I am sure I'll get flamed on and pummeled with laughter but I don't know what the answer is, but WANT TOO!!! I also realize that this is a Jet boat (oh man I love 'em!!) but this was the "best quality sound" I could find of what I am trying to describe. ----The SURGING of this motor.--- At :45 seconds in, you can see it surge from about 700 RPM to 2700 RPM instantly and continually. What is causing it to run that way?? I ABSOLUTELY FREAKIN' LOVE IT!!! It actually excites me when I hear it!!! I have built some nice running relatively fast engines in my time, BUT, I have not actually been up close and personal to a Blown, Alchohol, or Nitro motor. I am sure people around my area have them, I just haven't been around it. I have been to several "cackle fests" and experienced the sight, sound, and smell of these motors, but don't really understand the "phenomenom" of what is happening inside it to make that "beautiful exhaust note". I know that the alchohol/nitro mixture has a lot to do with the sound, the blower rotors, and also gear drives have an effect on the "sound" also and I understand that blown motors usually run lower compression ratios, but the surging sound is what I like and want to know about. Is this a "very rookie" question, or do other people wonder too but haven't asked?? I searched for a LONG time BEFORE I posted this!!! Another motor that sounds extremely close to this (at least to me) is Marky's Purple People Eater. Thanks for any info and "schooling" for me. Ken http://www.youtube.com/watch?v=_E73yfmqaQk http://www.youtube.com/watch?v=hxNc6U2f4lU
They tell you right there in the video. It has a cam with 4 lobes swapped, changing the firing order. GM used to have an odd fire V6 that behaved that way when you really worked it over.
I saw that and then it also talks about a barrel valve "rich-then-lean" mixture and didn't understand??? Then talks about the cam overlap. How is that 4 lobe swap ?? Is it specifically for high horse power motors? I have never heard about it before.Is it Common or not??? Also talks about the blower spinning faster than the motor, does that mean "under drive" or "overdrive"?? I realize the owner/builder of the motor would have the most info for me so I sent a message to the poster of that video but never got a reply so figured The HAMB would be the best place to ask.
The sound these engines make is related to the volume of fuel used, and thus the duration of the active combustion process. A nitro motors sound comes from two sources. First, the exhaust valve is opening in the middle of the combustion cycle (As opposed to a gasoline engine, whose charge is spent by the time the valve opens) and then, when the piston hits BDC, the still very active pressure wave bounces off the top of the piston and then up and out the pipe. You're actually hearing two separate pulses each time a nitro engines cylinder fires. Rod ratio and thus piston speed also have an effect on the sound of a fuel engine. Long rod ratio engines have a slow piston motion at the bottom of the stroke, and so they tend to sound deeper with not much snap. Short rod engines will sound very sharp and crisp...one of the big reasons Chevy engines sound so good on pop.
Idle check valve poppet bounce may be a contributing factor. A buddy of ours liked this sound so much he actually rigged an air conditioner idle solenoid on his street car to mimick this exact sound - worked really great, but was all fantasy.
You need to spend more time at your local Dragstrip and hear it in person. Alky is great but the sound of Nitro really gets me going.
The barrel valve is in the injector hat (the scoop on top of the blower). It is part of the fuel delivery system and functions sort of like a spigot in plumbing. First it's partway closed, then it comes open pretty fast dumping lots of fuel into the engine. It's adjustable, too - and is responsible for the quick and and down throttle action.
Oh I know that NITRO sounds INTOXICATING!!!! This video was just the best I could find with the quality of sound it had. Most videos with Nitro is so loud it actually sounds like crap when someone records it with a cam corder, or cell phone recording!!! Our "dragstrip"??.... HA HA . Just recently got back up and going, after quite a huge "he says, she says, taxes not paid, and it is now OWNED BY THE CITY OF SPOKANE, WASH. I have gone out there a few times but it is "mostly catered to" the younger euro/ricer racers. Not my thing but I don't put 'em down for doing what THEY like!!! We all started somewhere right? My sone has one of those "fast" tuner cars, he just has no interest in all the "cool traditional" <---read os OLD to him and his friends, stuff I have!!
Once you relocate to So Cal check out the CHRR event (Californis Hot Rod Reunion). You'll be deaf for two days when you leave but you'll have a whole new respect for the pioneers of drag racing. And who you calling Old!
The blown/injected gas burning engines have the same rolling idle as the boat. I think it's got something to do with running them rich at idle. There was an Anglia at The Gasser Gathering in St. Louis that comes to mind. I've also heard that George Boltoffs AA/GD would stand your hair on end at idle. Larry T
Those cams with the "swapped" lobes swap the 4 and 7. I think they are generally used in circle tracks if I ain't mistaken. I have heard cars on cruise night with that "exaggerated" lope. Come to find out a lot of guys where setting the timing for the sound. To me I would think it would break a cam. Those blown Alky and Nitro engines are intoxicating!
We used to run a blown alcohol dragster. That lope, or surge you hear can be tuned in or out of the idle with the barrel valve and idle speed adjustment. We had a standard cam, no lobe swap. Had to be careful backing up after the burnout as the car would accelerate, and then almost quit. Hard to manage with 30 + degrees of caster.
Exactly! When the barrel valve is set very rich at a slower idle, the loping sound happens. We used to run a Top Alcohol car too.
You can probably fix that idle problem by converting to EFI! The cam lobe swap is done to change the firing order, the logic being explained by exhaust theory, the pulses complementing each other better. Very small gain, you'd have to be playing at the top of the field and looking for a tenth to justify it.
At the strip that sound shows a good tuner from a bad one. That ones not tuned well. Think about how hard that would be to stage. When staging the car you need to be able to move the car foward an inch at a time. Each one of those surges would cause the car to leap foward FEET at a time. Hard to get it stopped in a 6" box. On a street car sitting at a stoplight in traffic would be pretty scary! Partially caused by barrel valve and part fuel pressure. Nothing to do with cam. This is what it should sound like,,, in fact this is music to my ears.
We loved that sound too and set up 'Psycho' to rump like that and couldn't drive the goddamn thing thru water to stage or burnout, the driver kept banging his head off the rollcage everytime it rump'd. Like the man said, sounds neat but... Just make it rich at idle and close down the butterflies. The worrysome part i heard on the video is the belt sounded tight to me and he'll have a harmonic balancer laying on the bottom of the boat before long. The GM 4/7 swap is very common, even the crate 502/502 motor has it, one of the things is to get the 5/7 exhaust pulse broken up and it tended to gather heat in that corner. You can hear it, the chevies have a double blip exhaust sound on the drivers' side and with the 4/7 swap it is broken up and sounds more like a ford.
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