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Hey Horsewidower,

It's hoffman900 from ST / YB. I forgot I had registered here eons ago - mostly just to lurk and see what other hemi engine builders were doing.

Anyway this plays into what Nick Smithberg (https://www.facebook.com/pages/Smithberg-Racing/224572780930205) and I worked on with my cylinder head.

Single cylinder, 2 valve, hemispherical combustion chamber.

90mm bore
Intake valve: 1.900" (49mm +1mm from OEM)
Exhaust valve: 1.475" (37.5mm , -1.5mm from OEM).
MCSA: 1.76in^2 (occurs at the carburetor throat/ flange at the head)
Calculated average velocity of the intake port at 7200rpm (power peak goal): 290fps

The exhaust valve is also sunk and 50* seats are used on both. Maximum valve lift with the cam offerings I can get are .540".

So why the 50* seat, smaller and sunk exhaust valve?

Well, in my case, I'm limited in the valvetrain department. In Nick's experience with the Chrysler Hemi's (this also seems to mirror Larry Meaux's), is that over scavenge is a problem with the hemispherical combustion chambers. Our intention is to hurt flow at overlap to minimize any over scavenge issues. By sinking the exhaust valve, making the valve smaller, and using 50* seats, we have done that. What we have now is a flow curve that while still strong at low lift flow, really starts ramping up at .250" valve lift.

An excellent post from Larry Meaux:
from PipeMax->
Trapped Ve% is the Measured CFM minus the Ring BlowBy CFM , minus the CFM
lost during the OverLap Period, then divided by the theoretical CFM

Theoretical CFM @ 100 % Ve = CID * RPM * .000289352

TrappedVe = ( MeasuredCFM - ( BlowbyCFM + OverLapCFM )) / TheoreticalCFM


the highest recorded VE% i've seen on my Dyno
was a Chrysler SS-Hemi 426 engine-> 140-142 VE%

That 140-142 VE was not true "Trapped VE% ".....
at least 20+ % of the 140-142 was lost out the Exhaust during OverLap

After those 1st 12 or 15 Pulls trying different things
we called Cam Motion and had another Cam ground
on 112 Centers from the original 104 Centers

With the new Cam Motion Cam in the Engine ( same Lobes , on 112 )
the VE dropped to 120's VE and gained around 40+ HP,
...still down from where we thought it should be.

Those Heads were Ported + Welded on Exhaust Ports,
and had tremendous Low-Lift exh flow numbers,
they Flowed more than the Intake Ports from .200-.300 Lift
...coupled with Cam's high Overlap Period ( 104 LSA )
and along with great set of Headers ,
the Engine's HP + TQ was terrible

the new Cam Motion 112 LSA picked up TQ + HP everywhere
and showed 20+ less VE ,..so a great deal of Mixture
was going out the Exh ports,
and with an already way too fast Intake Port's FPS w/epoxy in it,
was a disaster !

Here's a few Pics+Info on Indy Legend 426-1 alum Heads

notice the Exh at Low-Lift outflows the Intake side
..but unlike the SS-Hemi Heads, the Indy-Legends don't
have any too-fast CSA areas in the Ports,
so Over-Scavenging on these Heads will not matter anywhere near
what we saw on the Welded/SS-Hemi Heads we Dynoed .

Followed up by another excellent post from Calvin Elston (for those that don't know, he builds / designs Cobra Automotive's exhaust systems, besides all the NASCAR, NHRA, WoO, and IMSA championships under his belt).
always figured builders never worried about efficiencies much because most of them in my world gave up trying to measure air flow on the dyno. It wasn't untill O2 sensors became more available and running in cars as well as on the dyno that tuners actually had to deal with overscavaging. It presents the situation where the O2's tend richer and attempts to lean the system would make no change in O2's and power would drop.

I consider it a good thing for a tuner if he has time to work on things but bad if you don't. I run into it with customers quite often. The tuners ability to accept the reality of what is happening makes all the difference. Just as Larry mentioned, the Hemi was leaving a lot on the table and all it needed was a "smaller" cam to pick up bunches! Some builders have a hard time spending money on "smaller" cams.

So Dan, from my perspective, you should shoot for the shortest cam lobes as possible and let the intake and exhaust systems be responsible for the upper end of the power band. The better the intake and exhaust perform upstairs, the smaller lobes and the tighter you will be pulling the lobes together, The IC and EO determine the low end power and neither the intake or the exhaust can help there.

The ultimate fix is to make exhaust valves smaller and install bigger intake valves before you loop the cam lobes.
I've seen talk on here mentioning Vizard's 30* valve seats. Vizard quietly has been mum on the subject after most good cylinder head guys laughed at it. It's a trig problem, and as one can see, the 30* valve seat increases area open as the valve lifts off the seat vs. a 45* vs. a 50*, etc. In doing so, we have inadvertently increased the overlap area the engine sees. Not always a good thing.

