OK I was trying to get the correct pushrod length using the Miller Mid Lift Principle. This is with an L98 small block Chevy. The standard pushrod length for that motor is 7.200". I have Dart Pro One 200cc heads with valves .100" longer than standard and the heads have been milled about .030" and the block decked .015".

I made up a dial indicator to attach to the valve retainer so I could accurately measure the sweep of the Comp Cams Pro 1.6 Magnum rocker arm. Following the Miller Mid Lift Principle I was able to get the sweep of the rocker arm down to .030" on the valve stem which is pretty narrow. However in order to do this the pushrod had to be 7.485" long. I don't have any pictures but if you were to divide the top of the valve stem into thirds the center of the sweep would be 1/3 of the way from the exhaust side. Or another way to say it is that the sweep is on the exhaust edge of the middle third of the valve stem. Visually the sweep is quite narrow as I have not seen anything smaller in my limited experience and the pictures I have seen posted.

My engine builder who has never done a pushrod measurement that way says that is to far past center. I told him I thought it was OK but I would not like to see it any further. Also the valve lift at the valve is .560". This is with an the 218 XFI cam that has an advertised lift of .356" at the lobe but in actual measurement was a few thousanths less. So the real lift is not far off at all and in my mind quite close to what it should be.

So is the sweep ok being 1/3 from the edge of the exhast valve? Also anyone have a pushrod that was that much longer than stock? Any other comments welcome. I don't want to order the pushrods until I get some feedback.

 

I'm a fan of doing the pushrods to make the contact area as centerd as possible reguardless of width. That's going to load the valve stem the least amount.

Bret

 

I don't go for mid-lift. I go for the inflection point to be up around 2/3 lift. It results in more contact area on the valve tip, but generally a little closer to the intake side.

PLUS (here's the real benefit) most of the travel across the tip happens in the lowest 1/3 of the lift curve...... you know, where spring pressure is lowest. Up in the top 1/2 of the lift curve there is almost no movement of the roller across the valve tip (when spring pressures are highest).

When it doubt, go shorter. Little too short is almost always better than a little too long.

I just did a set of BBC heads from AFR and came up +.250 on the exhaust (exactly what AFR said they should be) but a massive +.400 on the intake to get proper rocker geometry. When I tried +.250 on the intake I could tell right away it was too short- the angle of the rocker just looked silly and a check of the contact point showed it almost falling off the intake side of the valve tip in the valve-closed position.

FYI- I'm sure others will disagree, but I've found that those cheap Moroso/Mr. Gasket plastic pushrod length checkers that you shove over the rocker stud are actually fairly dependable, beleive it or not. Usually get you within +-.050" reliably. I use one to get me "in the ballpark" and then I do my final tweaking from there. For someone who doesn't set up valvetrains on a regular basis I would have no reservations telling them to trust this simple/cheap tool.

 

Thanks for the replys. I do have one of those plastic pushrod checkers that slides over the rocker stud. Just for kicks I will see what it comes up with.

I have been doing a lot of research on the subject and there seems to basically be two schools of thought on the subject. One is to follow the mid-lift principle and get the narrowist scrub or sweep mark on the valve stem. With this method you let the mark fall where it may between +-.080 of the center of the 11/32 valve stem.

The other method that SStrokerAce uses is to get the sweep close to the center with the least amount of sweep. If I'm reading Damon correctly he uses a variation of this method.

When I go into the shop tomorrow I will have to evaluate everything. I have no pressure to get this thing done right away so I can take a day or two to look things over in detail.

 

I also do the "as close to center as possible and as narrow as possible". If ya do the mid lift by the book it will come out pretty close to the same.

 

When you shove on that plastic pushrod length checker your pushrods using the "mid-lift principle" will be too long. Usually by about .100-.150" or so.

Go ahead and try it. I already know roughly what you'll find. I've done this a few times.

You might be interested to know that factory stock rocker geometry is a lot closer to the 2/3 mark than to the 1/2 way point.

