BBC output overview - chapter 2

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David Vizard
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BBC output overview - chapter 2

Post by David Vizard » Wed Oct 17, 2018 4:26 pm

Don,
Thanks for the go-ahead on a continuation thread on this subject.

To pick up where I left off here is the last graph I posted on the subject before the thread got locked.

It shows how the flow goes up above the 0.25 D inflection point when bias is applied as previously shown.

so are there any downsides to this?

Answer - Yes - if the valve lift is too little to access that flow. Why? Because the increase in port volume to get the bias has gone up from 122 cc to 130. As a result the port velocity has drop and does not reach par until about 0.600 lift.

Just so you know I am edging toward the subject of port velocity and it's effect on the power curve.
Got to put a hold on this today as there is work I should have got to earlier.

DV
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Re: BBC output overview - chapter 2

Post by GARY C » Wed Oct 17, 2018 5:12 pm

Just thought I would put a link to Part 1 for future reference so new people can catch up from the beginning.
http://speedtalk.com/forum/viewtopic.php?f=1&t=54797
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Re: BBC output overview - chapter 2

Post by Carnut1 » Thu Oct 18, 2018 9:25 am

On my latest exhaust effort I spent some time getting the airspeed at the flange as even as possible to use as much of the port csa as possible. The port had a decent amount of port bias stock and needed as much expansion on the center of cylinder side and some ssr bias to really work. Thanks Charlie
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Re: BBC output overview - chapter 2

Post by David Vizard » Thu Oct 18, 2018 9:37 am

Carnut1 wrote:
Thu Oct 18, 2018 9:25 am
On my latest exhaust effort I spent some time getting the airspeed at the flange as even as possible to use as much of the port csa as possible. The port had a decent amount of port bias stock and needed as much expansion on the center of cylinder side and some ssr bias to really work. Thanks Charlie
Charlie, got any photo's of that???
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Re: BBC output overview - chapter 2

Post by Carnut1 » Thu Oct 18, 2018 11:09 am

I can post some later after work. Thanks
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Re: BBC output overview - chapter 2

Post by Carnut1 » Thu Oct 18, 2018 8:15 pm

20181018_200332.jpg
20181018_200600.jpg
A almost finished development exhaust. Airspeed is at flange Pitot @.7" lift coc stands for center of cylinder. You can see this example ( ls3) needed expansion on coc side to use the stock exit size more efficiently. The ssr needed bias and reshaping. Thanks, Charlie
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user-17438

Re: BBC output overview - chapter 2

Post by user-17438 » Thu Oct 18, 2018 8:52 pm

Carnut1 wrote:
Thu Oct 18, 2018 8:15 pm
20181018_200332.jpg20181018_200600.jpgA almost finished development exhaust. Airspeed is at flange Pitot @.7" lift coc stands for center of cylinder. You can see this example ( ls3) needed expansion on coc side to use the stock exit size more efficiently. The ssr needed bias and reshaping. Thanks, Charlie
That doesn't look like a BBC head. What is that?

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Re: BBC output overview - chapter 2

Post by superpursuit » Thu Oct 18, 2018 9:08 pm

MTENGINES wrote:
Thu Oct 18, 2018 8:52 pm
Carnut1 wrote:
Thu Oct 18, 2018 8:15 pm
20181018_200332.jpg20181018_200600.jpgA almost finished development exhaust. Airspeed is at flange Pitot @.7" lift coc stands for center of cylinder. You can see this example ( ls3) needed expansion on coc side to use the stock exit size more efficiently. The ssr needed bias and reshaping. Thanks, Charlie
That doesn't look like a BBC head. What is that?

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Re: BBC output overview - chapter 2

Post by Carnut1 » Thu Oct 18, 2018 9:12 pm

MTENGINES wrote:
Thu Oct 18, 2018 8:52 pm
Carnut1 wrote:
Thu Oct 18, 2018 8:15 pm
20181018_200332.jpg20181018_200600.jpgA almost finished development exhaust. Airspeed is at flange Pitot @.7" lift coc stands for center of cylinder. You can see this example ( ls3) needed expansion on coc side to use the stock exit size more efficiently. The ssr needed bias and reshaping. Thanks, Charlie
That doesn't look like a BBC head. What is that?
It is an ls3 head as stated.
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Re: BBC output overview - chapter 2

Post by user-17438 » Thu Oct 18, 2018 9:13 pm

MTENGINES wrote:
Thu Oct 18, 2018 8:52 pm
Carnut1 wrote:
Thu Oct 18, 2018 8:15 pm
20181018_200332.jpg20181018_200600.jpgA almost finished development exhaust. Airspeed is at flange Pitot @.7" lift coc stands for center of cylinder. You can see this example ( ls3) needed expansion on coc side to use the stock exit size more efficiently. The ssr needed bias and reshaping. Thanks, Charlie
That doesn't look like a BBC head. What is that?
Sorry I didn't see it was an LS.. It looked like an LS. That's why I posted

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Re: BBC output overview - chapter 2

Post by Carnut1 » Thu Oct 18, 2018 9:15 pm

An Example.
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Re: BBC output overview - chapter 2

Post by David Vizard » Fri Oct 19, 2018 12:31 pm

Carnut1 wrote:
Thu Oct 18, 2018 9:15 pm
An Example.
Charlie,

Thanks for doing as I asked here with a photo.
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Re: BBC output overview - chapter 2

Post by David Vizard » Fri Oct 19, 2018 1:28 pm

So we can see that bias is a useful tool. This applies not only to the exhaust but also to the intake.

In the graph below we have the 'weird port' (just as a reminder photo #1 is what I am refering too) head I recently did vs the big port Edelbrock head.

As can be seen from the graph the two heads have about the same area under the curve for a lift of 1.200 inches (we are actually shooting for 1.150.)

The thick line is the 'weird port' head and the thin the big port Eddie head. Ignore the Eddie exhaust as, at the time, it was not done. The port shown by the thick line can go into the Edelbrock head.

If we were just considering flow to access the power potential then we might reasonable conclude that there probably won't be that much difference.

But here's the kicker - the big port Edelbrock head that produced this flow curve had a port volume of 382 cc and a mean c/section of 3.99 Sq. ins and a C/section at the manifold face of a whopping 4.35 Sq.ins. The latter being bigger than the valve area - which is not good!

By contrast the 'weird port' head is 322 cc's for a mean c/section of 3.48 Sq. ins. and a c/section at the manifold face of 3.9. That's a whole lot smaller than the conventional Edelbrock port.

Now I don't what it to be concluded that this is the best Edelbrock port that can be achieved. I am pretty sure that MT Engines has better ports on the menu. Hopefully we will see some flow figs for these down the road.

In my next post we will look at how this impacts the port velocity.
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Re: BBC output overview - chapter 2

Post by JoePorting » Fri Oct 19, 2018 5:27 pm

If you carry that roof offset to the bowl, you might find some more CFM.
Joe Facciano

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Re: BBC output overview - chapter 2

Post by David Vizard » Fri Oct 19, 2018 10:05 pm

JoePorting wrote:
Fri Oct 19, 2018 5:27 pm
If you carry that roof offset to the bowl, you might find some more CFM.
Joe,

Thanks for the input. Will try that on a scrap casting I have.
DV
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