Tim Helgeson; via CarCraft.com: I’m putting together a Mercury Capri with a stroker 408. I have an older Holley 780 vacuum secondary carburetor. I just read your article on hydrogen fuel. What caught my eye was the reference to E85 fuel. Besides buying a new carb, is there a way to get my carb to run on E85?
Quick Fuel’s basic E85 conversion kit for a typical 750 Holley carburetor is metering bloc
Jeff Smith: You have amazing timing, Tim. We just finished researching a carburetor for John McGann’s 454 dyno test, and while leafing through Quick Fuel Technology’s catalog, we found a complete E85 conversion kit—appropriately colored green (PN 34-106, $130.94, Summit Racing). The kit consists of a pair of metering blocks and gaskets. But this isn’t the entire story. To do this correctly, you really need to also convert the carburetor over to Quick Fuel’s alcohol-resistant nitrophyl primary and secondary floats (PN 16-9, $20.65 each, Summit Racing). Because E85 releases less energy than gasoline when burned, this requires between 20 and 30 percent more fuel, so you’re going to need much larger main jets. Quick fuel recommends increasing the initial jet size by 10 numbers as a good starting point. Because of the greater volume of fuel required, it would also be a good idea to convert to Quick Fuel’s larger 0.130-inch stainless steel needle and seats (PN 18-10, $14.10, Summit Racing). The metering blocks have also had their power valve channel restrictors increased in size to 0.076 inch, which will help flow more fuel under load. The idle feed restrictors in the metering blocks are also larger at 0.043 inch and use stainless steel idle mixture screws.
Holley-style carburetors use both idle and high speed air bleeds that are located on the top of each venturi. On Quick Fuel E85 carburetors, both the idle and high-speed air bleeds are changed to compensate for greater fuel volume. It’s likely that your carburetor does not have replaceable bleeds. The bleeds will normally be larger for gasoline than for E85, so you will have to reduce their size. This can be done by removing, drilling, and tapping the idle and high-speed bleed holes for 6-32 style screw-in bleeds. This isn’t difficult if you remove the choke housing, but might be a bit intimidating. It’s much easier to just convert to a carburetor main body with adjustable bleeds. Both Holley and Proform make just such an animal for both vacuum and mechanical secondary 750-cfm Holley carburetors. The mechanical secondary Proform version is PN 67100C ($125.95, Summit Racing) and comes with squirters, jets, and bleeds. But all of these will need to be changed because of the E85. To get started, increase the primary main jets to 82 and 92 jets in the secondary. The high-speed bleeds (the ones closest to the accelerator pump squirter on both primary and secondary venturis) will need to be changed to 0.028 inch, while the idle air bleeds would need to be sized to 0.065 inch. This combination is intended for a 750-cfm mechanical secondary carburetor and is only intended to be used as a starting point for further tuning. Combining all the above mods into your existing carb will cost roughly $330, which is half the price of a new Q-series 750 E85 carburetor from Quick Fuel (PN Q-750-E85, $661.50, Summit Racing). Based on this, it’s still a pretty good deal.
It’s also a good idea to install some kind of air/fuel (A/F) ratio meter. We like the Inno
Of course, this is just the beginning of the exercise. Bolting on the carburetor and running E85 fuel will require some tuning to get optimal results out of the conversion. The first step is to install some kind of wideband air/fuel ratio meter in the exhaust. This will give you instantaneous results for the changes you are bound to make to tune the engine properly. Because E85 has a much lower Btu output per pound of fuel compared to gasoline, the air/fuel ratios you will be dealing with will be different. The included chart shows typical gasoline air/fuel (A/F) ratios that should be familiar. The term stoichiometric refers to the chemically correct A/F for ideal combustion that minimizes emissions. Most air/fuel ratio meters like the Innovate Motorsports meter we often use will give A/F in either the typical ratios for any fuel, or it can output in what is called Lambda. While not familiar to most enthusiasts, Lambda is a convenient way to look at A/F ratios, especially when dealing with multiple fuels because the Lambda numbers typically stay the same or are very similar. Note that the stoichiometric number for both gasoline and E85 are represented by the simple number 1. Maximum power-rich A/F for gasoline is generally around 0.85, and that number can be used as a starting point for E85, although we found some information on the Internet included in our chart that indicate a much richer 0.71 number. An easy way to remember these numbers is if you divide 12.5 by 14.7—the result is 0.85—so that would be a good number to start tuning for E85 as well. Another important point is that ethanol is very much like methanol, the A/F can be substantially richer than 0.85 Lambda and power will not drop off like it does with gasoline. According to the math, the gasoline equivalent of 0.71 Lambda is 10.4:1, which is stupid rich, yet E85 will make nearly best power at this rich setting and not cause a problem. The point is that you can overshoot the A/F with E85 with a rich mixture and not lose power. This also helps to prevent an overly lean mixture that could cause engine damage.
