Too big of a cylinder head...

General engine tech -- Drag Racing to Circle Track

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Re: Too big of a cylinder head...

Post by kimosabi » Sat Feb 10, 2018 4:35 am

randy331 wrote:
Wed Feb 07, 2018 11:11 am
kimosabi wrote:
Wed Feb 07, 2018 3:44 am
It shows the common knowledge nowadays that the cam they used were better suited to the 195 head than the others. Nothing else. Anyone questioning the importance of a custom grind to your combo after watching that test, should start thinking about doing something else than engine building.
How could you watch that video and come to that conclusion, or really any useful conclusion ??

Very similar results and the middle head have a slight advantage. When you have pretty much identical results between a 165 and 220 head, then you know the cam is not really optimized for either.

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Re: Too big of a cylinder head...

Post by burbfixer » Sun Feb 11, 2018 9:22 pm

travis wrote:
Mon Jan 29, 2018 6:02 pm
Hypothetical question...

If you use too “big” of a head on an application (planning for future growth for example), and want/need good part throttle low end and midrange would you cam it? Smaller duration and tighter lsa? Small and wide?
My knowlege & experience is limited compared to many here, but I had an experience with an engine I owned in the past that relates to this question.
When I first got my 396 back in the day, it was a stock 350hp chevelle engine with 10.25cr, stock 214/218, 461/480, 115lsa cam, stock iron high rise 'Holley' intake and stock exhaust manifolds. I put it in a 2wd 69 chev pickup with a th400 and 3.73 gears, and ran it on propane. It didn't last long because the exhaust valves recessed, but before that happened I loved the way that engine ran. I liked to play with this impressive 'torque surge' thing it would do. You could really feel it when you drove away from a stop light normally then right after the 1-2 upshift added another 1/4 to 1/3 throttle. The rpm was no more than 2000-2500, but I remember vividly how it would press you really hard back in the seat! Torque to weight ratio is what I was feeling, but here's the point : that engine was making plenty torque for the job at a fairly low rpm with oval ports that were something like 250 cc. For comparison, that port volume on a 396 is like a 220 cc port on a 350 - that's hot street/racing territory, isn't it? Makes me wonder if maybe it's a mistake to place too much emphasis on the importance of needing very small ports to have enough velocity for a fun daily driver....
When the stock small chamber heads went bad, I put some 454 heads on with similar size ports, but substantially larger chambers - about 20cc bigger. The engine ran fine, but that fun torque surge went away with that 1.5 or so points of compression loss. In other words, compression loss made a big, very noticeable difference in low rpm part throttle torque, but a fairly large port size had great low rpm part throttle torque when the compression was there. I never tried smaller port heads on that motor, so who knows, they may have made it even more torquey.

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Re: Too big of a cylinder head...

Post by F-BIRD'88 » Tue Feb 13, 2018 6:14 pm

You can always fill the intake port floor with epoxy or make floor filler inserts for each port.
We once got a set of Brodix track 1 heads in that someone had over ported them and made the port opening too big and the floor too deep. Thus the motor seemed to lack torque. (and did)
After filling the floors and flow testing Port speed really kicked up and flow only decreased a hair.
On the dyno it (a 355 SBC) then made a good bit more low/midrange torque and all the power it had before. It had a much better power curve. This thing was no slouch but the port fill made it even better.
The car picked up some performance too. It was a drag car. On the street with street tires it would never be a real difference. Don't sweat the big ports.

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