I agree with this 100%. I had my dyno tune done after changing cam, intake, carby and headers. I had it set up that first time, and since then do basic tune up my self. I havent had it dynoed again since.
One thing I did find usefull is with the Dyno tune was to set up my timing. My exhaust is too loud for me to hear it pinging, so that was set on the dyno, and since then I know where to get it basically right myself.
In Idollars case, he has been playing with the mixture adjustment screws, and does not know how to get it back to correct.

 

The problem with vacuum secondaries on a dyno is several. First it depends on the type of dyno, some provide very little resistance but just measure the rate of the roller's acceleration and calculate power based on a given mass of the roller(it takes x force to accelerate y mass at z rate etc). On these types of dynos the secondaries on a vacuum carb may never open fully because the engine is notloaded up as much in the same rpm range as when it's beingdriven. On dynos where there's an actual braking force applied to the roller this problem isn't as pronounced, because the car can be loaded up more heavily. Also, the problem with putting a lighter secondary spring in on a dyno run is that if the dyno isn't loading the car completely then the secondaries come on properly on the dyno, but under a harder load when driving they open more rapidly and can cause a bog as the secondaries rob the primaries of metering signal, and even if it doesn't bog it may later the AFR. The problem is just the nature of carburetion really, how a carburetor meters fuel and carries it in the airstream(both of which obviously effect combustion) is greatly dependant upon engine load and rpm, and in most cases people don't have access to a dyno that can accurately simulate the same engine load that will be seen while driving hard. That's why I say that in most cases dyno tuning a carbureted car isn't worth it, not because dyno tuning is bad but because it doesn't accurately reflect how the engine will be operating when the car is actually moving. With mechanical secondary carbs it's not as bad but still a problem, again because engine loading on the dyno is likely to be different than when the car is driving. Dyno tuning is great for EFI because the fuel management system isn't dependant on signal from engine load to operate like a carburetor is, or if someone has access to the proper type of dyno that can actually load the engine up to accurately reflect real world driving it works. But most of the people I talk to that build carbureted race engines have found that 9 times out of 10 dyno tuning picks up power on the dyno sheet, but the car actually runs more slowly on the track, and in some extreme cases the car runs like crap on the track after a dyno tune. I'm not against dyno tuning, I'm just against wasting time, effort and money on something that won't produce any useful gains. Why dyno tune a car if it's not going to actually make the car faster than it already is, or provide a better AFR? If the dyno tuning will work then yeah, have at it, but again, in most cases with carburetion that doesn't seem to be the case. I've even talked to people that have dyno tuned a carb'd car and got a great AFR and power went up on the dyno, but when they ran the car with wideband data logging, the AFR sucked and the car didn't run as strongly as it did before it was dyno tuned. Unless you know for certain the the dyno tuning for your car will make it perform better, then I don't see the point. Numbers on paper don't necessarily reflect how the car actually runs

 

I think this is where we are looking for a different end result.
Im using the dyno to get the car running properly after doing changes, or becuase its screwed up, not to try to get the biggest dyno number, or time at the track. And in the example given where Idollar has screwed up his mixtures and doesn't have the knowledge to fix it, the dyno is his best option, short of having a quarter mile master tuner get the absolute .001 of a second out of his car.
In terms of the type of dyno, 67mustang302,you are spot on. Here in Australia, the most common type of dyno is the fluid pump type, which can load up and engine for an extended period of time. The inertia type you are talking about, are really just dyno hero machines, and as you say, not very useful for tuning.
So note to Idollar, confirm what type of dyno you mechanic uses. Perhaps someone here can point him to somewhere local.

 

hey guys,
i am a little pissed right now because I just wrote a big assed long heartfelt response to all of you and my computer happily deleted it when I tried to post.
The gist of it was that I really do appreciate the sincere dialogue you are having on this subject.
You all make very good points.
The reasons I am wanting to do the dyno tuning are basically this:

1) I want what is best for my car. I want her to run the best she can.
2) I do not know what my hp/torque are ... I have no idea.
3) I would love to spend some time with the builders who know my car better than I do.
4) I dont mind spending a little cash to get her running the best she can if it will help her run strong for many years.
5) I know little about this car and am slowly learning. However, I do not have time or space to spend a whole lot of time tweaking her for the best performance based on those adjustments.

Several folks who have been following my thought process have helped lead me to the conclusion that dyno tuning would be appropriate in my situation. It is nice to hear a "Devils Advocate" perspective that I had not considered before.
I will look forward to hearing any further thoughts on this subject. Thanks for all your time guys. I really appreciate it.
Cheers!
Luke

 

Cheers Luke, and good luck.
Whichever way you, let us know how it end up.

 

It's not a devil's advocate position, just a realistic assesment. Like fast66 said, make sure if you do it it's on a fluid pump type dyno so that the dyno session is worth while. If you try to tune a car, especially a carbureted one, on an inertia type then you're just "spinning your wheels" Unfortunately from what I've seen(at least in this part of the country) pretty much everyone uses inertia type dynos, which do little more than produce an over-inflated horsepower figure. Get some books on carburetion too and read up on how it works, and how to tune your carburetor. That way in the future if you change anything or for some reason need to retune the carburetor, you'll know how. It's cheaper than a dyno and can be done without having to drive the car to a shop that has one.

 

67_302
Sorry if using the phrase "Devils Advocate" came out wrong. I guess it was just a little different perspective than others were giving me so thats where that came from. I really do appreciate you pointing out to me some things to look for in this process, because that is exactly what I was hoping to get.

fast66 & Aussie
I appreciate you helping me work through this stuff. I am lucky enough to not have any true "problems" with my engine right now but I really appreciate the pointers and insight you have offered on the subject.

Thanks to all of you for your help. Thanks for taking the time to understand my naivete on this topic. Of course many people would say I should have bought a late model car but I already have one (boring 04 Volvo) so this is how I am learning how a REAL engine works. love it!

Now I have more info in hand when trying to figure out what to do next. Thanks guys!