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Old 12-07-2018, 09:29 PM
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sbc bench build

Doing a bench build before buying all the parts. Already have the short block but need to know how to determine the max lift, duration etc that the build will allow before valves hit the pistons etc. I have a set of HRS aluminum heads to use for mock up.

It's easy to find cam suggestions but I need to know how to find the max lift etc that the build will allow before looking at cams.

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Old 12-07-2018, 09:58 PM
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See posts #28 and 29 on the thread "351 W Intake Choice"
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Old 12-08-2018, 12:04 PM
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Duration has more to do with what piston-to-valve clearance will be than maximum lift does, so looking at lift as the main factor is not the right idea.
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Old 12-08-2018, 12:29 PM
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Quote:
Originally Posted by ericnova72 View Post
Duration has more to do with what piston-to-valve clearance will be than maximum lift does, so looking at lift as the main factor is not the right idea.
Could you elaborate on that please?
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Old 12-08-2018, 12:54 PM
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When you go bigger on duration, you are increasing overlap, the time when both valves are open....and this is the same time the piston is approaching and leaving TDC having just pushed exhaust out and starting to pull intake mixture in.

As an example, these are just out of the blue numbers to illustrate a point...

Cam #1, small duration 220 duration @.050 and .450" lift, 108 lobe centerline - the intake valve starts to open at about 2 Before the piston reaches TDC, but is just open a tiny amount , less than .025" for sure. Piston clears it by a mile.

Cam #2 big duration 260 duration @.050" and the same .450" lift and 108 intake lobe centerline(we'll call it a "lift rule" cam ). Now the intake valve is starting to open a whopping 20 before the piston reaches TDC, and by the time the piston reaches 5 before TDC the valve might be open .150-.180" by then and the piston still has 5 to go before it starts heading away from that valve, while the intake valve will just keep on opening....now valve to piston clearance is a squeaker.

See how both examples are .450" lift, but valve to piston clearance is vastly different??

At peak valve lift, the piston is nearly 1/3 way back down the bore and heading away in relation to the intake valve.....it is at or near Bottom Dead Center and hasn't even started back up the bore if looking at it from a exhaust valve clearance point.....that's why looking at peak valve lift and trying to equate it to valve to piston clearance is not the correct way to go.
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Old 12-08-2018, 01:44 PM
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Quote:
Originally Posted by ericnova72 View Post
When you go bigger on duration, you are increasing overlap, the time when both valves are open....and this is the same time the piston is approaching and leaving TDC having just pushed exhaust out and starting to pull intake mixture in.

As an example, these are just out of the blue numbers to illustrate a point...

Cam #1, small duration 220 duration @.050 and .450" lift, 108 lobe centerline - the intake valve starts to open at about 2 Before the piston reaches TDC, but is just open a tiny amount , less than .025" for sure. Piston clears it by a mile.

Cam #2 big duration 260 duration @.050" and the same .450" lift and 108 intake lobe centerline(we'll call it a "lift rule" cam ). Now the intake valve is starting to open a whopping 20 before the piston reaches TDC, and by the time the piston reaches 5 before TDC the valve might be open .150-.180" by then and the piston still has 5 to go before it starts heading away from that valve, while the intake valve will just keep on opening....now valve to piston clearance is a squeaker.

See how both examples are .450" lift, but valve to piston clearance is vastly different??

At peak valve lift, the piston is nearly 1/3 way back down the bore and heading away in relation to the intake valve.....it is at or near Bottom Dead Center and hasn't even started back up the bore if looking at it from a exhaust valve clearance point.....that's why looking at peak valve lift and trying to equate it to valve to piston clearance is not the correct way to go.
Thank you. Now I have a better insight into it. Will take a little more thought to get it clear in my mind on how to prevent trouble but now I have some more info to work with.
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Old 12-08-2018, 04:24 PM
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What piston are you using?
What is the application for the engine?
Most aftermarket Pistons done have clearance problems when used with a appropriate cam, unless the heads have been cut .150" or something wild. BBC with
some cast Pistons are an exception.
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Old 12-08-2018, 08:43 PM
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Quote:
Originally Posted by lmsport View Post
What piston are you using?
What is the application for the engine?
Most aftermarket Pistons done have clearance problems when used with a appropriate cam, unless the heads have been cut .150" or something wild. BBC with
some cast Pistons are an exception.
Being a bench build I haven't chosen pistons yet. I wanted to try and understand the cam stuff 1st. Deck height and piston relief would play a part in the design. Not sure if I' explaining that correctly.
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Old 12-09-2018, 10:10 AM
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Piston to valve clearance can be overcome, so I wouldn't let it be a concern for part choices up front unless you're doing something atypical as lmsport explained. It will take a BIG duration and fast ramp rate before this becomes a concern with with 'shelf' parts and the cam installed within the cam card specs for advance. Obviously, advancing the cam (timing events)an extreme amount can cause issues as well.
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Old 12-09-2018, 12:17 PM
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Quote:
Originally Posted by 64nailhead View Post
Piston to valve clearance can be overcome, so I wouldn't let it be a concern for part choices up front unless you're doing something atypical as lmsport explained. It will take a BIG duration and fast ramp rate before this becomes a concern with with 'shelf' parts and the cam installed within the cam card specs for advance. Obviously, advancing the cam (timing events)an extreme amount can cause issues as well.
I'm aware that pistons can be cut if needed but what are some of the other ways that p to v clearance can be overcome?
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Old 12-09-2018, 01:03 PM
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Quote:
Originally Posted by Fang16 View Post
I'm aware that pistons can be cut if needed but what are some of the other ways that p to v clearance can be overcome?
Buying performance oriented pistons, rather than a more stock replacement type, as they will have bigger and deeper valve reliefs. Forged pistons are most likely to have bigger valve reliefs....if using hypereutectics, the Keith Black Signature series has very large valve reliefs