We have probably gone deeper than you intended when you asked about a simple carburetor conversion, but this is the kind of information you will need to tune your engine properly to get the maximum power out of both your engine and the fuel you will be using. It’s also important to note that you will need to know what the actual percentage of ethanol used in E85 fuel. Elsewhere in this issue, we have run through the steps to do this testing.
Gasoline and E85 Air/Fuel Mixture Ratios
|Max Power Lean
|Max Power Rich
Innovate Motorsports; 714/372-5910; InnovateMotorsports.com
Proform; 800/521-1005; ProformParts.com
Quick Fuel Technology; 270/793-0900; QuickFuelTechnology.com
Jeff Smith: On June 13, 2012, a Los Angeles man was killed and three others injured when a nitrous bottle exploded in a shop near downtown Los Angeles. At the time our story is written, the Los Angeles Fire Department has not released its findings on the cause of the explosion. However, a likely explanation is a common yet extremely dangerous technique of using a torch to heat a nitrous bottle to expedite the nitrous transfer process. The correct way to fill nitrous bottles is to use a nitrous pump that will pump nitrous into a smaller bottle to the appropriate weight and pressure. The problem with this approach is that these pumps are expensive. Another way to accomplish nitrous transfer is to safely increase the temperature of the main bottle with heated water while freezing the smaller bottle to expedite the nitrous transfer. Unfortunately, some enthusiasts take a more reckless shortcut by using an oxyacetylene or portable propane torch to heat the main bottle.
The problem with this is not that the nitrous gas is flammable, because it is not. Nitrous is not a fuel—it is an oxidizer. The real danger is that any kind of open flame creating localized heat applied to a highly pressurized container can aggravate even a minor surface abnormality—that can cause the bottle to facture. When this happens, the 1,200-plus psi instantly escapes with deadly results. All nitrous bottles are equipped with a high-pressure blow-off valve that is supposed to open before an over-pressure occurs in the cylinder. But exposure to a localized external flame can easily cause the bottle to burst long before the over-presure valve will work. When the bottle explodes, if the resulting pressure wave doesn’t instantly kill you, the shrapnel from the burst bottle will finish the job. In the Los Angeles case, the news reported that that man died from traumatic amputation of both legs. We mention this not to be gruesome, but rather as a object lesson about working with any highly pressurized containers like welding, nitrous, and even CO2 bottles. A typical working pressure for nitrous at 85 degrees F is between 950 and 1,000 psi. Even a small 10-pound bottle treated badly can cause grievous damage. Nitrous oxide is a common and acceptable path to increased power and is perfectly safe when used properly. The point here is to never exceed the maximum rated pressure for a container and never apply an open flame of any kind to any pressurized container. As that Los Angeles man unfortunately proved, the risk to your life is just too great.
Dished or Chambered?
Adam Hubley; via CarCraft.com: Let me start, like everyone else, by saying you have a great magazine. I really enjoy how your magazine is geared more toward guys like me on a budget who want to do a lot of the work themselves. I read in the last issue you wanted more tech questions, so I got one for ya. I see some cylinder heads are available with different size combustion chambers, and I’m wondering what would be the better way to go: larger chambers with flat-top pistons or smaller chambers with reverse-dome pistons? Assume that all else is equal on a pump-gas Chevy small-block.
