lowering the compression ratio

[Kamil S]

I've built a little computer model (Matlab), that would calculate how much more clearance volume is needed to lower the compression ratio a certain amount (I could send this to you if you would like).

I was wondering, if at all a good idea, if there are major consequences with "shaving" off a certain amount from the piston head, or doing that in conjuction with a thicker gasket.

Currently for the CBR600RR, it seems that to drop from a stock CR of 12.0 to about 10.5, you would have to trim those pretty pistons down about 3 to 5 /32 of an inch (which is a bit much, but I believe is doable without much jeopardy to the structure). The pistons aren't very aerodynamic either, just a few valve reliefs so I wouldn't think the internal flow would be killed by that.

I know that may sound like a really bad idea, I just wanted to hear some of you veterans' opinions on this. We want to lower the CR anyway, just wanted to see if there was a "cheaper" way to buying aftermarket ones.

Thanks.

[Kamil S]

I've built a little computer model (Matlab), that would calculate how much more clearance volume is needed to lower the compression ratio a certain amount (I could send this to you if you would like).

I was wondering, if at all a good idea, if there are major consequences with "shaving" off a certain amount from the piston head, or doing that in conjuction with a thicker gasket.

Currently for the CBR600RR, it seems that to drop from a stock CR of 12.0 to about 10.5, you would have to trim those pretty pistons down about 3 to 5 /32 of an inch (which is a bit much, but I believe is doable without much jeopardy to the structure). The pistons aren't very aerodynamic either, just a few valve reliefs so I wouldn't think the internal flow would be killed by that.

I know that may sound like a really bad idea, I just wanted to hear some of you veterans' opinions on this. We want to lower the CR anyway, just wanted to see if there was a "cheaper" way to buying aftermarket ones.

Thanks.

[Andycostin]

Why do you want to lower the compression ratio?? You gain power by increasing it not decreasing it, that is unless you're trying to run some fairly decent boost.
But maybe I'm not reading this properly??

[Kamil S]

yeah trying to get some boost out (turbocharged), sorry didn't mention that.

Theoretically you would only drop otto efficiency from 52.5 to 50 %, so not a whole lot of big deal there.

[UQ Turbo]

I am in the process of dropping compression ratio too. (Make sure you cc the combustion chamber yourself and don't trust published figures!!)

I have been advised not to remove any material from in between the inlet and exhaust valves in the cylinder head because of the temperature difference and associated thermal loading. Apparently you can quite easily crack the head there if you get too carried away removing material. The squish area should be kept as much as possible too: i.e Don't remove much (or anything) from the edges of the combustion chamber (piston and head). This doesn't leave very much to remove! Obtaining shorter conrods / pistons or a thicker head gasket removes the squish area as well, which would rule that out.

At the moment I'm considering exactly what you (Kamil S) are trying for: piston material removal or aftermarket items. Otherwise, a combination of shorter conrods and material removal might be the best bet and that shouldn't affect squish too much. Would be good to hear what other people are doing / have done.

UQTurbo

[Andycostin]

When talking to the guys from Wollongong last year, I asked them about what comp ratio they were running with the turbo, and the reply that I got was that it was standard. Apparently they reduced it, but then found that they could retain the original 12:1, whilst running around 15-18psi.
Anyone from Wollongong care to comment, or from any turbo team for that matter (Cornell, etc.)

[VFR750R]

Never reduced our compression ratio on FZR or YZF although FZR took boost better. With E85 we could run an easy 8psi of boost, and with good tuning as much as 12+psi without an intercooler. The FZR is rated at 12:1 but actually measures closer to 11.5:1 and with it's 59mm bore has a more efficient combustion chamber then say a 67mm bore F4I. I couldn't begin to guess a good compression ratio for a gas turbo FSAE car but 10:1 seems not out of line.
As mentioned, don't touch the flat areas of the head and the corresponding matching flat areas of the piston. If I was going to reduce the compression ratio, I would first work on the valve pockets blending and chamfering them and I would remove maybe 10% off the dome. If that reduced the ratio to 10:1, awesome, but I wouldn't go any further with stock pistons. With thicker head gaskets you could reduce it much easier, but if you were shooting for 10, you might as well shoot for 9 with the gasket, because the combustion process will be negatively effected by the gasket, and you'll need even less compression to run your boost goal.
Any team desiring big boost should seriously consider E85, which is making it to several gas stations around the country now.

