UCCR

[THE358BANSH]

What types of uncorrected compression ratios have you guys ran on pump gas with proper head design? I'm trying to decide on whether to run pump fuel or stay with VP110 for my new build. The design of the head is pretty critical so I am looking for people running tight squish(.036"ish), squish bands around 50%, an nice hemispherical domes(no tubs or toroidal stuff). I have heard of up to 15.76:1 with +4 timing @1000 feet of elevation in some drag applications. Albeit that application was not ran for long periods of time whereas mine will be.

 

Looking at your typical Banshee(64x54) with proper squish and a "20cc" dome our UCCR is 14.36:1 and is well known to run with +4 timing on 93 pump fuel with proper jetting all day long. How much further could that UCCR be stretched before having to reduce timing or start mixing fuel? I figure if I have to cut fuel I'll just say screw it and run straight VP110 with the timing and compression to match.

 

Also, since I touched on the full race gas builds. Where do you think the tradeoff occurs between high compression and pumping losses causing the engine not wanting to rev out? Where would the cut off of UCCR be in a full drag build with a good amount of exhaust duration(200+), decent timing, and 110 motor octane fuel? I have two different domes designed currently, one for 93 and one for 110 and both of them have great MSV numbers. What say you? Evan

[ticktock]

Might want to PM your question to Wildcardracing or Mullengineering. Both Brandon's are pretty hip to this.

 

Adam

[vulvafan537240]

Pm and ask them to reply in this thread. More people are interested in this topic and would like to see the answer.

[blowit]

Hey gents. I will try to help on a few points. Compression ratios do have some practical limits but relating them to RPM is NOT quite the right angle. I will try to apply some physics here for consideration. Compressing a gas causes that gas to heat up. The rate at which you compress that gas as well as how much (ratio) is what determines how hot that gas will get. As engine speed increases, the rate of compression goes up thus those gas charges in the head are hotter. This causes acceleration of flame front propagation thus the need to reduce ignition timing to maintain maximum cylinder pressures at the optimal point in the crank rotation.

 

As point of reference, you will notice most igntion curves will increase with rpm to a point, then decline. This is because the rapid compression of the gasses need less of a "head start" to hit the sweet spot of max performance because the charge burn rate is faster.

 

Many people relate ignition advance to power but realize that in a perfect world, starting ANY spark before TDC is counter productive and in a perfect world, we would really like to start that spark at about 5* ATDC. However, it takes time for fuel to burn and build pressure in the cylinder. The faster it will burn, the less timing advance you need.

 

The main issues regarding changes in fuel and compression ratios are that many will require difference curves than what is in the OEM CDI box. With today's digital ignitions, it is certainly possible to get it right. As it pertains to physics, the higher compression ratio you can handle, the faster you will be. However, regarding two strokes, expansion chamber reverse wave technology certainly throws a curve ball in there. Having a little extra room in the head is sometimes a good thing with well matched porting and pipes.

 

I personally am no stranger to heavy hitting compression. I have built several woods bikes for customers pumping up over 250psi of cranking compression. They have years on those engines and never been opened. The key is getting the timing right! When you feel a high compression engine hit the "brick wall" on the top end, that is a sign that you are starting the party too early and asking for engine failure. I cannot stress tuning your timing enough.

 

Many people get too wound up with numbers on timing. +4, +6, +10, etc. Every engine configuration is different and depending on porting, CR, fuel, ambient air, pressure, etc all makes a difference. In short, don't do what your buddy does, do what your particular engine wants. Unfortunately, those magic numbers above are ONLY the mechanical advance which is just a guess that the entire curve will be optimal in every rev range. +8 on the plate might be dead on in the bottom revs and way too much to get good overrev up top.

 

Brandon

Mull Engineering

[blowit]

 

Looking at your typical Banshee(64x54) with proper squish and a "20cc" dome our UCCR is 14.36:1 and is well known to run with +4 timing on 93 pump fuel with proper jetting all day long. How much further could that UCCR be stretched before having to reduce timing or start mixing fuel? I figure if I have to cut fuel I'll just say screw it and run straight VP110 with the timing and compression to match.

 

I would like to see your math on that. I am not calling you out, I just want to help you on the numbers since you seem to have the ambition to do it the right way.

[Nutwood 472]

Excellent thread

[THE358BANSH]

I would like to see your math on that. I am not calling you out, I just want to help you on the numbers since you seem to have the ambition to do it the right way.

 

Thank you sir, be right back with some info all fancy like...

