Don Terrill’s Speed-Talk

PackardV8 wrote:I notice you did a bit of work on the shallow side of the chamber wall. I no longer go there, as it seemed to lose more in compression and squish than it gained in flow. Other's thoughts?

I just noticed that too and believe you're absolutely right, Jack. Very little flow gained from total loss of whatever little sliver of squish band was there to begin with. That is a far less restrictive zone for short side flow but maybe slightly bigger radii pockets add potential for some additional pressure recovery on that side of the valve curtain?

It can also be a mistake to blow the chamber radii around the exhaust valve out to completely match the gasket too. Follow the gasket too long and can easily start to go backwards on average power gains. Bulk of exhaust restriction is in the throat sizing and SSR shape.. not so much in the chamber itself. And it's a HUGE mistake to use larger backside radii on the exhaust valve itself. Flatter style valves with one or even 2 backcuts(I settled on a narrow 38°into a wider 28°-30° and hand blended the edges away) works/worked pretty well in these little ports. Have to keep in mind this is a very space limited overly narrow(water) and near ridiculously shallow(more water) throat section to begin with and needs all the volume it can possibly get. Flow will increase everywhere in the lift curve as you get the valves backside profile closer to flat.

I do certainly agree that robbing away the ever important squish pads real estate sends you backwards on a running breathing engine but it's all a balancing act for what little you have to work with. I personally like to use preshaped stones to act as chamber hogs, one for roughing and another for finishing followed up by hand blending. Then I connect the dots by between the chamber hog cuts on both squish pads. I do stay away from digging out the area between the valves on the short side of the chamber though.. nothing to be gained from carving straight lines there.. only lost compression.

EDIT: I just remembered I have an old set of C5 dated heads buried someplace and IIRC they are also setup for a similar 1.84/1.6" valve combo. One of the very first SBF heads I did when I was a very young man over 30 years ago. Might be interesting to Charlie or any others if I could dig them out to look at my "apprentice stage" workmanship and overall thought process on port design. Good to be humbled back down to actual size once in a while too. lol I know I sure didn't have much of a knowldege base to begin with(no internet very few dedicated induction/head design books), but those little heads easily ran well past that smaller cams estimated power peak.. and good bit over 7,000 rpm on a 10.6:1 SCR 310" motor. Also carried well past peak without nosing over using those old speed junk of the day "Z/28 springs". IIRC they also still had TBC coatings on everything, intake and exhaust ports. Valves were coated too but are probably long gone after all this time. If I run across those or any other double back-cut exhaust valves I will try to snap a pic of those too.

Stock chamber 1.6 sized valve

Deshrouded chamber 1.6" valve.

Sorry, sick kids and internet issues here. The small deshroud of both intake and exhaust did improve flow and a noticable drop in intake swirl. Thanks, Charlie

Life happens whether we're busy or not. Just docter and love em' up and enjoy em' while you can, Charlie.

Did you test chamber mod's separately for each valve.. or only after both were deshrouded and short side chamber was recarved in between them?

groberts101 wrote:Life happens whether we're busy or not. Just docter and love em' up and enjoy em' while you can, Charlie.

Did you test chamber mod's separately for each valve.. or only after both were deshrouded and short side chamber was recarved in between them?

I did them both at the same time. This chamber is pretty tight and these will most likey go to 1.9 or even 1.94/1.6 but I want to see what I can get out of the 1.85" valve first. Thanks, Charlie

Chamber work and deshrouding flows. Mid range flow increase on the intake stalls swirl until .5" lift. Exhaust sees incresed flows from .15" lift and up. Thanks, Charlie

Deshrouding flows and flow per sq in valve area graphs.

As cast 289 vs. Rough ported, oversized valves and deshrouded chambers.

Flow per sq/in as cast vs. Rough ported, oversized valves and deshrouded chambers.

[attachment=0]0208172037_HDR.jpg[/attachmentAs cast vs. Rough ported, oversized valves and deshrouded chambers. Velocity - port energy graph.

Thanks Carnut.

Things are beginning to get interesting. Decent ssr cut on both ports. Better flow throughout the curve on the intake and practically the same on the exhaust. That would tell me that the small, curved exit may be holding the exhaust back. Nearly zero swirl up to .4" lift. Thanks, Charlie

Ssr cut flows.

As cast 289 head vs. Mods up to this point.