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Rotax 277 compression and head work. A little guidence please

40K views 245 replies 9 participants last post by  Daag44 
#1 ·
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?
 
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#2 ·
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
 
#4 · (Edited by Moderator)
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
 
#6 · (Edited by Moderator)
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?
 
#7 ·
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 ·
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
 
#223 ·
==============================

Looking back at this thread. Niko overseas who Races his 277 used a 38mm Mikuni Flat Side, with Head at 14.5cr and he turned it 9,200rpm with a Modified 670 Tuned Pipe.

277F = 268.8cc turned 9200rpm at 93% VE = 54.1hp and makes 81.2cfm!
277F = 268.8cc turned 9200rpm at 94% VE = 54.7hp and makes 82.1cfm!
277F = 268.8cc turned 9200rpm at 95% VE = 55.3hp and makes 83.0cfm!

Did you Measure your Intake ID?

Did you Measure the ID of your 34mm Carb?

My Intake sitting on my desk the ID = 37.6mm.

I recently bought a Mikuni 38mm Flat Side New off eBay for $81. The ID = 37.87mm and OD = 46.9mm! So very thick Walls, so you could Bore it for a True 38mm or overbore it to a 40/42mm.

The Mikuni 34mm Flat Sides New off eBay are cheaper Average $40. So I'm wondering if you couldn't just overbore a 34mm to a 36mm, or 38mm!

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#9 · (Edited by Moderator)
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
 

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#10 ·
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 26hp@6250rpm, and some were rated 28hp@6400rpm 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 25.4hp@6000rpm.

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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#11 ·
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 ·
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 · (Edited by Moderator)
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
 
#14 · (Edited by Moderator)
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 26hp@6250rpm with a 36mm Carb it Dynoed 25.4hp@6000rpm with Rotaxs Header/Muffler Exhaust. Switching it to their R&D Aero Tuned Pipe it dynoed 30.5hp@6250rpm. 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.wallaceracing.com/braking-hp.php

Off Rotaxs website. Interesting Info.

https://www.rotax.com/en/100-years/historical-vehicles-engines.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!
 

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#15 · (Edited by Moderator)
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.
 
#16 · (Edited by Moderator)
Math does not lie, only engine builders do.
Good advice
Especially when looking at data from a company that dyno'd its own pipes (R&D Aero) and then sold those pipes based on the improvements they claimed for them.

There are at least two generations of the 277/300 in snowmobiles -
The gasket heads - 11.8 theoretical and 6.7 effective
The O-ring heads. - 11.2 theoretical and 6.4 effective

The ultralight fly-boys never were sold the O-ring head engines.
 
#17 ·
Sightation
Your thoughts on compression numbers are a big help. 130-140 sounds about what I was guessing-realizng there are a lot of variables. The deeper I look into this stuff, the more I realize I don't know.

Aaen does different tunes and said he gave my motor the Trail Tune as I didn't want to compromise torque and reliability too much.
To increase RPMs, both the intake and exhaust ports were raised about .040" or 1mm.

Along with thinking the squish is a little much, the angle of it is also bugging me. Most pictures and drawings I've seen seem to match the angle of the piston or open up slightly toward the center. The angle on the squish on this head is at roughly 16 degrees. Here is a pix of a junk piston sitting on the head so you can see what I'm getting at and a pix of a protractor set at 16 degrees.

Radioflyer
I actually had the pipe made by NPP (Neil's Performance Pipes) in Canada and likely who you are talking about. He's the only one I could find that didn't need the sled for fitment. He said that it should gain me 2-3hp (10%) and a shop that did the porting on a 277 with his pipe said they dynoed about 4 hp if I remember. No other work except pipe and porting. Not sure what rpm.

The craftsmanship on the pipe is outstanding, but there were issues. Enough that I'm not sure I could recommend him, though he always made good on any issues.
Even with the porting, the stock pipe would hold the rpms noticeably better than NPP's when playing in the powder, climbing, and coming out of corners on the trail. I would still be using it but it is rusted out. The NPP pipe is faster on a wide open trail where holding RPMs isn't an issue.
I replaced the PB50 primary clutch with a P85 the end of last year in hopes that it will be more responsive and help keep the rpms in the powerband. It's promising, but dialing it in is whole other topic and I want to focus on squish and compression for now.