Retarding the cam will increase intake valve clearance, advancing it the opposite...the exact reverse is true for the exhaust valve.

In building SBC's, I've had to re-cut intake valve reliefs just once on a standard 23 off the shelf parts build, but it was a modified build and not a straight stock parts build. It was 5.7" rods in place of 5.565" rods in a 400 SBC, using stock replacement style TRW forged dish pistons of 1.560" compression height....the dish is .083" deep, with just a hint of valve eyebrows....we used a minimal .006-.007" deck cut to get things flat, an .053" thick head gasket, and then cut the piston from a dish to a near zero deck flat top to allow it to fit on the longer 5.7" rod.
The figure that sticks in my head is .017" was cut in a lathe from the actual bottom of the original dish, along with the entire .083" thick rim around the dish, and ended up with a flat piston at .015" above zero deck(.053" gasket, remember) for a nice .038" quench...but had minimal valve notch left, just a tiny little eyebrow that was originally in the bottom of the dish....since the piston was pushed up ..135" by the longer rod, I had to deepen the intake valve notch(exhaust still cleared by plenty). It was still a more radial clearance problem around a 2.055" intake valve more so than the notch not being deep enough.
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Old 12-09-2018, 02:02 PM
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Quote:
Originally Posted by ericnova72 View Post
Buying performance oriented pistons, rather than a more stock replacement type, as they will have bigger and deeper valve reliefs. Forged pistons are most likely to have bigger valve reliefs....if using hypereutectics, the Keith Black Signature series has very large valve reliefs

Retarding the cam will increase intake valve clearance, advancing it the opposite...the exact reverse is true for the exhaust valve.

In building SBC's, I've had to re-cut intake valve reliefs just once on a standard 23 off the shelf parts build, but it was a modified build and not a straight stock parts build. It was 5.7" rods in place of 5.565" rods in a 400 SBC, using stock replacement style TRW forged dish pistons of 1.560" compression height....the dish is .083" deep, with just a hint of valve eyebrows....we used a minimal .006-.007" deck cut to get things flat, an .053" thick head gasket, and then cut the piston from a dish to a near zero deck flat top to allow it to fit on the longer 5.7" rod.
The figure that sticks in my head is .017" was cut in a lathe from the actual bottom of the original dish, along with the entire .083" thick rim around the dish, and ended up with a flat piston at .015" above zero deck(.053" gasket, remember) for a nice .038" quench...but had minimal valve notch left, just a tiny little eyebrow that was originally in the bottom of the dish....since the piston was pushed up ..135" by the longer rod, I had to deepen the intake valve notch(exhaust still cleared by plenty). It was still a more radial clearance problem around a 2.055" intake valve more so than the notch not being deep enough.
What was the result of all this neat engineering? What was the engine intended for?
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Old 12-09-2018, 02:25 PM
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Quote:
Originally Posted by Fang16 View Post
What was the result of all this neat engineering? What was the engine intended for?
This was a street/strip 406 build, solid lifter cam, Dart 215 IE heads.
Started as a budget build, wanted a forged flat top but none were offered at the time (1992)for a 400 unless you went to JE or Ross or Wiseco, something expensive like that....so the stock replacement TRW forged dished for a 5.565" rod got cut to be a flat top for 5.7" rods. ARP bolted stock 350 rods, Elgin solid lifter cam, stock 400 crank, 4-bolt 400 block ...Torker II(Yuck!) manifold due to hood clearance.750 Holley DP.
This piston idea came from a couple of Joe Sherman magazine article builds and a build of his in the back of one of the Chevrolet Performance books you see sold in speed shops....he showed using a TRW 350 piston cut down in a 5.7" rod 383, I thought "why not do the same in a 406?". Once the short block was basically ready for assembly, one of the owners "Fuelie" heads was found to be cracked and the Darts were chosen as an alternative. So much for "budget" at that point, Ha Ha

Engine went into a '74 Z28 with a 4 gear Muncie and 3.73 gears, was about 450-460 HP or so and really moved well. Never made it to a track before the owner sold it to his son-in-law and I lost track of it from there....but it was a real scorcher on the street.
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