Best way to flow exhaust ports?

Pitot Style Bench discussions

Postby jfholm » Thu Aug 06, 2009 10:55 am

johno wrote:I wipped up a radiused flow guide similar to the one you showed me john, then i flow tested the DART PRO-1 head and got 160.2 cfm @ 10" on the pitot bench (did'nt quite have enough pull for a 28" test!!). I was surprised how much they flowed outa-the-box as the heads were quoted by DART at; 276cfm @ 28" / 164.9cfm @ 10" at 0.600". Of course, i tried to pick the worst looking port to flow-test and start from there but was thrilled with the improvement in readings and accuracy the radiused flow guide gave me!! Thanks heaps mate!!!! Can you elborate on which section of the head you are refering to as the 'CSA'?

CSA is the cross sectional area of the port and you are looking at the tightest spot or smallest area. I usually find that this area is at the pushrod pinch and also another tight spot is just about or near the SSR towards the intake manifold.

Getting in there to measure is a little hard to do even with snap gauges. This is why I like to pull a port mold using a 2 part urethane rubber. This link is probably the cheapest source: Smooth-On I would use the Reo-Flex 30. You can buy a trial size by clicking on the buy online button for $24.72 USD. You also need some release. I bought a spray can of the release that you spray the port with and the valve you have installed in the port.

The process is to spray the valve and port with the release. Install the valve in the port you want to get a mold from using a checking spring to hold it in place. position the head so the port entrance is horizontal and up. Mix your rubber in a quantity that will fill the port and pour into the port. Let it sit 6 - 8 hours before you try to remove the mold. Then remove the valve and push the mold out from port opening to valve pocket.

Once you have it out let it sit for a day before you play with it too much to let it settle down. Then you can get some dial calipers and measure the smallest areas. Some people will actually cut the mold at those spots or many spots in a port to get the CSA at many parts. After you cut the mold you can lay it on a piece of graph paper which usually has 1/4" squares and trace the outline of the port to calculate the square inche area of the port.

Now myself, I like to keep the mold intact to study the entire shape of the port. So I measure the height and width of the port at the tight areas and subtract for the radius areas. On most SBC that would be minus .187 - .200 square inches.

Most of the time a port will work better if the smallest CSA is closer to the valve. Ideally you would want it right under the valve. I will give you a hint, Won't happen on most SBC heads. Concentrate on the pushrod pinch area and near the SSR area.

If the area is too tight the engine will not RPM to potential and it too big will be lazy and not recover on shifts well.

This is where a good velocity probe helps out also. The PipeMax program by Larry Meaux calculates the needed CFM and CSA for you. It is well worth the investment. I think it is only $40 and he will email it to you.

John
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Postby jfholm » Thu Aug 06, 2009 12:39 pm

[color=#000000]This is an addendum to what I just wrote:
You are looking at velocities in the port. Don't expect great gaines in CFM. And further more don't worry too much about cfm. On my cousins where the peak HP is at 7500 rpm and shift points at 8300 rpm the CFM demand is 325 cfm through the head. Minimum CSA would be between 2.696 and 2.804 square inches. That would give a port velocity of 260 - 270 fps at 28" h2o.

His engine has a radical mechanical roller with 281
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Postby johno » Thu Aug 06, 2009 5:50 pm

Thanks for that info. mate! Thats an excellent idea making a rubber port casting, im in very tight with the local engine recon. guys over here (they love it when i walk in to get work done!!) so they 'donate' a variety of u/s cylinder heads to me for free to practice on. I'll scratch around over here for a similar product and give that a whirl!! Funny you should mention i should be more concerned with 'velocities' over 'cfm flow', as my software indicated that the valve efficiency of the DART PRO-1 heads was just above 95% and predicted a maximum 'potential' flow of 167.7cfm @ 10" (100% efficient). If DARTS bench says the heads flow 164.9cfm @ 10" and my bench 'sees' this as 160.2cfm (but really its 164.9cfm), would i be safe to assume that there is very limited 'extra' flow to be found from this valve/head combo or should i just ignore the software? Also, do you know what the best port velocities are at 10" test pressure?
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Postby johno » Thu Aug 06, 2009 6:04 pm

I almost forgot, iv been taking my practice heads and getting one of the end cylinders cut vertically straight through the valve guides on both an intake and exhaust valve so i can see the 'true' shape of the ports, valve/port inclination angles and how thick/thin the port walls are (similar to what your doing but far more destructive!!). This would also allow you to measure inside the ports! :;):
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Postby jfholm » Thu Aug 06, 2009 7:29 pm

yes cutting a head is very beneficial and you are very fortunate to be able to do that. I still like the mold as you can trace it straight to the graph paper if you want to.

