Making this SkiDoo 600RS engine competitive

Making this SkiDoo 600RS engine competitive
Local sled snow and hilldragracer Jake Lehnen has been coming to DTR for several years
looking for competitive HP with his SkiDoo 600 race sleds. In the past, Jake has tried
mod HO and RS engines but never hit on combination of engine mods/ compression/
ignition curves/ pipes etc to make the 165HP he was looking for to be competitive with
the best 600cc twins. He was stuck in the 135-145 HP range.
On this website (5/16/2012) we documented the yearly HP improvements of the Hentges
Racing Polaris 600cc factory snowcross racers from inception to 2012 which was the last
year of the full mod snowcross race engines in ISOC. Starting with 150+ HP in 2006 and
ultimately creating 170 HP at over 9000RPM. That final state of tune included Jaws high
RPM twin pipe stampings fit into the IQ chassis by DynoPort, with backpressure
optimized for maximum HP. Jake bought a set of those Jaws high RPM pipes made to fit
the SkiDoo chassis to compare to the lower RPM factory race pipes and the Jaws low
RPM high torque pipes that he had. And since most of the Hentges mod engines’
cylinders were ported by Justin at Full Power Performance, we were able to get him to
modify the SkiDoo 600 engine to see if he could apply his Polaris 600 port timing/ sizes/
shapes to try to get competitive power.
Phase I as shown initially was a stock 2010 600RS engine. The only modification was
higher-than-stock compression ratio (Bombardier high compression race domes) and
modified ignition timing curve. Jake had three different ECUs with rev limiters removed
and with varying timing curves along with a variety of offset keys to advance or retard
timing as needed. Sunoco Maximal fuel was used.
With the stock engine and factory race domes the high RPM Jaws twin pipes made 155
CHP, with timing and jetting maxed out to highest HP without detonation. The timing
was best with one of Jake’s custom ECUs with a 4 degree advance key added. This
power level is higher than any of the mod packages he’s tried so far. And as we had
hoped, these high RPM Jaws pipes made five more HP than either the low RPM Jaws
pipes or the SkiDoo factory race pipes.
Stock 2010 600RS engine, factory race domes, Jaws high RPM pipe, PHASE 1
EngSpd
RPM
7600
7700
7800
7900
8000
8100
8200
8300
8400
8500
8600
STPPwr
CHp
107.0
107.1
108.7
112.0
115.7
118.4
122.9
130.4
135.2
138.6
141.8
STPTrq
Clb-ft
74.0
73.0
73.2
74.4
75.9
76.8
78.7
82.5
84.5
85.6
86.6
BSFCAB
lb/hph
0.759
0.804
0.777
0.774
0.820
0.799
0.729
0.633
0.631
0.622
0.615
FulAB
lbs/hr
74.3
78.7
77.3
79.2
86.7
86.5
81.9
75.5
78.1
78.9
79.8
Air_1s
SCFM
174.2
176.2
178.9
181.9
185.1
189.9
191.7
194.1
197.2
199.8
203.7
CoolFw
GPM
38.02
38.38
38.84
39.37
39.84
40.19
40.81
41.89
42.67
43.34
43.89
AFRAB
Ratio
10.73
10.25
10.60
10.51
9.77
10.05
10.71
11.77
11.57
11.59
11.69
CoolOt
deg F
78
78
78
79
79
79
79
79
80
80
80
8700
8800
8900
9000
9100
9200
144.2
147.5
151.3
154.2
155.4
154.2
87.0
88.0
89.3
90.0
89.7
88.0
0.596
0.580
0.555
0.542
0.554
0.560
78.5
78.2
76.9
76.5
78.7
78.9
207.9
212.5
216.4
219.5
221.6
222.3
44.25
44.76
45.40
45.96
46.57
47.09
12.12
12.45
12.89
13.14
12.88
12.89
80
81
81
81
81
81
Next Jake brought the engine to Justin Fuller to take a crack at increasing airflow and HP.
The major difference between the SkiDoo and Polaris engine designs is the intake ports.
The Polaris uses a “case reed” design whereas the SkiDoo uses “cylinder reed” design—
similar to the Yamaha Vmax 4 800cc engines that Justin modifies to make up to 230hp.
So Justin modified each piston’s intake skirt with a “window” to allow the intake to
operate more like a case reed. Then he widened and recontoured the triple exhaust ports
to be similar to the design of the mod 600 Polaris’. But the transfer port design (timing,
sizes, aiming, etc) is quite different from the Polaris so Justin didn’t do much with them.
Since the timing of the stock RS transfers was higher than the mod Polaris, they wound
up with the thinnest base gasket then adjusted the exhaust port height as needed. This
time, the peak airflow CFM increased by about 6% but peak HP increased by only 3% at
peak. It seems likely that the 3% airflow that wasn’t contributing to added HP was
instead being “short-circuited”—passing through the now-open piston skirts, down into
the crankcase, up the transfers and out the exhaust ports while close to BDC. But this
large increase in airflow also increased the pipes’ backpressure, so larger diameter stinger
pipes and mufflers had to be fitted to create the average 2.6psi backpressure that this
engine made the most HP with. With the higher trapped airflow, the factory race domes
effective compression ratio was too high causing clicks of detonation. So Jake installed
slightly lower compression ratio Bikeman domes with narrower squish bands (but still
with fashionable .029” squish clearance).
