Lazair on Floats with Rotax 185's needs more power, Ideas?

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pictsidhe

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I love air die grinders. But only at work, where they have a suitably enormous compressor to run anything I want. 1/4" die grinders need far more air than home size cmpressors kick out. Anyone who's ever tried that would know...
Being a cheapskate dabbler, I use small, cheap 1/4" electric routers with the base removed. Works just like a 'real' die grinder but doesn't need a zillion CFM compressor.
 

n3puppy

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I love air die grinders. But only at work, where they have a suitably enormous compressor to run anything I want. 1/4" die grinders need far more air than home size cmpressors kick out. Anyone who's ever tried that would know...
Being a cheapskate dabbler, I use small, cheap 1/4" electric routers with the base removed. Works just like a 'real' die grinder but doesn't need a zillion CFM compressor.
X2 on the air grinders if you have a big enough compressor

One of the nice things about the cable grinders I showed is they have interchangeable heads .
The 45 and 90 degree are really nice for shaping transfer ports. Working on small bores like the 185 the 45 degree is perfect for getting in the bore and putting a chamfered edge on the ports to avoid catching a ring edge.

Obviously the shop had pro versions. But I bought the $80 one pictured to use around the house for almost any grinding chore I used to use a Mototool for. E4149B67-068E-4B59-BC33-A14C0B698586.jpeg
 
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proppastie

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They have always replied to me in the past
Well I asked non-standard questions.....like "can I run it direct without the reduction, or with the reduction would a wider blade allow me to use a 31" diameter prop.".....But I am not too upset about it , I really do not want to spend that much money.
 

Armilite

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Compression Wise
Raising compression as Armilite suggests will yield more HP without any need for change in carb size. The sedate 9.5cr of the 447 buys you about 1hp and the 11.6 of the 377 about 1.5Hp over stock 7cr (if that is actually the starting number)

======> The 447 is 9.6cr not 9.5cr. The 377 has been listed from 9.6cr to 11.6cr!

He will probably dispute this as he wrote in a previous post
He Said Maybe you should READ your own chart. A 100hp Engine with 8.0cr going to 12.5cr = 4.5 Points is 115.5hp or a 15.5 hp GAIN. Your Chart says 8.0cr to 8.5cr = 2.2hp for a +0.5cr for Big Car Engines. For these much Smaller CC Engines, it's more like 1hp Max for a +0.5cr Bump, as I said. How do I know this Info is that Skidoo made the same Single Cylinder Engine one year that used 8.0cr making 18hp@5500rpm and next year used 9.0cr making 20hp@5500rpm! So Max 2.0hp for a +1.0cr.

=====> When you use the same Spec's for a 302 Chevy V8 Engine in Desktop Dyno, but just change the CR used.

302 CHEVY v8 SB 7.0cr 390hp@6500rpm
302 CHEVY v8 SB 7.5cr 405hp@6500rpm +15hp
302 CHEVY v8 SB 8.0cr 418hp@6500rpm +13hp
302 CHEVY v8 SB 8.5cr 431hp@6500rpm +11hp
302 CHEVY v8 SB 9.0cr 442hp@6500rpm +11hp
302 CHEVY v8 SB 9.5cr 452hp@6500rpm +10hp
302 CHEVY v8 SB 10.0cr 461hp@6500rpm +9hp
302 CHEVY v8 SB 10.5cr 470hp@6500rpm +9hp
302 CHEVY v8 SB 11.0cr 478hp@6500rpm +8hp
302 CHEVY v8 SB 11.5cr 485hp@6500rpm +7hp

Going from 7.0cr to 11.5cr = 485hp - 390hp = 95hp Gain!