Here is a spreadsheet that was on the forum that converts velocity 2 ways - from your "H2O reading on your pitot velocity probe and gives you the velocity in FPS at you test pressure and then converts it to 28" H2O velocity.

If you are testing at a test pressure of 10" H2O and velocity probe gives you 6" H2O reading that is 162 FPS at 10" H2O test pressure and that converts to 271 FPS @ 28" H2O test pressure. 5.5" reading on your velocity probe would be 155 FPS at the 10" test pressure and converts to 260 fps @ 28" test pressure.

John
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Postby johno » Sat Aug 08, 2009 4:49 pm

Got a question about exhaust testing with my PITOT bench. Im running a Flow-Quick set-up (i will post pictures soon!!) that has the motors after the 'measuring pick-ups' at the very end of the 'flow circuit' which, during exhaust testing, applies heated air to 'both' the 'measuring pick-ups' AND the test item. Conversely a SF-110 pulls cold air through its 'measuring orifie' and heats the air before being applying it to the test item. I realise there is a need for a temp. correction of the SF-110 reading (due to temp. change from orifice to test peice & vice-versa) but if my Pitot bench is applying the SAME 'heated' air across the 'measuring pick-ups' AND the test item, (and exhaust is actually 'HOT' in real life!), why would i need to correct these figures?? Would'nt a 'hot' measurement be more REAL? ???
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Postby Tony » Sat Aug 08, 2009 5:18 pm

It might work if everything in the flow path (after the blower) was thermally insulated, because all that hot air is going to cool down fairly quickly as it comes in contact with the rest of your flow bench and the test piece. So where do you measure the actual temperature to correct with ?

Drawing in ambient temperature air, and flowing and measuring with that same ambient temperature air, everything remains at the same temperature. That vastly simplifies the while deal.

I suppose a lot depends on the type of blower too. Vacuum motors use the air to cool the motor windings, and they do run very hot. Every electrical watt going into that vacuum motor passes into the air.
A large single blower fitted with an external induction motor does not create anywhere near the temperature rise in the air as a vacuum motor will.

Just one more small detail to ponder I suppose.
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Postby johno » Mon Aug 10, 2009 3:28 am

Tony my bench is a Flow-Quick, which uses 2" rigid pressure pipe to connect a 'vacuum motor box' to a large box-shaped test plenum with adaptor (Flow-Quick pick-up is in the middle of the circuit). Would there be much of a temp drop, on exhaust testing, from my motors to the pick-up and test piece?
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Postby Tony » Mon Aug 10, 2009 4:04 am

johno wrote:Tony my bench is a Flow-Quick, which uses 2" rigid pressure pipe to connect a 'vacuum motor box' to a large box-shaped test plenum with adaptor (Flow-Quick pick-up is in the middle of the circuit). Would there be much of a temp drop, on exhaust testing, from my motors to the pick-up and test piece?

I really have no idea.
But perhaps you could measure some temperatures around your system to get some sort of a feel for what is going on.
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Postby hdwgfx » Fri Aug 28, 2009 10:45 pm

Johno in addition to this here great site and all it's members, if you want to read great in depth tech about porting heads go to --> http://www.speedtalk.com

tip ( if you don't already know ) go to google and type csa site:speedtalk.com
Thanks,
david
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Postby hdwgfx » Fri Aug 28, 2009 11:59 pm

John is the trial size of the Smooth On @ enough to do a intake 95 cc and exhaust port 73cc ??? thanks for the tip
Thanks,
david
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Postby jfholm » Sat Aug 29, 2009 1:22 am

hdwgfx wrote:John is the trial size of the Smooth On @ enough to do a intake 95 cc and exhaust port 73cc ??? thanks for the tip

should be no problem at all. My SBC has 193 cc intake ports and 75 cc exhaust ports and I did 2 intake port pours and 2 exhaust port pours with a kit that is the same size I got from TAP Plastics.

John
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Postby hdwgfx » Sat Aug 29, 2009 11:04 am

Thanks John :)
Thanks,
david
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Postby jfholm » Sat Aug 29, 2009 1:26 pm

btw I think there is an online calculator at that website to help calculate how much you need.

John
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