Yellow Bullet Forums banner

1 - 14 of 14 Posts

·
Registered
Joined
·
4,135 Posts
Discussion Starter #1
Anybody know the valve drop measurements on 243/799 heads. I have seen .155" for the intake said in a few places, but not the exhaust drop.
 

·
Registered
Joined
·
4,135 Posts
Discussion Starter #3
Hmm. I was under the impression that the exhaust had about .010" more drop. Anyone else measure theirs?
 

·
Registered
Joined
·
4,135 Posts
Discussion Starter #6
Yep. That is what I am finding. Most EFI guys don't measure or make note of the exhaust valve drop. I measured my PRC stage I Ls6 heads and it was .194" on the intake and .206" on the exhaust on an LQ9 but, of course those aren't stock. I know that the exhaust valve drop is often not important to the EFI guys, but for the carbed and single plane guys, a later EVC can be an opportunity to add some beneficial overlap.
 

·
Registered
Joined
·
1,603 Posts
What is valve drop and how is it measured?
Put the intake and exhaust valves in one cylinder without springs or retainers.

Put the head deck side on a flat surface, with a caliper measure from the top of the valve guide or seal to the valve tip, then measure again with the valve closed. The difference is the valve drop.

In some cases the valve head will be proud of the head deck surface, keep the deck surface up, and use a block deck bridge with a dial indicator in center position to measure the distance.
 

·
Registered
Joined
·
4,795 Posts
Put the intake and exhaust valves in one cylinder without springs or retainers.

Put the head deck side on a flat surface, with a caliper measure from the top of the valve guide or seal to the valve tip, then measure again with the valve closed. The difference is the valve drop.

In some cases the valve head will be proud of the head deck surface, keep the deck surface up, and use a block deck bridge with a dial indicator in center position to measure the distance.
Thank you
 

·
Registered
Joined
·
4,135 Posts
Discussion Starter #12 (Edited)
...and what is the purpose of such a measurement?
It gives you the ability to calculate the piston-to-valve clearance for a particular engine combination and camshaft so that you can order a camshaft that will fit without additional fly-cutting.

Piston To Valve Clearance Practical Limits and Calculations

So, now that we know the reasons why there are extra piston to valve clearance considerations in creating an proper camshaft for a single plain equipped LS engine with stock pistons, lets look at how we measure and estimate piston to valve clearances.

Valve Drop
The object of measuring valve drop is to determine how far a valve can open before it touches the piston while that piston is at top dead center in the cylinder. Knowing this figure is the only way one can estimate piston-to-valve clearance without actually measuring an assembled engine. Furthermore, knowing this allows engine builders to determine which camshafts might fit in a given engine without further modification. There are two ways to measure valve drop. Actual valve drop (assembled) and estimated valve drop (not assembled).

Actual (assembled) Valve Drop
The most fool proof way is to measure valve drop in an assembled engine. Just rotate the crank until the the piston is at top dead center and then lower the valve until it touches the piston. Whatever that distance is, that is your valve drop. Do this for both intake and exhaust valves as they are most often different.

Estimated (unassembled) Valve Drop
You can also estimate valve drop while the cylinder head is not on the engine. For this start by placing the cylinder head on a flat surface and lower the valve until it touches the surface in which the head rests. This measurement of how far the valve "drops" is your cylinder head valve drop. Next measure your piston to deck height relationship. Most LS engines have a positive deck(between .005" to .008" is most often reported by builders). Next measure the thickness of your head gasket. The formula looks something like this: Cylinder head valve drop + head gasket thickness - positive deck height = valve drop. this is very accurate for an engine equipped with flat top pistons. Here is an example:
.155" + .051" - .007" = .199"
Again, do this for both intake and exhaust valves as they are most often different. Also, engines with dome pistons or dished pistons might better be done assembled as it is difficult to account for the different piston surface shapes using this method.

