What makes a head "fastburn"?

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526FIREHAWK
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What makes a head "fastburn"?

Postby 526FIREHAWK » Fri Feb 17, 2006 2:53 pm

My question is What makes a head "fastburn"? is it the chamber design? can a head be modified to increse burn speed? and what are the advantages of faster burn speed?

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Postby beth » Fri Feb 17, 2006 6:10 pm

A fast burn chamber has most of it's volume close to the ignition source(s) and incorporates turbulence to spread the flame front within the chamber.

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Postby 526FIREHAWK » Fri Feb 17, 2006 7:16 pm

any real advantage to this besides less spark advance?

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Postby beth » Fri Feb 17, 2006 7:26 pm

less spark advance combined with a faster burn will result in less pressure working against the piston before TDC so there is more net power.

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Postby MadBill » Fri Feb 17, 2006 8:35 pm

Fsst burn also means less time for the end mixture to detonate. Such chambers usually will accept substantially higher CR, thus making more power.
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Postby SteveS » Fri Feb 17, 2006 8:51 pm

Depending upon the engine, I would think there would be an opportunity to increase the fraction of total combustion occurring ATDC ........thereby reducing pumping losses and increasing power.

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Postby MadBill » Fri Feb 17, 2006 9:12 pm

That comes with the territory when you fire the plug later. Since the peak pressure still needs to be at about the same angle ATDC for best power, there has to be more post-TDC burn.
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Postby 526FIREHAWK » Sat Feb 18, 2006 1:04 pm

thanks for the info.

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Re: What makes a head "fastburn"?

Postby SteveS » Sat Feb 18, 2006 4:47 pm

526FIREHAWK wrote:My question is What makes a head "fastburn"? is it the chamber design? can a head be modified to increse burn speed? and what are the advantages of faster burn speed?


In addition to the benefits that have already been identified, I would add reduction in heat losses.... to pistons, block and jackets.

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Postby bill jones » Sat Feb 18, 2006 5:23 pm

-I'm not an expert on fast burn heads but I experimented with a set of the early 1987 vortec heads that had the corkscrew intake ports.
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-Those heads had a LOT of swirl and the engine would not tolerate any more than about 28 total timing yet still wanted about 16 at idle (at 4500 elevation) when used on a 9.77:1 cr 400 sbc.
-When you would expect the engine to start to ping it didn't ping like normal----it hammered loud and hard.
-Then if you backed the timing down just one degree like from 29 down to 28 you could eliminate that hammering 100%.
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-The chamber didn't look to be significantly different than a 1000 other stock chevy heads---but the intake port was very obvious.

-Doing some simple wet flowtesting just using a siphon spraying solvent thru the port with the valve open at .400" lift:

A-a stock style intake port showed the most fluid and air coming around about 65% of the valve----with lots of slobber droplets near the shroud.

B-the vortec head spray pattern looked more like the wrinkle wall pattern on a drag tire except this pattern was virtually 100% of the valve circumference----showing a very obvious pattern difference between the conventional port.

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Postby Robert Kane » Sat Feb 18, 2006 5:32 pm

To determine a "fast burn" head, we must have a "slow burn" head to compare it with. In the case of a SBC, we have the "conventional" '461 "camel humps" and the Vortec head which is considered "fast burn"(1996 and later head). You will notice on the Vortec a heart shaped chamber with a much more centered spark plug location and a good sized quench pad which extends up to and slightly between the valve seat area. This promotes good mixture motion as the piston comes up it "squishes" the mixture out of the dead area and back toward the spark plug, causing a remixing as it moves. This motion helps homogenize the fuel and air mix, which allows the flame to travel much faster as it burns. Also keeping the mix near the plug helps to reduce "end gasses" which are the unburned fuel and air particles trapped near the edges of the bore and top ring. Since it all burns faster we need to reduce the timing advance so we don't start burning too soon, since the piston needs to see peak cylinder pressure between 7-15 degrees AFTER TDC in order to accelerate the piston down the bore faster. Also since most of the combustion pressure is finished pushing the piston down after only 1/3 of the stroke, even thought it is still burning well after that, a faster burn means we use more of the combustion pressure to move the piston, and less of the burn goes into just heating the piston and block. If you can imagine this all happening in a low rpm street engine, think how it benifits a high rpm racing engine where the burn time is super critical. we can increase burn time with piston domes, or dishes to raise compression to the verge of detonation, but the burn time needed to push the piston goes up dramatically as engine speed goes up, so we need these benifits to keep combustion up with piston speed. A lot of other factors then start to come into play, rod ratios, and lobe seperation angles, and so on. Take every opportunity to look at the progressions of chamber designs made by the factories and note the changes they made all for the sake of combustion efficiency (burn speed/ mix motion), you will learn boatloads.
There are some really smart guys who post on here who could fry your brain with their vast knowledge on these things, but I hope I have been able to shed some light on this, along with the others who posted.
This is a good question.
Robert

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Postby doctorpipe » Sun Feb 19, 2006 12:32 am

Just to add to the infomation. I talked to a GM engineer a few years ago about those 1987 vortec heads. He said they spent a few million dollars to find out it had too much swirl. It spun the fuel to the cylinder walls and there wasn't much left at the spark plug. He said they killed a lot of the swirl, picked up 20 hp. and a few mpg's. I have modified the pockets and have found similar results. Look at the Jaguar v-12 heads. early heads used the piston for the chamber, and got horrible fuel mileage. The HE head is a unique chamber (Porsche engineer design) that got better mpg and made more power. It only has around 28 degrees total ignition timing, and has 11.5 to 1 comp. ratio. Mercedes and some Porsches used that chamber desing for awhile. It can't flow air the best, but boy is it small.

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Postby Ape » Sun Feb 19, 2006 10:27 am

doctorpipe wrote: Look at the Jaguar v-12 heads. early heads used the piston for the chamber, and got horrible fuel mileage. The HE head is a unique chamber (Porsche engineer design)
John Aller
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Hi john,

if it´s the HE combustion chamber i have in mind it was a Volkswagen (not porsche engineer) development by a swiss engineer named michael may that also larry widmer quotes in his articles.

Here is an article on the subject.

http://www.jagweb.com/aj6eng/v12_performance.html

John do you have by chance any further papers on the subject, as im trying for about two years to get any further details especially on the volkswagen research as they supposedly ran the engine on 14:1 with outstanding HP-figures.
There is always advancement to be made.

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Postby 526FIREHAWK » Sun Feb 19, 2006 1:09 pm

What are some other heads besides the vortech heads that are faster burn? I was reading about the ProFiler 12* and some of the information I found reffered to them as fastburn. also In E/A pro If you build an engine and specify standaed burn run the simulation and then only change the burn speed to "somewhat faster" you pick up 60 or so HP. I know ther are alot more variables to whats going on, just trying to get a grasp of what it is. Thanks very much.


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