Unfortunately he did not read the directions for using the chart
The numbers on the chart represent a percentage increase from any HP to a new HP with a new compression ratio for ANY size engine.
From the chart

7 to 9.5 Cr = 2.5 points = 9hp x 109.6% = 9.864 HP
7 to11.5 Cr = 4.5 points = 9hp x 115.5% = 10.395 Hp

There is an online calculator that closely matches this number(certainly within rounding error)
http://bgsoflex.com/crchange.html
It does not ask for engine size
9hp - 7.5 to 11.5 Cr = 10 a 15% increase

=====> Your Cpmpression Calc doesn't hold True!
Computation Results:
  • Engine Horsepower (peak) is 390
  • Old Compression Ratio is 7.0
  • New Compression Ratio is 11.5
    Computation Results:
  • Computed New Engine HP is 450, a 15 percent change. (485hp).

Basing rule of thumb equations on decades old marketing data is problematic.
1) As he has said in previous posts , those manuals may contain typos
2) In the 60's there were no digital torque measuring devices. Claiming accuracy to 0.5 hp based on analog gauges of the day would be suspect.
3) Marketing guys inflate numbers- Briggs and others recently got sued and lost for inflating HP numbers.

======> Yes it is, but it's all we got to get some Ball Park Numbers! That 2hp difference on the same 335 Engine using 8.0cr(18hp) vs 9.0cr(20hp) might actually have been 1.6hp that was rounded up. I understand that and most other People do also. Your the one looking for an Absolute Number where there isn't any! Using a Smaller 185cc Engine it will probably be less. Just as your Compression Calc doesn't match to show +2hp. Most all of these Old Rotax Singles fall into 184cc to 335cc ranage.

Your Calc, Computation Results:
  • Engine Horsepower (peak) is 18
  • Old Compression Ratio is 8.0
  • New Compression Ratio is 9.0
    Computation Results:
  • Computed New Engine HP is 19, a 4 percent change. Manual said 20hp!

If you wish to use +1.0cr = 1hp, +0.5cr = .5hp than by all means do so. Till I see a Dyno showing the Numbers, I'll use mine.

If the 185UL is 9.4hp@5000rpm.
7.0cr = 9.4hp
8.0cr = 10.4hp
9.0cr = 11.4hp
10.0cr = 12.4hp
11.0cr = 13.4hp
11.5cr = 13.9hp
12.0cr = 14.4hp
12.5cr = 14.4hp
13.0cr = 15.4hp

The Lazair at MTOW of 450lbs needs 13.7hp for each Engine. That's why the SOLO 210 15hp was so well-liked.


For a gut check - his 2hp per CR rule of thumb would say the 4.5 point CR jump from 7 to 11.5 would yield a 9hp increase. That makes the 9hp 185UL engine 18hp with no other changes
Not very realistic. Plus why all discussion about carbs if you can get 9hp by simply raising compression.

For a good read on CR vs HP - try this article
https://www.hotrod.com/articles/0311em-power-squeeze/
It includes a similar chart and the mathematical equation used to develop itView attachment 93115
==========================================

I agree 2hp is Max you would see for a +1.0cr Bump on these Small CC Engines using the Manufactures Numbers. Are they 100% correct, nobody knows! Will they be different for Smaller than 335cc Engines, Yes, account you're using different CR's with different Bore & Strokes(CC) with different Size Carbs.

As can be seen on both the Hirth F-33(28hp) 313cc with 34mm Carb and the Rotax 277UL(26hp) 268cc with a 36mm Carb on both Power Charts, they both start losing power at around 5500rpm.

Just a Carb Change from 22mm to 28mm you would see a hp improvement. If you just use the Carb OD's 22mm to 28mm = about 27.5% more Air.
9.4hp + 27.5% = 11.985 hp. If you Bump the 185UL from 7.0cr to 9.5cr as I suggested with a 28mm Carb, using n3puppys rule 7.0 to 9.5 = 2.5 x 1.0 = 2.5hp + 9.4hp = 11.9hp + 27.5% = 15.1725 hp. By my General Rule 7.0 to 9.5 = 2.5 x 2.0 = 5hp + 9.4 = 14.4 + 27.5% = 18.36 hp. So Ball Park with 28mm Carb and 9.5 CR Bump upgrade that's what your looking at 15.1725 hp to 18.36 hp.

