High Compression Pistons

[Alan_RHIT]

Hello All I'm new to this forum but have been doing some extensive research on raising the CR of our engine which is a 600f4i. I've done some reading on high compression pistons. Is there a piston most people use? JE? Wiseco? Also by using a high compression piston does that increase overall volume of the engine? Is there a piston that increases the CR while staying under the 610cc limit? What kind of CR can we expect from such a piston?

[Alan_RHIT]

Hello All I'm new to this forum but have been doing some extensive research on raising the CR of our engine which is a 600f4i. I've done some reading on high compression pistons. Is there a piston most people use? JE? Wiseco? Also by using a high compression piston does that increase overall volume of the engine? Is there a piston that increases the CR while staying under the 610cc limit? What kind of CR can we expect from such a piston?

[Wesley]

Displacement is given by bore x stroke x number of cylinders...pistons don't change displacement. Just compression ratio.

[exFSAE]

A team once told me, and our engine guy, that they were using 15:1 CR in a F4i... along with 45deg spark advance across the board.

We were incredulous... and later saw them throw a rod at competition.

[Chris M]

Our team currently uses JE pistons, and has used Wisecos in the past.

Unless you've made other changes to the engine (like head, conrod or crankshaft) the CR you get will be what it says on the data sheet.

That all said, one of our older cars made plenty of power with stock pistons. So although high comp pistons are essentially a 'bolt-on' improvement (which is very appealing) the benefit is small when compared to getting the gas-exchange right.

[Superfast Matt McCoy]

High compression and spark advance should lead to a hole in a piston long before a thrown rod. In any case, 15:1 and 45° advance (at reasonably high revs) on an f4i shouldn't be a problem with 100 octane. I expect there were other strange things afoot in that motor.

You can take one layer out of the three layer head gasket on the f4i and bump the ratio up a bit for free. Any more and you get pistons/valve interference.

[exFSAE]

The 45deg advance, from what I recall, was at *all* revs. Whole map was 45.

In any event I wouldn't be surprised if there was a lot of other crap going on. It's FSAE afterall

Whatever the issue was... it dumped oil on their headers and engulfed the car in smoke and flames.

[Ockham]

It's worth mentioning that the same higher cylinder pressures which are the reason high compression pistons make more power will also lead to increased proclivity for, and sensitivity to, knock, and increased exhaust gas velocity and temperature. In short, you're more likely to severely damage the engine, and it will be louder, with increased risk of melting the headers. There's performance to be had, but one batch of bad gas could be the end of your engine. Anything beyond ~13:1 is a huge risk with pump gas, and still iffy with 100 octane. It all depends on your tuner; target MBT, and pray.

[Wesley]

Ordinarily I would agree with you Ockham, but in such a small and high speed engine, 15:1 is perfectly achievable on 100 octane.

If we were talking V8's or larger displacement engines, I would tend to agree, but the high degree of turbulence in the cylinder makes flame propogation much quicker and the quench of the F4i makes it pretty resistant to detonation.

That said, if you're running way lean, you'll have problems, so you have to have your tune ironed out, but if you're swapping pistons already, you should know what you're doing when it comes to tuning anyways.

And like Matt said, detonation is more likely to bust up your upper ring land or melt a hole in your piston crown, not that throwing a rod is impossible, but it's not the primary failure mode.

[Mbirt]

I like the amount of knowledge present in this thread, so I'll take the opportunity to bring up something that just popped into my head:

4-cylinders in FSAE are running safely at 65-70 degrees advance and/or 15:1 c/r. Compared to the singles, they have tiny bores and what seems to be more efficient port geometry. This should equate to faster flame propogation.

The megamanual states:
"total advance at WOT: should be from ~24° to ~40° depending on your bore size and combustion chamber characteristics. Older design engines (i.e. push rods, domed pistons, etc.), and those with large bores (big blocks, etc.) need more advance, about 36 to 38°. Newer designs (4 valve/cylinder, swirl port engines, etc.), and small bores, generally require less, about 28 to 32°."

and

"Note that the optimum amount of total advance is not necessarily the most that doesn't detonate. For example, with a modern cylinder head design, you might get maximum power at 32°, but might not experience any detonation until 38°-40°."

I haven't dared explore WOT advance greater than 40 degrees with our crf450x as of yet. Is anyone running 65-70 deg advance on a non-4cyl? And if not, why?