Joel5.0

Yep......just a little detail using an example:

If you measure and set for PtV, and get .120" intake clearance and .115" clearance exhaust at the closes point with 1.6 rockers, you should have ~ .100" on the intake and .095" on the exhaust with 1.7 rockers....... a change in rocker ratio shouldn't cause a clearance loss of .115" - .120".

cheetha94607

ok so why do people wont higher duration, what benifit does that have.

OVERK|LL

OK, lets use the "normal minimums" for piston to valve clearance to help you understand why the difference in peak lift (which is where the ratio change has it's greatest effect) is negatable between the two.

1st point: Minimum piston to valve clearance is typically thought of to be at LEAST 0.080.

So, lets plot a valve's opening, remembering that, when piston to valve clearance issues occur, they are no where NEAR peak lift; Interference occurs when:

1. On the intake valve when the the valve is chasing the piston down the bore
2. On the exhaust valve when the piston is chasing the valve up the bore

Peak lift in either instance has occurred when the valve is no where NEAR the piston.

So, assuming we need 0.080 minimum of piston to valve clearance, lets say that with a cam that has a maximum lift of .500 at the valve with 1.6's is being run with 1.7's. The 1.6's give you 0.090 clearance on both valves.

Lift at cam w/1.6's w/1.7's
0.030 0.048 0.051
0.040 0.064 0.068
0.050 0.080 0.085
0.060 0.096 0.102
0.070 0.112 0.119
0.080 0.128 0.136
0.090 0.144 0.153
0.100 0.160 0.170
0.110 0.176 0.187
0.120 0.192 0.204
0.130 0.208 0.221
0.140 0.224 0.238
0.150 0.240 0.255
0.160 0.256 0.272
0.170 0.272 0.289
0.180 0.288 0.306
0.190 0.304 0.323
0.200 0.320 0.340
0.210 0.336 0.357
0.220 0.352 0.374
0.230 0.368 0.391
0.240 0.384 0.408
0.250 0.400 0.425
0.260 0.416 0.442
0.270 0.432 0.459
0.280 0.448 0.476
0.290 0.464 0.493
0.300 0.480 0.510
0.310 0.496 0.527
0.313 0.500 0.532


Now notice, in the above table, where the differences are the most significant.........


Since we are dealing with a RATIO here, the effects in the difference between the two are the largest near peak lift. As you come down from peak lift, the difference becomes less and less......

In fact, when the valve is open .160 with the 1.6's, it's only open .170 with the 1.7's......... That's only 10 thou!!!! And you are supposed to have at LEAST 0.080 for proper piston to valve clearance on an engine with the factory RPM range.......

Now, take into account variances in lifter pump-up state and this difference is negligible. I'm sure Joel could plot some duration figures to accompany those lift numbers (at the camshaft) and then you could see the difference between the two ratios at that lift point........

Basically, if it doesn't clear with 1.7's, it's not going to clear with 1.6's. And if it does clear, the amount of clearance is SO minuscule, that it's far from the safe range and something as simple as lifter pump-up could cause a piston to hit a valve and catastrophic failure to ensue.

This is why it is important to CHECK piston to valve clearance rather than arbitrarily assuming that it's "good to go" because nothing went into bind when you rolled the motor over by hand........

AdderMk2

I like this guy ^^^

w8less

wow that has to be a first

AdderMk2

yea, imagine that... a new member that actually knows something about cars!

w8less

they're from a certain website

5spd GT

Adder - That is as simple as opening up Excel and typing in a multiplication formula. Give it a try...

Type in different lift points along the lobe, then multiply it by 1.6, 1.7, 1.8, 1.9, etc...now you are getting into NASCAR's range.

Very easy. I learned my multiplication tables in 3rd grade.

r390g

So if i change springs and rockers on my stock heads I can go higher than 540? I need to go 565.