[Kamil S]

Andrew,

Could you please give me more details on how you got 15 psi without altering CR? What type of fuel were you using, and were there any modifications?

My intuition tells me 12:1 with 15psi will most certainly detonate. I'm still new at this so I would really appreciate it if you gave me more insight.

VFR750R,

Any special gasket to use with E85? If we manage to get a thicker gasket that is. Also, given approximately AFR of 9.5:1 with E85, what do you think is a decent compression ratio?

Thanks guys.

[CornellGixxer]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Andycostin:
When talking to the guys from Wollongong last year, I asked them about what comp ratio they were running with the turbo, and the reply that I got was that it was standard. Apparently they reduced it, but then found that they could retain the original 12:1, whilst running around 15-18psi.
Anyone from Wollongong care to comment, or from any turbo team for that matter (Cornell, etc.) </div></BLOCKQUOTE>

One of the questions I had the most trouble answering at competition last year was, "So how much boost are you guys running?" There isnt a very good striaght-forward answer. At low RPM you can run big boost, choke the restrictor and put up big torque numbers. But as the revs go up the amount of boost you can run before choking out the restrictor has to go down also. So, at those low RPMs, for a short amount of time the engine can take much more boost than would be prudent for a pull on the dyno.

I dont know if Wollongong was referring to max boost seen during transient testing or steady state dyno pull figures, but if it is transient maximums Im not surprised.

[VFR750R]

I believe there was something about one austrailian team spiking to very high boost, which is probably fine, for very short periods of time. Steady state would be catastrophic.

Kamil S. It depends on the amount of boost you want to run. That is really close to Stoich, but I'd say no adjustment in CR up to 8psi with E85. If you richen it up, you could run 10-12. To be honest, I wouldn't adjust the compression ratio for a FSAE car running turbo E85. Too much hassle for too little gain. If you want to run more then 10psi boost get an intercooler.

[Kamil S]

thanks, i feel compelled to head in that direction too, and spend more time thinking about intercooling and adjusting the valve overlap.

[VFR750R]

I'd also like to recommend anyone to make an 'ideal' torque curve for different compression ratios and boost levels. can set a single value for compressor efficiency and mechanical losses but it will show you that CR will effect HP after choke, and boost effects HP before choke and by approximately how much. One thing you will notice is the more boost you run, the more avg hp you make at the expense of torque curve shape. Once you add a more realistic boost curve you will see a very peaky torque curve (regardless of turbo) which is what the driver feels. Picking a boost level is as much curve shape, and the ability of the engine to reliably run that boost, as looking for total output. A 7.5:1 30psi engine will get you the max airflow at almost all rpm points but I bet a 10:1 20psi engine will have more avg power. It is a compromise, and IMHO the ideal is more then 10psi and less then 20.

My gut feeling is that you want the torque output of the engine to be the same at every corner exit on the track. If the slowest corner exit is 4500rpm and the fastest 7000rpm, having the same torque from 4500-7000 makes the car easy to predict and drive. If you aren't traction limited, it doesn't matter what shape the curve is, then it's purely max avg.

[Chris Davin]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kamil S:
Andrew,

Could you please give me more details on how you got 15 psi without altering CR? What type of fuel were you using, and were there any modifications?

My intuition tells me 12:1 with 15psi will most certainly detonate. I'm still new at this so I would really appreciate it if you gave me more insight.

VFR750R,

Any special gasket to use with E85? If we manage to get a thicker gasket that is. Also, given approximately AFR of 9.5:1 with E85, what do you think is a decent compression ratio?

Thanks guys. </div></BLOCKQUOTE>

I would advise against changing your compression ratio, for the time being. If you shave the piston, the best case is that you will remove heat capacity, and you definitely want as much as you can get if you're going to be running a turbo. The worse case (and probably the more likely outcome, considering my experience with these pistons) is that you will compromise the piston structurally or thermally. Believe me, the pistons in a CBR600RR are not designed with large safety factors. If you could remove the kind of material necessary to see a noticeable drop in compression ratio, Honda's engineers didn't do a very good job designing it! A thicker head gasket will kill your squish and cause even more problems.

In my opinion, the only real way to change a compression ratio for these motors is by custom pistons, designed specifically for this purpose. But it's still possible to have a great engine with stock compression ratio. If I were you, I would use either E85, or 100 octane gas plus an intercooler, and spend some time (at least one year) sorting out your turbo system before you start thinking about changing your compression ratio. Don't ask me how I know, but I have this feeling you'll have your hands plenty full!