Technical Information

                                                                                             

Thanks for dropping by the tech support area. This support area is here to share all of the good and bad experiences that I have had with my blow-thru system. I am just like you guys, I had to learn from others and still am.

I would like to personally thank Bryan Sharer and Chris Waddell for their shared and helpful knowledge. These guys are first class and are great to talk with.

Blow-thru Engine Guidelines

The following specifications are general guidelines offered to AIDE in
the construction of a carbureted blow-thru engine for street / strip use. These are only general guidelines. For more detailed specifications regarding your specific application or for "strip only"  consult a professional engine builder.  

I will assume absolutely no responsibility for any failures or damages of any kind resulting from the information contained on this web site.

Pistons:
Forged pistons recommended for all applications. Cast and hypereutectic pistons can be used but should be limited to lower horsepower (approx 375-450hp) applications.

Compression ratio:
For pump gas (91-93 octane) applications, a compression ratio of 8.5:1 to 9:1 is recommended for boost levels of 8-10 psi.  Higher octane fuel will allow you to run higher boost levels, approximately 1 psi for every 2 points of octane. 

Heads:
The same rules for normally aspirated engines apply to turbocharged or supercharged motors.  Higher flowing heads will help generate more horsepower than stock heads.  Forced induction produces a percentage gain in horsepower; by starting with more base horsepower, a modified motor will receive a larger total hp gain (from the same percentage gain).Porting, especially on the exhaust side is recommended. Aluminum heads will allow you to run approximately 1 psi more boost than cast iron heads due to their ability to dissipate heat.

Cam:
Stock cam, E-303,F-303 and Crane 2031 cams are great off-the-shelf cams. These cams are known to generate 600+ RWHP with good heads.

Custom Camshafts: Lobe separation  112 to 116 degrees. Although 110 seems to be o.k.
Split pattern:  Exhaust duration and lift approximately 10 degrees and .010, respectively, greater than intake. Install cam straight up or degree it. Contact a cam manufacturer for lifts and durations that best suit your application.

Crankshaft and rods:
Cast up to 450-500 horsepower, forged steel for higher horsepower or for rpm levels above 6,000.

Exhaust:
Headers are recommended.  The size of headers are dependent on whether you are wanting to create more low end torque or high rpm horsepower. 1 5/8 seem to work best for all around performance, although 1 ½  and 1 ¾  are available sizes too. 

Intake manifold:
Single planes are HIGHLY recommended for better fuel distribution. Dual planes are also used in some applications to improve low end torque, however may require staggered jetting for good fuel distribution.

Carburetor Bonnet:

Get the biggest bonnet that you can. A bonnet with at least 2 ½ inches of height is recommended. The taller and wider the bonnet, the better the signal is that goes across the boosters. Smaller and shorter bonnets limit boost, causes bucking and a lot more of unusual characteristics that are not normal. ATI, West Coast Fabrications and gofastparts.com sell quality bonnets that work. Or you can build your own.

Carburetor:
Holley double pumper w/ mechanical secondaries.
600 cfm (#4776) for up to 600 fwhp
650 cfm (#4777) for up to 750 fwhp
700 cfm (#4778) for up to 850 fwhp
750 cfm (#4779) for up to 900 fwhp

All carburetor’s will require removal of the choke assembly and choke horn unless it is an HP series. Block off the choke arm lever hole. Replace the brass floats with the solid nitrophyl floats. Jet the carburetor up a few sizes to accommodate your motor. (NOTE): Use jet extensions on the secondary jets.

Some owners use a 50cc secondary pump kit. It doesn’t hurt, but it is not necessary that you buy one either. The standard 30cc pumps work just fine.

Fuel pump:
Your fuel pump must be capable of supplying the proper amount of fuel flow at the maximum operating pressure. To determine maximum operating fuel pressure requirement, add your maximum boost pressure to your initial idle fuel pressure.

Example:
(8 psi idle fuel press.) + (10 psi boost press.) = 18 psi max. fuel press.

Multiplying this figure by 1.2 (20% safety factor)  gives us 21.6 psi

To determine required fuel flow, multiply your total expected horsepower by a bsfc (brake specific fuel consumption) of .65.  This will give your fuel flow requirement in 1bs/hr.  To convert to gallons per hour, divide this figure by 5.87.

Example:
(500 boosted hp) x (.65 lbs/hr/hp bsfc) = 325 lbs/hr fuel flow

325 lbs/hr / 5.87 lbs/gal = 55.4 gals/hr fuel flow

Therefore, a fuel pump capable of providing a minimum fuel flow of 55.4 gals/hr at a pressure of 21.6 psi is required.

This can be accomplished by an all electric high pressure-high flow fuel pump (i.e. Magnaflow 300GPH, Mallory 110 or 250, Aeromotive 111-01 or similar type fuel pump) and a return style, boost sensitive fuel pressure regulator (Aeromotive 132-02, Mallory #4309 or similar). Or by using a boost referenced mechanical fuel pump only (up to 500+ hp) or a boost referenced mechanical fuel pump in conjunction with a low pressure electric pump (i.e. Holley blue, Comp 140 or similar).

FAQ

Over the past couple of years I have been asked many questions concerning my project, I have added the more common questions and answers below.