Cam Lobe Profiles, Ramp Rates and Lift
Another consideration in piston-to-valve clearance is lobe profiles and lift. One misconception that a lot of new builders have is that camshaft lift determines piston to valve clearance. While it is a factor, it is much less of a factor than the timing of the valve events. Consider this: A 5.7 liter LS1 has a stroke of 3.622". Your typical camshaft has a valve lift of .600". So, if the piston is at bottom dead center, the valve is still a full 3 inches way from the piston even and full open lift. However, when at top dead center, the piston is only .200" away from a fully closed valve. So, if that valve were to open full at top dead center, they would collide when the valve had merely reached 1/3 of its full lift. As you can see, valve timing is the main factor in piston to valve clearance in your LS.

With the above said, total lift is part of the equation in how fast a camshaft will open the engines valves. So, if the piston is leaving top dead center on the intake stroke and the intake valve is accelerating faster than the piston as it departs top dead center, as it often does at around 10 degrees ATDC, then the aggressiveness of the lobe can reduce piston-to-valve clearance. So, don't assume a different lobe profile will clear just because it has the same duration and lobe center angle.

Estimating Valve Clearance
Knowing your valve drop as measured by the 2 methods above is a key first step in estimating if a cam you wish to purchase will fit in your engine. The next part of the process is figuring your likely piston to valve clearance. There are two methods for this below: estimated TDC valve clearance and estimated actual valve clearance. Estimating your actual valve clearance is the most accurate way of predicting if a particular camshaft profile will fit or need additional piston flycutting.

How To Estimate Valve Clearance At TDC For A Given Camshaft
This is the process of estimating your valve clearance while the pistons is at top dead center. This is a great method for quickly estimating if your acceptable valve clearance is "in the neighborhood". This practice is an easy and great place to start because the cam suppliers can tell you the lobe lift of their profiles at various points. Comp Cams has these figure listed in their catalog. Here is an example of an LSL cam lobe:
Advertised Duration: 281
.050" Duration: 231
.200" Duration: 154
Total Lobe Lift: .363"
Lobe Lift @ TDC with 106 degree centerline: .081"
Lobe Lift @ TDC with 110 degree centerline: .067"

Lift with 1.7 Ratio Rocker: .617"​
In this example, you can see that if your intake lobe is on a 106 degree centerline, your intake lobe lift will be .081" at that point. So .081" x 1.7:1 rocker ratio yields .1377" or rounded to .138" valve lift. If your intake valve drop is .199" then your piston to valve clearance at TDC would be: .061". But, that is probably not your ACTUAL piston-to-valve clearance.
Your actual piston to valve clearance will likely not be at top dead center, but closer to 7-10 degrees ATDC for the intake and 7-10 degrees BTDC for the exhaust. So, be ready for your actual piston to valve clearance to be less.​
How To Estimate Your Actual Valve Clearance With A Given Camshaft
As you read above, your actual closest piston-to-valve clearance is not at top dead center. It might be between 5 and 15 degrees before or after top dead center. This is caused by the differences in speed between the valves open and close rates vs. the piston speed as it approaches and departs from top dead center. This complex relationship is difficult to visualize and even more complicated to plot easily. Fortunately, computers can do this job for us.
I have been trying out this great Excel based Piston to Valve clearance calculator. It appears to work properly. Give it a try and tell us what you think:
Piston To Valve Clearance Calculator
In the example above where the piston-to-valve clearance was .061" at TDC, the piston-to-valve clearance calculator predicts the actual estimated valve clearance to be .038" at 9 degrees after top dead center. So, you can see how if you only do the TDC calculations, you will likely be underestimating your clearance by a significant amount. In this case .023". While some might feel comfortable with .061" intake piston-to-valve clearance, not many are willing to brave .038" clearance.

How much is enough? Ultimately, that is up to you. The oft quoted old standard is .080" for the intake valve and .100" for the exhaust, but that does not seem to be the norm in the performance LS world. Several LS performance builders and cam vendors that I talk to more often quote between .040" - .060" intake and .060" - .090" exhaust in a well set up valve train system. Others push this even further. With that said, the ultimate responsibility is the vehicle owner's as he is the one who will be cleaning up the carnage and paying for the repairs.
 
1 - 14 of 14 Posts
Top