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Discussion Starter · #1 ·
A blown Gas engine usually has a compression ratio of 8 or 9 to 1,
while nitrous motors are usually 4 to 6 points higher. A tech article from the Reher Morrison web site on measuring Cylinder Pressure;described the burn characteristics of both types of engines.
The Nitrous motor shocked them with the extremely high cylinder pressure spikes and volatile nature of the combustion which is detonation prone as we well know..The Blower motor , on the other hand demonstrated smooth and more controlled combustion.. It is clear that the Nitrous motors make more power and live longer when combustion chambers are modified to slow down and or smooth out combustion. I am wondering if anyone has tried a purpose built low compression race motor for Nitrous?? Its required for mechanical supercharging...why not for the chemical method?
Anyone have any experience with this.?? or thoughts??
 

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A blown Gas engine usually has a compression ratio of 8 or 9 to 1,
while nitrous motors are usually 4 to 6 points higher. A tech article from the Reher Morrison web site on measuring Cylinder Pressure;described the burn characteristics of both types of engines.
The Nitrous motor shocked them with the extremely high cylinder pressure spikes and volatile nature of the combustion which is detonation prone as we well know..The Blower motor , on the other hand demonstrated smooth and more controlled combustion.. It is clear that the Nitrous motors make more power and live longer when combustion chambers are modified to slow down and or smooth out combustion. I am wondering if anyone has tried a purpose built low compression race motor for Nitrous?? Its required for mechanical supercharging...why not for the chemical method?
Anyone have any experience with this.?? or thoughts??
Although it was a long time ago. (late 80's) I had a 6-71 blown, street ,iron headed ,355, that kept removing harmonic balancers. Plain, single keywayed steel crank. Ended up selling the blower and bolting on a cheater kit, motor was 9.2 to 1. Don't remember the cam specs, but I remember it had quite a bit more duration on the exhaust side and I believe also more lift. Cam technology was not what it is today, by any means.

Long story short, hit it with 250 using 110 fuel. Never pulled any timing at all out of it. Ran it at 34 degrees. Never hurt a single piston or part. Of course, the nitrous jetting back then was not what it is today either. So, it was probably pig rich and the additional timing was covering it up. At the time, it was a daily driven street car and went 7.40. Which was plenty stout back then for a heavy street car, with nothing but some 9 inch slicks for traction aids.
 

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"Darkness"
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I know a guy that was over 15 to 1 with a big block Pontiac. The car would hurt pistons all the time with anything bigger than a .040 pill in a fogger. He wanted to lower his compression some and ended up at 11.7 to 1 by mistake. His engine builder claimed it would be a pig. The car now runs it's best ever on a .038 pill lol. He has yet to hurt it with up to a .046.

I'm not sure anymore how important compression is and how low you can go before hurting performance. I'm sure each combination can be different. I would like to have the time and money to test a engine from 9 to 15 to 1 with different tunes. A lot of us have learned that high compression is not the way to go.
 

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Discussion Starter · #4 ·
Mike...That is what I was driving at....Low compression for a purpose
built Nitrous motor seems logical.....Not many folks offering an opinion...;);)...maybe that means something. Yeti who posts here sometimes may be able to shed some light on this.. The data from
WWII war planes may not be helpful even though there was a lot of R&D, as those engines only used Nitrous to supplement their already
high performance aircraft engines which had some decent compression...
 

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I'm at 8.27 dynamic on a little 357sbc street/strip engine, purpose built to spray on 87 octane (700 to the tires). It's not much NA, but knocks down 20mpg with 28" tires, 3.73 and no overdrive. This is the 3rd one i've built along these lines, next one is going even lower on comp.
 

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I'm right at 11:1 thinking of uping the compression to get a little more but I'm happy so far with it and every time I want to crank it I don't have to put race gas in it.
 

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I know a guy that was over 15 to 1 with a big block Pontiac. The car would hurt pistons all the time with anything bigger than a .040 pill in a fogger. He wanted to lower his compression some and ended up at 11.7 to 1 by mistake. His engine builder claimed it would be a pig. The car now runs it's best ever on a .038 pill lol. He has yet to hurt it with up to a .046.

I'm not sure anymore how important compression is and how low you can go before hurting performance. I'm sure each combination can be different. I would like to have the time and money to test a engine from 9 to 15 to 1 with different tunes. A lot of us have learned that high compression is not the way to go.
Every combo has its breaking point ,the question is do you want to try your luck finding it ? 15 Is too high for your buddy but 13 may have been the sweet spot and now he is at 11 and prob left HP on the table....Will never know unless he makes a piston change,but I am all about reliability over a temperamental motor..
 

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Wasn't purposely done in my case but iam in the 8:1 range with my turd. And on motor it'd exactly that, a turd. 13.0's on motor and 10.90's on a 150 shot. She likes the juice....haha. Good post as I often wondered about my low compression and spraying.
 

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funny, just had this conversation with Charlie Buck when I picked up my heads the other day. I was shoting for 14-1 and we ended up at 13-1. I told him I will sell my brand new pistons and start over,and he said why?? I know from experience that the lower the # the wider and more forgiving the tuning window is. so long story short,we are at 13-1 and we will be fine.i wanted to make a little more power on motor(alky) but I do not think I will loose any sleep over this.i know every motor has a sweet spot and have seen really low comp motors go like hell when they are sprayed.seems the lower the HP,the bigger gains are made, with a small shot,to a certain point.
 

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To answer the OP, A supercharged engine you are pumping air into the chamber before the piston moves upwards so even though the static compression ratio may be much lower the actual PSI at TDC will usually be at least as high as a NA engine with 4-6 points more compression.

All engines (NA, Supercharged, Gasoline, Alcohol, etc.) have an ideal compression ratio, nitrous is no exception the "ideal" compression ratio's for various combo's has been researched already, sadly there are no undiscovered secrets in this regard.
 

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Discussion Starter · #12 ·
good info and perspective Bottlefed.....so you are saying that optimum C/R have been established for various Nitrous combos? i.e. BBC..Conventional, Big Chief, SBF etc ??
 

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good info and perspective Bottlefed.....so you are saying that optimum C/R have been established for various Nitrous combos? i.e. BBC..Conventional, Big Chief, SBF etc ??
I think to a large extent yes most big name engine builders have a pretty good idea what will work with a head,cam,etc, at a given power level. That's not to say that there is total agreement on an exact CR, rather that I doubt someone will find out that on a common engine combo (BBC, SBF whatever) that 9 to 1 works better than 13.5 to 1 by using totally different valve events, 40° of timing, or some other change to take advantage of the compression.

Who knows though not many would have predicted the amount of nitro you can burn with lots of spark and LOTS of lead.

I can't see it happening with no other changes to the nitrous system since with a conventional nitrous system we can burn as much N/F as we can ingest... but.....Maybe by totally changing the delivery of the fuel and nitrous and delivering both in very large cold droplets to delay the expansion of the nitrous till its in the cylinder and on the fuel to delay the burn time you would be able to run twice the PPH and increase performance.

Wait a minute I just contradicted myself...kind of :confused:
 
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