Exhaust port flow

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moparian
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Exhaust port flow

Post by moparian »

580 BB 14.1 comp methanol drag engine deal,I sent the heads to a shop to give the heads some freshen up and a bit of port work they opened up the exhaust entrance to much and killed a lot of flow.how much will this hurt power ?
intake/exhaust 200 lift 167/105 300 258/141 400 323/174 500 372/194 600 424/206 700 446/213 800 455/220 exhaust valve is 1.70 and no flow pipe on it
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Re: Exhaust port flow

Post by WeingartnerRacing »

Exhaust flow numbers don’t mean a whole lot. If you shrink the the throat they usually flow better down low but make less power. This is hard to describe but on the flowbench the sound tells you really how good it is. It should quite down as valve opens. If it pops and bangs or is loud the port needs work.
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Re: Exhaust port flow

Post by Abbottracingheads »

Are you sure about the exhaust valve size? Haven't seen anybody put a 1.7 in a 580 bbc.
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Re: Exhaust port flow

Post by moparian »

Abbottracingheads wrote: Tue Aug 13, 2019 10:26 am Are you sure about the exhaust valve size? Haven't seen anybody put a 1.7 in a 580 bbc.
Yes it was a 1.760 but the guy said it was to big and 1.70 would work better ,seat are 45 s and throat is 91%
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Re: Exhaust port flow

Post by moparian »

WeingartnerRacing wrote: Tue Aug 13, 2019 10:10 am Exhaust flow numbers don’t mean a whole lot. If you shrink the the throat they usually flow better down low but make less power. This is hard to describe but on the flowbench the sound tells you really how good it is. It should quite down as valve opens. If it pops and bangs or is loud the port needs work.
Thank's i will check that out, cam is 285/300 on 115 l/c
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Re: Exhaust port flow

Post by jarmoyp »

Mopar bb have a smaller exhaust. Maybe it flows better now with a pipe.
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Re: Exhaust port flow

Post by moparian »

jarmoyp wrote: Tue Aug 13, 2019 10:40 am Mopar bb have a smaller exhaust. Maybe it flows better now with a pipe.
I have not flowed it with pipe it will need 2 1/4 pipes just to cover the hole then maybe stepped up from there
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Re: Exhaust port flow

Post by PRH »

B1 heads?

I haven’t had too many of those through the shop.

The ex port exit at the flange is quite large, but the bowl area of those heads are pretty small.

I’ve tested a few ported by other shops that had pretty nice ex flow....... others not so much.

I realize the ex flow numbers don’t seem to count for much, but I can get 280 out of an Indy 440-1 ex port(no tube) way easier than a B1.
Somewhat handy with a die grinder.
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Re: Exhaust port flow

Post by moparian »

PRH wrote: Tue Aug 13, 2019 12:38 pm B1 heads?

I haven’t had too many of those through the shop.

The ex port exit at the flange is quite large, but the bowl area of those heads are pretty small.

I’ve tested a few ported by other shops that had pretty nice ex flow....... others not so much.

I realize the ex flow numbers don’t seem to count for much, but I can get 280 out of an Indy 440-1 ex port(no tube) way easier than a B1.
Yes B1 s they were 270 cfm before they cut the valve by 060 and opened it up,they got new inserts too and throat is 91%
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Re: Exhaust port flow

Post by Walter R. Malik »

WeingartnerRacing wrote: Tue Aug 13, 2019 10:10 am Exhaust flow numbers don’t mean a whole lot. If you shrink the the throat they usually flow better down low but make less power. This is hard to describe but on the flowbench the sound tells you really how good it is. It should quite down as valve opens. If it pops and bangs or is loud the port needs work.
Exactly ... we have seen that getting the exhaust port to flow well gets the shape correct then blowing out the bowl area to about 91% losses airflow but, makes more power and goes faster down a dragstrip.
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Re: Exhaust port flow

Post by PRH »

moparian wrote: Tue Aug 13, 2019 9:58 pm
PRH wrote: Tue Aug 13, 2019 12:38 pm B1 heads?

