Arctic Cat 800/ 900+cc big bore dyno test sessions We occasionally have substandard modified snowmobile engines that fail to develop the expected horsepower, but I typically don't discuss them on DTR since low HP may not be the fault of the engine modifier. Out of phase crankshaft? Tight crank bearing? Mouse nest in the muffler? But this one is different--large bore Cat 800 cylinders that were purchased as a package by Snowmobiler X with billet head, full matching exhaust system and a Power Commander V with timing control from a major aftermarket engine modifier. He was promised 200+ HP and unfortunately made 10% less as delivered, which was unacceptable to Snowmobiler X who lives 200 miles from DTR. There are several Cat big bore 800-- 900+ cc engines posted on this website, and all are in the mid 190 HP range which is what I suggested to Snowmobiler X to expect. He reluctantly was good with that. But when all we could get out of the 900+ cc engine was 185 HP in session one (a bit more than the 180+ HP 800 engine he had sold to buy the higher power big bore), he was unhappy. Rightfully so since I had tested a Cat 800 big bore previously from the same company that was well over 190 HP! We noticed in the dyno test data that computed airflow SCFM (Standard Cubic Feet per Minute) was low, and pipe backpressure was higher than what we typically see with a properly tuned single pipe. Session 1 EngSpd STPPwr STPTrq BSFA FuelA LamAF1 LM1Air ExhPrs FulPrA RPM CHp Clb-ft lb/hph lbs/hr Ratio SCFM psig psig 7000 154.8 116.1 0.573 84.6 13.09 248 3.1 43.9 7050 157.3 117.2 0.567 85.1 13.06 249 3.2 43.8 7100 160.0 118.4 0.562 85.8 13.02 250 3.2 43.8 7150 162.2 119.1 0.559 86.5 13.01 252 3.2 43.8 7200 164.4 119.9 0.557 87.3 12.96 254 3.3 43.8 7250 166.0 120.3 0.554 87.8 12.91 254 3.3 43.8 7300 167.5 120.5 0.553 88.4 12.86 255 3.4 43.8 7350 168.7 120.5 0.555 89.3 12.84 257 3.4 43.7 7400 169.5 120.3 0.562 91.0 12.80 261 3.5 43.7 7450 170.0 119.8 0.570 92.4 12.71 263 3.6 43.7 7500 170.2 119.2 0.574 93.3 12.62 264 3.7 43.7 7550 170.2 118.4 0.577 93.7 12.55 263 3.7 43.7 7600 170.6 117.9 0.577 93.8 12.53 263 3.8 43.7 7650 170.8 117.3 0.575 93.8 12.53 263 3.9 43.7 7700 171.2 116.7 0.573 93.6 12.61 264 3.9 43.7 7750 172.3 116.8 0.570 93.6 12.66 266 3.9 43.7 7800 173.4 116.7 0.568 93.9 12.70 267 4.0 43.7 7850 174.4 116.7 0.567 94.2 12.70 268 4.0 43.7 7900 175.3 116.5 0.566 94.6 12.67 268 4.1 43.7 7950 176.3 116.5 0.564 94.8 12.63 268 4.1 43.7 8000 177.5 116.5 0.562 95.1 12.58 268 4.2 43.7 8050 178.8 116.6 0.561 95.6 12.54 269 4.2 43.7 8100 180.0 116.7 0.558 95.8 12.52 269 4.2 43.7
