rod lengths in top fuel (1 Viewer)

[flapjack]

What is the purpose of differing rod lengths in top fuel? I know that the rods come in various lengths, but I don't know if different rod lengths are used in the engine at the same time, or if they are consistent. Is this for altering the compression ratio? I've also heard that head gasket thickness comes into play, but I am not sure how that works, either...

 

[Ramos]

Different compression heights in some cylinders. So to achieve it you use either different pistons or rod. Depending on your inventory. The air doesn't flow equally into each cylinder and these cars are pretty on edge so you tinker with heights to make the piston survive without pinching a ring or breaking a piston. Its not much but from front to back on a block the airflow is different. We use 3 different heights in combo now, It was 5 but when we put on set back blower, it equaled out airflow a bit. I really don't have time to truly explain it. But if your at a race come talk to me and I can show you better in person.

 

[MM]

What is the purpose of differing rod lengths in top fuel? I know that the rods come in various lengths, but I don't know if different rod lengths are used in the engine at the same time, or if they are consistent. Is this for altering the compression ratio? I've also heard that head gasket thickness comes into play, but I am not sure how that works, either...

Different rod lengths give you different compressions. Teams will run different rod lengths in different cylinders to control cylinder temperatures. The closer the cylinder temps are to each other, the happier the motor will be. You want all the cylinders to burn at a consistent temp. This temperature can also be controlled by adjusting the down nozzles in the cylinder heads for the particular cylinder you want to adjust. Generally, the front two cylinders (#'s 1 and 2) burn the hottest because they get the most air. Usually tuners will run these two holes with less compression to help even the motor out. Every tuners has a different method though. Some will run different rod lengths in many different cylinders. Head gasket thickness adjusts for the compression ratio of the entire motor. The thinner the gasket, the more compression you have. This is where water grains and barometric pressure come into play. The more moisture there is in the air, the more compression and blower you need, because their is less air getting into the motor because of the increased moisture present. Again, these are pretty general statements. Each team and crew chief has their own philosophy on how and why to adjust compression for the conditions. This is how we have always approached things on the teams I have been a part of. Hope this helps.

 

[Ramos]

ya what he said.

 

[flapjack]

Thanks Mike and Ryan. It is what I though it was, but confirmation is good. I guess the cylinder temperature and compression ratio aspect of changing rod lengths also explains the set back blower as the roots style blowers push more air to the front cylinders. That is, I assume the front cylinders on the block get shorter rod lengths...

 

[.]

Interesting, learn something new every day. I knew they ran different compression in some cylinders, had no idea they were using up to 5 different rod lengths to do it.

 

[FINAL EFFORT]

Alot of team will run different hieght pistons also the hi-comp low % motors will run anywhere from .220 out to .230 out low comp motors with higher % in the tank will run anywhere from .210 out to .220 out. and to tell you how different teams and combos are we bought a combination from Kalitta back in 84 for out top fuel hydro and funny car that had the piston at .300 very very touchy combo

 

[flapjack]

Alot of team will run different hieght pistons also the hi-comp low % motors will run anywhere from .220 out to .230 out low comp motors with higher % in the tank will run anywhere from .210 out to .220 out. and to tell you how different teams and combos are we bought a combination from Kalitta back in 84 for out top fuel hydro and funny car that had the piston at .300 very very touchy combo

I was wondering about piston heights this weekend when it was mentioned that Jim Dunn had made sweeping changes in his engine program, one of which is different piston heights. I take it that it's either differing rod lengths, or differing piston heights, but not both.

 

[Nitrohelper]

..... differing rod lengths, or differing piston heights, but not both....

We're looking for a certain height out of the top of the block ...
That can be achieved by changing one or both, depending on what you have in stock...

 

[Larry jr.]

Changing the rod length also changes the piston speed which can be an aid to tuning by increasing or decreasing port volocity in the intake tract.

Changing piston compession height will change compression in the combustion chamber, and help even out heat and do pretty much what has already been said, but piston speed changes combined with compression changes are more versatile, that is were rod length variations come into play.