Alky & Air - mass flow velocity & energy calculation . . . . .
- Thread starter Jerryl
- Start date
imjoesnuffy
I just break stuff!
- Joined
- Sep 13, 2005
I have ran as high as 32psi on 93 octane, shifting anywhere from 6200 to 6600 rpm, 231 ci, with a M15 and M10 nozzle with zero knock on a stock TB. No wonder I had such a hard time keeping the up pipe from blowing off. 
I have asked before and I will ask again, what affect does compression ratio and camshaft duration play in an gas/alky engine?
I have asked before and I will ask again, what affect does compression ratio and camshaft duration play in an gas/alky engine?
- Joined
- Feb 18, 2006
Boost raises compression. As boost is raised so is compression. The more compression means more octane that is needed to keep from detonation.
Well, there are some things you may be able guesstimate. I don't know what the compressor map is for that turbo but I'll pull one up that maybe is reasonably close (a 57trim Turbonetics):
If you are making 375WHP (with 15% drivetrain loss = 440HP) and lets use a standard guesstimate of 10HP per lb/min so you are flowing 44 lb/min which agrees with the guess in another post.
You are making 20psi boost but the CAS V2 has a pressure drop of around 3psi so you are running 23psi at the turbo. Your pressure ratio is ((23+14.7)/14.7 = 2.56. Looking at the compressor map above you might be in the 60% efficiency range.
Using efficiency to calculate turbo outlet temps with this formula (http://www.gnttype.org/techarea/turbo/turboflow.html) we get an a turbo outlet temperature of 332F for the ambient temp of 80F. Does this sound right? If we look at the data from the Bob Dick intercooler test (http://www.vortexbuicks-etc.com/ICtest.htm) on Table 3 page 15 we see that the CAS V2 had a turbo outlet temp of 319F when the TB inlet temp was 99F for their test at 21psi boost on a TE51 turbo. So the numbers seem within reason for a very quick guess.
So what can we gather from this? Well, we are out of the sweet spot of the turbo (highest efficiency) but the IC is doing a good job. If you had a perfect intercooler with no pressure drop we'd still be out of the sweet spot of the turbo. However, lowing the TB temps with alky would create more power. Airflow is proportional to density and density is inversely proportional to temps (but temps must be converted to K). Lowering the TB inlet temp by 30 degrees would increase the density by (99+460)/(69+460) = 1.056 or almost 6%. So there's power to be had by lowering the inlet air temp.
You can also see that by using a stock location IC with a turbo that has this sort of compressor map that without a good temperature drop through the IC or with heat soak in hot weather the TB temps would be very high and power would go down. Alky would definitely be a benefit for that type of combo and is the reason it works so well on the smaller turbos and/or SLICs as it lowers inlet temps at the TB.
Now if you went to a larger turbo with one like the compressor map below:
For the same 44lb/min and the same pressure ratio of 2.56 you are in a better efficiency range. Outlet temps from the turbo maybe only be 280F (a drop of 50F). So you could get away with a smaller IC or be more likely to running ambient temps after the CAS V2 IC (more power at the same boost). You wouldn't benefit from alky as much as with the smaller turbo. But in really hot weather it would still lower inlet temps after the IC. By looking at the compressor maps you can see that the larger turbo is much more efficient at the larger air flow numbers. Of course, the engine has to be able to make use of that air (such as ported heads).
I'm only referring to alky as it relates to TB temps here. Beyond that it still raises the octane and suppresses knock in one way or another which is probably how it helps people with the more efficient combos.
imjoesnuffy
I just break stuff!
- Joined
- Sep 13, 2005
Thanks for the response Rick. I understand that, but I guess my two questions would be:
1. How does overlap on a camshaft affect an alky/gas motor if at all?
I think my engine likes high boost and high rpm because it is at a relatively low CR and a small 206/206 roller cam. Just looking for some of the science behind it I guess. 32 psi with a 70 p-trim turbo, 6200-6600 rpms shifts, 231 ci, 93 octane and alky, 26 degrees timing in 1st and 2nd, 23 degrees in 3rd, with no knock. I am honestly afraid to push it any further, because according to everyone here, it shouldn't be able to handle that. The time that both valves are open, the alky spraying in, has to have an effect, is it just cooling? Will a longer duration cam with alcohol injection reduce chances of detonation even further because of the longer cooling affect?