8150 181.0 116.6 0.555 95.8 12.51 269 4.3 43.6 8200 182.0 116.5 0.551 95.6 12.51 268 4.3 43.7 8250 183.0 116.5 0.546 95.2 12.55 268 4.3 43.7 8300 184.2 116.5 0.539 94.7 12.60 267 4.4 43.7 8350 185.0 116.4 0.532 93.7 12.69 266 4.4 43.7 8400 185.5 116.0 0.525 92.8 12.78 266 4.4 43.7 8450 185.7 115.4 0.520 91.9 12.91 266 4.3 43.8 8500 185.3 114.5 0.516 91.1 13.07 267 4.3 43.9 8550 184.2 113.1 0.514 90.2 13.26 268 4.4 43.9 8600 183.4 112.0 0.511 89.1 13.37 267 4.3 43.9 We shared the session one dyno test data with the vendor, and they suggested that he might have been shipped and exhaust pipe and muffler for a stock bore engine. So Snowmobiler X shipped the exhaust back to the vendor for modification. Back to the dyno for session two, we had a major problem with the sled's coolant temp sensor. All it would read (indicated on the Dynojet laptop) was about 40F which kept the ECU in "protect-me" mode and HP was even lower than session one! No meaningful test data was created this day. Session 2 EngSpd STPPwr STPTrq BSFA_B FulA_B LamAF1 LM1Air ExhPrs Exh_1 7000 146.4 109.9 0.659 91.6 11.87 244 2.5 967 7050 148.4 110.6 0.659 92.8 11.82 247 2.6 970 7100 150.2 111.1 0.662 94.3 11.77 249 2.6 974 7150 151.7 111.4 0.666 95.9 11.70 252 2.7 979 7200 153.0 111.6 0.667 96.8 11.65 253 2.7 983 7250 154.1 111.6 0.667 97.6 11.60 254 2.7 987 7300 155.1 111.6 0.668 98.3 11.57 256 2.8 991 7350 156.1 111.5 0.674 99.8 11.54 259 2.8 995 7400 156.7 111.2 0.688 102.3 11.45 263 2.9 1002 7450 156.7 110.5 0.704 104.7 11.29 266 3.0 1007 7500 156.4 109.5 0.713 105.7 11.16 265 3.1 1010 7550 155.7 108.3 0.718 106.1 11.10 265 3.1 1013 7600 155.7 107.6 0.717 105.9 11.10 264 3.2 1015 7650 155.4 106.7 0.717 105.6 11.13 264 3.2 1019 7700 155.6 106.1 0.712 105.0 11.20 264 3.2 1027 7750 156.5 106.0 0.706 104.6 11.25 265 3.2 1034 7800 157.8 106.2 0.698 104.3 11.32 265 3.3 1047 7850 159.3 106.5 0.690 104.2 11.37 266 3.3 1057 7900 160.7 106.8 0.685 104.2 11.40 267 3.3 1065 7950 161.8 106.9 0.680 104.2 11.43 268 3.3 1073 8000 162.8 106.9 0.676 104.2 11.46 269 3.3 1081 8050 163.9 107.0 0.671 104.1 11.51 269 3.4 1092 8100 165.3 107.2 0.667 104.4 11.55 271 3.4 1103 8150 166.8 107.5 0.665 105.1 11.55 273 3.4 1113 8200 167.4 107.2 0.666 105.5 11.53 274 3.4 1118
8250 166.7 106.1 0.669 105.5 11.54 274 3.4 1120 8300 162.9 103.1 0.684 105.4 11.68 277 3.4 1118 Snowmobiler X replaced his coolant temp sensor and it was back to DTR again, and the reduced backpressure of the pipe/ muffler improved airflow but it was obvious that even tuned to 13/1 we wouldn't get over 190 HP. Session 3 EngSpd STPPwr STPTrq BSFA FuelA LamAF1 LM1Air ExhPrs Exh_1 7000 157.3 118.0 0.545 83.0 13.70 250 2.8 921 7050 159.8 119.0 0.546 84.5 13.62 253 2.8 921 7100 162.5 120.2 0.548 86.2 13.53 257 2.9 924 7150 165.0 121.2 0.547 87.4 13.47 259 2.9 929 7200 167.3 122.0 0.546 88.5 13.42 261 2.9 937 7250 169.3 122.7 0.545 89.4 13.38 263 3.0 945 7300 171.3 123.2 0.546 90.5 13.33 266 3.0 954 7350 173.0 123.6 0.551 92.3 13.28 270 3.1 962 7400 174.4 123.8 0.561 94.7 13.21 275 3.2 972 7450 175.5 123.7 0.566 96.2 13.13 278 3.2 978 7500 176.2 123.4 0.572 97.5 13.00 279 3.3 987 7550 176.7 122.9 0.576 98.6 12.87 279 3.3 994 7600 177.3 122.5 0.578 99.2 12.78 279 3.4 998 7650 177.9 122.2 0.579 99.7 12.70 279 3.4 1002 7700 178.7 121.9 0.580 100.3 12.64 279 3.5 1005 7750 179.8 121.9 0.578 