The best thing to do is make one of them Club Props for 15hp, hook a Tach up on engine on a Test Stand, and Test to get a Baseline, and then start by taking off the Muffler and see what it does to get a Baseline. Then add a Straight Pipe and Test to get a Baseline. Then add a Bigger 28mm Carb and Test. Picking up an extra 185 Head would be nice account you could Mill it for 9.5cr to do CR Testing, then mill the original for 10.5cr till you meet your goal of 15hp.

If you use the Peak Flow/HP Calc, 50% VE is as low as it goes: 1hp has a variance of 3%. So if only making 9.4hp@5000rpm it's probably around 45-46% VE. My 670 Dynoed 114.7hp@7750rpm = at 93% 114hp, at 94% 115hp, so probably 93.7% VE.

185UL 184cc at 5000rpm at 50% VE = 11hp and makes 16 cfm.
185UL 184cc at 5000rpm at 100% VE = 22hp and makes 32 cfm.

At 6500rpm, 184cc/7cc = 26.2 hp. 184cc at 6500 making 26hp falls into the 91%-94% VE range. The Calc only works with whole numbers. Most 2 Strokes that use 11.5cr, the proper Size Carbs, and turned 6500rpm falls into that 91-94% range.
 

Armilite

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From a breathing efficiency stand point - an engines required carb airflow is based on the Volumetric efficiency of the engine. 90% is used in examples below since that is realistic for a MODERN carbed, muffler equipped two stroke. (The 185 is hardly modern...)

=======> First of all the Stock 377F/380F is rated 36hp@7000rpm used on Sleds with a 30mm Carb. 368cc making 36hp@7000rpm = 59-60% VE. The 377UL was rated 35hp@6500rpm using a 36mm Carb which comes in at 62-63% VE. Engines making 90-94% VE are using a Tuned Pipe not a Muffler! <====

Armilite might dispute these numbers based on his previous post
He said 184cc and you always Size a 2 Stroke Carb at least 10% over what you need! on Avg a +1mm Carb = 6-7% more AIR. On these Engines, we have a set Max 6500rpm, and I wouldn't run over 11.8cr so you have a Max CR to use also. These Smaller Engines can only use so Big of Carb probably 38mm Max for 277UL to 503UL. You will never see a 100% VE let alone 110% VE.

See the attached dyno sheets that show 90% is reasonable.
377F @ 82CFM @ 6900 rpm = 91% VE
550F @ 132cfm @ 7005 rpm= 96%VE

======> No your 377 isn't making 91% VE, it's only making 45hp! 368cc making 45hp@6900rpm dials in at 75-76% VE. A Mild Tuned Pipe added to the Stock Engine 36hp + 25%(9hp) = 45hp! The other Dyno Sheet doesn't have a Label, so we don't really know what it was for. If for a 550F it looks like 65.26hp. 553cc at 7005rpm making 65hp is 71% VE. The 550F uses 9.6cr. That sheet is dated 2003, and I talked to a guy who had his New 2004 Dynoed which had 34mm Carbs and he said it made 70hp@7000rpm. Later 550F's used lower rpm and Smaller 30mm Carbs. Aaen sells Tuned Pipes that show a 6-9hp Increase at Stock rpm. If you compare the Stock 380HO 368cc. It was 48hp@7000rpm = 79% VE, and with a better-Tuned Pipe it made 57.26hp@7000rpm = 93% VE. CFM is based off Bore & Stroke & rpm, Hp is based off Toque made, VE is based off Hp made at a Specific rpm. <====

Using Armilites calculator and his +10% rule
185cc @ 90% VE @ 5000 rpm = 29cfm +10% = 31.9
185cc @ 90% VE @ 5500 rpm = 32cfm + 10% = 35.2