This is a late-model hemi combustion chamber. Notice both the twin spark plugs per chamber
Jeff Smith: I think it’s a function of the quality of readers this column enjoys that the questions are becoming increasingly challenging. When I first began working for Car Craft (yes, Jimmy Carter was president), I wrangled a yearlong series of afternoon sessions with Jim McFarland when he was Edelbrock’s vice president and director of engineering. I called them our Friday Afternoon Club. One of those highly condensed mechanical engineering sessions included a discussion that I thought at the time was a bit obscure. It had to do with surface-to-volume relationships of the combustion space in spark-ignition engines. As an example, we looked at Mazda’s rotary engine, which made a lot of horsepower (relative to its size) because of its extreme rpm potential. But Jim pointed out that it suffered from excessive hydrocarbon emissions because of its inherently large combustion surface area. While emissions and horsepower might seem like strange bedfellows, they both relate to combustion efficiency. What I learned was very simple: Reducing the volume of the combustion space also reduces the surface area, and that will help produce a more efficient combustion process. Within the wide range of domestic V8 engines, there are winners and underachievers when it comes to this relationship. A larger surface area allows heat to transfer into the water jackets or through the piston and be lost, as opposed to converting that heat to pressure and torque. An example of a big chamber would be something like a true hemispherical chamber. This design employs a large volume chamber and surface area. To compound the problem, a hemi generally requires a large domed piston (more area) to create decent compression. While hemi-headed engines clearly make power, they are not necessarily very efficient. While you didn’t specifically ask about efficiency, that is really the nature of your question. Let’s zip ahead from the second incarnation of the Hemi in 1964 (the first Chrysler hemi, dubbed the FirePower, debuted in 1951) to today’s version. Note that this latest Hemi improves upon the design slightly with twin quench areas on opposing sides of the chamber. While this improves mixture motion, it really doesn’t improve on the surface area question. Plus, because the valves are so large, this latest hemi requires two spark plugs per chamber. The spin is that the second plug reduces the hydrocarbon emissions, which is both true and understandable given the hemi’s large surface area and the fact that, because of the positioning of the valves, a single spark plug cannot be centrally located. The most recent incarnations of the Chrysler hemi combustion chamber merely pinch the sides in, creating flat, quench areas, which means they are hemispherical mainly with regard to the name.
To return to your specific question on a small-block Chevy, it hopefully is somewhat obvious by now that a smaller, more compact wedge-style chamber offers a smaller volume and therefore surface area. Older small-block Chevy heads had 64cc chambers, but they were virtually forced into this size because those early engines used very short strokes. A 3-inch stroke 302 with a 4.00-inch bore, a flat top piston, 0.040-thick gasket, zero deck, and a 64cc chamber will create a 9.55:1 compression. As strokes increase, the chamber size must become larger to maintain a similar compression ratio. Stretch that stroke to 3.75 inches and the compression increases to 11.7:1. This demands an increase in combustion chamber volume (either the chamber or a dished piston) of an additional 18cc’s to maintain the compression at 9.55:1. So this illustrates the relationship between displacement, surface area, and volume. Yet, with the latest LS3 engine (6.2L, 375ci) with a 4.060-inch bore and 3.62-inch stroke, the stock combustion chamber size is only 70 cc. Compare that to the older 350 small-block Chevy’s 76cc chamber throughout the ’70s, and there has been movement in the right direction. This smaller space creates a stock LS3 compression of 10.7:1, which also improves efficiency—as long as you can avoid detonation.
This brings us to the critical aspect of chamber shape. Compare those old ’60s and ’70s small-block bathtub chambers to current LS engine shapes, and there are significant clues to chamber development. If you study the photo of the LS7 combustion chamber, you can see how the intake valve port is designed to twist (or swirl, if you prefer) to push the inlet charge over toward the exhaust side of the chamber. The heart-shaped design is intended to improve combustion activity, which will improve the rate at which combustion occurs. Improving the rate of combustion means the engine requires less ignition lead, which also improves efficiency. If the chamber shape improves combustion speed, total ignition timing only needs to be about 28 degrees before top dead center (BTDC), compared to older Chevy engines, which required about 42 degrees. Less timing means the engine is doing less negative work, while more timing means that early combustion activity is occurring, increasing cylinder pressure before the piston arrives at TDC. A longer ignition lead demands engine power to overcome this rising cylinder pressure, which is lost power to the flywheel. With less timing required, that power can now be applied directly to the flywheel. It’s very close to free horsepower, so combustion chamber shape is now nearly as important as the compression ratio itself.
