Arctic Cat 800/ 900+cc big bore dyno test sessions

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Robert Simon
5 years ago
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1 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 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

2 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_

3 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_ 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,

4 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_ 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

5 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_

6 ============================================================= 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.

2017 Polaris Axys 800 pipe update new questions and some answers

2017 Polaris Axys 800 pipe update new questions and some answers It required 40+ full throttle dyno sweep tests to bring this brand new 800 engine s HP up from the low 140 s to the low 150 HP level which