2010 600RS engine, FPP ported pistons/ cylinders, BMP race domes, Jaws high
RPM pipes with enlarged stingers, PHASE 2:
EngSpd
RPM
7600
7700
7800
7900
8000
8100
8200
8300
8400
8500
8600
8700
8800
STPPwr
CHp
111.7
113.5
115.8
118.6
121.1
124.2
127.5
130.7
134.6
139.1
142.6
146.4
152.7
STPTrq
Clb-ft
77.2
77.4
78.0
78.8
79.5
80.6
81.6
82.7
84.2
85.9
87.1
88.4
91.1
BSFCAB
lb/hph
0.665
0.677
0.683
0.684
0.672
0.660
0.643
0.632
0.619
0.597
0.589
0.576
0.563
FulAB
lbs/hr
71.7
74.1
76.3
78.2
78.5
79.1
79.1
79.8
80.4
80.1
81.1
81.4
83.0
AFRAB
Ratio
12.22
11.96
11.86
11.63
11.74
11.87
12.02
12.09
12.17
12.37
12.44
12.61
12.52
Air_1s
SCFM
191.3
193.7
197.6
198.8
201.4
205.1
207.7
210.5
213.6
216.5
220.2
224.3
227.0
ExhPrs
psig
1.9
1.9
2.0
2.0
2.1
2.1
2.2
2.2
2.3
2.4
2.4
2.5
2.5
Exh_2
deg F
504
522
537
554
567
584
598
615
631
647
659
671
692
8900
9000
9100
9200
9300
155.6
157.8
159.0
159.0
156.7
91.8
92.1
91.8
90.8
88.5
0.570
0.578
0.560
0.565
0.571
85.6
88.1
85.8
86.6
86.3
12.31
12.12
12.57
12.53
12.57
230.0
233.1
235.7
237.0
236.9
2.5
2.6
2.6
2.6
2.6
707
719
725
730
735
Next, Jake installed a newer (2012-13) 600RS crankcase and crankshaft and came back to
the dyno for phase 3. This bottom end has reduced crankcase volume due mostly to a
crankshaft that’s fitted with plastic “stuffers” designed to displace air without added
weight. This single change increased airflow another 7% at peak (with a 4% reduction in
lower RPM airflow), but netted us very little extra peak HP—and we lost a few HP in the
midrange from 7500-8500 RPM where airflow was lower. Strangely, the pipe
backpressure actually dropped while the high RPM CFM increased! This required Jake to
tighten the stingers just slightly to bring the backpressure back to optimal. The
backpressure that created maximum HP now was .2 psi lower than before! As we can see
from the following graph the airflow “crisscrossed” at 8600. Once again, it’s likely that
most of the extra airflow was created as the pistons reached BDC, and that tighter
crankcase may have pumped most of the added CFM out the mufflers.
2012 600RS engine, FPP ported pistons/ cylinders, Bikeman domes, Jaws high RPM
pipes with enlarged stingers, PHASE 3:
EngSpd
RPM
7500
7600
7700
7800
7900
8000
8100
8200
8300
8400
8500
8600
8700
8800
8900
9000
9100
9200
STPPwr
CHp
108.9
109.2
112.4
114.6
116.8
119.3
121.4
124.1
129.1
133.7
138.8
144.6
149.0
152.2
155.0
157.7
159.4
159.6
STPTrq
Clb-ft
76.3
75.4
76.6
77.2
77.7
78.3
78.7
79.5
81.7
83.6
85.8
88.3
89.9
90.8
91.4
92.0
92.0
91.1
BSFCAB
lb/hph
0.717
0.733
0.720
0.707
0.698
0.675
0.652
0.638
0.615
0.609
0.600
0.585
0.578
0.565
0.572
0.562
0.564
0.579
FulAB
lbs/hr
74.0
75.7
76.6
76.8
77.2
76.2
74.9
74.9
75.1
77.1
78.8
80.0
81.5
81.4
83.8
83.8
85.0
87.4
AFRAB
Ratio
11.19
11.18
11.24
11.37
11.47
11.84
12.22
12.46
12.68
12.58
12.60
12.65
12.70
12.98
12.90
13.15
13.02
13.03
Air_1c
CFM
185.3
189.6
192.6
195.3
198.2
202.0
204.8
209.0
213.0
217.1
222.3
226.6
231.6
236.6
242.1
246.8
251.7
254.8
DenAlt
Feet
1067
1067
1066
1066
1068
1069
1068
1069
1069
1069
1069
1069
1069
1070
1071
1071
1072
1072
ExhPrs
psig
1.3
1.2
1.7
1.7
1.8
1.8
1.9
2.0
2.0
1.9
2.0
2.1
2.3
2.4
2.3
2.4
2.4
2.4
The current airflow is very close to the 259 SCFM that the Hentges Polaris mod 600s are
pumping with very similar pipes at 170 CHP. So we can assume that this SkiDoo 600RS
is pumping 170 CHP worth of air, but only trapping 93% of the air/ fuel mixture making
HP with the Polaris mods. That suggests that this engine is “short circuiting” 7% more air
than the Polaris mod engines. Justin Fuller has reviewed this data, and he and Jake must
now strategize on how to try to trap some or all of that 7% and make power instead of
added hot exhaust gases burning inside the pipes. Justin is thinking that the timing/
aiming of the transfer ports may be the culprit, but revising those to match the Polaris’
transfer ports is major surgery. The tentative plan is to cut the bottom of the cylinders so
transfer timing more closely matches the mod Polaris, then modify the exhaust ports and
combustion chambers to bring them back to spec. We also have some different reeds/
cages to try. Justin wants to try larger carbs, too. The next session will be Phase 4.