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 it
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