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Rotax 277 compression and head work. A little guidence please


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#1 stevetundra

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Posted 03 October 2020 - 12:37 PM

My 06 Tundra 300 isn't vintage, but I'm thinking I might get a little more help here with the motor.

 

While it's not the most likely candidate for modifying, I've been enjoying playing with my Tundra and it's keeping my brain cells active learning new stuff.  Now I'm looking for a little help on the Rotax 277.  Keep in mind this is a newer version with CDI.  

 

I hadn't planned on engine work but the original exhaust rusted out and at $650 for OEM, thought I'd just get an aftermarket pipe..... and if doing that, might as well get it tuned for a higher rpm, which meant the engine would need a little porting as well... and of course might as well do some head work if I'm doing all that.....  Funny how that works.  

 

So, it's ported to 7,500 from 6,800 and .040 was removed from the head and .020 from the squish.  This was done by Aaen Performance.  There was improvement, but the timing was at about 6 degrees at 7,000 rpms and 16-18 was recommended with the increase in rpms, so after running it with no issues one winter, I sent the CDI out to get reprogrammed.  I decided on 12 degrees at 7,500 rpms with was a little more conservative. I started running into what appears to be detonation issues on hard pulls, even with 93 octane.  Damage to the very edge of the piston causing scoring and then seizing up.  Nice thing about a single is it doesn't take much get it back running.  

At first, I thought I needed to take a little off the squish to reduce compression, but after doing some reading, it seems that the squish is actually to much at .075.  So now I'm thinking another .010-.015 off the head, but I'm concerned as the compression will be higher yet.  I borrowed what I hope is an accurate compression gage and it reads 125 psi with the modded head and 110 with a stock head.  Seems like 110 is about right stock, but I can't find info on that and 125 seems low for the modded head?  

 

Any thoughts on all this? 



#2 sightation

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Posted 03 October 2020 - 06:30 PM

Just for some more background to make sure I am reading this correct

1) Ran fine with stock timing of 6 degrees - problems started with 12 degrees. No other changes correct?

2) Current squish with Mod head is .075 - What is squish with stock head?

3) Have you checked mod head volumes to determine current compression ratio?
Flat Plate head cc and full installed cc

4) 125lbs for the modded head is not necessarily a problem - if Aaen raised the exhaust port height for more Rpm, it will lower your compression reading

#3 Mendomotox

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Posted 04 October 2020 - 05:43 AM

I'm no expert but isn't 75 a little high for squish? was running lean when it sized?
'71 Elan,'74 Elan,'79 Elan,'95 Powder Special 580,'01 800 RMK 144",'07 M8 162" Boondocker Turbo,'08 M8 162" sled engined CVT TimberSled in progress

#4 stevetundra

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Posted 04 October 2020 - 09:10 AM

No other changes and I’m running the stock piston.

Stock head .095 About 26cc
Mod head. .075 About 24 cc

The mod head definitely has more squish area too.

If memory serves, I believe we got 26 cc on the mod head assembled but I need to find my notes to be sure. The mod head has a larger squish area for sure.

Shouldn’t have been lean. I’ve tuned the carb using plug chops and used that to set the temps on an ETG I have on the pipe. I also went up at least one size on the MJ after the first seizure.

I can post pix of both heads this evening.

And thanks for the replies
Steve

Edited by stevetundra, 04 October 2020 - 11:21 AM.


#5 stevetundra

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Posted 04 October 2020 - 07:35 PM

Here’s pictures of the heads. Right one is the one that was machined.
The squish has a little bit more of an angle to it and there is a small flat about .080 wide right were it meets the step.

Attached Images

  • 6D355E92-0BF7-46F5-8FB8-479AD63E202A.jpeg
  • 40505B55-EDBC-42CF-B45D-AFC4883DE454.jpeg


#6 sightation

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Posted 04 October 2020 - 09:10 PM

So - .075 is not as tight as I might like for performance - It's not bad for a stock engine. I don't think its causing the seizure.
The wide Squish band looks about the same as the one on my 277 race sleds.

With the 2cc diff in heads - the 11.2 stock compression value would be about 12 with the mod head. Not that high. The original 277's in the 80's were 11.8

I think the root cause is somehow related to your changing the ignition programming from 6 to 12. That's where the problem started after a year of running ok.

With the reprogram - did they just raise the tail end by 6 degrees? - or did the entire curve get shifted up 6?

Edited by sightation, 04 October 2020 - 10:47 PM.


#7 stevetundra

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Posted 06 October 2020 - 02:50 AM

Here is a cut n paste from a thread I started on timing.
________________

“Timing is limited to 8 points. You can change the timing and the rpm of these points and the line between them is linear.
The original timing is as follows.”

Original RPM TIMING
800--------------10°
1200------------15°
1400------------18°
2500------------18°
3000------------22°
4000------------22°
5000------------17°
8000------------11°

What makes this a little more complex is there is also an offset added for engine tolerances such as keyway, stator, pickup, etc. on this one it was a retard of 3 degrees. We had double checked the timing at 7,500 rpms and found it at around 6 degrees. So a little more than the 3 degrees off.

