[Diy_efi] Throttle body size

[Tlsalt at aol.com]

Eric Byrd wrote,

The SDS folks I just got done reading from their site say this about
throttle bodies:  if the vacuum taken off the MAP sensor is higher at
full throttle than at idle, your throttle body is too small.

I found out some more info.  An engine needs 5.7 cc/min of fuel per hp. 


Hello Eric,

Read that again......another impossibility.  MAP sensors read manifold 
pressure, vacuum in an NA engine.  Just what kind of engine could have higher 
vacuum at WOT than when the same engine is sucking against a closed throttle 
plate?  No chance. What they mean is if vacuum start climbing after reaching 
a minimum at WOT under load.

The amount of fuel an engine needs is not 5.7cc/min per hp.  It varies for 
every engine and is the BSFC.  How much ?  Engines from the same manufacture 
can vary by as much as 18% (high comp Heron vs low compression Hemi) and 
there are no accurate sweeping generalizations about BSFC. In fact, most of 
these subjects are far more complicated than you suggest.

I'd speculate that the main reason some Hondas are less detonation prone is 
not the bore size, it is the design of the combustion chamber at and near 
TDC. The difference in distance is very small on these little engines. There 
are plenty of small bore detonation prone designs.  Small bore engines with 
large, open combustion chambers (like Hemis), that need tall piston domes to 
have decent compression ratios, are very detonation sensitive. Their 
combustion chambers at TDC are long thin crescents.  Many Hondas have small, 
compact combustion chambers, with almost flat top pistons and are much less 
sensitive.

I agree, speculation is not a crime, but offering erroneous, sweeping 
generalizations as "advice" on this list should elicit some response.  What 
ever happened to Bruce ?   I'm sure he would have something to say about this 
subject........

Paul

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