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 http://fsae.com/groupee_common/emoticons/icon_smile.gif

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?

[Kirk Feldkamp]

There are basically two 'limits' for ignition advance when tuning for full load. They are knock limits and minimum best timing (MBT). Knock limited is what it sounds like. If the engine is not knock limited, there is a point past which more advance actually nets zero or diminishing returns. Once you reach this point, you should back off the advance as far as you can so that you can still make that peak torque. This is how you set minimum best timing. The idea of the ignition advance is you're timing the cylinder pressure rise due to combustion so as to optimize the torque (ie work) output. Past a point, the pressure is rising too early in the cycle to be of any benefit. If you're working with a supposedly non-knock limited engine, it's still advisable to exercise extreme caution. It's a bad feeling when you do in fact find those knock limits. http://fsae.com/groupee_common/emoticons/icon_wink.gif There are a few auxiliary knock sensing products out there, and a few ECU's have built in knock control algorithms. The Pectel ECU's comes to mind for this functionality, but I'd guess there are others available. If knock sensing devices aren't your cup of tea, good ol' plug reading is a reasonable (and inexpensive) way to gauge your max advance. You'll start to see silver specks (they look like little pin points of light) of aluminum from the piston and/or head on the spark plug ceramic long before you ever get into full blown detonation. You can use a spark plug light with magnifying glass to see this.

-Kirk

[Mbirt]

I have absolutely no 4-cyl dyno experience due to my joining the team after they switched to the 450X. So please excuse my ignorance. But have teams really found MBT in the 65+ deg range? Or is this due to engine guys tuning to knock at 67 deg and the forgiving nature of a well-breathnig, small-bore engine?

I'm just trying to wrap my head around why a single, with a much larger bore and poorer breathing, doesn't need just as much or more timing. Maybe MBT for the 600's and the singles is closer, while the 4-cyls can go much further until knock occurs?

Time to get the dyno stand back together!

[Kirk Feldkamp]

Based on many discussions over the years with FSAE engine tuners, many don't even know what the MBT concept is. They will bemoan their engine keeps breaking, and chalk it up to manufacturing shortfalls, rather than a direct cause of their tuning. That said, there are plenty of *reports* of running 45 degrees or more on sportbike engines without mechanical problems. Is it ideal? Not necessarily, and only the dyno will tell you what the "right" answer is. Is it possible? Certainly.

-Kirk

[Xeilos]

I can say for sure (I was present for the tuning http://fsae.com/groupee_common/emoticons/icon_biggrin.gif) that we were running 78 dgerees timing at high loads and high rpms. At first we thought there was something wrong with the engine and with each time we advanced the timing, the engine just kept on making more power. We were quite freaked out, but nothing went wrong so we went with it.

I have no idea as to the reason why. We were dyno'ing with a stock RR engine (CR of 12.2:1 if my memory serves me correctly) and 93/94 octane pump gas.

[Ockham]

@Wesley: A clever tuner can probably pull off 17:1 with E85 and steady nerves, but I maintain that you're into diminishing returns by that point. What's the power difference between a well-tuned 12.5:1 CBR600RR on 93 octane, and the same engine at 15:1 on 100 octane? Of course, I suppose it's not necessarily about whether something is reasonable, but whether it can be done at all. http://fsae.com/groupee_common/emoticons/icon_wink.gif This is racing, after all. My words were meant as a caution; new teams will be surprised how close to the top they can get with stock hardware and plenty of tuning time, so why screw around with new pistons when you're starting out?

Regarding engine damage under hard knock; we killed two engines this year, one when we accidentally hit and stayed at the engine's natural frequency, and another with a single, catastrophic knock which broke the connecting rod into three pieces and wedged the piston in the bore at about a twenty degree angle. That said, we've found the CBR600RRs are remarkably knock-resistant.

@Mbirt: Is it safe to say that FSAE's combination of tiny chambers and ridiculous piston velocities means "extremely high" spark advance numbers are actually reasonable? This would explain why the single-cylinder guys don't see such high advances.

@Xeilos: What ECU were you using? Presumably not the PE (limited to 50 degrees). Are you sure your timing wheel was set up correctly? We had ours clocked 30 degrees backward at one point, so 50 degrees of advance was actually 20.

[RANeff]

We saw no extra power far below any of those outrageous advance numbers! Thats awesome some of you can get so much advance, we just didnt see the need to go farther with it. Woder if thats true advance though, thats really high... Ill take a safe, knock free tune that makes good power over running on the ragged edge to make 5hp more any-day http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Xeilos]

@Ockham:

We are using the Wolf V500 and could not advance the ECU any further. Our timing may have been out as much as 20 degrees (we estimated it was less than this during checks) but still, that is 60 degree of advance at the minimum.

We heard no distinct knock at these spark advances (we were listening and praying http://fsae.com/groupee_common/emoticons/icon_biggrin.gif) so we assumed it was good.

And I agree that the CBR600RR is remarkably knock resistant. It is almost idiot proof (well bullet proof) except for running with little/no oiling pressure http://fsae.com/groupee_common/emoticons/icon_confused.gif.

[Boffin]

Just to add in here for every one's reference, we're running an 04 CBR600RR with the compression bumped up to 13.5:1 running on 98 octane, which last year we tuned to MBT which was 38 degrees under WOT.

We could have gone to 40, but it only made a poof teenth more up top where we weren't going to use it.