18 degrees was recommended at 7,000 but I didn’t want to go quite that far. Also, as the limiter was set at 8,000 rpms and I hit that occasionally I had that changed to 8,500

Time I had it changed to.

RPM TIMING

800 10
1200 15
1400 18
2500 19
3000 23
4500 23
7000 19
8500 12

This actually gave me about 12 degrees at 7,000 rpms. Which was about what I was looking for.

So with taking some off the head, porting and piping for 7,500 and advancing the timing, it put the little guy closer to the edge.

_________________

So I did change the timing mainly from 4500 on but the only time I had a failure is on a WOT pull after say about ¼ mile or so.

I probably should have done a better job in explaining what I was up to in my original post. I’m pretty sure you are right in regards to the timing change being the cause or the straw that broke the camel’s back so to speak. Aaen is the one that did the original work and he had told me that the timing should be around 18 degrees. I am a little hesitant to give up on the timing yet since I don’t THINK I went that aggressive, especially with what is involved in getting it changed or the cost of a new CDI.

I had been thinking of making the squish bigger thinking the compression was to much, but after some reading, I started pondering the idea of tightening up the squish a little bit to shorted the combustion to see if that would help and give me a little more hp at the same time. I wanted to question Aaen on the squish, but he’s a hard one to get a hold of. I’m wondering if there was a reason he didn’t want to go further with the squish? Perhaps it was because it was kind of limited by the amount of compression the motor would have without opening up the dome?

Also, after looking back at some notes, I think I will go back and check my timing from scratch just to make sure I’m not way off on something before I do anything. I’ve checked it before, but it seems odd that the original timing was at 6 degrees……

#8 stevetundra

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Posted 16 October 2020 - 09:14 AM

Well, I've got the motor on the bench and discovered the carb boot has a crack in it, so it could have been lean.  The carb boots for this sled have a reputation for splitting and I keep a spare on hand.  I've replace it 3 times for cracks over the years so I'm a little bummed I didn't catch it.  It was last winter but if I remember, I did flip the choke on an off a little while running to check for lean and also sprayed ether around the engine looking for leaks.  I didn't notice anything on the EGT either,  The boot is definitely cracked so these tests obviously didn't catch it.     

 

I did a leak test and the engines still tight, so that's good.

 

 

Sightation

You indicated you use this engine for racing?  Out of curiosity, what are you using for a carb?  Mine came with a vm34.  I've heard of guys going to a 36mm but also heard you might lose some low end performance.  Haven't really considered it with fitment issues. 

 

Do you think the compression is low enough that I can get away with taking .010-.015 off the head to tighten up the squish some?  

 

Again I appreciate the help and comments.

Steve

 

 

  



#9 sightation

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Posted 17 October 2020 - 04:57 PM

Good catch on the carb boot.

We race the early 80's Citation 3500's in a super stock oval class.
Means gasket head vs your o-ring and still must use stock exhaust.

We run the 36mm carb because it does give us a little more on top end. Don't know if its much different on low end because we use higher clutch engagement RPM. (4500)

As far as tightening up the squish a little more, here is what we do. Get the squish where we want first, then cut the resultant band back to get the volume/Compression we want. Pure guess you are around 12 right now.

1) measure head cc before
2) measure head cc after squish change

Then put an angle cut on the chamber side edge of the Squish band to raise the volume back to where it was. Will keep compression where it was

Attached Images

  • 267646A4-6470-4A4F-887A-234279FE1AE3.jpeg

Edited by sightation, 17 October 2020 - 04:59 PM.


#10 RotaxFlyer

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Posted 18 October 2020 - 10:22 AM

Isn't your Squish Band to be between .040 - .060?

 

277F in Sleds came with 34mm Mikuni Carbs.

 

The 277UL came with a Bing 54 36mm carb and some were rated [email protected], and some were rated [email protected] depending on what Exhaust was used.

 

As far as I know, All 277's used Theoretical 11.8cr, effective 6.7cr.

 

These guys make some of the best Carb Boots & Gaskets.

http://jbmindustries.com/GASKETS.html

 

​When R&D Aero Dynoed the Standard 277UL rated 26hp it actually Dynoed [email protected]

 

If you look at Rotaxs 277UL Power Graph, you can see where it starts losing hp after 5500rpm.

 

Standard 277UL with Stock Exhaust vs R&D Aero's mild Tuned Pipe.

 

Stock Rotax 277UL Compression. A perfect built engine at Sea Level 11.8cr x 14.7 = 173.46 psi! For each +1000ft - 3% off.  So 173.46 - 3% = 168.2562 psi.

 

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  • ROTAX 277UL vs Tuned Pipe.jpg


#11 RotaxFlyer

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Posted 18 October 2020 - 10:50 AM

No other changes and I’m running the stock piston.

Stock head .095 About 26cc
Mod head. .075 About 24 cc

The mod head definitely has more squish area too.

If memory serves, I believe we got 26 cc on the mod head assembled but I need to find my notes to be sure. The mod head has a larger squish area for sure.

