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Bubba Gump Shrimp
I don't see why the powers to be cant see that the turbines need something to make them competitive again, it be great to see the general back out and running -- that is an awesome piece of equipment thats just setting simply because the higher ups won't give them whatever they need to compete, just don't understand that - they let the V8s improve on technology but not the turbines or Allison's --- i guess thats why alot of people love watching the mod classes in Europe !!!

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???
I remember when the class was clear full of turbines and that was all that won. Didn't NTPA make a rule that no one could build another turbine to compete with them? Until it's your money being invested I would say be happy with what you have. It is truly an amazing class to watch.

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Sascha
The biggest problem with the Allison isn't actually the max rpm but the "usuable" torque range on mechanical fuel injection in combination with that supercharger.

Some things we have learnt here at the Green Monster stable, where, as some of you have probable figured, we have built some competetive and winning Allison machines:

The engine is equipped with a super charger, which delivers air by the square related to its rpm. So at low rpm that thing does basically nothing and then drastically increases its air delivery.
However, there is a max of air it can transfer, as at a certain speed (according to our test results at app. 37,000 rpm / 3800 on the engine with a 9.6 blower ratio) the air at the end of the tips of the blades gets into stall speed and from that point on it stops to increase the air flow, but it starts to turn the air REALLY hot.

So - little performance in low rpm, and depending on the gear ratio in your supercharger, kind of a wall it runs into at high speed.

To make things worse, the heat you create at high rpms, actually shows as "lean" on your plugs (plugs show heat, not mixture) and you keep adding fuel to not melt pistons.

So you're basically throwing methanol at the hot intake air, to cool it down and make it rev higher. You're not getting any more air though (whilest adding more and more fuel) and are not making more power.

So while you're actually capped in power at the top end and have very little boost in low rpms, due to the super charger characteristics, the mechanical fuel system and this "throwing fuel at it to keep it alive at higher rpm" makes things even worse.

Given the fact the mechanical fuel pump delivers fuel "linear", the blower delivers air "by the square" and you then even "add fuel on top" (= just moving the linear fuel supply line up by putting a smaller bypass in) you basically kill it with fuel in the mid range.

To sum it up: Useable power only between 3200 and 3800 rpm.

Below that the engines just fall "flat on their face" and above that there is no extra power (Actually a loss. Your typical Allison might rev 4500 - but as soon as it sees a little load, it'll be back +- 3800).
On tracks where the engines only work in that rpm range, everything is fine and an Allison with mechanical fuel injection and stock blower wheel can compete.
When it takes wider rpm range - chance less.

So, the two changes we have made were:

a) Efi - not because it makes more power - but we have a lot more control over the fuel and can give the right amount at any condition (compared to a mechanical system, we can now "pull" more fuel in mid range where the blower is not creating excessive amounts of heat and the engine doesn't need as much methanol for cooling).

b) Putting a bit more modern blower wheel on it, that doesn't get into stall speed that quickly.
We now have a useable rpm range of app 2800 to 5200 rpm - compared to 3200 - 3800 before. The engines don't bog down that easily anymore on tough tracks and when it's becoming a speed race, on a little looser dirt, the engines will go up to high rpms, too without loosing air flow.

There was an interview with Mr. Veldhuizen a while back, who runs twin JD "Alky Burner" engines with turbos in the mod class and he described this "lack of useable torque curve" as his major issue. Works on some tracks, but when it takes low end torque - no chance.

The roots style blowers supply air linearly. They don't have all those issues (as it matches the characteristics of their fuel supply pump). They have full boost and pretty correct fuel in the complete rpm range and will supply full power once you open the butterflies.

The most simple way to get Allisons to compete in the mod classes with mechanical fuel injection, would be to put roots blowers on them. Not sure how the maths adds up there, but if you could run two Allisons with 14-71s in a class with three 8-71s - that might be a step in the right direction.

[youtu.be] if you check the video, engines rev pretty high, but they also "hang in there".

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OK2NV
Wow, didn't know that about Allison's. Great post!!!

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Glenn mc
I believe it was ken venheison from morrison il,if memory serves me right

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Larry
How would these vehicles at 9,200 pounds compare to todays Unlimited Mods at todays weight as far as distance pulled and performance? Would they be 10' or 50' or more off today's mods. What is the HP difference from these machines to todays true unlimited mods? What are the biggest technology advances from the top tier 80's mod.

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