Tuned like this we were making incredibly good power and torque

[Mikey Antonakakis]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Xeilos:
@Ockham:

We are using the Wolf V500 and could not advance the ECU any further. Our timing may have been out as much as 20 degrees (we estimated it was less than this during checks) but still, that is 60 degree of advance at the minimum.

We heard no distinct knock at these spark advances (we were listening and praying http://fsae.com/groupee_common/emoticons/icon_biggrin.gif) so we assumed it was good.

And I agree that the CBR600RR is remarkably knock resistant. It is almost idiot proof (well bullet proof) except for running with little/no oiling pressure http://fsae.com/groupee_common/emoticons/icon_confused.gif. </div></BLOCKQUOTE>
Is this timing checked with a timing light?

[Drew Price]

Does your timing light go up to 80??

http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Best,
Drew

[Boffin]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
Does your timing light go up to 80?? </div></BLOCKQUOTE> Pretty sure he was asking if the base ignition and advance had been checked, which you can do on an RR as they have 0 deg and 18? deg marked on the crank trigger wheel.

The more I think about this, base 60 and max 78 don't make sense. The stock bike ecu idles at 15deg.
My thinking is to re check both your CRIP (crank index reference position, motec thing but wolf would have to have similar) and tooth ratio.
But if tooth ratio was wrong, so would your RPM.

So yeah. Did you check it with a timing light?

And Xeilos, RR aren't idiot proof. Ask that to our 08 engine guy who destroyed 3 RR's

[Charlie]

This is the major issue.

I'm quite certain that many FSAE teams don't really know their base timing. The trigger wheel fab, or ECU setup is incorrect.

That's why you get such a wide range of MBT.

Of course, I'm sure some teams are tuning to knock, being ignorant of what MBT is, but I think the main discrepancy is base timing.

[Mbirt]

Thanks guys! I'm glad I was able to ignite such a conversation.

In the future, when Indian and Chinese teams come knocking asking what MBT is for their engines, they can be linked here. Or, ideally, find the thread with a click of the search button.

[Ockham]

I'm surprised no one makes an ECU which works with stock CBR600RR (or another engine) timing signals, ie, the sum of the crank and cam wave forms. Using totally stock timing, which is a perfectly good reference, would remove one of the biggest unknowns many teams face.

Fortunately or unfortunately, the advance shown on your computer screen is not necessarily what's happening in real life. Tuning software doesn't care whether the hardware's off; if your computer reads 78 degrees or -118 degrees where the dyno reports best torque, who cares? As long as you have best torque. Though this transparency does complicate these discussions. :P

On the upside, if you need more advance than the engine computer gives, you can advance the timing wheel, moving the spark event's reference further forward. On some old-fashioned engines, physically rotating the timing trigger wheel was how you adjusted advance. Ah, the bad old days...

[Kirk Feldkamp]

If they did, it would be for the stock ECU plug. Anyone who has tried to source OEM connectors, pins, and tooling knows that's either impossible or very cost prohibitive. If this was possible and easy to do, I would think everyone would use the stock ECU and a PowerCommander. Clearly, that's not the case. Unless you make a serious piece of zip tie art (or spend a lot of time modifying a stock harness), the "stock ECU" route is not usually the preferred route.

I have seen trigger setups pre-configured on an ECU before. I believe it was with a DTA, and I think it was for a 600F4i, if memory serves. Getting trigger settings hashed out is really inconsequential if you take the time to understand what you're doing, and what the ECU is expecting to see. Plus, once you've got it set, it *shouldn't* change on you. I've seen some less than ideal trigger wheel (and attachment method) designs over the years, so sometimes they CAN move on you. Haha.

This all assumes you have the ability to adjust the trigger timing and tooth configuration. ECU's like the Motec's can... but I think the PE's (and possibly more) require a single trigger configuration. I would not want to have to monkey with the trigger system if a perfectly good one already exists. In the case of a previously carbed engine (think offroad and older motocross), I would make damn sure that did my homework on what the ECU I was using would be expecting before I design my trigger wheels.

-Kirk

[Mikey Antonakakis]

Just make sure your VR sensor stays bolted in and you should be fine once you work out the wheel settings. It's easy to find a 12-1 wheel for a F4i. Or to make one.

[Wesley]

@Ockham: I completely agree that the returns aren't worth the risk. The most compression change we've made is as Matt said, removing the middle layer from the MLS gasket to bump it up a little bit to 12.5.

Also, do that many teams really not tune advance for power? I mean, there obviously is a knock limit, but you shouldn't run at that limit just because you can.

I think what makes these engines so responsive to the increases in advance lies in the effect of the restrictor on rarifying intake charge. When you have such reduced cylinder filling, flame propogation would tend to slow down I would think, and so when you have that lower flame speed you see a benefit from increased advance versus the stock unrestricted motor.

I know that the restrictors DO tend to cause the engine to benefit from increased advance, but that being the cause is just a theory at this point. Any takers?

[Hector]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I think what makes these engines so responsive to the increases in advance lies in the effect of the restrictor on rarifying intake charge. When you have such reduced cylinder filling, flame propogation would tend to slow down I would think, and so when you have that lower flame speed you see a benefit from increased advance versus the stock unrestricted motor. </div></BLOCKQUOTE>
You hit the nail on the head. The engines DO see a benefit from an increase in timing.

We've double- and triple-checked our timing to make sure that what we think we're running is accurate, and we're running quite a bit more than stock.

Remember, you'll typically see a plateau where you you keep increasing timing but fail to see any increase in power. Set your timing to where this plateau starts: any more timing and you're just increasing stress and adding heat to the motor.