Compare this GM LS7 combustion chamber to a photo of an old-time small-block Chevy chamber
Next, we add quench into the equation. Earlier, we mentioned that the new hemi design included quench areas. This is the flat portion of the combustion space directly above the piston. In a well-designed engine, this will match up to a flat portion on the piston. By creating matching flat areas between the piston and the chamber, as the piston approaches TDC, this creates a very tight area (perhaps around 0.037-inch clearance piston-to-head with a steel connecting rod) that pushes or squishes the air/fuel mixture into the chamber, helping to more thoroughly mix the air and fuel. This helps to combust more of the total air and fuel in the chamber and create more power. This quench is part of the surface area, but the net effect is still very positive. This works so well that minimizing the piston-to-head distance with a wide quench area not only increases compression, but it will also reduce the engine’s sensitivity to detonation by more thoroughly mixing the air and fuel. This increased mixture motion and activity is essential to creating small fuel droplets. Small fuel droplets burn much more quickly than large ones, speeding up the combustion process. The result is usually a gain in terms of efficiency and power. Often, tightening the quench reduces the total ignition timing the engine requires to make peak power because of the improved mixture motion in the combustion chamber.
So for a small-block Chevy head with a 4.00-inch bore engine, you’re looking at using a small chamber size, between 60 and 70 cc. Using a 355ci engine, a 60cc chamber with a flat-top piston with 6cc’s worth of valve reliefs, a piston 0.005 inch below the deck, and a 0.041-inch-thick gasket would produce a 10.5:1 compression. This also creates a piston-to-head clearance of 0.046 inch. You could tighten this even more by pushing the piston perhaps 0.003 or 0.004 inch out of the deck to get it down to that 0.037-inch figure, but this would probably require a slight dish if we were to maintain that 10.5:1 compression ratio. If a dish is required, choose a piston that uses a D-shape dish where the shape matches roughly the shape of the chamber.
There are many pistons that use a large, open dish that eliminate the quench area. Avoid these pistons because they hurt combustion efficiency, reduce compression, and increase the surface-to-volume ratio. You might even look into coating the piston and chamber to improve efficiency by reducing the amount of heat lost through both areas. Chambers with that heart shape is an important criteria for selection. This also reinforces the concept that the chamber cannot be viewed by itself, but ultimately as part of the entire combustion space enclosed on the opposite side by the piston. A good job of matching the two components along with attention to the quench will deliver a crisp running engine that will run strong and efficient.
To finally answer your question, an ideal combustion space would be the tightest piston-to-head clearance you could safely generate with a small, efficient combustion chamber, and (depending upon compression requirements) a small D-shaped dish in the piston while still retaining the full quench portion of the piston. Minimizing the dish will reduce the surface area, but compression cannot be ignored. With a very good chamber, it’s possible to build an engine with 10.5:1 compression (or more) on pump gas, but that is also affected by cam timing. If I haven’t completely bored you to tears, this should at least point you in the proper direction.
Chevrolet Performance; 800/450-4150; chevroletperformance.com
The Continuing Saga of the BHJ 409 Torque Plates
This is what Mark Ascher’s BHJ 348/409 torque plate looks like.
Mark Ascher; Mondota Heights, MN: I was shocked to read in the latest issue of CC that Don Barrington owned the only BHJ torque plate in existence, and had commissioned its creation. I walked out to my shop, and pulled from a dusty shelf the other existing BHJ torque plate! I purchased this sometime in late ’70s or early ’80s. There was no apparent voodoo to making these, as it was listed in their catalog at the time. I can send you my address for my free T-shirt.
Jeff Smith: When I ran that quote from Don Barrington, I half expected somebody to produce another 409 torque plate. That’s actually good news for 409 fans. They don’t have to send their block all the way to California if they live in Michigan now. I sent Mark’s email to Don Barrington, and he said that when he requested his, BHJ had problems making it, and he was just going by what they told him. More than likely, there are many sets out there.
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