Carb. Flow values based on Venturi size
22mm = 32 cfm Ok at 5000rpm. 3.25CFM short at 5500rpm
28mm = 52 cfm/ 35.2 = 48% buffer @ 5500 rpm
30mm = 59 cfm/ 35.2 = 67% buffer @ 5500 rpm

The 22mm HL sized carb on the 185 is borderline @ 5500rpm IF the engine breathing efficiency is really 90% (That is not very likely)

=====> Maybe, I didn't explain myself as clear as I should. 110% is just that.
184 cc at 100% at 5000rpm = 32cfm, Stock 22mm = 32.3cfm so you should Size the Carb for at least 110% which is 36cfm @5000rpm!
You want your Max HP Power at your 5000rpm.

22mm 379.9 0.085 32.3 cfm
23mm 415.3 0.085 35.3 cfm
24mm 452.2 0.085 38.4 cfm
25mm 490.6 0.085 41.7 cfm
26mm 530.7 0.085 45.1 cfm
27mm 572.3 0.085 48.6 cfm
28mm 615.4 0.085 52.3 cfm
29mm 660.2 0.085 56.1 cfm
30mm 706.5 0.085 60.1 cfm

184cc at 100% at 5500rpm = 36cfm, so you want at least 110% = 38cfm Carb. Carbs are usually Sold in even numbers, odd numbers are Special Order. Air Density affects all these numbers!


The 377, 1/2 = 185UL used a 30mm Carb, the 377UL used a 36mm Carb. Newer 380's use a pair of 30mm Carbs.

BHP of 9.4 at sea level and Elevation of 1000 feet is 0.28 HP loss.
BHP of 9.4 at sea level and Elevation of 2000 feet is 0.56 HP loss. <===

But before any time and energy is expended adopting a larger carb, other mods would likely be needed to increase the amount of air the engine can draw to achieve the 90% or more VE.View attachment 93118View attachment 93119
=====================================

The only way you will ever see a 185UL make 90% VE is a Tuned Pipe and Bigger 28mm+ Carb and raise the CR. It probably has small Ports, so Porting also. The engine has to breathe in and breathe out.
 

n3puppy

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Messages
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==========================================
When you use the same Spec's for a 302 Chevy V8 Engine in Desktop Dyno, but just change the CR used.
302 CHEVY v8 SB 7.0cr 390hp@6500rpm
302 CHEVY v8 SB 7.5cr 405hp@6500rpm +15hp.
302 CHEVY v8 SB 8.0cr 418hp@6500rpm +13hp.
302 CHEVY v8 SB 8.5cr 431hp@6500rpm +11hp.
302 CHEVY v8 SB 9.0cr 442hp@6500rpm +11hp.
302 CHEVY v8 SB 9.5cr 452hp@6500rpm +10hp.
302 CHEVY v8 SB 10.0cr 461hp@6500rpm +9hp.
302 CHEVY v8 SB 10.5cr 470hp@6500rpm +9hp.
302 CHEVY v8 SB 11.0cr 478hp@6500rpm +8hp
302 CHEVY v8 SB 11.5cr 485hp@6500rpm +7hp.
Going from 7.0cr to 11.5cr = 485hp - 390hp = 95hp Gain! 1.2436

If you wish to use +1.0cr = 1hp, +0.5cr = .5hp than by all means do so. Till I see a Dyno showing the Numbers, I'll use mine.
Thank you for Again proving my point.

