Conrod weight vs Crown height

[Donfrondo]

Im looking for someone really clever to give some detailed advice on pistons and conrods of a race motor.

Ive two options, i can use a 6" titanium conrod weighing 326g with a std crown height (~1.5inch iirc) forged piston weighing 295g. Or i use a steel 6.25" rod (435g) with a low crown height (1.25") piston which is a little lighter too.

The engine has a 68.26mm stroke, 73.5mm bore, 16valves, and is expected to rev to 10000+rpm

The engine builder is suggesting option 1 as I already have the ti rods, but the rpms vs the crown height is bothering me. Does anyone know if there's cause for concern

[Donfrondo]

Hopefully. I know there are some engineers and some students on here, hopefully one of them specialises. If not a suggestion on where to ask would be well recieved.

[Donfrondo]

All the components are off the shelf, so it's just a case of mixing and matching to create the combo that's going to work best. The motor can be pulled apart as often as I like, I anticipate I'll have it apart over winter once a year to check it over.

[Donfrondo]

An A-series with a sprint spec 16v conversion. The omega forged pistons already use a smaller than std pin which mate to the arrow rods which have a little end only suited to the smaller pin. If i didn't already have the ti rods i probably wouldn't be asking the questions, i would have just gone with the longer rods. If the suggestion is there's nothing in it between the two options then I'll use what I've got. Its more a case of if I'm loosing out by using the combo?

[Donfrondo]

Yea MED. Steves been great and his advice was to go with the ti rods because that's what I have, he said he builds loads of race engines with the full height pistons and never has any issues, I've no reason to doubt him. Im using his 68mm short stroke crank which I'm starting to think would suit the shorter height piston. But then the complication is the fact the rod is over 100g lighter, a weight I'd never save with a shorter height piston.

[Donfrondo]

That's superb knowledge, thankyou all very much.

[Donfrondo]

KiaDiseasel said:

Lovely mathematical technical question. If only there were someone that way oriented on this site who could answer it

Donfrondo said:

Hopefully. I know there are some engineers and some students on here, hopefully one of them specialises. If not a suggestion on where to ask would be well recieved.

Well now i feel like a nob. Excuse my ignorance. Thankyou

[Donfrondo]

As mentioned above, it's a 16v sprint engine. Works out at about 1155cc, with the bmw 16v conversion. Brett Simms doing the head, ported with bigger valves, tougher springs, 300 degree cams, 40mm throttle bodies and 12.5:1 CR. MED have done the short block, with their arrow 68mm crank, titanium rods, 73.5mm forged omegas, balanced up with their lightest flywheel and clutch assembly. Gearbox is the only part undecided so far, I'm probably going to put a clubman-spec SCCR 'box on it off my road car to dyno it, see how it makes power before i decided which gearset and final drive to put with it in the car.

[Donfrondo]

Was at the engine builders yesterday, the short height piston is only 10g lighter than the full height. 297g vs 288g with pin. Is there anything else to consider with a short crown height piston? Like less friction because less skirt? or tipping effect because of the center of gravity in relation to the pin?

[Donfrondo]

Funny you should mention the rod bolts. When I was talking to MED, I cant remember what he said they were made out of, but the rod bolts that come with them are £375 per set!

Love learning about what goes on inside engines, its fascinating, thanks for your time.