Questions about David Vizard 128 Rule and Dyno Tests

[rpetrowsky]

I'm a new member here and am trying to fill in my knowledge gap on something that seems overlooked when discussing David Vizard's "128 rule" and overlap. I apologize as I realize this topic has been discussed ad nauseum. Unfortunately, all of the dyno tests I find show simply tightening the LSA while maintaining the same duration. This results in an overall increase in torque, for some obvious reasons. However, in his, books, David Vizard talks about how those wider LSA cams have too long of a duration. To actually test his theory would require treating overlap and IVC as a fixed event and adjusting duration with the change in LSA. The tests treat duration as a fixed event and adjust overlap and IVC. Therefore, the LSA dyno tests aren't really showing anything about his theory.

Is there any test out there to show dyno tests of camshafts with identical overlap and IVC closing points that would validate David Vizard's theories? Doing this type of test would mean the narrower LSA was ground with less duration and advance that the wider LSA.

I'm in the process of planning an engine build and am greatly interested in the results of such a test before I order a cam. One cam designer has come said 285/289 on 113.5 LSA for a 408 Windsor with Trick Flow 11r 190 heads and about 10.5:1 compression. This gives 60 degrees overlap. Using the David Vizard formula gives me closer to 273/277 on 105 LSA and 65 degrees overlap. There is obviously a world of difference between these two cams. I would really like to see how a dyno test of his formula keeping overlap similar plays out before making up my mind.

Thanks!
Robert

[F-BIRD'88]

The David Vizard formula does not give you the overlap.
It givs you the LSA..
The overlap has to be chosen to feed the intended engine application. purpose and required rpm range.
From these you get the duration and can then calc the IVC event.. You can tweek that by adv/retard on install.
DV does give a rough overlap application pie chart as suggestion.
Yes DV does say that most people want way too much duration, thus cam catalog overlap is kept in check with wider LSA typical on catalog cams.
How have you determined , selected the overlap for your cam? What makes you think your choice is the right amount?

[F-BIRD'88]

DV gives a formula for in line wedge heads for sbc and another for SBF windsor.. He says nothing about twisted wedge hesds nor other valve abgle heads for the SBF windsor engine. Where how did you calc 105°LSA for this head on a 408.?

[rpetrowsky]

I'm considering the overlap chart as part of the David Vizard formula. I'm looking at the right side of the chart given that I have a 408 small block. I'm selecting about halfway between section 2 for mid range and section 3 for street performance. This gives me something a few degrees either side of 62.5 degrees. Erring to the street performance side gives me 65. Assuming the chart is correct, this should give me a streetable cam.

You make a good point on the IVC. I am assuming keeping it fixed in the test. You could also install everything on the same advance and test it. My thinking is that keeping the same IVC will keep the dynamic compression the same throughout the test. The smaller the cam, the more retarded it would end up. I would expect the final appropriate centerline would be somewhere between.

You are also right that there isn't a formula for the overlap on the twisted wedge head. I know his formulas have applied across the board with better flowing heads and different valve angles and that he varies the starting number to subtract from. I think I have seen somewhere that LS heads use a 132 formula, or something similar, that results in a wider LSA. Perhaps the twisted wedge heads need a wider LSA too. Either way, I would like to see a verification of the formula based on my original post.

[Tom68]

If you test with a fixed ivc you are simply testing how much overlap your motor likes.
What's the motor getting used for, mostly street ? Converter ? Vehicle weight ? Rear gear ? Compression ? Full exhaust ? Idle desired ? Intake runner length ?
These are all the things that you use to determine what overlap will suit.
Unless you want to use the formula then tailor the vehicle to it, that'd be an odd way to do a build.

[F-BIRD'88]

Are you trying to make best power/torque or get a warm fuzzy feeling? The (WOT) powerfulll cam will give up fuzzy feel "drivability" and idle quality.
Forget the fixed IVC point. Forget "dynamic compression"
Forget another cam company..

Wether the best cam is single pattern or a dual pattern cam will also tweek things like IVC and insyallwd position.
-Drivability" is very subjective. Thats why cam companies tend to keep the LSA wide (er) knowing most buyers will tend to also want more duration choice VS less, most of the time.
The catalog is to sell you a cam. VS get you the most powerfull cam for the job.

