This was a relatively stock rebuild on a spare 401 I had. The machinist checked the main bores and said they were okay. Heads got a light cut to remove a few marks from the head gaskets. Block surface looked nice, so it didn't get surfaced. With the replacement 0.020" pistons, they averaged 0.045" below the deck surface. Based on that, the block appeared to be square. Heads varied between 125-127cc. Next engine will get built 'more than stock':Brow:
Glad to hear they are making you a new set! My Ross pistons list a C.H. (compression height) of 1.93". That would be from the pin centerline to the flat outside portion of the piston top (not the dome). You could have them increase that figure to move the piston up in the bore to increase CR.....instead of decking it more. Make the domes big enough....my stockers measured 46cc, tho I've heard some say they are 49 cc. Will valve notches be cut deep enough for your cam? ....or will you do that later? Gary in NY suggested having valve notches tailor cut....the exhaust notch should be cut to a smaller diameter. Not a big 'one size fits-all' notch... the same size for all 16 valves. so...you will have 4 pistons notched one way, and 4 notched the opposite. What cam are you using? A cam with more duration should have more CR than an engine with a stock cam.
the machine shop will not let them make the new pistons. so he took another .017 off the deck, piston is .021 in the hole. gives me 9.75 comp. that a total of .035 off the block and .015 greg cut the heads. .050 total off. using a .038 compresse head gasket, stock was .015 they say.so I picked up .023 there,.050-.023=.027. so -.027 lower total than stock .do you follow me?. .027 lower between deck, gasket and heads should not hurt anything? hope the intake will not move much. the valve notchs were already there.
Walt I am looking at a comp cam, 218-223 at 50. 504-493 lift on a 110 Lsa,they said its a fast ramp cam. or would you go bigger. thanks Joe
Joe, even with the "Seemingly" high lift you will be probably only end up with around .430"-.440" at the valve. More lobe lift on the cam doesn't necessarily give you more lift at the valve because of the short, stubby "Stock" rocker arm. Anything above a .250" lobe lift at the cam & the stock rocker loses ratio rapidly. Ask them what the fast rise lobe rate is. Stock they are .007" per degree of CAMSHAFT rotation. Almost an average roller cam specs. I would like to see you get a 112* or 114* lobe spacing. This will give you more a usable & a wider Torque & HP band of usable RPM's. I'll almost guarantee the pistons will hit the heads in the corners if the corners of the pistons closest to the combustion chamber are squared off & not rounded. Have the machinist only install one piston & rod assembly & check this with clay or some other method. Check the thickness of the pistons below the valve notches & the side of the dome. On ours we needed more side clearance on the dome. We checked with JE & they said there was plently of thickness to be able to accomplish this. After we cut .022" off the side of the piston dome we cut a hole through the piston at the very bottom. They said it was our fault & had to pay again for pistons. At .022" thickness with the compression we were looking to run, even at stock we would have melted a hole in the piston. They made really nice pistons, rounded corners instead of square, for better flame propagation, etc. Now we have a set of pistons that can't be used. 1K thrown away!!!!!!!
Joe stock is about 208 degrees duration, The big isky cam in my blue GS is 245 degree, so you are in the middle, basically a street performance cam. If you are going to add headers, bigger carb(s), and more gear ratio, you cold go bigger...it depends on how what you want to use the car. I hate the cam in my blue car, it's sluggish around town, and doesn't get going until it's over 4000 rpm, then it rips!:grin: So I'm going milder on the red 66 I'm doing now. Your Comp cam is similar to a TA 25: 218/228 duration with 110 lsa. Poston NH400 is also 218/228 duration but with a wider lobe separation of 114 degrees. If Tom says the 114 lsa work better, I'll believe him! I haven't done the extensive testing he has.:TU: Carmen Faso has his own grinds avail. I'd suggest you call him and get his recommendations. Let us know what he recommends! Valve/piston clearance is tightest during overlap...the exhaust valve is closing, and the intake is starting to open....usually between 10 degrees BTDC and 10 degrees ATDC. So on a cam with a tighter lobe separation (lsa) of 110 degrees, both exhaust and intake valves are open more at the 10* BTDC to 10* ATDC, than a cam with a wider lsa of 114*... hence V/P clearance will be tighter on the 110* lsa cam. (wish I could draw you a picture!) I went with the Poston NH400 and 114*lsa. With a 0.15" head gasket, and 0.007' cut on the heads, vp clearance is snug at 0.070 to 0.090". Your engine is only 0.009" tighter than mine. Not a significant difference. I have pistons 0.045" down, with 0.015" head gasket, and 0.007 head cut...total of 0.053" You have 0.021" down, with 0.038" gasket, and 0.015" head cut....total of 0.044". There are variables, ranging from piston design, to how deep the new valve seats are cut. Just check vp clearance carefully, I suspect you will need to have the pistons cut, or use a different cam. One of the primary reasons I went with the 114* lsa of the Poston NH400 cam is because of recommendations like this from Tom. Thanks Tom!:TU: I have another cam I want to try in my blue 66...similar specs to the big Isky that's in there now, but wider lsa. I'm very curious to see what the difference will be.
