i know this might be out of the realm for most, but i figure i can catch a few insightful responses. when looking at a motor build with the same displacement (107), compression ratio (10.5:1), stroke, and bore, what would be the preference and why for head volume and piston type between these two:

(1)flat top piston with larger head volume (86cc)
(2)pop up piston (11cc) and smaller head volume (75cc)

 

Well, if the only difference between the 2 builds is the pistons being flat top or domed, then the compression ratios wouldn't be the same. A domed piston would increase your compression ratio given the same head used in both set ups.

 

The most important thing to remember is to be sure to re-calibrate your Flux Capacitor to be able to handle the 80,000 Jigawatts generated by the Johnson Rod.

 

OP stated that the heads were different, keeping the compression ratio the same. Question was, given this, which is preferable. I would think the flat tops would be preferable due to the torque generated, at least for low end. You need torque to get hp. The domed piston setup probably is better at higher rpms. Just my humble opinion.

 

Your example does not create "like" conditions. I ASSume you've been you've been looking at someones online compression calculator, but it appears you have listed the pistons with opposite chamber size to create similar resultant compression ratio.

"Generally speaking", for what we are doing, it really does not make a huge difference on which way ya get the the desired end.

**However**, some piston dome configurations compliment chamber design. Is it a measurable or intelligible difference? For street motors... not really.

There is also a caveat to domed pistons in certain configs too. Depending on chamber size & shape, valve size, dome size, and cam profile (not just lift, but timing as well), one can experience piston to valve interference "challenges".

Which is "better"?

Simple!

It depends....

Talk to the folks "spec'ing" your engine package. If they are competent, they will have the experience to make the proper recommendation for you.

Clear as mud?

 

for those providing the light hearted responses, thanks for the entertainment value ;-)

philm: thanks for the well thought out response. most of what you described relates to 'mechanical fit' of the components and you bring up some good points. you also make reference to the performance differences and that was more of what i was looking for. i was just wondering along the lines of ignition and flame propagation as to whether there was a difference.

i have been conversing with tman and that is where this stems from. he offers two different configurations for the 107 and its a package deal, so the mechanical fit issues you addressed should be managed. no science projects, just two different ways to get there. fyi, he prefers the domed option because it requires less to be taken out of the chamber area on the heads.

 

TR's a sharp cat.

If you are considering one of his packages, listen to what he tells ya.

One thing about his 98 & 107 packages.... He really likes the RevPerf cylinders. Been there done that... stick with the bored stockers!

Gonna get ya Stage 3 Elite HO?

 

This question certainly brings back fond memories of a 4th year thermo lecture with a guest prof. As "wannabe" performance engineers we all attended the lecture and were promptly lost in about 5 minutes.

We were fortunate enough to cone away with some basic summary point, namely:

stroke vs bore affects torque and top end performance

top end chamber geometry (including number of valves, valve location, plug position, piston shape and TDC clearance) was all about rate and even spread of flame propogation and the ability to expel exhaust gas.

That was enough to make me realise that I didn't have a clue and this is best left to the experts.

 

I have a very good friend who knows that stuff, and I was going to post that flame propagation and expelling exhaust gas are important considerations--I have no answers--but he's talked/explained the issues with me from time to time.

Go with the builder's recommendation for what you want to accomplish from the build and possible future modifications.

 

might be something like this in the works ;-) haven't ruled out HQs but i like the fact that tman is right up the road, has a pretty good tuner, and will tune both of my devices...lol (tmax and sert) i know both sides of that one, but we can discuss that at a later time..lol