Regardless of what you decide to do in the end it's always good to try out several different methods and understand what each is doing to your valvetrain and why. Experimentation almost always makes you think about WHY you are doign things a certain way. Thinking is usually a good thing. But if you're dumb like me, thinking can get you into trouble real fast if I don't run it past people more knowlegable than myself before I put it into action.

 

With the Miller Mid Lift theory I would have wound up with a pushrod 7.485" long plus .015" for the lifter preload. Final length would have been 7.500" long amazingly. The sweep or scrub was exactly .030" and about 1/3 of the way from the end of the valve tip on the exhaust side.

Doing it your way I wound up with a pushrod 7.350" plus .015" for the lifter preload and a total length of 7.365". The scrub or sweep was just a tad over .040" and the middle of the sweep was just a little past center toward the exhaust side. The valve lift amazingly was exactly .570" which was as advertised from Comp Cams for the 218XFI. This was with the 918 beehive springs installed and using the Comp Cams adjustable pushrod no less.

I figured that was also close to what Bret and 1Racerdude recommended. Decided to call it good and ordered from Terry Manton #404 7.365" pushrods with 3/8 diameter and .083" wall. Also wound up with a spring bind clearance ranging from .050" to .060". Hard to get closer with .015" shims.

This motor buildup has taken a lot of unexpected zig zags. Like has been said many times you have to measure and check everything. If I had just assembled the motor with the bare minimum of checking I'm sure it would have turned out OK but I can guarantee I would have been down on power from what it would have been and not have my .036" squish.

Two things that come to mind are the compression and cam degreeing. If I had not checked everything I can tell you I would have been at least a 1/2 point down on compression if not more. Also Comp Cams recommended to install the cam at a 109 intake centerline. If I had installed it straight up without checking it would have been on a 106 intake centerline.

I had originally wanted to intall the cam on a 112 ICL but Bret recommended against it. I wound up installing it on a 110 intake centerline to give me a tad more top end and it was easy to do with the 4 degree retard slot. Thanks to all for their advice and help.

 

Sounds good. Amazing what you can learn about your motor when you spend some time actually checking things instead of just reading specs and screwing it together. I think you made a wise choice on the pushrods. If my suggestions put final pushrod length close to what Bret and 1racerdue suggested in the first place then I guess I can live with that.

Totally understood about actual compression vs. what you THINK you have using specs or "estimators." True head gasket volume is one of the biggest sources of errors in most people's calculation, followed by actual piston deck height vs. spec.

 

Just did a check on the compression by using the parts I was going to use. The Dart heads were advertised as 64cc and so marked on the heads. In fact they were over 69cc. The pistons I was going to use had a 6cc valve clearance. So if I had just bolted on the heads and used the pistons I was originally going to use without checking I would have wound up with a compression ratio of 9.83:1. With the corrected parts I wound up with a compression ratio of 10.75:1 and a DCR of 8.52:1.

The specs on the cam lift sweep turned out this way. At mid lift the sweep was about .028". The rest of the sweep was about .013". Damon is this not close to what you were talking about?

 

Yeah, me, Bret and 1racerdude all came to roughly the same conclusion on puhshrod length, but by different methods. What you're seeing lines up with what I would expect you would see.

 

Good work '89.

Sweating the details is what seperates the fast from the real fast.

BTW, I'm using 7.350 Trick Flows and Crower .050 backset rockers for a real nice sweep on my +.100 valves and plenty of retainer clearance.

Dean

 

Deleted

 

Damn!

What intake you running?

 

Highly modified TPI. Here is the link for more information for those interested.
http://www.thirdgen.org/techboard/tp...-tpi-dyno.html

I want to say something further. Bret Bauer, his father and others on this board have given out some free advice. It is very good advice. We(myself and my engine builder) followed it with my valve train components and other parts of my build over the last year. When they say every part of the valve train is a spring they are not kidding. You need to use the stiffest parts you can buy. I know there are personalities involved but please look past that.