HP vs compression ratio changes are dictated by the engineering accepted equation.
E=1-(1/R^K-1)
E=efficiency,R=static(nominal) compression ratio, and K = 1.4

While I have never seen the Desktop Dyno Program it looks like it simply uses a different number for K than the chart. That is quite normal because 1.4 is for a theoretically normal engine. K will vary because of things like Air cooled vs Liquid cooled affecting losses due to different combustion chamber temperatures. (Heat soaking)
The chart I showed uses K=1.25

Your chart still follows the rule of diminishing returns of the Otto Cyle engine. As compression increases, the % change per point of compression decreases.
Using Desktop Dyno's numbers you provided
302 CHEVY v8 SB 7.0cr 390hp@6500rpm
302 CHEVY v8 SB 7.5cr 405hp@6500rpm = 3.85%
302 CHEVY v8 SB 8.0cr 418hp@6500rpm = 3.21%
302 CHEVY v8 SB 8.5cr 431hp@6500rpm = 3.11%
302 CHEVY v8 SB 9.0cr 442hp@6500rpm = 2.55%
302 CHEVY v8 SB 9.5cr 452hp@6500rpm = 2.26%
302 CHEVY v8 SB 10.0cr 461hp@6500rpm = 1.99%
302 CHEVY v8 SB 10.5cr 470hp@6500rpm = 1.95%
302 CHEVY v8 SB 11.0cr 478hp@6500rpm = 1.70%
302 CHEVY v8 SB 11.5cr 485hp@6500rpm = 1.46%
Per your chart,
390hp@7.0 vs 485@ 11.5cr = 24.36%

For the 185UL @ 4.5 point CR increase
7.0CR = 9.4hp
11.5cr = 9.4x1.2436 = 11.69 hp (Desktop dyno)
11.5cr = 9.4x1.1150 = 10.48 hp (Chart)
Both of those numbers are significantly below your "New Rule of Thumb"
1 hp per point of compression increase
7 to 11.5 = 9.4hp + 4.5hp = 13.9 Hp

If your rule of thumb number were actually correct - we could stop any discussion about carb size because 13.9 hp x 2 = 27.8 hp with 22mm carbs and current exhaust. EF11AE2E-5773-4924-AB0E-B6F0F3AA94A2.jpeg 7A64B2E2-9F3B-434D-A309-53676229B6BA.jpeg
 
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n3puppy

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==========================================
The best thing to do is make one of them Club Props for 15hp, hook a Tach up on engine on a Test Stand, and Test to get a Baseline, and then start by taking off the Muffler and see what it does to get a Baseline. Then add a Straight Pipe and Test to get a Baseline. Then add a Bigger 28mm Carb and Test. Picking up an extra 185 Head would be nice account you could Mill it for 9.5cr to do CR Testing, then mill the original for 10.5cr till you meet your goal of 15hp.
That is a very good idea

From your album I see you have two engines
Rotax 335@ 8.0CR
Rotax 340 Tnt @ 10.5 CR

You could make a club prop -(If it would work at 5500 rpm)
Run the 335 with the 8.0 head.
Then swap the 10.5 TNT onto it to see which method of estimation is closer.

8.0cr = 18 hp stock
10.5cr = 18 x 1.096 = 19.3 hp (chart %)
10.5cr = 18 x 1.124 = 20.2hp (Desktop Dyno %)
10.5cr = 18 + 2.5 = 20.5hp (new 1 pt = 1 hp rule of thumb)
10.5cr = 18 + 5.0 = 23.0 hp (old 1 pt = 2hp rule of thumb)


A5D12670-EB74-4519-9895-DB8BE8C1DB24.jpegD537ADE9-FC74-4ED9-9F7F-C8ED22D3CF4A.jpeg
 
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n3puppy

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==========================================
If you use the Peak Flow/HP Calc, 50% VE is as low as it goes: 1hp has a variance of 3%. So if only making 9.4hp@5000rpm it's probably around 45-46% VE. My 670 Dynoed 114.7hp@7750rpm = at 93% 114hp, at 94% 115hp, so probably 93.7% VE.

