This article was prompted by my coming across yet another number of web site postings concerning the controversial subject of lift flow. It is commonly accepted among many successful two valve competition engine builders that too much low lift flow hurts power. Although commonly accepted by many that is not to say it is a correct assumption. Those engine builders that claim too much low lift flow hurts power are basing their conclusions on what seems to be irrefutable evidence supporting such a claim – namely back to back dyno tests. If this is so the case for seeing a reduced output when low lift flow goes up seems clear and beyond any argument. Non-the-less in this feature I am going to argue this issue.

In this feature I want to make the point that just because the dyno shows more power it does not mean that the move just made was right. Here I am going to throw in what I think is a great example of exactly the reverse.

Just before the turn of the millennia I received a call from a then well known Nitrous pro-mod engine builder. He had some questions he thought I might know the answers too that were not directly related to nitrous Pro-Mod engines but the subject got around to that anyway. During the conversation I asked him what the life expectancy of one of his record setting race winning nitrous engines was. The answer – one pass or about 8 seconds – which ever came up first. What shocked me here was the cost per second to run an engine like this. Think about it – an SR71 Blackbird costs a quarter million per hour or, in round figures, 70 bucks a second. Even if most of the Pro-Mod’s engine parts survived and the engine could be rebuilt for say $15,000 the cost, per second, would be in the order of $1875. In other words it was 23 times the cost per hour to run than an SR71!

I was intrigued at the short life and after some questions I managed to get faxed to me an in-cylinder pressure graph. This was something of an education because outside of my own nitrous motors I had not seen what goes on in the cylinder of a winning nitrous motor. What shocked me most was the 5000 plus psi very near TDC and it prompted me to ask what the compression ratio was for this motor. Answer – 16/1! My response to this was “why so high’? Not unexpectedly the answer was that this was what showed the best results on the dyno. Now you would think at this point the argument for a high CR was an open and shut case. If the power went up as the CR went up that must be the way to go.

What I said next to this engine builder sort of floored him. In so many words I said that the fact the dyno showed the power was going up as the CR was increased was a clear indication that he was going the wrong way. His answer here was – “OK – I really want to see you talk your way out of this one”! So I did – and here is how, a step at a time, it all came about.

Let’s start here with cylinder pressures and expansion ratios. Let’s say that our engine has enough structural integrity to hold out against a peak cylinder pressure of 3000 psi. We will say that so long as we don’t exceed 3000 psi our engine will be reliable.

Figure 1. C/R vs Cylinder Pressure Decay

Figure 1. C/R vs Cylinder Pressure Decay

Next step, with a nitrous engine we can achieve that 3000 psi either with a relatively small amount of nitrous and a lot of compression or a lot of nitrous and a lot less compression. Let us assume we can go either way without any combustion difficulties. If this is the case then there is a clear cut argument for going with a low CR and a lot of nitrous. And it goes like this: The higher the expansion ratio (the downward stroke equivalent to the compression ratio) the faster the cylinder pressure decays. To make the point I will take two extreme compression ratios (and thereby expansion ratio’s) – these are 15/1 and 2/1. If the 15/1 cylinder has 3000 psi at TDC (or thereabouts) that 3000 psi has decayed to about 300 psi by the time the piston has travelled half way down the bore. So there is only 300 psi times the piston area pushing the crank around. The same scenario for the 2/1 cylinder is that from the starting point of 3000 psi it has only decayed to 1350 psi by the time the piston is half way down the bore. This means there is a lot more force there to turn the crank and the average torque throughout the expansion stroke is about 400% more than with the 15/1 cylinder. But as my Pro Mod guy pointed out the dyno shows the reverse – so is the theory wrong? No – the fact that the theory is unimpeachably right and the dyno gave results exactly to the contrary told me that the fuel manufactures (at that time and I don’t know if things have change yet) really did not have a handle on the brews they were selling to the nitrous guys.

Here’s how all that plays out. For the combustion of a typical petroleum distillate (gasoline) there has to be a certain fraction that has turned to vapor by the time the spark is about to be delivered. I really don’t know what that fraction is just prior to combustion but some temperature measurements I have done while testing fuels and carburetion atomization indicated that about 15% of the charge needs to be vaporized by the time the mixture has reached the vicinity of the intake valve. Any less than this under normal conditions appears to start compromising the charges ability to ignite properly. The intake temperature in a normally aspirated engine running in an atmosphere of 80 degrees is about 50-60 degrees due to a combination of the latent heat of evaporation of the fuel and the manifolds heat absorption from the rest of the engine.

Now let’s throw in a bunch of nitrous oxide. With smaller amounts the initial response from the engine is to deliver a healthy increase. But keep on stepping up that nitrous and the gains stop coming. Back off the nitrous and increase the compression and further gains in output are seen. Why is this? Well we can see that with a lot of nitrous the charge temperature is going to be down around the -128 F mark and none of the fuel will vaporize at this temperature. So the induced charge is not very combustible hence the poor results seen. When the compression is raised the heat of compression causes more of the charge to vaporize thus improving the combustibility of the charge and hey-presto – more power results. What is all this telling us? In short it tells us that the race fuel manufactures at that time universally did not know, or could not brew what was wanted for nitrous engines.

I remember a conversation I had at about this time with Tim Wusz who was the tech guy at Union 76 – they made all the fuel for cup car events at the time. I did not really know Tim but knew of him and I suspect the situation was the same the other way around. My opening gambit here went like this – ‘Hi Tim, David Vizard here, I called to tell you guys you are brewing your fuel wrong for nitrous engines’. It was quiet the other end for a few seconds and then when Tim started talking I knew I had got his attention 100%. Tim was generous with his time and listened to the evidence/data that I had to support my claim. After about 15 minutes I finished and his words were ‘Well it looks like we are brewing our fuel wrong for nitrous motors’. I hawked this notion around to several major fuel companies as I felt I had a handle on how fuel for nitrous motors could be substantially improved. Unfortunately all my efforts were of no avail.

Unlike a regular motor what a nitrous motor needs is a fuel with lots of highly volatile components in it. The heavy, harder to ignite fractions that make power in an engine that does not have a sub-zero intake temperatures have no place in a fuel for a nitrous engine. I don’t know how much things have changed here but I did point out this shortcoming the several well known fuel blenders and backed it up with some very pertinent test result and all I got was a lot of my time wasted. Even five years ago (about) I saw an ad for a fuel blend that claimed it was ideal for nitrous and turbo engines. These two circumstances have diametrically opposing requirements so who were they kidding?

So what we have here was the fact that the power increase seen on these nitrous engines was the result of the application of a band aid fix in one area when the real problem was in another. If the fuel was fixed the engine would make more power the lower the CR as you could keep on putting in more and more nitrous until our supposedly limiting pressure figure of 3000 psi was seen.

So hopefully I have made the point that just because the dyno results are one way or another does not mean the move made was categorically right or wrong. No one item within an engine works in isolation to another. That old saying about changing only one thing on the dyno at a time and testing is really a good plan for novices who need to stay out of trouble – but for a pro who needs to understand in the first place what the bigger picture might be – umm.

Let’s get a little nearer to home here and by home I mean high flow at low lift – good or bad and what the dyno tells us. Let me make one thing clear – even a moderate change in low lift flow can show up as a negative result on the dyno if the cam and port sizing was optimal for the before test. Unless cam events and port sizes are re-optimized any back to back dyno test of low lift versus high is totally bogus!!!! My engines have won many many races because the heads involved had significantly better low lift flow than the other guys – and just so you know as often as not the ‘other guys’ were F1 guys. Hopefully I’m not mincing words here!

David Vizard.

 

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