What octane gas do big bore high compression guys run?

[HoneyBadgerRy]

I am unaware of anybody running aftermarket pistons in their pregen, so I will ask you guys. What bore size and compression ratio do you run (if over stock) and what octane gas do you use?

[genosr1]

new gen 13:1 89 octane ran great no detonation

[verboten1]

Big bore high compression? My 685 KLR at 10.5:1 runs on 89 unless it's super hot out...

[Racer x]

Nothing more than 110. More like 100. Above 110 and the engine will not even start. I have gone with 13 to 1 and found 110 was good. From a detonation point. But the power was not as good as the lower. That was all using Rocket brand fuel. Since switching to VP I have not done much testing and not done anything but nitrous and lots of ignition advance.

Using the Brt ignition I can pull timing below 7500. That is the area I had problems with. 44 deg or even 42 deg caused detonation during the lower gear acceleration. Still pull a little timin at 10000+ but don't know if it is detonating or not. I can't hear anything but wind and exhaust.

[HoneyBadgerRy]

Its interesting that you can push these motors so much with such low octane. They must have great cooling and great valve cooling. I was scared that a 13.5:1 would need 110 or maybe 118 e85.

[subxero]

I would see no reason to run straight 110 in just about any modded sport bike, I would be mixing 110 with 93 to end up somewhere closer to 100 if I had a machine that needed the extra octane.

What about 100LL av fuel? I know it is popular in the dirtbike community but I don't know if it would be safe/good to run in a 4t sport bike?

[HoneyBadgerRy]

I belive av fuel doesn't make as much power as pump gas, but it should be ethanol free and leaded, which is nice.

Idealy I would want to run pump gasoline, and If I required a higher octane, I would try a COP swap, ignition advance, and run e85. I would also consider running 93 with a leaded octane booster. (Or I could polish the combustion chamber and the piston tops to help prevent detonation, but this is irrelevant, because if you can run 13.1:1 on 89, I'm sure you can run 13.5:1 on 92/93.)

[subxero]

there are commercial bikes that run 13.5:1 and some higher, I assume they are rated for 93

doesn't exactly mean much but well there it is lol

[HoneyBadgerRy]

Compression ratio alone really means nothing as far as gas octane, its a combination of so many variables to determine anything, but ninja 250s to ninja 250s is a fair comparison, ninja 250's to zx10r is apples to oranges.

As RacerX said before, just by playing with cam timing he got his engine to go from some crazy low compression to 300 psi.

[jkv45]

Originally Posted by HoneyBadgerRy

Its interesting that you can push these motors so much with such low octane. They must have great cooling and great valve cooling. I was scared that a 13.5:1 would need 110 or maybe 118 e85.

There are a lot of factors going on (chamber/piston design, timing, mixture, etc) but there are numerous hyper-performance cycles on the market that safely get by on Premium pump gas.

In general, to make max power, you want the lowest octane fuel that just barely keeps detonation away, and will lose power with any octane over that point. Detonation tends to start at lower RPMs and high loads, so you may be able to hear or feel it when cruising at a lower RPMs/speeds and opening the throttle significantly (heavy load).

[HoneyBadgerRy]

Originally Posted by jkv45

There are a lot of factors going on (chamber/piston design, timing, mixture, etc) but there are numerous hyper-performance cycles on the market that safely get by on Premium pump gas.

In general, to make max power, you want the lowest octane fuel that just barely keeps detonation away, and will lose power with any octane over that point. Detonation tends to start at lower RPMs and high loads, so you may be able to hear or feel it when cruising at a lower RPMs/speeds and opening the throttle significantly (heavy load).

I'm well aware of detonation. My Xc 250 detonates on most premium gas, but 93 octane from BP does not. I typically mix their 93 with some leaded 110 (VP).

[Fred659]

I'm about to put 66mm 12.5cr wiseco pistons in my 300.

The ecu is re-flashed, but is still stock timing (From 10° BTDC at 1 300 r/min (rpm)31° BTDC at 7 000 r/min (rpm)) if we check in the manual. it has about 43 hp to the wheel at the moment with stock piston and the reflash.

I would prefer to use pump gas (91 shell v-power) but can go with some race fuel and make my own mix (Sunoco 98). Also, can I put a +XX% on the power commander to start the dyno test (ex: 335/296 = 13%)?

It's a track only bike.

Any advice here from racers?