2. If my engine were to be at 7.25:1 at 32 psi, is that the equivalent of an 8.5:1 engine at 25 psi? There has to be an optimal area where compression ratio comes in to play for a purpose built 93/alky motor, my engine was built 6 years ago ( gotta be 25,000 miles on it as well) and I have flogged the living daylights out of it almost every time I drive it. I have changed a few headgaskets along the way, but no other damage yet.
1. I don't know if overlap and/or the amount of time the valves stay open would be any different for alky as long as the alky mixture stays constant. With the longer duration cams and the higher rpm range you need more overlap but I don't think adding alky changes that dynamic. Alky seems to work well wiith a lot of cam ranges as long as the rest of the combo matches.
For your combo with the smaller duration cam, maybe you don't have as much air per cycle compared to a longer duration cam at the higher rpms. And also if the the rest of the motor doesn't flow as much, say you have minimally ported heads and minimal supporting components maybe thats why you can run higher timing and boost. When I ran a similar 70 turbo but with a 224 cam and Al heads and other supporting components I only needed to run 24psi at low timing to run low to mid 10s in the high 120s mph.
2. Running lower CR allows you to run more boost. Generally, you gain more power from the extra boost vs the power loss from the lower CR ratio. There are calculators for effective compression ratio to compare boost at one CR to higher boost at a lower CR. I don't know how that relates powerwise to our engines and how much boost you can get away with at the lower CR compared to the higher CRs. I wouldn't be surprised if there are gains to be had from the lower CRs with 93/alky but traditionally people with the high power combos have run leaded gas so maybe it wasn't as critical. Also, at the higher boost levels people have to watch out for the decreased output from stock fuel pump setups at the high fuel pressures. So by the time the setups are modified to handle the higher boost levels you could run at lower CRs they probably switch to leaded gas anyway so maybe its moot.
But running a low CR combined with the smaller duration 206 cam along with maybe not having a very high flowing combo (minimally ported heads, etc) can explain why you can run the high boost and timing at those rpms as compared to others.
- Joined
- Feb 18, 2006
That or the reading is not accurate.
Jerryl
Tall Unvaccinated Chinese Guy
- Joined
- Dec 14, 2004
You definately know what you are talking about and have proven it as well.
Just to throw a bone out there. The RJC plate and its distribution on a running engine vs a non running engine.You definately know what you are talking about and have proven it as well.
And we still havent discussed the exactly "how much" is being sprayed vs how much is actually needed to be sprayed.. coupled with the different intakes, different IC, different plumbing, different throttlebodies, different air fuels people are targetting, back pressure, etc..
I'll stick to my guns on this statement. There is no "Ronco" set it and forget it when it comes to tuning a boosted engine. The higher the squeeze applied to the motor, the more you need to be vigilant as to whats happening. If you look at a professional race car.. they will rent the track days before an event and rework the fueling and maps. Why would they do this if it was perfectly tuned??? The answer is once climatic conditions change.. so will the tuneup on the motor. When your at 21 PSI you have tons of leeway for mistakes. and probably can set it and forget it year round. Once your at 30 PSI.. game changer.. and if you step up to that poker table.. you need to be prepared for the outcome.
Pre IC spray cant be done on cars with BOV's. That will typically be most cars with "large turbo's"
One thing thinking from afar.. why not mount nozzles into the frount mount tanks. That would increase time for the alcohol to make it from point A to point B. This may be a simpler more effective way.
Murphster.. ditto on monitoring pressure and priming the system. This eliminates 99% of the problems. And if your having problems with slosh and pressure.. maybe time to switch out the tank for something else.. Which would only affect 1% of the TR communitty.
Great thread.. Jerryl. I'm never afraid of information..
