Destroke engines to limit speeds (1 Viewer)

[Paul]

A quick and cost effective way to limit the nitro speeds and get back to 1/4 mile racing could be the following:

I think Dale Armstrong's original suggestion to reduce power by having a max 6 to 1 compression ratio made sense, and the initial testing that was done on it apparently looked good.

Why not enforce a max 6 to 1 compression ratio while also requiring that the engines be de-stroked from their current stokes to accomplish the reduction?

The only change required would be new cranks, all the current pistons, rods and cylinder head gaskets would be retained.

Making a quick estimate shows that reducing the current crank stroke from 4.5" down to 4.43" would knock about half a compression point off, or down from the typical 6.5 ratio to the required 6.0 ratio. This also knocks the displacement down from 500 to 487 cu in, not huge but it will knock the speeds down a little more than a compression ratio change alone.

So the rule would be max compression ratio of 6 to 1, max stroke of 4.43" and max displacement of 487 cu. in. The advantages are only a crank change is required, all the current pistons, rods and head gaskets are still useable.

Paul T.

 

[Rajunz]

And then they would spin them even faster?

 

[5.0 Junkie]

Rear gear is fixed. You slip the clutch more? If so, slower speeds which is what we are looking for.

I think the crankshaft is an excellent idea. It involves the least amount of change. I also think they should run 100% with the lower compression. Still get a little over 300mph runs, with less breakage and 100% makes the show better.

And then they would spin them even faster?

 

[JT]

If you de-stroke the crank they will just go to an even smaller bore to make up for it. Then again most of the top a-fuel motors run around 420 cubic inches right now. Just add a blower and they will run just fine. It would slow them down, but not enough.

 

[Wes]

There are a million ways (well...maybe not that many...but a lot!) to get a short-stroke motor to RPM. In fact...the shorter the stroke, the higher the power band. Just look at Pro Stock and Comp cars...stroke is non-existent ini those classes. And when you reduce stroke you reduce the rotating weight of the bottom end...even more propensity to RPM.

Now you could go the other way...longer stroke with a smaller bore to get 500". That would reduce the cylinder volume. Couple that with a maximum 6:1 C/R and you've drastically lessened the engine's ability to consume as much nitro and it will RPM less...resulting in lowered top end speeds with a fixed gear ratio and spec tire.

 

[Bmoy]

If the higher the power band, the more reason to stay in it. By that, I mean they would take slightly longer to get to the powerband and might just stay there slightly longer. In that, the cars might not over rev quite so much and therefore a possibility of less detonation.

??????????????????????????????? It's just a theory.

 

[JT]

The key to running a fuel motor is the longer stroke. It takes a long time to burn the nitro...hence so much timing. If you shorten the stroke it will mean the tuners can't burn as much fuel as they can't get it all lit before top dead center.

 

[Nick Name]

Trying to rpm a nitro motor is not easy as the fuel burns too slow. When the NHRA limited the rear gear, the top crew chiefs were calling Elmer Trett and some of the other bike guys to find out how they were able to burn it at 11,000 RPM.

Jay

 

[Mooseman]

It will be interesting to see what they do in the long run , the crew chiefs will come up with something I mean they build these engines fuel time for a living so if anyone knows how they work and how to slow em its the crew chiefs .

 

[Toejam]

Trying to rpm a nitro motor is not easy as the fuel burns too slow. When the NHRA limited the rear gear, the top crew chiefs were calling Elmer Trett and some of the other bike guys to find out how they were able to burn it at 11,000 RPM.

Jay

Isn't that what started the Heavy Volume pumps??

 

[Toby]

Thats the $100k question Johnny!!!

You shorten the stroke, the piston speed goes down. Will that make for a happier nitro engine? Its not as long in a physical dimension, yet there is more time for the nitro to burn at the same RPM. Anyone familiar with A/FD combos, chip in. When the A/FD guys run a smaller engine, do they run a smaller bore or stroke?

The key to running a fuel motor is the longer stroke. It takes a long time to burn the nitro...hence so much timing. If you shorten the stroke it will mean the tuners can't burn as much fuel as they can't get it all lit before top dead center.

 

[Paul]

Interesting comments fellas, so far nobodies shot the idea out of the sky so it seems like its something that should be considered.

I still think its the most cost effective way to drop compression and the reduced cubic inches that comes with it helps reduce power also.

The ideas being suggested to limit power by reducing fuel flow (single pump or restricted line size) concern me because the high dollar teams can spend mega bucks to try to work around the limitations, like what happened with the restrictor plate engines in NASCAR.

Paul T.

 

[Nick Name]

Interesting comments fellas, so far nobodies shot the idea out of the sky so it seems like its something that should be considered.

I still think its the most cost effective way to drop compression and the reduced cubic inches that comes with it helps reduce power also.

The ideas being suggested to limit power by reducing fuel flow (single pump or restricted line size) concern me because the high dollar teams can spend mega bucks to try to work around the limitations, like what happened with the restrictor plate engines in NASCAR.

Paul T.

How would you "work around" a restricted flow fuel pump?

Jay

 

[Paul]

How would you "work around" a restricted flow fuel pump?

Jay

If a particular pump model was specified, high dollar teams could "stealth" blueprint the pumps for better flow. This typically requires buying a big batch of the pumps and selecting the best individual parts. Remachining parts (while carefully matching the stock machining marks) to put them on the edge of legality is another typical trick. This all comes after you spent a fair amount of time and money on a pump dyno figuring out the the best way around the rules.

These kind of games have been played heavily in other forms of motorsports (Indy, Nascar) that have tried restricting carbs or inlet flow, which is not that different from restricting a pump.

Could it be made to work? Maybe, but there would always be questions and rumors about some teams working around it with questionable methods.

You can't really do high dollar "stealth" cheating on the stroke based compression reduction.

Paul T.

 

[clwill]

If a particular pump model was specified, high dollar teams could "stealth" blueprint the pumps for better flow. This typically requires buying a big batch of the pumps and selecting the best individual parts. Remachining parts (while carefully matching the stock machining marks) to put them on the edge of legality is another typical trick. This all comes after you spent a fair amount of time and money on a pump dyno figuring out the the best way around the rules.
<<snip...>>

So you have a spec fuel pump that is built for NHRA, and they hand you a new one each day -- you return your old one, get a new one, they are serialized, you have to run one of theirs, you think it's bad, you swap for a new one, etc. Pumps are owned and supplied by NHRA. I dunno, just thinking out loud here...

 

[You!]

....?!!uh.

 

[Thrill]

I think Armstrong's compression idea has merit, along with Dupuy's idea to limit the inlet line size to a spec. Using stroke to lower compression just isn't a very practical way to lower compression. You do that with dome volume and rod length. Losing 13 cid, the car won't even notice that a bit. There's such a surplus of power right now, plus on a supercharged motor, your cubic inches are bolted to the intake with a pulley. One could argue it might even make more power by making the blower more efficient. I don't think CID is the answer.

Yes, lower stroke with the same rod/piston would lower the compression but it's just not the right way to do it. Not to mention you would make all the crankshaft inventory out there obsolete, and cranks are a hell of a lot more expensive than rods.

In reference to piston speed on the A/FD's, most have went to .500 short deck blocks with a shorter rod to get the piston speed back up.

 

[clwill]

....?!!uh.

OK, don't like that, how about just a limit on fuel line size. Couldn't possibly be easier to check, nearly impossible to cheat, can't put more down the line no matter how big the pump is...