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Username Post: Speed Density        (Topic#355210)
4dr 57 
Valued Contributor
Posts: 4629

Loc: Texas Hill Country
Reg: 11-10-04
02-12-19 06:14 AM - Post#2759705    

Recently a man I met with said his early Gen II LT1 motor (sic) was a speed density system.
Are GM TBI systems called speed density systems?
What are the Gen II LT1 systems called, Gen III systems called?
Thanks.
Stan

It's all good. mostly




 




Rick_L 
Honored Member
Posts: 27661
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
02-12-19 09:57 AM - Post#2759740    
    In response to 4dr 57

The alternative to "speed density" is to use a MAF (mass air flow) sensor. The tune can use both too.

GM TBI systems are speed density. There were TPI systems that were MAF and later speed density. Early gen2 LT1s were speed density, no MAF. In 94 they started using a MAF, the tune runs mostly off the MAF but in some conditions is speed density. LS engines are the same way.

A general statement is that speed density is better at transient conditions and MAF is better for steady or near steady conditions.




 
4dr 57 
Valued Contributor
Posts: 4629

Loc: Texas Hill Country
Reg: 11-10-04
02-14-19 03:53 AM - Post#2759901    
    In response to Rick_L

  • Rick_L Said:
The alternative to "speed density" is to use a MAF (mass air flow) sensor. The tune can use both too.

GM TBI systems are speed density. There were TPI systems that were MAF and later speed density. Early gen2 LT1s were speed density, no MAF. In 94 they started using a MAF, the tune runs mostly off the MAF but in some conditions is speed density. LS engines are the same way.

A general statement is that speed density is better at transient conditions and MAF is better for steady or near steady conditions.





ok, Rick cool summation too-thanks

The key is the general statement, as it turned out that "speed density is better than..." is the title of a book, thank you very much!

Oddly, on page 18 is a quantifier relating that speed density is used but is not "self correcting" for conditions such as altitude and other motoring changes. The MAF system does so when necessary.

It seems like the title of the book is a purposeful misnomer
Thanks.
Stan


https://books.google.com/books?id=6IIBnOV1v1gC &...



As an aside, I'm pretty sure TBI/Speed Density systems are both batch fire systems where as MAF systems are sequential fuel and spark systems. This right?
The mpg on the former used to be around 17 in trucks no matter the load (within reason) or with out, where the MAF systems would get a lot more better mpg.



 
Rick_L 
Honored Member
Posts: 27661
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
02-14-19 07:44 AM - Post#2759913    
    In response to 4dr 57

  • Quote:
speed density is used but is not "self correcting" for conditions such as altitude and other motoring changes.



Not true, speed density IS self correcting for elevation and weather changes.

  • Quote:
I'm pretty sure TBI/Speed Density systems are both batch fire systems where as MAF systems are sequential fuel and spark systems. This right?



Not true. TBI is going to be batch fire because the fuel is introduced to the entire intake manifold. TPI MAF systems were batch fire, as were the later speed density systems. Early LT1 speed density as well as the later MAF/speed density combo were sequential.

A sequential MAF system is the most precise, so potentially it gets the best fuel economy. Not to say that the others couldn't do well.



 
jktucker92 
Silver Supporting Member
Posts: 318
jktucker92
Loc: West Richland, WA
Reg: 02-05-17
02-20-19 09:54 AM - Post#2760357    
    In response to Rick_L

There are books written out there that can give you the advantages / disadvantages of each system, but I'll try to keep it brief. In order to run as efficient as possible, you need to mix fuel and air at a specific mass ratio. 14.7:1 is the Stoichiometric ratio that is ideal, but richer mixtures can provide more power. With a MAF sensor, you measure the mass of the air flowing in the intake, which makes the calculation of how much fuel simple and accurate. The problem is the MAF sensors are more expensive than a simple pressure and speed sensor, especially early on. Also, the MAF sensors can be restrictive when you want to increase performance, so they are often removed in high performance applications. A speed density system uses the speed of the engine, manifold pressure, and temperature to calculate the mass of the air flowing into the intake and into the cylinder. This is pretty accurate, but not as accurate as a MAF sensor. As a result, the engines generally are tuned to run a little richer than the MAF systems to avoid damaging the engine by running too lean all the time.
Whether an engine is batch or sequential injected is a different, but related topic. In order to have sequential injection, the engine must be port injected, and the injector fires on each cylinder as each valve is opened. Batch systems can fire all the injectors on each revolution, or half one one revolution and the other half on the next. As it comes out of the injector, it's a fine mist, and the longer it is in the manifold, the more that mist becomes larger droplets, which burn less efficiently. Port injected systems are better than throttle body systems because their injectors are close to the ports and the fuel stays in the fine mist better.
The most efficient system is a MAF sequential injected system, which is why all new vehicles are MAF sequential injected systems.



 




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