So true! We have a local shop owner that holds numerous NHRA records, some are over 20 years old. He was the first to run in the 6's with a naturally aspirated small block, first to go over 200 mph. Some of his records haven't been beaten, how weird is that?
- Joined
- May 25, 2001
Wanted to make a quick point on the discussion of rich vs. lean on the rear cylinders with alcohol.
When spraying, we assume a homogeneous mixture through the throttle body. While not 100% true, this is probably a valid assumption for first-order effects. If this is the case then each cylinder sees the same ratio of air:alcohol *regardless* of airflow distribution. While the ratio is constant, the mass flow to each cylinder is not. Therefore, the fuel injectors have to spray different amounts per cylinder to maintain the same ratios of air:alcohol, air:fuel, alcohol:fuel.
The alcohol spray shouldn't be causing a rich or lean condition anywhere. Instead, since the homogeneous mixture passes through the plenum, it's the fuel injectors that modify the gas ratios in the cylinders.
In reality there is not 100% atomization of the alcohol. Larger droplets maintain a more constant velocity and would end up closer to the rear of the engine, if we assume that's the highest flow rate. I think this isn't a leading order effect.
Just some food for thought. Very interesting thread.
When spraying, we assume a homogeneous mixture through the throttle body. While not 100% true, this is probably a valid assumption for first-order effects. If this is the case then each cylinder sees the same ratio of air:alcohol *regardless* of airflow distribution. While the ratio is constant, the mass flow to each cylinder is not. Therefore, the fuel injectors have to spray different amounts per cylinder to maintain the same ratios of air:alcohol, air:fuel, alcohol:fuel.
The alcohol spray shouldn't be causing a rich or lean condition anywhere. Instead, since the homogeneous mixture passes through the plenum, it's the fuel injectors that modify the gas ratios in the cylinders.
In reality there is not 100% atomization of the alcohol. Larger droplets maintain a more constant velocity and would end up closer to the rear of the engine, if we assume that's the highest flow rate. I think this isn't a leading order effect.
Just some food for thought. Very interesting thread.
Jerryl
Tall Unvaccinated Chinese Guy
- Joined
- Dec 14, 2004
I am just a student . . . . thinking about stuff . . . .
You're up to something Jerryl, and you haven't sent me an email about it yet.
Jerryl
Tall Unvaccinated Chinese Guy
- Joined
- Dec 14, 2004
Charlie,You're up to something Jerryl, and you haven't sent me an email about it yet.
I am resending it.
Just wondering why you said temps have to be converted to K? I've done many calculations with the gas laws, Kelvin is used for Celsius and Rankine for Fahrenheit, and all pressures are in absolute. And you did use Rankine, so I'm assuming it was a misprint.
Not a misprint. It doesn't matter which one you use for my example since its a ratio. To convert from Rankine to Kelvin you multiply by 5/9. Using Kelvin in the temperature ratio:
[(CurrentTempF+460)*5/9]/[(NewTempF+460) *5/9]
the 5/9 cancels out so you are left with the same ratio of:
(CurrentTempF+460)/(NewTempF+460) = (99+460)/(69+460) = 1.056
to find how much your density changes. In my work, radiation physics, I use gas law calculations on a daily basis and the temperatures are always converted to Kelvin. Everything is in standard SI/metric units for any type of calculation. You might see degrees F used to compare to something the real world, but once its used in a calculation its always Celsius/Kelvin.
Jerryl
Tall Unvaccinated Chinese Guy
- Joined
- Dec 14, 2004
I really need to take some time and read the articles . . . . carefully.
Some GREAT info. Thanks for sharing.
Charlie . . . still waiting brother!
Rebuildng a shredder (bush hog) Jerryl, a 1950's model and hopefully I'll be done tomorrow, but with all the rotted metal so far you never know. I hope to have some time late tomorrow if everything goes right but I've never seen metal rotted this bad before.
I hope to have some time late tomorrow if everything goes right but I've never seen metal rotted this bad before.Similar threads