the je piston dome is now rounded they fit right up into the head. like Walt said I will have to check to see if the valves hit now.
I belive the comments on the dome hitting the sides of the heads only pertain to the 425's, the 401's have a little more room. Also from what I've seen, dome's on the 401's tend to to be a little higher to get the compression since we all use the same heads Roger
talked to comp cams again and told them the ex ports do not flow well, so they went with a 218-230 duration 504-490 lift on a 110 Lsa with a 108 centerline. I do have ta performance headers.
I know you like a wider Lsa 112-114,but the cam is on a 108 centerline with only 2 deg built in. gives a little more top end. I ask comp about a wider Lsa. only going to 5300-5400rpm
Right, exactly, your only going to 5300-5400 RPM's, not 8000!!!! +. 5300-5400, in our cases anyway, is not any where's near close to top end. When we used a 108* lobe separation the engine maxed out at 5800. Next was 110* & the engine maxed out at 5800. Last was 112* & again maxed out at 5800. On all 3, after max, at 5950 or so the line fell like it fell off a cliff. This is because the engine RAN OUT OF AIR!!!!!! The only time we saw more is with "My Rockers" where it still maxed at 5800, but up to 7100 only lost about 30HP. Never did find where it dropped off, was afraid to go any higher.
Just got around to cc'ing the domes on one of my Stock 425 pistons: 40 cc's (Dome height is .320) One of my Stock 401's came out at 47 (Dome height is .370). Again makes sense since the 425 is moving quite a bit more "charge" into the same combustion chamber size. (If you believe the Compression ratios were the same.) As for cam selection. Like everyone else has stated, it all depends on what you want. Big lumpy lump to sound cool and lose low end (great for idling through a car show, etc.) , go with duration and 110, more streetable 114 Roger
On another point. The exhaust is not the major problem in a normally aspirated combination as thought for the last 60 or so years. IT'S THE INTAKES!!!!! Ask Walt again how much he seemingly dislikes the cam he has in his '66 GS. Yes, my "Rockers" will wake up ANY cam. It's all about getting the AIR IN to begin with. The exhaust will find it's way out it has NO choice. Granted the exhaust could use help but the only way the exhaust is over burdened is when the intake is pressurized. On most of our cars they are HEAVY compared to others. Low end torque is what makes street cars FUN to drive!!!! Having to slip the bejesus out of the clutch to get the car to start moving is NO FUN!!!! Having an Auto trans. that has to idle at 1200 in neutral then put into gear with a BANG & idles rough is also no fun. Being standard shift does make it easier, but I'd be more than willing to give up some higher RPM HP & a rup/rup/rup idle to more focus put on low end torque. Drive ability, MPG & all around more pleasurable driving & throttle response is what I look for. Again, just my nickels worth as everyone has their own thoughts.
thats smaller domes then what I saw before. 46 for 425 and 53 for 401. thats makes the stock comp even lower from the factory.
Just got my old TRW forged pistons back from getting TBC and SDF coated, cc'ing the domes on the 401 .060 over: 56/57 cc's (Dome height is .461) (Stock 401's came out at 47 (Dome height .370)). the Heads cc'd at 123/124. The pistons are .045 in the hole. I'm using a .015 gasket. Without hurting my brain, what would the static compression come out to? Back in the day (1970) the pistons where supposedly "12 1/2 to 1 Hi-compression" pistons probably marketing numbers...(I did have to run Sunoco 260, everything else spark knocked something terrible) who remembers Sunoco 260?? Roger