Thanks
Steve
 

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#18 · (Edited by Moderator)
Wow - I see what you mean about the squish angle. Not what I would have expected.

We normally cut to match the piston. Usually some divergence towards the middle due to the piston dome not being the same angle all the way.
When you measured the squish - was the .075 the edge? Middle?

Did Aaen have a piston when he cut the head? Almost looks like he just followed the same angle as the stock head.
 
#19 ·
I found it.

Good morning.

We have worked with the 277 fan in snowmobiles. I will attach a few pics. Yes, we dyno test all of our stuff...

It is best that you call us at the shop so that we can discuss your project in detail.

Neil

Sent: Saturday, August 8, 2020 5:34am
To: info@npowerperformance.com
Subject: Tuned Pipe?

780-444-7472

http://npowerperformance.com/
 

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#20 · (Edited by Moderator)
<Along with thinking the squish is a little much, the angle of it is also bugging me. Most pictures and drawings I've seen seem to match the angle of the piston or open up slightly toward the center. The angle on the squish on this head is at roughly 16 degrees. Here is a pix of a junk piston sitting on the head so you can see what I'm getting at and a pix of a protractor set at 16 degrees.>

Squish band is at 16 deg.

What is the angle of the piston crown?

I understand the crown is a curve, but you should be able to get a good idea for the portion that matches the squish band.

By the way, cool picture of the piston in the head. You chose the best viewing angle to see the angles. Are you able to post 800x600 or 1024x768? If you use Irfanview I can show to strip much of data to get large high def pictures in their smaller sizes.
 
#21 ·
Radioflyer
That pipe looks mighty familiar lol. Note the stinger insert on the left for pipe tuning that he suggested I play with.

Sightation
The squish measurement is at the edge, at the tightest spot. I think I sent the piston, but can't be positive as it was over 2 years ago now. I might have an old email with that info. I do know I sent lead readings along with the jug and head. I'm trying to get a hold of him to see what his thoughts are but that's been an issue in the past. Busy guy.

This has turned into a mixed bag. On the one hand, I'm stoked that there are some potential improvements I can make. On the other hand, I'm not sure this head is salvageable. I'm just throwing numbers out, but does it seem reasonable that I would need to cut About .030 off the head and maybe take 0.15 off the squish at the proper angle to get a reasonable size squish band? Squish would still be .050 That would mean a total of .070 off the head overall. Would this compromise its structural integrity at all?
I'm assuming that I would need to take a fair amount out of the dome to bring the compression back some. Originally, I was hoping to just take some off the head, this makes things more complicated. In a past life, I was a Die Maker so I have an idea what needs doing, but I have limited access to equipment. A lathe with a 3 jaw chuck and a mill with limited time available. Setting up on the lathe will take some effort, figuring out a jig and getting the head centered and square.

I do have a spare stock head. I was hoping to save it and experiment on the already modded one, but perhaps not. I could leave the modded one as is which is usable and play with the stock one.

Daag
The angle at the piston's edge is roughly 10 degrees. So a 6 degree difference.

I did some editing on the pix for better detail and added one of the stock head with the piston on it as well.
 

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#22 ·
I do have a spare stock head. I was hoping to save it and experiment on the already modded one, but perhaps not. I could leave the modded one as is which is usable and play with the stock one.
That's a tough one.

I think I would work with the extra stock head - keeping the mod head as a working spare.
That way you can cut it correct to start, vs trying to modify an already iffy head.

Truth be told - if it were not for the extra holes for the locating pins, I would try to find an early gasket type head to work with. (I wonder how necessary those pins are)

It starts out as 11.8 to one. Then you automatically tighten squish because on your O-ring cylinder you don't need the head gasket.

You do need to flat mill out some sealing grooves used by the gasket - Milling those off to get a true flat surface for the O-ring buys you another few extra thousands.

You would have the factory squish band and angle - just tighter. Maybe even too tight. But then you could follow the angle of the factory squish and make it deeper.
 

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#23 · (Edited by Moderator)
I think you're right, I'll probably leave the modded head as is. Likely less work that way and I will be able to use it in the mean time. I did put the stock one on for a quick spin last winter and the modded head definitely has more spunk even if the squish isn't right. To be fair though, the carb was a little richer than stock for the test run though.

I see a head off a Citation 3500 for $64 shipped. Have to think about it though. Have to ponder the pin issue and weigh cost vs work involved.