[THE358BANSH]

Stock Banshee numbers:

 

64mm bore x 64mm bore x .7854 x 54mm stroke x 1 cylinder = 173,717.9 cubic mm

 

173,717.9 cubic mm ÷ 1000 = 173.72cc swept volume for one cylinder

 

Flat plat volume of CoolHead dome is 20cc with spark plug crevice volume

 

Piston dome volume of 64mm bore piston with 120mm dome radius = 7.0cc

 

20cc flat plate volume - 7.0cc dome volume = 13cc trapped volume @TDC

 

(173.72cc swept volume + 13cc trapped volume @TDC) / 13 cc trapped volume @TDC = 14.36:1 uncorrected compression ratio

 

 

The other option if you have a super computer...

 

 

[blowit]

Everything looks good minus the piston dome profile. Assumption there is a constant radial profile over the dome, which is incorrect. You will notice playing with your value inputs that any small change in that dome volume has a radical effect on calculated UCCR. Once you have the volume for the piston dome, it will come back into alignment.

 

Great work so far though. Wish more people would take to the books before guessing and hoping...........

 

 

By the way, where did that calc come from? It looked familiar so I went through some of my old files and there it was. I have no data on it so just curious where that ever came from.

 

 

Brandon

[blowit]

Good God man, I was clicking around and due to the "terminology" all being the same and the same values, I suspect even some of the engine builders on here are using the wrong shit!

 

I even saw where some were "rounding" these values +/- .5cc. That is a hell no and exactly why some stuff "should" work and doesn't. I guess that is the magic of the Inet. Once something is started and passed around, but wrong, it is like Herpes.....Trendy but still ultimately not cool....

[THE358BANSH]

Everything looks good minus the piston dome profile. Assumption there is a constant radial profile over the dome, which is incorrect. You will notice playing with your value inputs that any small change in that dome volume has a radical effect on calculated UCCR. Once you have the volume for the piston dome, it will come back into alignment.

 

Great work so far though. Wish more people would take to the books before guessing and hoping...........

 

 

By the way, where did that calc come from? It looked familiar so I went through some of my old files and there it was. I have no data on it so just curious where that ever came from.

 

 

Brandon

 

I don't remember exactly where it came from. I want to say either the old MacDizzy site or a RZ350 forum, one of those two almost positive.

 

As for the dome profile, I just went next door and looked at a 795 series for a 65mm bore because it's all I have around now. I measured it up and I am coming up with 10.32cc? Seems a bit far out and in the wrong direction to me...

 

As best I can measure here is the cross section of the piston dome profile...

 

[Wildcardracing]

I come up with 6.68cc dome volume for that piston. Also don't forget to calculate deck height, piston clearance and top ring height into your figures as these will change your ratio's quite a bit.

As for your question on max UCCR to run on pump fuel (or any given fuel for that matter):

There are too many variables to say "this maximum value will work", this is why recommendations are usually on the conservative side. UCCR, CCR (trapped ratio), MSV, elevation, and port/pipe efficiency are just some of the factors that will dictate what octane of fuel your engine will require.

 

But, as a direct answer to your question: With properly designed domes I have run UCCR approaching 15:1 on pump fuel at our elevation of 5,000"

 

-Brandon

[THE358BANSH]

I come up with 6.68cc dome volume for that piston. Also don't forget to calculate deck height, piston clearance and top ring height into your figures as these will change your ratio's quite a bit.

As for your question on max UCCR to run on pump fuel (or any given fuel for that matter):

There are too many variables to say "this maximum value will work", this is why recommendations are usually on the conservative side. UCCR, CCR (trapped ratio), MSV, elevation, and port/pipe efficiency are just some of the factors that will dictate what octane of fuel your engine will require.

 

But, as a direct answer to your question: With properly designed domes I have run UCCR approaching 15:1 on pump fuel at our elevation of 5,000"

 

-Brandon

 

 

Thanks for the reply Brandon. Would you care to share how you came about that figure? If you don't want to I understand...

[Wildcardracing]

I can do it longhand just like you, just add the crevis volume to your chamber or dome volume. But, there are some handy calculators that can do it for you. Most are engineered towards 4 strokes, just substitute the intake closing abdc for exhaust closing abdc and it will work great for a 2 stroke. Here is a link to one I like:

 

My link

[mopar1rules]

Question for the original poster, how do you like that BiMotion software? I have and use TSR and like it. I never found the computer calculations of cc volume to ever be what the actual cc volume is that i measure with my burette filled with kerosene. I just do it the trusty burette way and know the actual. Some the calculations on paper or in theory, don't quite seem to equal what the real world measurements are.