I'm wondering why your not seeing more gain with the 7MM vs. stock 3/8" valve stock port comparison. I consistently see 10% or more increase throughout the whole range over the 3/8" stem valves when using 7MM stuff before porting.

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There's no such thing as too much cam....only not enough engine!

He flowed it with no spark plug in the hole.
I am pretty sure he sonic checks to make sure he isn't going to hit water. That allows a little more leeway with cutting.

But seriously, there are so many variables it is hard to say. Valve size is a big part, and obviously on bench the bigger valve will flow more. But, what does it do when it is on a motor and has tge shrouding of the cylinder wall thrown in? How many hammaswingas have been on here claiming big flow numbers and huge valve sizes but the car is either a pig or the motor never gets built?

Reality is flow numbers are just like dyno numbers. Good for tuning purposes and arguing on the internet, but that's about it. Unless you flow the heads on the same bench, same day, same operator there are just too many variables.

For me, the only numbers that tell a true story are ET & MPH on a time slip. The eyes don't lie.

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Official Cookie and Mater Tormentor.

That Tilley head was right around a 1.78 intake valve and he made over 350 HP at the crank w/12:1 and webers. Yes, very hard to compare head flows on different machines. Tilley uses a sonic checker to max it out, and fills holes with epoxy if he breaks through. I plan to get a sonic checker at some point and push harder on porting.

To me, if you have a 3/8" valve with the stem backcut to 11/32" or a bit smaller, I don't see how you are going to see much difference going to 7 mm. I need to measure the engnbldr valves and see how small is the narrowed stem.

Slant on,
Lou

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Home of Slant6-powered fun machines since 1988

I just dropped my Engine builder valves off so I can't measure them but they are definitely undercut at least 1/32" maybe more.

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Now I am wondering if maybe the bigger gain is in making the guide and guide boss smaller, and not so much in the valve stem diameter.

Lou

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Home of Slant6-powered fun machines since 1988

I did this on my last two heads and saw some gains in my ET, the first one I reduced the cast iron shrouding and smoothed the bronze guide intrusion into the runner...the final head I removed the guide material to the roof of the runner and blended it, this gained a bit more time...Of course I had deduced that the shortened guide might cause quicker guide wear and inaccurate seating over time, but since this was only going to be raced I figured the short term gain would be fine...possibly this improvement and a thinner stem might be all that can be gained here...that being said this would not be a good "street" mod as the bigger port tends to lack umph at low rpms off idle...

Yes, you get gains from a better profiled thinner guide for the 7MM valve also. But I'm comparing the gains from a bone stock non-undercut stem 3/8" valve with stock guide to a race flow undercut 7MM stem and tapered reconfigured guide, but unported on both and seeing a consistent 10% gain just from that. Porting yields a much higher gain as you also increase valve sizes obviously.

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There's no such thing as too much cam....only not enough engine!

I had a Carpenter head for a Z-1 Kaw motor that was done in the mid '70s. The most noticeable thing on it were the guides being cut down and smoothed out. Yes, it went through guides pretty quickly on the street, but it did make quite a difference over a stock head.

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Official Cookie and Mater Tormentor.

The valves have been un-shrouded and the exhaust side is done. Intake went up to 192 cfm @ 0.500" lift.

Exhaust went from stock at 100 cfm @ 0.500" lift up to 132 cfm @ 0.500" lift. I don't have any of the lower lift numbers yet. The total puts the exhaust at 68.8% of intake flow.

I know the slant exhaust is typically weak, but this is worse than I expected.

Any tips on where to pick up a few more cfm? Do I just make up the difference in duration on the cam? On the exhaust port, is it time to break out the sonic tester and just go for port volume?

I'd really like to get the flow up to 70-75% of intake...

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Brooks

'72 Dart Swinger
'71 Plymouth 'Cuda

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Pick 2!

My experience has been that the exhaust flows proportionately better than the intake. Three different heads show 80-86% of intake flow. Rob Simmons posted a bunch of flow charts on a thread about his engine a couple of years ago and I believe they showed the same thing. The head that Glen Knowlton flowed showed 166@.5 with a 1.46 valve. That being said the exhaust bowl is also where you tend to see holes if your casting has unfavorable core shift. The head that Mike Jeffrey did that I use on my street car and is not aggressively ported shows 145@.5.

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If you didn't drive it there, it's not a street car.

In my mind the smaller valve stem would flow more. If the "valve curtin"(at lift) flows less than the bowl with a 3/8 stem then a 7mm stem will not help. Hope that makes sense. Mine has 3/8 stems and flows 196 @ .600 1.8 valve.
Will

Its very difficult to tell someone how to port a head over the phone or internet. Without seeing a visual of what has been done its still more difficult to tell you where you need to go next. If you still have the stock valve guide diameter protruding into the port, that can be a lot of it there. With a 7MM valve, the OD of the guide needs to be a lot smaller than the OEM Slant guide, or Chevy or Ford, whichever head you happen to be modifying.

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There's no such thing as too much cam....only not enough engine!

Yes, % exh flow is usually very high relative to other motors.

Lou

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Home of Slant6-powered fun machines since 1988

What size do you have the exhaust port opening opened up to? Length and height.....

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There's no such thing as too much cam....only not enough engine!

What year head? I guess the 81-up heads have smaller exh ports relative to intake than earlier ones.

Lou

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Home of Slant6-powered fun machines since 1988