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Tractor Pulling Discussion Forum

LP compression limits

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BT

04-08-2005 18:34:02




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Hey guys, I know that LP engines have higher compression than gas, but what is the limit before anything bad happens? Can they survive something as radical as 11 or 12:1 or higher? Just a theoretical question I have been wondering about. Thanks for your insight. BT




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Smta puller

04-10-2005 18:51:29




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 Re: LP compression limits in reply to BT, 04-08-2005 18:34:02  
My farmall has 13.6 static compression, With the Lp 450 head; Cranking compression is 270.Running a 5.75 stroke and 4.310 bore.Runs and lugs good,On 110 raceing fuel



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Andy2

04-09-2005 18:59:18




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 Re: LP compression limits in reply to BT, 04-08-2005 18:34:02  
The LP I get for farm use is 105 octane and it varies a bit. We are running 230 lbs compression in one tractor and it runs great. I have heard,(but never witnessed)of people putting plugs in a diesel head and running diesel compressions with LP. I would think, like anything there is a point of deminishing returns;I'd be curious as to what that would be.



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G/MAN

04-09-2005 07:33:35




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 Re: LP compression limits in reply to BT, 04-08-2005 18:34:02  
I'm not an LP expert, but there's no hard and fast compression ratio/compression pressure limit for any octane fuel. The design of the engine's combustions chamber, size of bore, operating temperature, the material the engine is made out of and a bunch of other factors dictate how much compression a specific fuel can stand up to without preignition.



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LW

04-08-2005 19:18:04




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 Re: LP compression limits in reply to BT, 04-08-2005 18:34:02  
YES!



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?someanswermaybe?

04-09-2005 09:07:10




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 Re: LP compression limits in reply to LW, 04-08-2005 19:18:04  
I believe LP is like 100 octane or 115 octane fuel, depending on the fuel. maybe someone else knows more. it should be good up to say 200psi.



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LW

04-10-2005 10:34:01




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 Re: LP compression limits in reply to ?someanswermaybe?, 04-09-2005 09:07:10  
The gas it self can go from 100 to 130 octane. It varies from domestic heat gas to motor fuel, and all levels in between according on suppply and demand I have had some in the past that didn't run worth a crap when it got gone and we refueled the tractor went back to its old self. I have run as high as 13:1 but 11.5:1 seems to be a good stopping point on a moline,If I was talking about a stock rpm puller I would stay below 10.5:1 in a moline, others will be similar but like G-man said above there are a lot of other variables that effect an engine's pre detonation point. Compression is not the only factor in making power don't get hung up on psi in cyl. there seems to be alot of that on this board thats the last thing that crosses my mind when building an engine. I will check the static compression ratio mathmatically, but as far as psi in cyl. here again alot of variables effect it also, cam overlap, point of centerline, etc.

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someanswer

04-10-2005 20:45:48




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 Re: LP compression limits in reply to LW, 04-10-2005 10:34:01  
We dont' run molines, but we have a small cube JD A that has a little bit of compression. I don't know what the ratio would be, but it has over 200psi. we can't run it on premium cause it spark knocks like crazy. 110 does it ok, but i dont understand what the ratio has to do with the ocmpression, because your ratio could be 15:1, but if it isn't at optimal performance, you could run it on regular. I like to do things in psi because you can better tell your motors fuel requirements then some ratio. am i wrong to say that?

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G/MAN

04-11-2005 13:35:48




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 Re: LP compression limits in reply to someanswer, 04-10-2005 20:45:48  
The compression ratios isn't the ONLY factor dictating compression pressure, but it's one of the major ones that does. Valve overlap on the camshaft is another. If you have a lot of overlap, the engine will tend to "bleed off" compression pressure, which can fool the engine and fuel into thinking the compression ratio has been lowered, and reduce the tendency for preignition. And no, just because an engine may not be in optimum tune and condition does not mean that it's going to be able to run on a lower octane fuel with the same compression pressure. You're not going to run any 15:1 CR engine on regular gas. 15:1 is a pretty ungodly compression ratio for ANY spark-ignition fuel.

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someanswer

04-11-2005 14:54:26




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 Re: LP compression limits in reply to G/MAN, 04-11-2005 13:35:48  
not really, have built some motors in dirtbikes up to 23:1, and ran it on 115 oxeygenated fuel. The ratio isn't quite what people think it is if its not optimized. CR is a mathmatic formula based on cylinder volume compressed and uncompressed, not a 100% realistic value if the motor don't breath properly. The valve overlap thing is because the valves can cool easier with more.



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G/MAN

04-11-2005 17:52:31




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 Re: LP compression limits in reply to someanswer, 04-11-2005 14:54:26  
There's a little difference in a small-displacement two-stroke engine made to turn umpteen thousand rpm and a long-stroke/big-bore four-stroke tractor engine made to turn a hot 1500 rpm. I know what the compression ratio is - the ratio of the volume above the piston at BDC to the volume above the piston at TDC. That's simple engine math, but I can tell you that you're not going to build any 4-stroke engine to 23:1 and make it run on any spark-ignition fuel I've ever heard of. I do have a question. In those motorcycle engines, are you actually building compression from the very instant the piston starts traveling up the bore, or does the piston travel up somewhat before the intake is closed off and pressure can build? If the latter is happening, calculating the compression ratio the old-fashioned way wouldn't be technically accurate, would it? As there is a loss of volume before compression begins?

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someanswer

04-11-2005 18:20:18




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 Re: LP compression limits in reply to G/MAN, 04-11-2005 17:52:31  
I didn't say a big tractor or something, i was just making a point that over 15:1 is possible in gas spark ignition motor. and yes, you are right on the two stroke motors and the ports that are open until the piston passes them. THey stay open about 1/3 of the compression stroke/exhaust stroke up, so yes your right. That also pertains to the PSI it has instead of the ratio. like you said about the ratio, there are different variables and them variables effect the compression pressure and temperature of the motor.

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someanswer

04-11-2005 18:31:27




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 Re: LP compression limits in reply to someanswer, 04-11-2005 18:20:18  
I guess the point i was tryin to make is to not get all gungho on cr's..... focus on teh performance of the motor and getting the fuel and air to make a good cranking pressure. THats all i can say. I know cr's can make a base for stuff, but theres alot to build off a compression ratio. i guess take it for what its worth.



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LW

04-11-2005 15:54:16




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 Re: LP compression limits in reply to someanswer, 04-11-2005 14:54:26  
Well bring that dirt bike to the next pull and we can hook it to the sled!HAHAHAHAHA



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Flogger

04-11-2005 22:11:02




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 Re: LP compression limits in reply to LW, 04-11-2005 15:54:16  
Careful what you wish for there LW... Somebody'll come up with a 500 lb dirtbike class and we'll all have to sit through it, lol.



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buickanddeere

04-10-2005 21:42:42




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 Re: LP compression limits in reply to someanswer, 04-10-2005 20:45:48  
Both mechanical compression ratio and cranking psi together. Will give better guide to what an engines optimum rpm range and octane requirments are.



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