Measuring P to V vs valve drop @ TDC
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Measuring P to V vs valve drop @ TDC The whole point of measuring P to V is to ensure that the piston doesn't hit the valves. I would think that the easiest way to do this would be to just check valve drop @ TDC. Bring the piston to TDC, then take off the valvesprings and measure the amount of drop until it hits the piston. If it has more than .070 INT & .090 EXH you should be fine since this would be the absolute closest the valves would ever get to the piston. I realize that this wouldn't give you actual piston to valve clearance when the motor is running (it would be more) but if you have enough clearance with just checking valve drop then you should have enough clearance when the motor is running.
Am I wrong?
Am I wrong?
If you have to use Plastiguage to measure PTV clearance it would be best not to ever run the engine if clearances are that close, LOL. Plastigauge is used to measure bearing clearances. You would some about the size AAA battery to measure PTV clearance. 
Last edited by Steve H.; Feb 14, 2005 at 02:09 PM.
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From: W.P.B. Florida
Guys, your missing the point. Yes, when cam timing/valve events are taken into consideration the 8* before and after figure is correct. But when you take valve timing out of it, the piston is closest to the valves at TDC simply because it's closest to the head at TDC. So if you measure the valve drop at TDC and have enough clearance then you should be OK. What I don't know is will you ever have enough valve drop @TDC to know if it's safe.
Example; For a .560 lift cam you would need .650 valve drop (.560 + .090) on the exhaust and .630 (.560 + .070) on the intake to know for sure that your safe. So my question is has anybody measured valve drop @ TDC and gotten anywhere near those figures?
Plastigauge for P to V???
Example; For a .560 lift cam you would need .650 valve drop (.560 + .090) on the exhaust and .630 (.560 + .070) on the intake to know for sure that your safe. So my question is has anybody measured valve drop @ TDC and gotten anywhere near those figures?
Plastigauge for P to V???
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I dont care who disagree's with this..but clay it.Oil the clay and run a light test spring..rotate the motor 2 or 3 times..remove the head and measure the clay..simple and very accurate due to actual engine running simulation at a very low rpm point (30-40rpm) It will take into account before and after tdc timing as well..just use a thick tough clay so it will not distort
Carter
Carter
I would have to agree that the piston would never get any closer to the valve than at TDC. The point I think most are thinking is that the cam actually pushes the valve out further at 8* before and after tdc, but if you take the timing out like aftica said and take in account the total lift of your camshaft it should be a safe bet that you will have clearance. It is a safe way to do it just not a custom way to do it, most want as close as they can get safely, either to get more compression or higher lift out of a cam, so the clay on the piston would still be better imho.
Nothing wrong with claying at all. I think piston drop done the right way is a little more accurate though.
Valves are not fully open at TDC, so why measure only TDC? You should do it in several spots like stated, at least 10* before and after, because this will be your actual clearance of your actual cam (which will vary depending on ICL and advance ground in) not just TDC where your valves won't be fully open. The Comp degree kit comes with springs you use in place of you valve springs so after your valves are fully open, you just push them down to the piston and measure the PTV.
Valves are not fully open at TDC, so why measure only TDC? You should do it in several spots like stated, at least 10* before and after, because this will be your actual clearance of your actual cam (which will vary depending on ICL and advance ground in) not just TDC where your valves won't be fully open. The Comp degree kit comes with springs you use in place of you valve springs so after your valves are fully open, you just push them down to the piston and measure the PTV.
The piston is not closest to the valves at TDC. Like everyone else is saying, the valves open and close much faster than the piston moves. Take this for example, the exhaust valve is open and the piston starts screaming tward it. Since the lobes of a cam are graduated, (Ramp Rate) the valve takes time to get moving. So as the piston is moving at a constant velocity tward the valve, the valve is accelerating away from the piston. The point in time where the valve is closest to the piston depends on your duration, lift, and timing of valve events. It has nothing to do with TDC. If you measure your clearance at TDC, and then rotate the motor back about 8-10 degrees, you will find that the piston and valve are actually closer at that point in time.
Originally Posted by Lvmyz28
The piston is not closest to the valves at TDC. Like everyone else is saying, the valves open and close much faster than the piston moves. Take this for example, the exhaust valve is open and the piston starts screaming tward it. Since the lobes of a cam are graduated, (Ramp Rate) the valve takes time to get moving. So as the piston is moving at a constant velocity tward the valve, the valve is accelerating away from the piston. The point in time where the valve is closest to the piston depends on your duration, lift, and timing of valve events. It has nothing to do with TDC. If you measure your clearance at TDC, and then rotate the motor back about 8-10 degrees, you will find that the piston and valve are actually closer at that point in time.
what he is saying is take all of the timing, and everything out of the equation. put at tdc and then determine the furthest the valve will EVER be out and then calculate how much clearance you would have. not a great way to do it but it would work. Do you get what he is trying to say now?
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Originally Posted by Blue02Ws6
what he is saying is take all of the timing, and everything out of the equation. put at tdc and then determine the furthest the valve will EVER be out and then calculate how much clearance you would have. not a great way to do it but it would work. Do you get what he is trying to say now?
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From: W.P.B. Florida
Originally Posted by Blue02Ws6
what he is saying is take all of the timing, and everything out of the equation. put at tdc and then determine the furthest the valve will EVER be out and then calculate how much clearance you would have. not a great way to do it but it would work. Do you get what he is trying to say now?
I know my small cam will clear the heads I'm installing. (Shaved .020) What I want to do is measure something that won't change if I put another cam in it. Cam timing will change with a new cam later on, valve drop at TDC won't, so I wanted a constant so that I wouldn't have to pull the heads to check P to V after the new cam. Some people have stated you need x amount of valve drop @ TDC to have enough clearance for x cam. What good would claying (thus using current cam timing) do me if I already know my current cam will clear? Later when I go to a sponser and say I have .xxx clearance with my B1 cam, will your 230/230 cam (example only) clear? They wouldn't be able to answer that because valve events will probably be different. However if I give them valve drop figure @TDC they may be able to tell me if it will clear because that figure won't change with just a cam swap.
Yeah, I was not thinking lastnight, god I hate birthdays. This thread got over complicated way too much. The problem with what he wants to do is to measure full lift at tdc and make sure there is clearance. If you did this the stock cam would even be too big measuring like this. That is why people can even change their valves by the tdc method because there is so little clearence. I guess I was jsut trying to buy into his agruement which seemed like a smart idea until you think about it...
There is no way arround doing the clay check method to make sure you have enough p/v clearance.
There is no way arround doing the clay check method to make sure you have enough p/v clearance. 
You cannot do it via the valve drop method -- even if you were to take readings at 8-10 degrees before and after TDC because you need to know how much lift your cam has at those points. Different cams have different ramp rates/lift characteristics. One cam may only have .100 lift at 10* and another one may have .300 lift at 10*. TDC has nothing to do with clearnace issues unless you tried to do this crazy checking method and bent all your vavles allready.
you need to know the TDC lift of the cam
Then the drop method works fine.
typical drop on a stock car is 200 thou. If you have a cam with TDC lift of .080, X 1.7= 136 thou.
Minus the 200 drop= 64 thou of theoretical clearance.
Then the drop method works fine.
typical drop on a stock car is 200 thou. If you have a cam with TDC lift of .080, X 1.7= 136 thou.
Minus the 200 drop= 64 thou of theoretical clearance.
PTV needs to be check on the intake side between 8 and 12 degrees ATDC and the exhaust side between 8 and 12 degrees BTDC.
By the way, these clearance are with a Texas Speed MS3, Heads are milled .020 and Diamond pistons with -2 cc valve reliefs.