I haven’t had too many of those through the shop.

The ex port exit at the flange is quite large, but the bowl area of those heads are pretty small.

I’ve tested a few ported by other shops that had pretty nice ex flow....... others not so much.

I realize the ex flow numbers don’t seem to count for much, but I can get 280 out of an Indy 440-1 ex port(no tube) way easier than a B1.
Yes B1 s they were 270 cfm before they cut the valve by 060 and opened it up,they got new inserts too and throat is 91%
So, if I read your first post correctly, the heads currently have 1.70 ex valves and flow 220@.800 lift....... and they had 1.76 valves and flowed 270 before?

That really doesn’t seem like an improvement to me.

91% of a 1.70 valve is 1.547....... which seems like a pretty small hole to be used for a 580” racing engine.
Somewhat handy with a die grinder.
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Re: Exhaust port flow

Post by moparian »

PRH wrote: Wed Aug 14, 2019 10:19 am
moparian wrote: Tue Aug 13, 2019 9:58 pm
PRH wrote: Tue Aug 13, 2019 12:38 pm B1 heads?

I haven’t had too many of those through the shop.

The ex port exit at the flange is quite large, but the bowl area of those heads are pretty small.

I’ve tested a few ported by other shops that had pretty nice ex flow....... others not so much.

I realize the ex flow numbers don’t seem to count for much, but I can get 280 out of an Indy 440-1 ex port(no tube) way easier than a B1.
Yes B1 s they were 270 cfm before they cut the valve by 060 and opened it up,they got new inserts too and throat is 91%
So, if I read your first post correctly, the heads currently have 1.70 ex valves and flow 220@.800 lift....... and they had 1.76 valves and flowed 270 before?

That really doesn’t seem like an improvement to me.

91% of a 1.70 valve is 1.547....... which seems like a pretty small hole to be used for a 580” racing engine.
The improvement was on the intake side ,But the exhaust was very disappointing ,What size throat do you think it should have? and maybe i should go back to the original size valve?
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Re: Exhaust port flow

Post by PRH »

I can’t say that I have any first hand experience of taking a set of heads off a good running motor that made the expected power, and ran good numbers on the track........ that used heads with what I would consider “good” exhaust flow......... then, blew the ex throats out, lost flow....... then made more power and the car went faster.

I’ve heard this same scenario from multiple sources, so I don’t doubt it happens........ but I’m stuck in the past where you went after good flow.

I haven’t tried to get one of my customers to be the guinea big with their heads that they’re happy with to do that experiment with...... so I don’t have an answer.

However, what was done with your heads doesn’t follow that line of thinking.

The premise is that you take your great flowing exhaust port that has a smaller than 91% throat diameter, and bring it up to 91%....... which will likely cost some flow.
So in your case that would mean taking the existing bowl that was done for the 1.76 valve and bringing it up to 91%(If it wasn’t that big already).
What your heads got is a smaller valve installed, then a throat of 91% for the new valve.
Somewhat handy with a die grinder.
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Re: Exhaust port flow

Post by JoePorting »

Small exhaust ports/ valves make more power than large. The reason [theory]: A secondary combustion chamber is developing in the exhaust port which is creating reversion. (I.e. more exhaust gases going in the exhaust port than going out). This creates a secondary combustion process that makes reversion and a cool flame out the exhaust. The flame is confirmation of unburned air/fuel going thru the motor caused by the reversion pulse.
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Re: Exhaust port flow

Post by jarmoyp »

1.76" 270 cfm.
1.70" 220 cfm.
Maybe that is not the correct flow numbers, you double check everything in the flowbench? Before and after.

"maybe i should go back to the original size valve?"

Maybe you should go first in the dyno and then go back to the original valve size. If it needs to go.
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