whamo

Sorry Arne, I remember you and I talking about the exhaust system diameter affecting torque output. I guess it is more accurate to say that exhaust velocity will affect power output. The summary of the article is that backpressure robs horsepower. It doesnt follow that a larger exhaust system automatically produced more horspower, because exhaust velocity may suffer. In summary, a well designed exhaust system that reduces backpressure while keeping exhaust velocity high will increase horsepower and torque output.

Excuse me if your not a motorhead, but I thought those of you who are might be intereseted in the following. Its often been repeated that if you dont have enough backpressure the torque output of your engine will suffer. I have even repeated this statement. However I have never heard any proof of this "fact". I read an exhaust system comparison in one of the automotive magazines where they dyno tested on a V8 engine, back to back 2.5,3.0 and 3.5" exhaust systems. Torque and horsepower increased with each increase in exhaust system diameter. A quick google search revealed this article. Which I thought was quite revealing, and dispels some commonly held myths about exhaust system backpressure. "xhaust backpressure the myth -------------------------------------------------------------------------------- Backpressure: The myth and why it's wrong. I. Introduction One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion. II. Some basic exhaust theory Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream. III. Backpressure and velocity Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate. The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM. Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM. IV. So how did this myth come to be? I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity. V. So why is exhaust velocity so important? The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s)." __________________ Steve Frisby 1971 Datsun 240z-under restoration(wifes car) 2004 Nissan 350z (wife thinks its her car) 1971 455 Stage 2 skylark custom

The floats actuate the needle and seats, also called fuel inlet valves. They can easily become stuck open by debris in the fuel.

One of your fuel inlet valves are stuck open. You should remove the air cleaner to determine which carb the fuel is coming from. Its unlikely both are stuck. It shouldnt require replacement of the valve, just cleaning debris from it. However it may be worn beyond the point of cleaning, in which case replacement will be nessesary.

3-4k in parts should make a fair start, f you already have the engine. Add in machine shop costs $1200-1500, a professional to port the head $400-$600 and you assemble it yourself. 6 grand plus your own labor, that stock 6 begins to look pretty nice, or a V8.

If you plan on having the head ported you should seek the advice of someone who has experience porting L6 nissan heads, or at least someone who has experience porting performance cylinder heads in general that has a flow bench. This is not something that the average machine shop does, or at least does well. A poor job of porting a head can decrease performance, bigger isnt always better. Cutting the valve seats for larger valves is something a competent machine shop can do, but to take advantage of the larger valves a blend of the bowl area behind the valve would be adviseable. An experienced head porter should be sought out for this task.

Typically an aftermarket harmonic balance is added for safety, not for performance. They are required by some racing organizations at higher levels of performance. The electric waterpump is typically used on drag racing cars that are not driven on the street, they are not relaible enough for day to day use.

Are you having the head ported or just machined to accept the larger valves? Or both?

I am no expert on turbocharging but I am becoming a huge fan of the technology. You can safely turbocharge your engine. You may not be able to run a lot of boost, but you still should be able to increase your power considerably, even with low boost levels.

I'm putting my wifes 240z together. I am a musclecar guy, but plan to build a hopped up Z car for myself someday. We are running a stock 240 engine with a 5speed zx transmission. The car was pretty darn quick before and it had an automatic. My car is a 1971 buick ( ) with a 600hp 455( ) Its mean and nasty.

Oh, yeah.... and its cheap too. http://www.autopartswarehouse.com/details/QQNissanQQ280ZQQNOSQQNitrous_SystemQQ19751978QQN2305029.html

I have your solution.... its called nitrous oxide. In the correct dosage its perfectly safe, is easy to conceal, and will not affect your normal drivability, and can beinstalled in a weekend.

In your initial post you indicated that you were interested in turbocharging your car. If you want to turbocharge your car (a great idea if you want to build serious power and retain driveability), it would be far cheaper if you used the 280zx engine, fuel injection and turbo from a donor car. It still will not be inexpensive, no matter how you slice it. If you turbo charge your engine or swap a ZX turbo motor in, the car will still look the same. Im not sure what you mean by a "mod car" If you try to build serious power without a turbo, the drivablity of the car will suffer. Performance is a trade off. (performance-driveablity-inexpensive) you can have 2 of the 3..... its rare to have all 3. My plan is to build a 280zx engine, used aftermarket forged pistons, a BIG aftermarket turbo, intercooler,fuel injection system and really sticky tires. I should have the money saved up in about 10 years......

The real question is what do you want out of your car? That question can only be answered by you. When you decide what you want from the car, then you can start to plan a buildup that will achieve your goals. Just putting an engine that produces max horspower in the car will not get you there. Plus, I can assure you that an engine that produces maximum horsepower wouldnt be much fun for long. Big numbers arent really good for much other than bragging rights.

Thanks for the link.... a quick google search found this. http://www.triumphexperience.com/article/how-to-blow-through-an-su.html

Has anyone ever heard if it is possible to use a blow through system on an SU carb? It doesnt look like it would be difficult to install a centrifigal blower on an l6 engine, but are the carbs "blow through" friendly? I know you can blow through a carter or holley 4bbl carb. If possible to install a blower without going fuel injected, Id like to try it.

Does anyone have a factory service manual? That would/should put this issue to rest.