More thoughts and questions......
Based on the two E Class scenarios below, which one would be the better choice from a piston compression height?
If I wanted to turn the engine at 8000-8500 RPM would I choose the lightest rod & piston weight or some other combo?
Bore Stroke # Cyl Rod Length Cu In Deck Height Rod Ratio Calculated Piston Compress Height
3.5512 3.250 8 6 257.52 8.937 1.85 1.312
3.5512 3.250 8 6.125 257.52 8.937 1.88 1.187
Excuse my ramblings....just trying to get a little bit smarter
Skip
OK, here is the part where it gets
"complicated" . . . . . . . .
1) Class displacement limit 260.99 cu. in, I calculate your displacement @ 257.52 cu. in. also, so 98.67% of legal displacement. That's very good.
2) Rod length/stroke ratios vary by 2%, probably not much there, plus the engine is blown. No worries.
3) With a shorter compression height, one might
presume a piston would be lighter. This is not necessarily true. The weight (mass) can vary, based on manufacturer and/or application. Generally, (as in normally aspirated) lighter is better. For blown engines with high boost pressures, you definitely want to add mass (weight) to cope with the higher thermal loads on the piston. Piston mfg's (or Dynoroom,) can advise you here. Piston cooling may also be in order . . . . .
4) 8500 rpm with a 3.25" stroke = mean piston speed of 4604 feet/minute
high = max piston speed of 7232 feet minute
very high = max piston acceleration of 142,668 ft/sec^2
extremely high I have not calculated the rod load yet, and I want to suggest you consider a shorter stroke . . . . . .
OR, an rpm change as below:
5) 8000 rpm with a 3.25" stroke = mean piston speed of 4333 feet/minute better, 5.9%
= max piston speed of 6807 feet minute better, 5.9%
= max piston acceleration of 126,662 ft/sec^2 WAAY better, 11.2%
And of course the rod load would be lower. If it was mine, I'd run 7800/8000 rpm max.
For comparison, a 3.25" stroke Cup engine. Keep in mind that Cup engines use very light components, are not blown and have very short service intervals . . . . .
6) 9500 rpm with a 3.25" stroke = mean piston speed of 5146 feet/minute extremely high, 5000 is a typical maximum
= max piston speed of 8083 feet minute extremely high, 8000 is a typical maximum
= max piston acceleration of 178,614 ft/sec^2 beyond extremely high, 150,000 is a typical maximum
Very light weight (as in unobtanium) components and short service life are what makes this combination work. The very thin rings and very tight ring grooves required
to make this combo work, will work against you with a blower engine . . . . . .
7) And of course the big question: How much blower pressure or absolute pressure? ?There are others on the board that have more experience than I do with blown engines. I would seek out some further advice from them.
Fordboy
edit note: I added in some additional numbers for comparison, segments 5, 6, 7.