100.7 12.59 279 3.5 1010 7800 181.0 121.9 0.578 101.3 12.55 280 3.5 1018 7850 182.2 121.9 0.577 101.8 12.51 280 3.5 1025 7900 183.4 121.9 0.576 102.3 12.47 281 3.6 1031 7950 184.3 121.7 0.576 102.7 12.42 281 3.6 1037 8000 184.9 121.4 0.577 103.1 12.36 281 3.6 1045 8050 185.3 120.9 0.578 103.6 12.31 281 3.6 1053 8100 185.5 120.3 0.580 104.0 12.27 281 3.6 1061 8150 185.5 119.6 0.583 104.6 12.26 282 3.6 1064 8200 184.0 117.8 0.590 104.9 12.30 284 3.6 1064 After Snowmobiler X's third session I encountered an issue of low HP on a mod Cat 800 with a PCV that seemed to sap HP, and create a similar oddly flat HP curve--peak torque low and nearly 1000 RPM lower than HP peak RPM. So I wrongly suggested that he remove the PCV, add a big timing key and try a Boondocker EFI tuner. With a four degree key, and a muffler inlet restrictor (1 5/8" ID) to bring pipe backpressure close to 4 psi, we tapped out at 188 HP. Snowmobiler X is now frazzled and p'd off. He paid me $300 for a two hour R&D session but had no where near the promised HP! This is one of those times where I could have just slightly tweaked the dyno software (slight of hand,
indiscernible by anyone but another dyno operator) and got Snowmobiler X his 195 HP but I've never done that, never will. Session 4 EngSpd STPPwr STPTrq BSFA FuelA LamAF1 LM1Air ExhPrs Exh_1 7000 160.6 120.5 0.585 93.0 13.20 265 3.0 918 7050 162.3 120.9 0.584 93.9 13.11 266 3.0 926 7100 164.4 121.6 0.581 94.6 13.06 267 3.1 933 7150 166.6 122.3 0.580 95.6 13.02 269 3.1 940 7200 168.6 123.0 0.578 96.4 12.97 271 3.2 946 7250 170.5 123.5 0.575 97.0 12.94 272 3.2 954 7300 172.0 123.7 0.570 96.9 12.97 272 3.2 962 7350 173.5 124.0 0.566 97.1 13.06 275 3.3 972 7400 174.7 124.0 0.568 98.0 13.12 279 3.3 984 7450 175.5 123.7 0.572 99.2 13.08 282 3.5 996 7500 176.3 123.4 0.572 99.6 13.01 281 3.5 1003 7550 176.9 123.1 0.571 99.7 12.98 281 3.6 1010 7600 177.7 122.8 0.567 99.4 13.02 281 3.6 1024 7650 178.7 122.7 0.562 99.1 13.07 281 3.6 1039 7700 179.7 122.6 0.556 98.6 13.08 280 3.6 1049 7750 180.7 122.5 0.550 98.0 13.09 279 3.7 1062 7800 181.8 122.4 0.543 97.4 13.10 277 3.7 1072 7850 182.6 122.2 0.539 97.0 13.13 277 3.7 1083 7900 183.1 121.7 0.536 96.7 13.17 277 3.8 1098 7950 183.6 121.3 0.535 96.7 13.20 278 3.8 1109 8000 184.3 121.0 0.534 96.9 13.15 277 3.8 1122 8050 185.2 120.8 0.533 97.1 13.09 277 3.9 1129 8100 185.8 120.5 0.534 97.7 12.99 276 3.9 1138 8150 186.7 120.3 0.537 98.5 12.93 278 3.9 1146 8200 187.5 120.1 0.538 99.1 12.91 279 3.9 1153 8250 188.2 119.8 0.537 99.3 12.90 279 3.9 1159 8300 188.2 119.1 0.534 98.8 12.90 278 3.9 1167 8350 188.1 118.3 0.532 98.3 12.96 278 3.9 1173 8400 186.1 116.3 0.535 97.7 13.15 280 3.9 1175 So now we have made exhaust system happy, eliminated the PCV as the culprit, then the only thing left is the big bore cylinders. Are they ported like the same vendor's Cat big bore that made mid-190's in earlier testing here? I told Snowmobiler X that Rob Schooping of HTG Racing (who's