Shouldn’t have been lean. I’ve tuned the carb using plug chops and used that to set the temps on an ETG I have on the pipe. I also went up at least one size on the MJ after the first seizure.

I can post pix of both heads this evening.

And thanks for the replies
Steve

 

If Stock Head is 11.8cr with 26cc, a Head with 24cc = 12.8cr.

 

If your Stock Head is 11.2cr with 26cc, a Head with 24cc = 12.1cr.

 

At Sea Level 11.8cr x 14.7 = 173.46 psi

 

At +1000ft = 168.2 psi

 

At +2000ft = 163.2 psi

 

At +3000ft = 158.3 psi

 

At +4000ft = 153.5 psi

 

etc!



#12 sightation

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Posted 18 October 2020 - 12:52 PM

Stevetunda

Just to be clear we only went to 36mm because Dyno tests showed with a PORTED motor 36mm helped.

Stock Motor - 34/36 virtually same dyno results.
The PORTS limit stock 277/300 not 34mm carb

Ultralights used 36mm Bings for parts commonality (277,377,447,503,582 all used 36mm)

#13 sightation

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Posted 18 October 2020 - 12:55 PM

Stevetundra

Don't be concerned if your cranking compression is lower than the numbers someone gives you with an equation like CRx14.7 = PSI

That equation doesn't take exhaust port height in consideration. The cranking PSI on a gauge will change based on how high and wide the port is since the cylinder is bleeding air until the port is fully closed.

Depending on how far AAEN went with the port - 130-140ish is not uncommon with 12:1

Edited by sightation, 18 October 2020 - 06:45 PM.


#14 RotaxFlyer

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Posted 18 October 2020 - 04:13 PM

I have seen some Manuals list the 277F hp at 7000rpm and some listed it at 7500rpm. With all the Type O's in Manuals you never really know.

 

While some Others here may tell You not to be worried if your Numbers are Low, Low means less hp made!

 

NASA, sets Atmospheric Pressure at 14.7 psi as the Standard to use. It can vary a little in different Places in the World! Math does not lie, only engine builders do.

 

All Engines HP made are rated at Sea Level, unless Specially built like for the Military, which like my Military 10kw Generator Motor is rated 20hp at 10,000ft so it can be used in 98% of the World, Air Dropped in on a skid. So Just as your HP made varies at different Altitudes account of Air Density, it also affects your Compression PSI Numbers. That's why Mountain Sleds use +2-3 Points higher cr Heads.

 

Companies like to usually round up hp numbers and use whole hp Numbers, hence Dynoed 25.4hp became 26hp.

 

On the 277UL rated [email protected] with a 36mm Carb it Dynoed [email protected] with Rotaxs Header/Muffler Exhaust. Switching it to their R&D Aero Tuned Pipe it dynoed [email protected] So the PORTING was not the problem, it was the Exhaust Type used. But the Higher you turn them eventually the Porting does come into play as does the Carb Size.

 

The fellow on Youtube Hollaendaren that does some wonderful stuff with his 277 Race Engine and other Skidoo Engines, turned his 277F 9200rpm using a 38mm Mikuni Flat Side Carb, using 14.5cr, making around 50-55hp he said. He adapted a 670 Tuned Pipe. He adapted an Aermacchi Dual Plug motorcycle head off a Motorcycle.

 

277UL Rated 26hp at Sea Level.

BHP of 26 at sea level and Elevation of 1000 feet is 0.78 HP loss.

BHP of 26 at sea level and Elevation of 2000 feet is 1.56 HP loss.

BHP of 26 at sea level and Elevation of 3000 feet is 2.34 HP loss.

BHP of 26 at sea level and Elevation of 4000 feet is 3.12 HP loss.

BHP of 26 at sea level and Elevation of 6000 feet is 4.68 HP loss.

BHP of 26 at sea level and Elevation of 7000 feet is 5.46 HP loss.

etc.

http://www.wallacera.../braking-hp.php

 

Off Rotaxs website. Interesting Info.

https://www.rotax.co...s.html#type-277

 

Type 277 Snowmobile Engine 1978 - 2008, came with 34mm Carb.
POWER AT RPM: 20 KW at 6,750rpm 1/MIN and 20 KW = 26.82044 hp.
 
A Used 277UL 11.8cr Cranking Compression!

Attached Images

  • ROTAX 277UL - 14.jpg
  • Aermacchi Dual Plug motorcycle head - 1.jpg
  • NICO 277 Nico Aermacchi twin plug head.jpg

Edited by RotaxFlyer, 18 October 2020 - 04:23 PM.


#15 RotaxFlyer

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Posted 18 October 2020 - 04:38 PM

There is a guy in Canada, I don't remember his name at the moment that I ran across a while back that makes Sled Tuned Pipes for the 277F. He sent me a few photo's, they must be on my Laptop. I don't think Aaen, Jaws, SLP, etc. makes any for the 277F. Using Sightations formula to match up Engine Durations you could adapt one from another Engine like the guy did with his 277 using a 670 Tuned Pipe.


Edited by RotaxFlyer, 18 October 2020 - 04:40 PM.





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