How tight is your PTV?
How tight is your PTV? Ive heard the Trex cam with unmodified heads and stock gaskets leaves .020 on the intake and .030 on the exhaust which is way tighter than what ive read clearance should be at minimally which is .080 intake .100 exhaust. ive done alot of searching and seen pretty much everything in between. I know when i bought my first cam from TSP which was a 233/239 .600 .600 on a 113 i was told to expect .060-.070 with my 62cc 243s and stock gaskets. this leaves one a little confused what to shoot for, it almost seems as long as there is some clearance its good to go. im going to clay my motor once i have heads for it im just really unsure how far to push it with my max effort kind of thinking. I want to take every last thousandth off the new pair of 243s im getting but where do i stop? Feel free to post your setup and ptv measurements, cam specs, gasket size, machine work to the heads, etc
Even on a motor that is built to exacting tolerances and has amazing valvetrain control, things will expand, flex, stretch, etc. You have to make sure you have clearance at the rpm limit, not just when you have the spring checkers and dial indicator rigged up and turning it by hand.
Add the potential of being on the edge of loosing valvetrain control at high rpm, wear creating larger tolerances, etc... and perhaps the catastrophic occurance of a mis-shift and over-rev...
you want the most clearance you can get.
I mean, you can run less than .080/.100 ... many knowledgable folks have done it, but they really knew what they were doing.
If you're willing to really crunch some numbers to see if you can get away with it, go for it.
I would play it safe and flycut.
Add the potential of being on the edge of loosing valvetrain control at high rpm, wear creating larger tolerances, etc... and perhaps the catastrophic occurance of a mis-shift and over-rev...
you want the most clearance you can get.
I mean, you can run less than .080/.100 ... many knowledgable folks have done it, but they really knew what they were doing.
If you're willing to really crunch some numbers to see if you can get away with it, go for it.
I would play it safe and flycut.
Ive heard the Trex cam with unmodified heads and stock gaskets leaves .020 on the intake and .030 on the exhaust which is way tighter than what ive read clearance should be at minimally which is .080 intake .100 exhaust. ive done alot of searching and seen pretty much everything in between. I know when i bought my first cam from TSP which was a 233/239 .600 .600 on a 113 i was told to expect .060-.070 with my 62cc 243s and stock gaskets. this leaves one a little confused what to shoot for, it almost seems as long as there is some clearance its good to go. im going to clay my motor once i have heads for it im just really unsure how far to push it with my max effort kind of thinking. I want to take every last thousandth off the new pair of 243s im getting but where do i stop? Feel free to post your setup and ptv measurements, cam specs, gasket size, machine work to the heads, etc
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Summit recommends a minimum of .060 intake and .090 exhaust (checked with head gasket in place). The extra on the exhaust is helpful when the engine encounters mechanical over-rev from a bad downshift.
FWIW - I've known of professional racing engine builders will go .000 on intake on a cold motor because when the running the engine expands creating some small amount of P to V in true max effort builds.
My 99 T/A h&c set up had IIRC .070 on the intake & .090 exhaust with a 224/224 xer cam on a true 113 LSA. The heads had been decked and the valve job was sunk for better flow etc. That was less than expected for the cam in question. Ran great was pulled for a 416. My personal comfort limit is .070 & .090. My other set ups had good p to v with custom pistons plus heads didn't have a sunk valve job etc .090 + etc
My 99 T/A h&c set up had IIRC .070 on the intake & .090 exhaust with a 224/224 xer cam on a true 113 LSA. The heads had been decked and the valve job was sunk for better flow etc. That was less than expected for the cam in question. Ran great was pulled for a 416. My personal comfort limit is .070 & .090. My other set ups had good p to v with custom pistons plus heads didn't have a sunk valve job etc .090 + etc
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We know that minimum piston to head is about .034 with an iron block. The block does grow away from crank C/L with heat, but so does the crank and rods and pistons. Leading to that .034 piston to head number when the carbon is basically scraped off. This can give you a guide to minimum piston to valve.
We know that minimum piston to head is about .034 with an iron block. The block does grow away from crank C/L with heat, but so does the crank and rods and pistons. Leading to that .034 piston to head number when the carbon is basically scraped off. This can give you a guide to minimum piston to valve.
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The aluminum blocks grow another .010 or so, but we don't know the rate of growth of the crank/rod/piston in relation. Potentially an engine that was pre-warmed every time before starting could get away with less piston to head and piston to valve clearance, but that's not a reality for most of us. So we stick with the .060/.090 number in general.
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Originally Posted by Summitracing
Summit recommends a minimum of .060 intake and .090 exhaust (checked with head gasket in place). The extra on the exhaust is helpful when the engine encounters mechanical over-rev from a bad downshift.
Originally Posted by Summitracing
The aluminum blocks grow another .010 or so, but we don't know the rate of growth of the crank/rod/piston in relation. Potentially an engine that was pre-warmed every time before starting could get away with less piston to head and piston to valve clearance, but that's not a reality for most of us. So we stick with the .060/.090 number in general.
I've never understood how you can bend a intake valve unless you drop it or have a catastrophic failure, because the piston is either at TDC or JUST before TDC when it opens.
Wouldn't the intake valve issue be based on cam timing?
I know from reading on here for years you definitely know cams, and your stuff.
I am a little curious on this, as my experience is opposite. The piston is always chasing the exhaust valve, and with valve float, back in my 5.0 days with a E or X cam and milling the heads, I seen it happen several times.
I've never understood how you can bend a intake valve unless you drop it or have a catastrophic failure, because the piston is either at TDC or JUST before TDC when it opens.
Wouldn't the intake valve issue be based on cam timing?
I know from reading on here for years you definitely know cams, and your stuff.
I've never understood how you can bend a intake valve unless you drop it or have a catastrophic failure, because the piston is either at TDC or JUST before TDC when it opens.
Wouldn't the intake valve issue be based on cam timing?
I know from reading on here for years you definitely know cams, and your stuff.
A third consideration is assymtrical lobes. As many cams now are. The valves are faster off the seat and slower back on the seat. So at TDC, the intake valve is chasing the piston.
I guess the short answer to your question is yes, it is based on cam timing and valve size, but most cams are such that the intake valve is closer to the piston to begin with. So less needs to go wrong for them to contact.
That said, I know very little about the 5.0. If that head is set up with the exhaust valves closer to the pistons than the intake valves, or the angles favor the intake valves, etc. I can think of lots of ways a head could be done that would favor hitting the exhaust first. Most of what I posted is SBC / LS experience