  1. What has to be done to the carburetor for a blow-thru set-up?
    1. Mill the choke horn or cut it off. (unless you buy a Holley HP series carb), they do not have choke horns or a hole to plug up.
    2. Plug the hole where the choke lever arm used to be.
    3. Add Nitrophyl solid floats.
    4. Add jetting of about 85/95. This will most likely be way too fat, but it is safe. Take jetting down to get the motor to start running crispier and smoother. The best way to get the best results for your engine is get on a dyno with a wideband for an A/F ratio reading.
    5. Add secondary jet extensions.
    6. If you already know what your idle vaccum is, then add a power valve that is half of you engine vaccum plus a half. Say your motor has 14 inches of vaccum at idle, then you would need a 7.5 power valve.
    7. Some use the secondary 50cc accelerator pump kit. Although it is not needed, the stock 30cc units work just fine.
    8. Although this modification is not a standard, has to be done issue, the Power Valve Restrictor Channels(PVRC’s) can be drilled for more fuel as well. The idea behind this is that you can run a smaller primary jet, which will help to run cleaner and save gas mileage but give you the needed fuel under boost as if you had the stock size PVRC holes and a bigger jet. The PVRC’s can be seen right after you take the power valve out. They are the two little holes. I take a paper clip and stick down in the holes and measure how deep it is and then mark a 3/32” drill bit the same depth and then drill the two holes to that size. DO NOT  try any bigger than a 3/32” bit. Also do not feel intimidated to do this mod. It is as easy as I have explained it.
  1. What type of bonnet to use?
    1. I made my own bonnet because there were no available bonnets at the time that fit my specifications.
    2. Gofastparts.com has a very good bonnet that works great for $159. It has lots of volume area and has been known to support 1000 HP.
    3. If you decide to make your own, make sure you have at least 3 inches of area above the carburetor. Anything less may cause severe turbulence and cause your motor to stumble and act  all kinds of weird ways.
    4. Enclosures that Vortech/Paxton sells seems to work well too, but I have no experience with one of those at all.
  1. What type of fuel system do I use?
    1. The fuel system is just as important for a blow-thru carburetor as it is important for a EFI forced induction system.
    2. Some people have been known to get away with a EFI fuel pump and regulate it down to 6-7 psi of fuel pressure at idle. I personally do not recommend this due to the fact that a EFI pump was designed for 38 psi at idle, therefore regulating it to 6-7 psi would seem to do harm to the pump and may hurt your motor in the long run.
    3. Some have also used mechanical fuel pumps but you would have to boost reference the fuel pump to raise fuel pressure under boost.
    4. My recommendation is to get an electric fuel pump like an Aeromotive, Magnaflow, Mallory or an equivalent pump with the Aeromotive 132-02 boost referenced regulator. The Holley blue can work, you just have to add the higher pressure spring to it to make more fuel pressure under boost but the Holley blue to me is only recommended for up to 400 HP in a blow-thru set-up. The other fuel pumps can go in the 800-900 HP range without incident.
    5. Fuel lines should be at least –6 or –8 feeding to the regulator and –6 from the regulator to the carb bowls. The return should always be the next size bigger than your feed. The return system for a carburetor is extremely sensitive to fuel back-up much unlike a EFI system.
  1. What type of intake will work best?
    1. A single plane intake is always recommended because the fuel can be better evenly distributed. The intake of choice that will be trouble free as far as blowing head gaskets is the Weiand X-celerator(7515). This intake can produce up to 800 RWHP in a blow-thru system. It has much, much straighter corner runners and can feed the corner runners a lot better than any of the other aftermarket intakes offered. It’s the best to use for a 302.
    2. The Victor Jr.(302) intake has been known and proven to lean out the corner cylinders because the runners have a 90 degree turn and are long. Fuel always gets to the middle runners first and can’t get to the corner cylinders fast enough therefore you go lean and/or blow head gaskets. Another single plane intake such as the Edelbrock Torquer II would not be recommended at all either. It too has sharp bent corner runners that are a potential problem.
    3. I would not recommend a dual plane intake at all because again of fuel distribution problems. Some dual plane systems have been known to work, but none of these systems were making over 350 RWHP either with about 5 psi of boost.
    4. The Victor Jr. Intake for a 351 may be fine. The corner runners are not as sharp of a bend for fuel to travel. I do not have any experience with a 351 motor, so you will have to R&D on your own. Weiand does not make a X-celerator intake for a 351.
  1. What type of fuel mileage do you get?
    1. That all depends on your HP goal because of the jetting that you use. I did drive with 85/95 jetting 280 miles one way on a tank of gas and had 1/8 tank left before filling back up.
    2. On average around town with 85/95 jetting, I get usually about 15-17 MPG while not getting on it at all.
    3. I would get lots better gas mileage if I put in 60/60’s and just plain cruised, but who wants to keep changing jets just to cruise and then to race.
  1. What type of ignition do you use and what about a boost retard?
    1. I use a MSD 6AL ignition box without any types of boost retard. I have a MSD8582 distributor that I locked out at 20 degrees and that’s it.
    2. Boost retards are great for when you want that feeling of being safe on the street and you have the retard on 2-3 degrees, but I didn’t buy one at the time.
    3. Holley makes the Pro strip annihilator that has total individual cylinder timing control which is very neat, but costly. This ignition system will help enable you to use the Victor Jr. intake because you can take more timing out on the corner runners, but then not make as much power as if you were able to leave all that timing in there as well.