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Old 02-02-2004, 12:50 PM
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Flow Benching Blues

I am working on a new 351w stroked to 410ci. A motor that I have spent some time on buying the best of products, and giving some care because it will be my personal motor. Anyhow we set it up on the dyno, ran a few pulls adjusting everything in, and got it dialed in at 513hp. Shooting for 575 I was unhappy and thought about changing the heads. Instead I thought I would play with the heads I already had so, I pulled the heads and sent them off to flow bench for a base line figure, which I should of done to begin with but didnt. Anyhow, Once I had the base line figuers, I was Guessing the ports were too small for a 410ci motor, I started my port work. Having done lots of porting in the past, this set of pro topline heads were no big deal to port, alot like the victor jr heads. Anyhow doing what I felt was a very promising port job, and I new these were gonna kick some ***** on the flow bench, I took the heads into the machine shop with a smile on my face. I did a little bragging about my port work, and waited the next day for the results. when the results came back it was a 3%(exh) loss all the way from .100 to .700 lift except for .500 which gained 1.5cfm.. ok so now my heart is broken, and very pissed I just wasted alot of time, I said heck with it, and planned to just put the heads back for now and deal with it later. Putting the motor back together, and wanting to know how much I killed my HP, back to the dyno I went.. This time it dynoed at 528HP and had 13 more torque then before. So go figure..

Has anyone had this happen before? It would seem that sometimes flow bench testing and the real world may have diffenent results sometimes.
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Old 02-02-2004, 01:00 PM
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I wouldnt trust a flow bench test. It doesnt take a genius to figure out that bigger and smoother passages will allow more air to flow. Bigger valves wont help unless you grind away the metal behind the valve seat to take advantage of your new larger valves.
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Old 02-02-2004, 01:10 PM
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I gather that you did the intakes as well. Possibly you picked up more flow on the intake side, and that was really the weak link. So the exhaust ports could loose a little flow, but the overall results were an improvement.

Guess number two is that the shops conversion figures were off for the temperature and pressure readings for one of the days.
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Old 02-02-2004, 01:11 PM
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I would guess that the little bit of opening up of the runners allowed the heads to flow a little better at high rpm. That may be why you got a higher peak number. I would guess that at lower RPM, the numbers went down a little.

Adam
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Old 02-02-2004, 01:56 PM
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I would say that the cam is to big and when you hurt the flow numbers it actually caused the cam to be more effective with the cylinder heads.

Because of the highflow cylinder heads we have today most people do over cam. If a head is moving good air then the camshaft does not have to do alot of work. ..therefore it can be smaller. I have had engines where a cam 20 degrees smaller on duration made 80 mor HP.

Flowbenches are wonderful tools that are needed to gather data to build combinations, but they are not the end all tell all that some people make them out to be.

Intake ports should not be smooth, the rougher the better.

Chris
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Old 02-02-2004, 06:34 PM
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Flow Bench

Brains boy first question did you use a section of ex. pipe when you flowed the heads if not you need to use one it straights the air out when it exits the ex. If you only did the ex. what you probably may have done with out knowing it was to get the cross sectional area in the wright place in the ex. port. I have found out that the super flow flow benches don,t work real good on the ex. side it seems like you have to do a ton of grinding to get any improvement.



mike
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Old 02-02-2004, 06:51 PM
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thats not all true smoother ports flow better to an extent. you dont see people with mirror finished intake ports for a reason
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Old 02-02-2004, 07:00 PM
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ports

The reason you don,t mirror finish intake ports is because liquid will stick to it and cause puddling. Try this take a piece of sand paper and put water on it and try blowing it off then take apiece of glass and try it again you will find that it is easier to blow the water off the sand paper. Now on the ex. side you want a mirror finish because you only have air to deal with and it likes a smooth surface as possible.There is what they call a boundary layer off air as it goes Thur the port were the air dosn,t touch the port walls about .020 thick i believe


mike
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Old 02-03-2004, 11:01 AM
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Mike no I didnt use a exhaust pipe when testing it.
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Old 02-03-2004, 01:53 PM
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What crank, rods, pistons, rings, did you use? I'm thinking about stroking my 351W someday and was looking for some prices and difficulty figures. Any advice would be greatly appreciated. Thanks.


MoocH
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Old 02-05-2004, 06:54 AM
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Mooch,

In my situation I was building a nitrous motor. I had to buy a new crank, and all the cranks were the same cost for a 393,408,418,427. I went with the 408 as the 427 tends to have a short life span and doesnt last long. With the 427 the pin usualy ends up in the oil ring lands and has more oil consuption. For the most part people usualy only do the 427 for drag only. Although some are used on the street, I prefer the 408. which is acutaly 410ci @ .040 overbore.
I was also building a nitrous motor so I wanted to try running a 6" rod because in my opinion a shorter rod seems to make a little more power with nitrous on the windsors, not to mention I had a set of H beam Carrillo rods I wanted to put to use. The rods were a 2" rod journal so I had to machine the crank down to the chevy 2" journal. At the sacrifice of rod ratio, a 6" rod and 4" stoke leaves a little more room for rings which is important with running nitrous, you want to run the top ring at least .300 to .400 from the top. I had the 11:1 pistons made by venolia and requested a total of .006 tapper. I used a 69 block but anything from 69-73 has a high nickel content and thicker webbing. in 74 the webbing is the same but the nickel content is lower, and in 74 they switched to the 9.5" deck height. In 75 the block casting changed and lost alot of strength.
Well this is a little bit of what I had done on this motor, you can PM me if you like.

Ben
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