Injected alcohol comp ratio
 

Looking for some input regarding maximum compresson ratio on an Alcohol injected comp eliminator SBC.
The engine I'm working on is a 410 SBC B/ND engine ,not a very quick one.
The engine currently has a c/r of 17.3 to1 ,which I have no problem with if it was a gas engine.
I've always observed on other engines a lack of improvement when going over 15to1 on alcohol .
My concern is that the volume of fuel to burn at the afr required for alcohol will be greater with the lower c/r ,theoreticaly making more power.
does this make sense or am I mistaken about the benefit of fuel volume vs. c/r ?

Re: Injected alcohol comp ratio
 

Tom
Well Tom If you think about it compression ratio is just an expression of a mechanical relationship and isn't affected by the air fuel ratio or the fuel type.(i.e. it doesn't change)
Your concern that the volume of fuel to burn at the afr required for alcohol will be greater with the lower c/r ,theoreticaly making more power.doesn't make sense to me as the fuel itself is not oxygen bearing and the fuel itself in basically not compressable only the air the fuel is suspended in is compressable.
If you look at how Alcohol dragsters are running an 11:1 CR with thirty plus pounds of boost their effective compression ratio's are much higher that the 17.3:1 naturally aspirated combo your working with. My guess is that in the other engines you were working with that their lack of improvement was due to the chamber design the amount of squish and swirl which may result in some of the fuel falling out of suspension which resulted in detonation.
So based upon what the dragster guys are doing I think if you can get the air fuel mixture to stay homigized you should be able to produce more power with a higher compression ratio.

But then of course I could be wrong!

Re: Injected alcohol comp ratio
 

Bill ,that why I'm asking.
I want to believe tha the motor wants the higher c/r , for the same reasons you pointed out.
I began questioning the higher c/r after talking with some sprint car guys I know who told me they tried higher c/r's than 15 to 1 and saw no more power.
I realise there is still a benefit to the 17 to 1 ratio when dealing with poor atmospheric conditions and higher altitudes, but since the engine needs pistons anyway ,I thought it would be worth questioning.

Re: Injected alcohol comp ratio
 

I discussed this at length with an engine builder friend of mine (who builds mostly gasoline drag racing motors) and for reliability reasons he “wouldn’t let a motor leave his shop with at 17:1 C/R’ It was his opinion that that’s too close to the ragged edge and if he built such a motor it would surely come back to bite him”.
So our theories aside, based upon what your sprint car guys and my buddys input maybe you should look elsewhere for power, obviously air flow and fuel mixture quality would be the logical place to start looking there may also be some improvements to be found in spark/timing management as well.
Good luck.

Re: Injected alcohol comp ratio
 

Hey Tom.. give Steve Levine a call.. he has been working on that SBC combination for years and is very close to doing something good.. seems his machinists just let him down in some areas..

That being said, he can probably give you all the info you might need.. 845-527-5822.. I believe that is his cell..

Re: Injected alcohol comp ratio
 

Found the site below and gives a good outline of alcohol theory

http://www.turbofast.com.au/racefuel2.html

Re: Injected alcohol comp ratio
 

Thanks for the link, some interesting reading.
Charlie, it's a naturally aspirated SBC not a blown combo like Steves.
I've spoken to a few knowlegable builders recently ,who confirmed the desirability of the 17to1 ratio,even with alcohol. as fuel ...their reasoning follows some of the statements on the turbofast site, treat the engine the same as it was a gasoline fired Comp engine.
So it will be going back together with the higher comp ratio.

Thanks to all for your thoughts on this, This is my first Comp Eliminator engine build in nearly 30 years and even tho I keep my eyes and ears open in the pits, there is a mountain of learning to do on this combo.

Re: Injected alcohol comp ratio
 

Actually Methanol is oxygen bearing and that is the reason the A/F ratio is 6.4:1 compared to Gasoline at 14.7:1. Higher compression ratios can be used due to the higher octane of alcohol, but they are also needed to ensure enough heat to burn the fuel since alcohol burns cooler than gasoline. Due to the lower BTU/lb of fuel, you have to burn more alcohol to get the equivalent BTU of gasoline. But given the A/F ratio, this is possible. The real key is in the released heat energy as that is what pushes the piston down. Because of the high air requirement to combust gasoline, you can only burn .066 lb. fuel/lb. air, giving you 1359 BTU/lb. air. With Methanol, you can burn .156lb. fuel/lb. air, giving you 1524 BTU/lb. air. Couple this with the higher octane allowing higher compression and more ignition lead it sounds like you will have a monster Tom. Good Luck!

Re: Injected alcohol comp ratio
 

Compression is used to ensure the fuel is burned as complete and as fast as possible, however anything more than needed to reach that goal is wasting energy to compress the mixture. You might be surprised to find a Pro Stock engine runs at or less than 15 to 1... You may want to talk to Chad Speier, he did the heads for the Parks ND cars and might give you some advice on your project. He has a tech forum here.
http://1320techtalk.com/

Re: Injected alcohol comp ratio
 

Unless someone has changed the rules, Pro Stock does not run alcohol, therefore they are more limited on CR. When cup cars could use qualifying motors, they ran as high as 18:1 on race gas. Why? While there is increased resistance due to higher compression ratio on the compression stroke, there is also a considerable increase in efficiency on the power stroke which you are missing. The following does a decent job of explaining: "Cylinder pressures and output will increase as the CR is raised, but what is less obvious is that the increase in compression also increases the engines thermal efficiency. Thermal efficiency is a measure of how effectively the engine converts heat into mechanical power. To appreciate this it is better to consider the engines expansion ratio (ER). This is the opposite side of the coin to the CR and describes what is occurring as the piston moves down on the power stroke rather than what happens as it moves up on the compression stroke." Read more: http://www.popularhotrodding.com/tec...#ixzz1jxVXExeV

Alcohol and gasoline are two different animals with their own specific requirements. You can't just look at one particular race engine because its fast and apply that to every other engine. While some Pro Stock engines only have approx. 15:1 to 16:1 static compression ratios, you are forgetting that with their cam and head profiles, the dynamic compression is approaching 18 to 20:1. Any more static ratio, and bad things start to happen.