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
Does this hold true for a FiTech 8-injector throttle body? These are wet-flow injected above the throttle blades.
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.
What about a carburetor before the supercharger? Remotely mounted and piped through intercooler up to intake. So basically premixed charge air through intercooler…good or bad?
Hello Jordan,
Several or our inquirers have proposed this in the past, even referring to other supercharger systems being run this way. The fuel would cool the atmospheric temperature slightly, then the supercharger would re-heat it, justifying the intercooler you mention. However, we have not run a TorqStorm in this manner, so I cannot estimate the gains. Yet, it seems more complicated than the simple blow-through arrangement.
I have a TorqStorm supercharger on my 351 Windsor with a QuickFuel blow-through carburetor. In the summer heat around 80-90 degrees it runs a little rough but in the winter around 40° it runs beautifully. I don’t really want to buy a intercooler, should I?
Hello Peter,
Can you email me at sales@torqstorm.com and we can investigate your drivability issues? Happy to help and if needed we have a former QFT engineer that can lend his knowledge on your project.
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.
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.
Would the same principle apply for a draw-through supercharger?
Hello Steve: Draw-through supercharging shows one of its weaknesses in this area; that is fuel being added before the supercharger. So, the entire air-fuel mixture is heated during compression from the supercharger. Some draw-through set ups use after-cooling to lower the charge temps.
I’m starting to build a supercharged car using carb induction. My idea is to get cool natural air to the supercharger with the supercharger in the cold air stream. Then to intercooler before the blow thru carb to the engine. Your article is against this on the basis that air into carb might be 100deg cooler (and not vaporize so well) and will only be 10deg cooler below the butterfly if I understand what you’re saying? So, as physics means we get nothing for nothing, what has the biggest loss or gain? Is the 100deg cooler at vaporization more or less offset by the gain of 10deg cooler entering the engine?
The other concern I have with no intercooling (as I’m going vintage racing with no electronics of any sort) is that in the heat of battle on a hot day on an engine that I am squeezing as hard as I can on pump gas, there is no protection and no system in place for protection of the engine. Therefore, if the air gets too hot, the power will get less but is there a risk of detonation or similar destructive forces due to very hot fuel/air mix? It was common on old cars in hot weather to have trouble with fuel vapor locks etc.
I’d appreciate any comments or science or better still dyno results from anyone whos tested these ideas?
Actually on re-reading my own comments above, it would seem reasonable to have the air to the supercharger, then thru the carb to vaporize before the intercooler? Are there any known problems with an intercooler being full of fuel/air mix?
Hello Murray,
There will always be situations that defy performance statements. However, we are firm believers in blow-thru carburetion without the use of intercoolers.
Hi Chris,
I have no issue with your system of blow-thru without intercooling, I’ve seen it work fine. Also, there’s an abundance of suck-thru carburetors before the supercharger and no intercooler too. As you guys have done it and tested it rather than just write a theoretical article, I am trying to search your brain for things you found or things to avoid and variations you may have tried and thought about? I’m hoping you may have heard of pitfalls with my idea, which might save me one or two steps in the learning-the-hard-way process. Or perhaps another reader can offer something from experience.
Hello Murray,
I have never experienced blow-through fueling with after-cooling. I have seen after-coolers used on Roots-style supercharging systems, the fuel being drawn from the carburetor through the supercharger then cooled before entering the cylinders. Other than its height, the packaging is convenient with a relatively short run from fuel dispensing to intake valve.
I could see a benefit to after-cooling for temperature control. This is assuming that conventional direct blow-through cannot handle the task. I do not see a lot of road race blow-through carburetor set ups to draw reference from. For simplicity, I would look at a conventional blow-through system with water/methanol as an add-on if needed—or possibly blending fuels to incorporate some alcohol in the air charge. It will cool better than race gas alone.
Hi Chris, Now you mention it I have heard of after-coolers used on Roots-style supercharging but it’s certainly not common, making me more wary that any gain comes at a price? The idea of alcohol injection to intercool no doubt works but I’d like to avoid it as it’s another hassle with rules/safety requirements and just another concept to have organised for race day. Plus I’d imagine it adds another factor in fuel/air mixture management?
There’s no argument that intercooled managed turbo systems produce much better results, but they don’t put fuel thru the intercooler or throttle prior to the intercooler. I see two potential problems in learning about known pitfalls. First, the effects of fuel/air mix in the intercooler and what containment/flow/cooling may happen or do to it? Second, the big volume between the butterfly and the intake valve and its effects on engine behaviour? This discussion can go around in circles which is why I’m looking for others experiences.
Whatever system I choose will take some sorting, especially in minimising the number of remakes or alterations. Small changes can make big improvements and big changes small improvements. Obviously, I’m keen to avoid big changes that make no difference or make it worse!
Murray,
Perhaps you could call Chris Beardsley direct on (616) 706-5580.
Cheers,
-Vm
Hi Chris, Can I run the supercharger air to non-blow-through carbs? Mine is a 1964 Corvair with two carbs, like forced air? Thanks, John.
Hello John, I’m unaware of any naturally aspirated carburetor operating as blow-through without some kind of modification. If you have other questions, email me directly at sales@torqstorm.com
John,
If you wanted to use a non-blow-through carburetor on a blow-through type of supercharged application, it can be done but involves drilling air bleed holes in your throttle blades. I am unsure what size or positioning they need to be, as that differs for each application, but it is possible.
Sorry about my couple of years’ delay in responding to your question.
Regards
Steve
As a longtime reader of MGI, I find the older Q&A tech sessions just as entertaining and informative as when the topics were originally discussed. That’s a true sign of quality automotive journalism. Thanks for the memories and Moore!