Something for all to chew on and think about how it applies to the Alfa engines....

Pictures of my cylinder head:


Note: nothing was tightened down in this shot. It's a smooth run when everything is bolted tight together.

Valve profiles after trying many shapes and backcuts:




Left side is just the head and intake spigot with a radius plate, the right side is with the carburetor and velocity stack attached to the head.

Exhaust port. The .700" value is the .600" lift value with a stub pipe. CFM isn't really a concern on the exhaust port. Velocity, shape, and port stability are.


Cheers,
Bob
 

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In a perfect world, I would be able to run a much more aggressive rocker ratio, dial back the seat to seat timing to something much more ideal, and then we could work on making the port strong everywhere. Unfortunately, I'm limited to 1.3 ratio rockers (SOHC and their location's are fixed), so in order to get the lift I need, the duration has to be such for valvetrain dynamics that I can't get real crazy, so this was our approach.

As a side note, the larger intake valve did not increase flow, but it made the port more stable (as seen as flowing it on the flowbench and watching the manometer). Nick spent a lot of time making the port stable (remember, a running engine could see 100" of depression or so vs. what's seen on the flow bench). Numbers being equal, a head that shows more flow stability will always make more power, which I'm sure you already know. Our main limiting factor was the material around the port sides - the short turn can only be 'blown out' so far. I mean, take a look at how far guys like Nick and Chad go with some of theirs heads that have a shallow approach to the valve. Downdraft designs can dial that back some and don't have to slow the air up as much around the short side to get it to turn.

We used a Mikuni 38mm flatslide (this is our MCSA) to size my power peak. I'm looking for something in the 7200rpm range with a power curve similar to a EMC deal . The material is there to use a 40mm Mikuni, which would have ultimately allowed more air to flow through.

Cheers,
Bob
 

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Here is another Calvin Elston post as it relates to overscavenging:
If you keep dropping the pressure in the header, and that is not hard, you will eventually get to the condition which you are seeing which is overscavaging. Changing the camshaft around is effectual only to the degree that it was too large or too tight in the beginning.

Lets try stepping back further from the situation. There is a basic amount of duration you need to get to a certain rpm and power level with a given engine combo. There is a wall there that you hit when your just dropping header pressure and correspondingly reducing exh duration and overlap, (and all the other tricks there are). It does not allow you to really make too much more power with the same size intake valve. It is a diminishing return. The adjustment is to reduce the size of the exhaust valve. This allows you to keep the duration or cam timing where it really wants to be and still exhaust well enough. The smaller exhaust valve allows most engines to use a larger intake valve and this is the ultimate purpose in using a properly tuned header. The well tuned exhaust side allows a larger intake valve which is the easier way to move more air into the cylinders than sucking from the back door. If your header is not functioning well, you will not be able to get there. :)
Very few engine people get this.
I believe if you look at PS type 2valve engine development over the last 10 years, you will see a very large increase in power and rpm, but exh valves are the same or smaller in diameter and the intakes are larger.The merged collector and corresponding header and valve sizing has been a important factor.
I think a while back I spoke about the "established" thinking about valve diameter relationships, ratios and all...and how they were all based on exhaust systems that did little more than duct gases in the proper direction. In just one area, exhaust headers, if you can make a large difference in what the engine "see's" on the "out" side, one has to back up and stumble through all the other "tuning" aspects of the given project...if you have enough time and money. I am glad you have a relationship with an outfit that is willing to pursue this area. The Hemi (I like to call them Hemroids...) has some distinct problems in regards to overscavaging and that makes it a better platform to learn about this issue
He also has a great blog. Exhausting 101 ? Understanding your exhaust system www.elstonheaders.com
 

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Discussion Starter #5
Bob, thanks very much for posting.

Nick and I are Engine Masters team-mates. I led the team after Danny passed away.

Currently he's working on my Volvo head that we are adapting for a Ford block. Hopefully it'll be in our Bonneville truck next year.

Thanks for sharing all the information.

Bob
 

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Bob, thanks very much for posting.

Nick and I are Engine Masters team-mates. I led the team after Danny passed away.

Currently he's working on my Volvo head that we are adapting for a Ford block. Hopefully it'll be in our Bonneville truck next year.

Thanks for sharing all the information.

Bob
Bob,

Didn't realize this was you, I know of you (don't worry it's a good thing). :thumbup:

I'm really excited to see how the Volvo head conversion comes out!

Cheers,
Bob
 
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