[rpetrowsky]

The engine will be mostly street. It is going in a 67 Cougar and I expect it to weigh around 3,200-3,300 pounds. I'm still planning and the pistons and head combos I'm looking at will give me either 10.1 or 10.6:1. It will be with 1 3/4 full length headers and 2.5" dual exhaust. I would like an idle around 800-900 rpm, or less. I don't know the intake runner length, but believe I am limited to a Performer RPM due to hood clearance issues. Because I'm building the car, I still get to choose a few of the other things. I can set the converter and gear ratio where I want. I would prefer a slightly lower cruise RPM as I'm putting in a 4r70w and want to take advantage of the overdrive. This is why I chose an overlap between the two different sections.

[F-BIRD'88]

All the complaints on all the "Thumpr cams" is evidence how most hot rod cam buyers feel. Especially if with limited engine tune skills.

[F-BIRD'88]

Cruising in overdrive wants a very mild duration and overlap. DV's 128 formula is for getting best WOT torque/power. Not "cruising in overdrive" nor idleing.

[rpetrowsky]

Are you trying to make best power/torque or get a warm fuzzy feeling? The (WOT) powerfulll cam will give up fuzzy feel "drivability" and idle quality.
Forget the fixed IVC point. Forget "dynamic compression"
Forget another cam company..

Wether the best cam is single pattern or a dual pattern cam will also tweek things like IVC and insyallwd position.
-Drivability" is very subjective. Thats why cam companies tend to keep the LSA wide (er) knowing most buyers will tend to also want more duration choice VS less, most of the time.
The catalog is to sell you a cam. VS get you the most powerfull cam for the job.

You've nailed the big problem on the head. Cam companies want to sell a cam and don't want people to be mad that the 290 cam they put in a 302 won't idle so they compromise on specs. That is really why I'm interested in a real world comparison of David Vizard's theory, so that I can see how small of a cam I can use to keep my torque up (though I doubt low end torque will be an issue on a 408) while having enough RPM range to make it fun to drive.

I would ultimately like the best possible power and torque while still having a cruising rpm range in the low to mid 2,000s RPM. I don't need or want something that will rev to the moon because it will have the stock 351w mains and a 4.000 stroke. I want the short block to have some longevity. I also don't want something that drives like it has a truck engine.

Doesn't the overlap chart, combined with the 128 formula, give the best WOT torque and power while still considering the RPM range for the given section of the overlap chart? It seems as if the formula applies as equally to a low RPM stump puller as it does a circle track engine.

[Stan Weiss]

What RPM do you want peak HP @? What is the intake valve size for your heads and what do they flow @ 0.600"?

[rpetrowsky]

I'd like peak HP between 5,500 RPM and 6,000. The valves are 1.055 and flow around 304 CFM @ 0.600" according to the manufacturer.

[rpetrowsky]

Somehow, this has turned into my cam choice rather than validation of the David Vizard formula. I have already given a lot of specifics to the cam company and I trust their choice will meet my requirements. I really am curious, as I'm sure a lot of other people are, how a wide LSA cam with longer duration compares to a narrow LSA cam with shorter duration with overlap being equal.

The original question is would a smaller cam on tighter LSA give the same results as predicted by the 128 formula and are there any dyno tests to show this? All I can find are the same duration cams tested on different LSA.

[digger]

Somehow, this has turned into my cam choice rather than validation of the David Vizard formula. I have already given a lot of specifics to the cam company and I trust their choice will meet my requirements. I really am curious, as I'm sure a lot of other people are, how a wide LSA cam with longer duration compares to a narrow LSA cam with shorter duration with overlap being equal.

The original question is would a smaller cam on tighter LSA give the same results as predicted by the 128 formula and are there any dyno tests to show this? All I can find are the same duration cams tested on different LSA.

Generalising alot, for the same overlap then if assume EVC and IVO are fixed so we are actually testing IVC and EVO

Normally the later EVO and earlier the IVC the more lowend and midrange but falls on its face, and the earlier the EVO and later IVC the more very topend to a point.

Normally the IVC is more dominant than EVO

[Stan Weiss]

I don't have answer for you on dyno tests /results. Use entering your numbers I do get a little different results.

Stan

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