. See my comments
on switching of the AC compressor for those so equipped.
To summarize, the hit from tire size changes can easily exceed 10% of the
available excess thrust (net torque) at the wheels. The change in torque from
a small underpowered engine, driving the vehicle through a 5.87 vs. a 4.86
real drive final ratio should be amazing. I'm still waiting to hear about the
results obtained by the individual with the diesel westy who was advised to
run the AC trans in order to reduce revs!
More transmission comments coming.
Frank Grunthaner
(PS. I purposefully quoted Larry's original post to keep the topic in
context.)