Thanks

[Bob KellyIII]

in my years of Modifying bikes (mostly 2 strokes) i discovered most bikes gain very little benefit from increasing compression.... oddly enough as v-8 engines really do gain alot.... I believe that is due to mutable cylinders...
...I think there is more to be gained in a properly tuned carburetor than raising compression, or even changing the cams is better than compression
simply because you put so much more stress on the parts when increasing compression for very little gain...
Larger carburetors and better cams would be my suggestion for motor hop up on a ninja.... not increased compression...
.... my 2 cents worth !
Bob.......

[Fred659]

in my case, I'm going from 10.6 to 12.5 (ninja 300)

according to this picture, it would be like a 4.35% ish gains

[Mohawk]

You need to ignore static compression ratios & work out the dynamic compression ratio based on the cams you are running. 95RON is good for 11-11.5/1 dynamic ratio. Most unleaded fuelled bikes run 10.5-11/1 dynamic ratios. If your dynamic ratio is higher then you will need higher octane fuel. There are calculators on the net for this. Wallace Racing do one.

[Bob KellyIII]

But you are not looking at the stress factor you are adding when you increase the compression ratio .
if you want a reliable bike this is a major consideration...
Typically when increasing compression ratios on big V-8's the mechanics usually change the connecting rods for stronger ones this is necessary to keep the engine from blowing up and destroying itself...
for just a few percent change in compression ratio obviously this is not a concern but consider what it is also doing to the main bearings....so now instead of the main bearings being hit with a 1lb hammer every time the piston fires it is being hit with a 1.75lb hammer every time....a small change in compression ratio changes to a large change in the main bearings and connecting rods.......
you obviously know this, but seem to be disregarding it as a non-issue...
let me assure you it is not a non-issue!
Bob....

[Mohawk]

I ran this up to compare loads for the 300 Ninja engines I'm big boring.

https://www.dropbox.com/s/xfm3qns1da...Loads.JPG?dl=0

Rev & weight increase the load much more than combustion. And most engine have 10-20% over engineering for failure free longevity. By this I mean they design/test it to find practical limits then add material &/or strength to components to ensure they don't fail at the prak expected loads. If you don't go too crazy you can use some of that spare capacity & still have a reliable engine.

Note that I am disappointed i the weight of the Wiseco pistons & should have just gone with ZZR600 ones.

[Bob KellyIII]

that is probably quite true. especially given the RPM these bikes turn at.
but their is a surprising amount of weight you can remove from a piston with a die grinder.... but on doing twins they have to be the exact same weight,...
i mean down to the nat's ass ! you just have to be real careful not to weaken
the piston in the process !
....
Bob.....

[Fred659]

Originally Posted by Mohawk

You need to ignore static compression ratios & work out the dynamic compression ratio based on the cams you are running. 95RON is good for 11-11.5/1 dynamic ratio. Most unleaded fuelled bikes run 10.5-11/1 dynamic ratios. If your dynamic ratio is higher then you will need higher octane fuel. There are calculators on the net for this. Wallace Racing do one.

I just calculated the DCR for my engine:

12.6 SCR (from all my measurements) (Stock 10.6; 08 zx-10 = 12.9)
int cam closing at 56° (08 zx-10 = 77°)
Rod Length of 3.770"
10.56 DCR (stock would be 8.912; 08 zx-10 would be 9.028)

From: https://www.omnicalculator.com/every...pression-ratio

could you please tell us where you got the 95RON information and where did you find that most unleaded fuelled bikes run 10.5-11/1 dynamic ratios?

Thank you

[Mohawk]

OK so just re-read this thread to get back up to speed, it's been a while.
The Pre-gen 250 has always had 12.5:1 SCR. Running 10.6:1 DCR.

The 300 because it was sold in South East Asia as well where fuel quality is generally lower they lengthened the stoke but kept the same bore & reduced the SCR to 10.5:1 so it will run on any fuel.

The engine bottom end is the same as the pre/new gen 250 except for the longer stroke.
The pre-gen with heavier pistons revved to 14K/rpm, the 300 to 13K/rpm due to the longer stroke but similar conrods. The 250 had 10gram heavier pistons & produced approx 8% more average inertia than the 300 with a peak 14% BDC inertia, so the 300 has some leeway to play with.

If you add a bigbore, the piston weight will have the biggest impact. Check the table I added. If the DCR does not change much then apart from adjusting fuelling to match the increased air volume to keep the AFR in a good range then everything should be fine.
The closer the new pistons are to the weight of the old ones the lower the additional stress level on the motor. YMMV.

Stock 300 SCR @10.6:1 gives 8.93:1 DCR, my 64mm 12.5:1 SCR gives 10.5:1 DCR which is right on the money for 95RON fuel.

