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ASP 21 vs OS Max 25FP

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Graham Cramp22/08/2019 11:03:38
41 forum posts

Good day all,

Quick question regarding the two above engines. I'm intending to begin a build soon from a slightly older plan which requires an OS Max 25FP. This is a discontinued engine and I was wondering how the ASP 21 compares. I know OS have a more recent alternative, but i want the round cylinder head as I want to replicate to give the illusion of a replica radial engine.

Thanks in advance

Dave Cunnington22/08/2019 11:08:46
181 forum posts
62 photos

Graham PM sent

Denis Watkins22/08/2019 11:23:07
4555 forum posts
125 photos

These are ballpark figures from my notebook Graham

But I made a similar comparison for a small Hobbyking Edge conversion and chose the ASP 21 as the numbers added up

I have a note of the ASP21 giving 550W top end and the

OS25 at 600W top end

These numbers are from my notebook so no science involved

The ASP21 is very loud, so consider an add on silencer, or in my case, I unbolted the existing silencer and added another 1" of pipe


Edited By Denis Watkins on 22/08/2019 11:23:48

Jon - Laser Engines22/08/2019 12:49:16
5566 forum posts
271 photos

I would expect the ASP to beat the 25fp. The 25fp was a great engines but it was a plain bearing engine with (if i remember rightly) cross flow porting. It was never a powerhouse.

The ASP 21 is ballraced with a much higher performance porting setup and larger carb. It is likely to offer similar or greater performance even with the smaller capacity.

Both engines are loud by modern standards on stock silencers but that can be easily corrected.

Braddock, VC22/08/2019 13:05:40
1655 forum posts
82 photos

25FP were schneurle ported but no great shakes, even on 15 or 20 % nitro, best easily obtainable 21 by a country mile is/was the irvine 20. There are more powerful ones but asp isn't one of them and cost could become an issue. The silencers of most 21s leave a lot to be desired.

Back to the OP the asp is a better bet than the 25 FP unless you already have an FP as they are good reliable engines with an OS no 8 plug and probably suited to your model if the design recommends it.

FWIW OS claimed a much higher output than was good for them for the FP series, in real terms the asp is about 15% more powerful and 15% more noisy, especially if the the os was fitted with the super silencer.

I have the 25FP, the OS 25 FSR and the Irvine 20, the fsr leaves them all for dead but they are a bit like rocking horse poo at present.

Jon - Laser Engines22/08/2019 14:14:48
5566 forum posts
271 photos
Posted by Braddock, VC on 22/08/2019 13:05:40:

25FP were schneurle ported but no great shakes

Ah fair play, i thought the fp35 was crossflow and by extension the 25 but i could have been thinking of the earlier max series as i know they were crossflow. Its been some time since i dismantled any of them. I think all of mine are sat unloved in a drawer somewhere.

Graham Cramp22/08/2019 14:26:13
41 forum posts

Thanks for the replies everyone. I had thought that the ASP would likely be similar in performance but had nothing technical to back it up. Might noise be an issue... maybe... do I make it an electric setup...? I'm just not sure I could to that to myself!

Thanks again everyone!

Peter Christy22/08/2019 14:58:59
1839 forum posts

If noise is an issue, Just Engines do a range of "quiet" and "super-quiet" silencers for small 2-strokes. They have the OS fitting dimensions, but will also fit the clones, such as ASP.




Nigel R22/08/2019 17:37:12
4012 forum posts
691 photos

"i thought the fp35 was crossflow and by extension the 25 but i could have been thinking of the earlier max series as i know they were crossflow"

All FP were Schneurle, but without the the boost port, I believe. They started off as cast iron and then went to ABN. I read a rumour the ABN types had slightly larger ports and consequently a bit more beans.

Just to confuse things, OS labelled all sorts of their 2 strokes "max" at various points!

Those JE silencers look suspiciously like ASP items - get them while you can.

GONZO22/08/2019 18:24:24
1380 forum posts
14 photos

As a point of interest the piston/liner set from the Thunder Tiger 25GP is an exact fit in the OS 25FP and has a boost port. I have two OS 25FP so modified, as yet un run. Everything else also appears to be interchangeable. My TT 25 GP turns a master 9X6 @12,000rpm on 10% fuel.

Nigel R23/08/2019 08:51:05
4012 forum posts
691 photos

And the Magnum/TT GP were true ABC vs the OS ABN.

GONZO23/08/2019 09:41:44
1380 forum posts
14 photos


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