Misunderstandings about intercoolers and carbureted superchargers

Misunderstandings about intercoolers and carbureted superchargers

By Chris Beardsley:

 

Unlike port fuel injection systems, carburetors have a unique advantage while operating on boosted engines without an intercooler.

Chris Beardsley

In carbureted applications, the air charge from the supercharger is significantly warmer than ambient air. When warmer air is forced through a carburetor, the *vaporization process is enhanced as the cool fuel mixes with it. Ever try starting your carbureted engine in the dead of winter? Now compare that to a hot August afternoon. The warmer air of the supercharger blowing through the carburetor amplifies the vaporization process. The result of superior vaporization is a cooler, denser air charge under pressure.

The warmer air mixing through the carburetor does something else just before it cools. The heat acting on the fuel causes the fuel particles to disperse—a chemical explosive process that sends fuel in every direction with violent force. When this occurs at the entrance to the plenum, each intake runner is filled with a more evenly balanced mixture of fuel and air that enters the cylinders. Naturally, cylinder-to-cylinder distribution affects horsepower. For these reasons, the ample performance of carburetors incorporated in boosted projects without an intercooler is evident. Even common pump fuels generate impressive power, and increasing ignition timing can further the power potential using these principles.

“But intercooling is better,” I hear you say. “What if I add one of those?”

While intercoolers have their place in boosted performance, for most carbureted applications adding an intercooler works against you. It looks fantastic and its associated plumbing enriches any engine compartment.

But, by directing the air charge through an intercooler to feed the carburetor, we lose its natural heat and associated benefits during the vaporization process. The intake air charge lacks the density, the distribution or the balanced fuel mixture of a simple direct flow system.

“Yet, with the intercooler the air is colder, right?” Yes, technically the air charge might be a few degrees colder. Air reaching the carburetor without intercooling typically reflects intake temperatures of 60-70 degrees below the throttle blades compared to intercooled temperatures that may be 10 degrees cooler. But they enter the carburetor around 100 degrees cooler. Relinquishing this amount of heat weakens the vaporization process and the distribution. The slightly cooler air temperature does not recover the loss of these other consequential factors.

In the case of operating a supercharger with a carburetor, run the air straight in and enjoy its inherent hidden benefits, including the cost saving and complexity of the intercooler.

*Author correction: The earlier use of the word atomization has been replaced by vaporization.

Source:

TorqStorm Superchargers
TorqStorm.com
Telephone (616) 706-5580 or visit www.TorqStorm.com or e-mail the knowledgeable Beardsley at: sales@torqstorm.com

4 Comments

  1. Does this hold true for a FiTech 8-injector throttle body? These are wet-flow injected above the throttle blades.

    Reply
  2. Hello David: The effects are similar when using throttle body efi systems. Aside from the pros and cons of these versus a carburetor, the fuel acting on the warmer air will cool the air charge, and the intercooler is unnecessary. Thank you for raising this question.

    Considering port fuel-injected engines for a moment, the intercooling benefits still remain, as the injector, spraying directly at the valve, gives no time for regular fuels to cool the air charge. To clarify some of the misinformation that confuses these scenarios, we will discuss them more extensively in future tech releases.

    Reply
  3. Interesting stuff, and I’m sure you have the dyno results to back it up, but for me there are one or two problems…

    The first is simply the fact that OEM intercoolers are universal on forced induction engines and it’s hard to believe that the manufacturers put them there because they look nice. It’s certainly true that most of these engines are port injected and turbocharged and it is of course inescapable that a turbo imparts more heat to the intake charge than does a supercharger. However, because the gas speeds are relatively high I believe the difference is not that great, and intercoolers are also used in OEM supercharging applications (e.g. the VW G60 engines of a few years back).

    The second is the claim that atomization is better at higher temperatures. There’s no doubt that fuel doesn’t vaporize so well at low temperatures, but whether temperature has any effect on atomization I’m not so sure, and anyway it is the vaporization that matters in terms of charge temperature. However, we’re not achieving ‘dead of winter’ charge temperatures with the addition of an intercooler, so the fuel is still going to vaporize just fine, which surely means the overall charge density must always be higher with an intercooler.

    Another issue is the reference to ‘a chemical explosive process’ which presumably means Brownian Motion. This is scarcely explosive but it does speed up with temperature, but to claim that this materially improves charge distribution to the cylinders seems fanciful at best. I’m sure there must be something else going on here. Maybe it’s the friction of the additional plumbing or relatively low boost pressures not needing an intercooler, but I’m pretty sure it’s not as simple as it’s being presented.

    Finally – the FiTech question. You say that intercoolers are a benefit to port injected engines (agreed – they all have them) but you claim that due to the position of the injector there is no time for the charge to be cooled (by the latent heat of vaporization). Sorry, but this just cannot be true. Either the fuel vaporizes or it doesn’t – if it does (and it does) the cooling effect delivered must be the same, regardless of how far down the intake port it takes place. I would propose that port injected FI systems with forced induction have intercoolers not because they somehow don’t benefit from the vaporization of the fuel, but because the charge temperature rises too high and the charge density is correspondingly too low for good power without them.

    Reply
  4. Bill,
    Thank you for your comments. With regard to your heat observations, the hotter air from the supercharger has an undeniable effect on the air-fuel mixture. Both cylinder balance and distribution is elevated with heat, which causes the mixture to scatter or expand to better fill the runners. The effect of higher temperatures on this type of fuel system improves combustion. My scientific description may be deficient, however, given the results I have seen, I cannot argue with the fact it works regardless of the wording.

    Reply

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