Regarding the fuel, please note in Europe we use the RON measurement system, in US & US influenced areas they use the MON system. The numbers are NOT the same & the anti-knock levels are thus different. In Europe there are generally only two levels of unleaded fuel available 95RON is the basic unleaded & 97-98RON is the "Super" unleaded. There are a few sources of 99RON & or some street side higher octane rating fuel pumps, but these are generally only near race circuits, where the rules state many classes MUST run pump fuel, hence pumps near race circuits ! Plus you can buy octane booster to mix with Super for a higher octane rating again YMMV.

As to the info on DCR, I have run the numbers through the DCR calculator for many bikes over the years & regardless what SCR the maker claims the DCR's have all come out between 10.5-11:1 based on the valve timings. You will notice that any bikes with greater than 12:1 SCR will all have fairly large valve overlaps & you can't compress a gas in a cylinder until the valves close & as engines are designed to run on standard unleaded, then they can't have a DCR higher than 11-11.5:1 max without knocking.

Have fun running numbers ��

[Fred659]

Originally Posted by Mohawk

OK so just re-read this thread to get back up to speed, it's been a while.
The Pre-gen 250 has always had 12.5:1 SCR. Running 10.6:1 DCR.

The 300 because it was sold in South East Asia as well where fuel quality is generally lower they lengthened the stoke but kept the same bore & reduced the SCR to 10.5:1 so it will run on any fuel.

The engine bottom end is the same as the pre/new gen 250 except for the longer stroke.
The pre-gen with heavier pistons revved to 14K/rpm, the 300 to 13K/rpm due to the longer stroke but similar conrods. The 250 had 10gram heavier pistons & produced approx 8% more average inertia than the 300 with a peak 14% BDC inertia, so the 300 has some leeway to play with.

If you add a bigbore, the piston weight will have the biggest impact. Check the table I added. If the DCR does not change much then apart from adjusting fuelling to match the increased air volume to keep the AFR in a good range then everything should be fine.
The closer the new pistons are to the weight of the old ones the lower the additional stress level on the motor. YMMV.

Stock 300 SCR @10.6:1 gives 8.93:1 DCR, my 64mm 12.5:1 SCR gives 10.5:1 DCR which is right on the money for 95RON fuel.

Regarding the fuel, please note in Europe we use the RON measurement system, in US & US influenced areas they use the MON system. The numbers are NOT the same & the anti-knock levels are thus different. In Europe there are generally only two levels of unleaded fuel available 95RON is the basic unleaded & 97-98RON is the "Super" unleaded. There are a few sources of 99RON & or some street side higher octane rating fuel pumps, but these are generally only near race circuits, where the rules state many classes MUST run pump fuel, hence pumps near race circuits ! Plus you can buy octane booster to mix with Super for a higher octane rating again YMMV.

As to the info on DCR, I have run the numbers through the DCR calculator for many bikes over the years & regardless what SCR the maker claims the DCR's have all come out between 10.5-11:1 based on the valve timings. You will notice that any bikes with greater than 12:1 SCR will all have fairly large valve overlaps & you can't compress a gas in a cylinder until the valves close & as engines are designed to run on standard unleaded, then they can't have a DCR higher than 11-11.5:1 max without knocking.

Have fun running numbers ��

Excellent reply here, thanks!!!

FYI, I put Carrillo rods too (I'll post the engine build when i'n done), so i'm not too worried about the heavier pistons ( I would've been with the stock con rod though).

As for the Fuel, I bought Fuel Factory 2WL race fuel which is RON: 103, MON: 85, AKI: 94 and will dyno tune the bike with this fuel. If you think I should use a fuel with more MON also, please say so. I have access to higher octane fuel also. Do not worry for the injectors / gas pump, those are upgraded too.

Thanks again for the great reply.

[Mohawk]

NP what DCR did you end up with ?

[Fred659]

Originally Posted by mohawk

np what dcr did you end up with ?

10.56


20240415_084251.jpg

[Mohawk]

So you should be able to run pump gas at that DCR & save a fortune on expensive high octane fuel ��

[Fred659]

Originally Posted by Mohawk

So you should be able to run pump gas at that DCR & save a fortune on expensive high octane fuel ��

I don't mind the race fuel as it's a race bike... The main goal is to put power to the ground (the most I can). This is why I selected that fuel.

What was confusing for me was that all DCR information I could find was for big V8 and they were all talking about 9.5 being the max for pump gas. I guess bikes and bike muscle car are not an apple to apple comparaison.

Thanks again.