185UL 184cc at 5000rpm at 50% VE = 11hp and makes 16 cfm.
185UL 184cc at 5000rpm at 100% VE = 22hp and makes 32 cfm.
Once again, you are using the 1.5cfm /Hp number that the author of the program has repeatedly said is inaccurate
It consistently will inflate the HP numbers of a 2 stroke engine

Published Dyno results have shown 2-stroke engines in the range of-
1.7 - 2.0 CFM/HP based on things like pipe vs muffler
2.0 is the number produced by a 550F Trail sled

Using that number of 2.0CFM/HP
185UL 184cc at 5000rpm at 50% VE = 16cfm and makes 8hp.
185UL 184cc at 5000rpm at 100% VE = 32cfm and makes 16hp.

As previously noted the 22mm carb flows 32cfm
Using your rule of thumb that carb must be 10% bigger than required
Means the 22mm carb should be used at only 90% of rated flow
32x 90% = 29cfm

With a 22mm carb @ 2CFM/HP
185UL at 5000rpm at 57%VE = 18cfm and makes 9 hp
185UL at 5000rpm at 90%VE = 29cfm and makes 14.5hp
282EABF1-3786-4C06-835D-B9B527A7C3F8.jpeg
 
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n3puppy

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Jun 25, 2019
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=====================================
The only way you will ever see a 185UL make 90% VE is a Tuned Pipe and Bigger 28mm+ Carb and raise the CR. It probably has small Ports, so Porting also. The engine has to breathe in and breathe out.
It appears you must have typed your values in wrong when you used the Peak Flow calculator

Using your carb flow number chart
22mm = 32cfm
28mm = 52.3 cfm

Using the ACCURATE portion of the calculator for VE% at Flow rate
185cc @ 5000 rpm @ 32cfm = 97% VE. (22mm carb)
185cc@ 5000 rpm @ 52cfm = 158% VE (28mm carb)

Porting and a pipe are still likely required for a 185UL to fully utilize the 22mm carbs full flow of 32cfm giving 97% VE.
It would seem unrealistic to believe any amount of porting and piping will fully utilize the 52cfm flow of a 28mm carb and achieve 158%VE

As I previously stated - while a 28mm is oversized for the 185UL, it should be ok in the application. Additionally it provides a starting point for Jetting using the specs for the Skidoo Elan 250 that came equipped with a 28mm carb69CD85AC-BBFD-4553-B890-4D966C8DB089.jpeg7C551845-C93C-4B23-A663-0F48C717D2B7.jpeg
 
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Dave Hodges

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Reidsville, GA
Thanks for that information
I have been struggling to figure out how much faster the identical prop will turn when jumping from 9 to 12hp

If I am understanding you correctly -
Cubed root of 1.3333 =1.100642416
5000rpm@9hp x 1.100642416 = 5503 rpm @12 hp using the same prop.

Likewise, 150rpm increase with exhaust change calculates as
5150/5000rpm = 1.03 , Then cubed = 1.092727
9hp x 1.092727 = 9.834543 hp to gain 150 rpm

That's a 9% increase in hp from poking holes in the muffler!

Did I do that all correctly?
That's the way I figure it too.
 

Dave Hodges

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Reidsville, GA
==========================

Dave, we're trying to change the rpm, if any maybe +500rpm. The 185UL 7/8" PTO in Direct Drive won't handle much Weight, that's WHY they used such a Small Plastic Prop originally. At 9hp@5000rpm the upgrade was a Carbon 32" x 10.

185UL PTO Failure with Original Crank which was I think Machined wrong, No Filet. Upgraded Crank with Rust doesn't Help either.

View attachment 93184

View attachment 93185
I can't tell you anything about this particular set up. But theoretically speaking, 5500 rpm would be a 10% increase above 5000 rpm. Torque is proportional to the square of rpm. So the torque on the propeller shaft would be increased 21% (because 1.1x1.1=1.21). The horsepower needed to increase the rpm 10% would be 1.1x1.1x1.1=1.331 times the original horsepower.
 
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