I have a 59a flathead with big bore and stroke (296) Navarro 10.0 heads. When I calculate the compression ratio I get 10.86:1. That’s seems high. Will it work on a flathead? my only unknown is the deck clearance? Assuming it .040” but it could be less since I decked the block. Mahle says I should be at 9.4 with 64cc heads but I have 58cc heads.
The actual deck clearance needs to be measured for an accurate calculation. Can you borrow a depth mic? With only a 12.8cc dome, the deck clearance will affect the CR.
You can calculate deck clearance nominally if you know the rod length, deck height and piston pin height. Actual measurement is always more accurate.
On those heads, is it flat on the area above the pistons? If so, I think you're going to have some severe "dome kissing the head" issues. How far above the deck is the top of that piston dome?
Do you have some used gaskets you could assemble the heads with to check your clearance? The old clay ball method. Then you could also check valve clearance.
Those heads have been well used, abused and milled a lot - which can be seen by how shallow the piston dome areas are and also you can tell that the "last guy" had to fly-cut the heads a lot just for valve clearance. Given the shape of the deck surfaces, they'll need to be milled again - making the chambers even smaller. Now, some folks would re-dome the chambers and rework the valve areas . . . but then a lot more material is removed and they're already thin (decks, above the valves, etc). Truthfully, you'd be much better off with a new set of heads. Hate to tell you to spend a chunk of money, but I'd not run those heads for the build you're doing. Typical gasket thickness is about .052 for most gasket manufacturers, but you can buy Cometic MLS gaskets at different thicknesses. One thing that is very important is to tune your final squish/quench above the piston to be .040 to .045 (including the gasket). When you get a new set of heads, you may find that it is much greater than this . . . so you'll usually need to mill even a new set of larger chamber heads.
To give you an idea as to what the chambers looked like on these same Navarro heads - new ones, see the picture below. Notice how much deeper the valve pocket areas are. This will give you an idea as to how much has already been milled from your heads. My guess is the dome areas have been reworked (probably by hand) as well. They're pretty much "used up" in my mind.
I like the Edelbrock block letter heads. I wanted to buy them original but Edelbrock closed down. I think they are back in business and reproduce them again? I’ll save the Navarro heads for a stock flathead. Even with the new heads at 65cc’s and .070 compressed gasket thickness, I’ll be at 10.81:1 compression. Is that going to work?
You can't use the calculator that you're attempting to use. It has no idea as to the shape of the chamber/head-gasket, valve pocket volumes, etc. You'll need to determine the volume of the head gasket manually - given a .052 gasket thickness. I don't have my 3D CAD system here in SD, so I can't calculate it for you. Also, you have some CC's in the sunken valve area. You'll have to do the calculations manually - using the right numbers. Note: I recently built a 302 cubic inch engine with the Eddy heads - and it runs just fine without detonation. It is relieved though - so that does lower compression. I don't have my calculations with me . . . so I can't tell you the final CC "chamber" volume (which obviously includes the relief).
You’re right. I didn’t consider the shape of the gasket. It isn’t round so the calculation are going to work. Mahle said I should have about 9.4:1 with 65cc heads. They are probably closer to the true compression ratio than I am. Thanks for explaining this.
You'll probably be just fine with that CR. You might need to run premium, but who cares these days - its a hotrod! Best of luck, B&S
https://www.cometic.com/products/C15089-098 These can help lower the compression a little bit. Cheaper than new heads.
Always well to remember on flatheads there is an inverse relationship between compression and top end power. Gaining compression chokes off breathing. The Harley-Davidson KR 750cc flathead V-twin was dynoed and raced by the best for more than fifteen years. They tried everything possible and made the most power with 6.75 compression. On the street, more compression will make more low speed torque, but will choke off breathing that much lower RPM. jack vines
While this does lower compression, it also increases the quench. You want the quench/squish at .040 to .045 over the piston. It is a bad idea to increase this to lower compression . . . you're far better off adding a relief to the transfer area (in the block) or reworking the chamber areas behind the valve "eyebrows" and over the transfer area in the heads. Having a tight quench is super-important for all flatheads.
This is true to some degree (as I have Harley KR heads, jugs, cams) - though there is a very important thing to consider. The KR was a purpose-built race engine that had extensive design and dyno time, years of testing, fantastic ports, etc.. It was designed to make power at much higher RPMs (like 4500 - 8500) - where the FLOW really counted more than compression. Due to the higher RPM power-band, the roller cam designs in the KRs had HUGE duration and very tight lobe-centers . . . everything was designed for power on the top-end. Also, the pistons popped up into the head - such that when the piston crown came down the whole transfer area was exposed, and flow could be maximized. I've used the KR type chambers and pop-up piston designs in our Bonneville flathead Cadillac - which is also blown and injected. I believe the design has helped us make horsepower . . . though everything is custom (including the .510 lift roller cam). Bottom line . . . it is difficult to compare our flatheads to the KR . . . they are quite different in all aspects of their design intentions as well as application (race only).
Just for shits and giggles - here are the KR components as well as the head design from our Bonneville engine:
Going off memory, new Edelbrock block letter heads are 65 CC. When I checked them, I got 62 CC consistently. https://www.edelbrock.com/flathead-ford-cylinder-head-with-block-letter-logo-1126.html
Yeah, I really don't love their "newer" CNC chambers in the valve area . . . their original "as cast" chamber designs were a LOT better.
Somewhere recently, I came across a discussion on what made the modern LS engines/heads work so well. I forget the exact term, but it seemed to me, IIRC, that it had something partially due to creating a “swirl” affect. I noticed how sharp and distinct the section is between the valves. Without giving up too much, is this part of the “secret” to making more power?
I don't believe the "eyebrow" shape has a lot to do with it - as that just follows the original block/head shape up top.
Dale, I may have missed it, but is the head in the picture for a Cadillac or Ford flathead? If Ford, I'd love to hear more about the spark plug placement.
That is for a 1942 Cadillac flathead - modeled after the Harley KR style chamber and running special pop-up pistons (.040 quench at the top). Lots of folks might say the spark plug is in the wrong place, but I figure that Harley did it on the KR, so I might as well do it. I've pondered their design --> with the angled "shelf" which angles away from the intake valve -> I that that "maybe" the flame travel would naturally flow towards the far side of the shelf (which also means that there is an OPEN area above that part of the cylinder. Maybe the combustion process "swirls" into the cylinder for a more complete burn? I have no way of knowing this to be true, but it is a hunch of mine. Same goes with the intake charge - it has an open path down into the cylinder on that side . . . and it might swirl as well. Also, the plug would stay pretty cool in that location - so with a blown alky motor under 21 lbs of boost, this also seems like a good thing. We have plenty of ignition power --> with either a MSD ProMag 20 or a MSD Power Grid . . . so igniting the alcohol is no problem. I will say that having a ProMag on the beast sure seems to produce more power over even a hopped up 4 amp Vertex mag with neo magnets. If only I had an engine dyno, unlimited time and really unlimited money! I'd like to try this design on a flathead Ford, I believe it would have potential. B&S
Interesting. If I was building an all out flathead race engine, my gut tells me to have that sharp point between the two valve reliefs vs the smooth transition shapes below. Lol!
Ziggster, Look closely at the well used heads. I don't believe that is a sharp point, but more so marks from where the valve was kissing the roof of the chamber.
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