A question. What's the clearance machined around the head were the studs go through for? Cooling? More pressure for sealing? Wondering if that would need to be machined back to speck. I know Aaen did.

Also, with the ports raised .040, Do you still think I could get away with 130-140psi compression?

Thanks again
Steve
 
#25 ·
I find this post really interesting as a younger man i did a lot of 100cc kart engine building in a few classes and we did a ton of port and head work. back then on those we found that lowering the transfer ports gave us a broader power band raising the ex port raised rpm and power to a point if we went to far we would burn pistons we ran the squish as tight as we could as we found no loss there. as for timing it really depended on the track more gave us better pick up less would top out more. interesting thing is the guy who helped me learn my way with two strokes was a factory can am moto cross rider. he helped develop the bikes he was crazy but a great guy! we race direct drive and some clutch classes good times!
 
#26 ·
Sightation
Your thoughts on compression numbers are a big help. 130-140 sounds about what I was guessing-realizng there are a lot of variables. The deeper I look into this stuff, the more I realize I don't know.

Aaen does different tunes and said he gave my motor the Trail Tune as I didn't want to compromise torque and reliability too much.
To increase RPMs, both the intake and exhaust ports were raised about .040" or 1mm.

Along with thinking the squish is a little much, the angle of it is also bugging me. Most pictures and drawings I've seen seem to match the angle of the piston or open up slightly toward the center. The angle on the squish on this head is at roughly 16 degrees. Here is a pix of a junk piston sitting on the head so you can see what I'm getting at and a pix of a protractor set at 16 degrees.

Radioflyer
I actually had the pipe made by NPP (Neil's Performance Pipes) in Canada and likely who you are talking about. He's the only one I could find that didn't need the sled for fitment. He said that it should gain me 2-3hp (10%) and a shop that did the porting on a 277 with his pipe said they dynoed about 4 hp if I remember. No other work except pipe and porting. Not sure what rpm.

The craftsmanship on the pipe is outstanding, but there were issues. Enough that I'm not sure I could recommend him, though he always made good on any issues.
Even with the porting, the stock pipe would hold the rpms noticeably better than NPP's when playing in the powder, climbing, and coming out of corners on the trail. I would still be using it but it is rusted out. The NPP pipe is faster on a wide open trail where holding RPMs isn't an issue.
I replaced the PB50 primary clutch with a P85 the end of last year in hopes that it will be more responsive and help keep the rpms in the powerband. It's promising, but dialing it in is whole other topic and I want to focus on squish and compression for now.

Thanks
Steve
I showed you what the Cranking psi was on a used 277UL with Stock 11.8cr was at 150 psi. It's my understanding that the Squish Angle is 7 Degrees, not 16 degrees which has lowered your PSI! The OD of the Head Chamber for the Stock 72mm Piston should be 73mm, where 1mm = 0.03936996" that's .040" per side. Yours looks Wider which also lowers your PSI.

If the Stock 277UL with 11.8cr actually Dynoed Max 25.4hp@6000rpm, Rated 26hp@6250rpm!

With R&D's Tuned Pipe it Dynoed 30.5hp@6250rpm, it made +5.1hp. 25.4hp + 20% = 30.48hp! 25.4hp + 21% = 30.734 hp. So 25.4hp = 20.1% = 30.5054 hp. I would call that a very Mild Tuned Pipe. A Porting Expert told me if I used 6500rpm, 11.5cr, with Proper Size Carb, it takes on Avg 7cc to make 1hp with a Good Designed Tuned Pipe. 277UL is 268.8cc / 7cc = 38.4hp@6500rpm. 268.8cc/25.4hp = 10.6cc with Rotaxs UL Header/Muffler. The Stock 277UL Engine Avg 2.8hp per +250rpm till about 5500rpm where it starts losing Power. So is the Exhaust too Restricting, the Carb to Small, or is it the Porting, or a combo? 277UL used on Scat Hover Craft were rated 28hp@6400rpm probably with a different Exhaust.

A Simonini Victor 1 Super, uses 9.5cr, and a Tuned Pipe, a 400cc Single and makes 54hp@6200rpm! 400cc/7cc = 57.1hp@6500rpm. If you Minus 2hp for going from 11.5cr to the 9.5cr that would make it 55.1hp if Perfect. Turn it +300rpm it would make the 55hp.
 
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