From digest.v7.n438 Tue Oct 14 18:44:47 1997
From: Ron Katona <ron3b_at_cris.com>
Date: Tue, 14 Oct 1997 18:25:30 -0400
Subject: Re: Supercharging
Prakash Maggan wrote:
> Does anyone here have any experience or comments regarding Sebring
Experience no, comments yes. I looked into this and did a lot of
snooping around on Miata mail lists, web pages, and news groups to get
the low down. The final verdict on the Sebring was somewhat mixed. The
quality of the kit was generally praised, as was Downing Atlanta's
(distributor) service and willingness to stand behind their kits.
Performance, however, varied.
The problem with their kit comes from a lack of intercooler combined
with the least efficient compressor available - the Roots type
supercharger. A lot of people assume turbos create more charge heat.
They do not. Compressor efficiency is the key to keeping the heat down
because physics determines how much air will heat up as it is
compressed. 8 psi is 8 psi, but a less efficient compressor will cause
more heat to be developed during the compression process. Turbos are
still more efficient than the best superchargers, thus producing less
intake charge heat. Notice I don't say underhood heat, a turbo may
create more heat there, but not in the intake charge. A good cool air
source outside the engine compartment is therefore a must for a good
Sebring claims they've overcome these shortcomings, but I wonder. Some
of the things Miata owners related to me was that the Sebring has
inconsistent performance in weather. That's a sign that intake charge
heat is a problem. Another was that aftermarket ignition retard systems
were needed to prevent detonation in some cases - another indication of
charge heat problems. The M42/44 does have knock sensors that should
make such ignition mods unnecessary, but they will be working overtime
to keep up with the demands of the inefficient supercharger. In other
words, Sebring is relying on the stock BMW electronics to keep
detonation from occurring. Performance will be inconsistent as a result.
Companies like ERT generally use more efficient compressors,
intercoolers, and custom electronics to re-map the fuel curve to prevent
detonation. The Sebring kit relies on a rising rate fuel pressure
regulator on the fuel return line to increase fuel flow during the
injector pulses. It's imprecise, and given the potential charge heat
related problems, it's not an elegant solution.
Sebring advertises something like 8 psi and 200 HP for their kit.
However, with the heat problems, I doubt you'll see that on anything
other than ideal days. In hot weather, the knock sensors will be doing
their thing, and power will be reduced. I can't comment on reliability
since the stock BMW electronics are pretty darn good. They may just keep
everything together. I don't want to risk it.
I've driven a 318ti with the Mosselman Turbo kit. (In fact, it was the
very turbo pictured on the Turner Motorsports 318ti Club Sport, sold
used to a third party) Very nice. Yes, it still relies on a return line
fuel pressure regulator, however, its' more efficient compressor,
intercooler, and lower boost give a lot more peace of mind. Power is
seamless and lag is nil (really). Feels like a 2.5l 6 in both smoothness
and power curve. Low end torque was dramatically improved.
A Mosselman with custom Jim C. electronics replacing the fuel pressure
stuff would be the hot setup, although the car I drove was doing quite
well without a new chip.
I wonder also how close some old standard hot-rodding things like
porting, polishing, cams, chip, intake, exhaust, could come to 180-185
HP naturally aspirated. Would this be cheaper?? Anyway, YMMV, etc., but
I would not put a Sebring on my BMW.
[ Help ]
Ron Katona mailto:ron3b_at_cris.com
97 318ti Active