downsized, modding engines at his home shop) was coming to test Kevin Cameron's tight squish recommendation on his Polaris 1000cc triple the following Saturday, and as he's done many powerful Cat engines tested here, he might take a look at Snowmobiler X's cylinders. That day when Rob was here dyno testing near-interference squish on his engine, Snowmobiler X drove 200 miles to DTR to have Rob evaluate the Cat big bore cylinders. Immediately, Rob noted that the exhaust ports were lower than stock (as you bore any cylinder larger, the tapered exhaust port becomes lower, requiring extensive port grinding to bring the ports back up to spec), and
that the transfer ports were horribly ground--misaligned from one another (see TCD "colliding streams" in the scanned printed issues). These cylinders were surely ported by someone promoted from the shipping department! Rob reported the cylinders--raising the exhaust ports a full 1.5mm and squaring up the transfer ports so the A/F mixture streams entering the cylinders would collide properly. Here is the final result, which surely pleased Snowmobiler X! Session 5 EngSpd STPPwr STPTrq BSFA FuelA LamAF1 LM1Air ExhPrs Exh_1 7000 161.0 120.8 0.573 88.0 13.67 268 2.8 822 7050 162.6 121.1 0.572 88.7 13.66 269 2.9 841 7100 164.4 121.6 0.566 88.8 13.70 271 3.0 852 7150 166.4 122.2 0.562 89.2 13.77 273 3.0 863 7200 168.8 123.1 0.559 90.0 13.83 277 3.1 877 7250 171.1 124.0 0.555 90.6 13.86 279 3.2 890 7300 173.5 124.8 0.549 90.8 13.87 280 3.2 902 7350 175.6 125.5 0.544 91.1 13.88 281 3.3 914 7400 177.7 126.1 0.553 93.6 13.80 288 3.3 930 7450 179.1 126.2 0.558 95.2 13.67 290 3.3 938 7500 180.2 126.2 0.561 96.4 13.52 290 3.4 945 7550 181.3 126.1 0.561 96.9 13.41 290 3.4 952 7600 183.1 126.6 0.560 97.6 13.31 289 3.5 964 7650 185.5 127.3 0.554 97.9 13.29 290 3.5 976 7700 187.8 128.1 0.549 98.2 13.30 291 3.6 987 7750 190.0 128.8 0.543 98.3 13.34 292 3.6 1001 7800 191.8 129.1 0.538 98.2 13.39 293 3.6 1011 7850 193.4 129.4 0.532 98.0 13.45 294 3.7 1026 7900 195.0 129.7 0.526 97.7 13.50 294 3.7 1040 7950 196.1 129.6 0.524 97.7 13.49 294 3.7 1055 8000 197.1 129.4 0.523 98.0 13.44 294 3.7 1063 8050 197.4 128.8 0.525 98.6 13.32 293 3.7 1073 8100 197.8 128.2 0.527 99.1 13.20 292 3.7 1079 8150 197.6 127.4 0.532 100.0 13.05 291 3.7 1084 8200 196.6 125.9 0.541 101.0 12.98 293 3.7 1082
============================================================= So it's obvious that Snowmobiler X got severely shortchanged by the vendor, and it took five dyno sessions and optimizing by Rob Schooping to get the HP we expect. This might happen with anyone, and it would be wise to document stock port heights with a $10 Harbor Freight digital caliper, then ask what port heights would be expected with any big bore cylinders. Then, ALWAYS tell whoever is modding your engine that you're coming to DTR to tune and evaluate (even if you're not planning to do that), and you're likely to get the most experienced porting guy to do your engine.