I need advice on fuel type for DASA motor

[Harbor]

I just found out my local booze store sells race gas 98 through 118 octane. Could I take the 118 and mix it with 93 and be good? It was about $100 for 5 gallons of sunoco 118

I just went thru this… . Each of the fuels ABOVE the SS 100 unleaded have different stoichiometric fuel ratios and slightly different specific gravity’s and can require jetting/mixture changes…

Stoichiometric fuel ratios are as follows
Pump gas with 10% ethanol is around 14.1
Didn’t look up pump with 0 ethanol
SS 100 unleaded is around 14.2
110 is around 14.9
112 is 15.1

I mistakenly bought the 112, thought I was pumping 110…. then went for a Sunday ride and brought some along to add when I was getting low…. Well my ski ran like poop….down 50% on power and won’t rev out. Then i wondered if it could be the fuel… looked up the stoichiometric ratio and specific gravity and found it different than the pump gas my ski I used when I tuned my ski…. Switched back to pump gas and resolved the issue… called Sunoco tech and confirmed my findings…

The SS 100 unleaded is the closest stoichiometric ratio to pump gas. If you already tuned your ski mixing the SS100 would require almost no tune changes..

When I added the 112 I was roughly 50/50 race and pump.. or 60/40 race/pump and it ran like crap as far as power goes…

If you get anything other than the SS 100 unleaded.. retune your ski to your new fuel mix…

My ski is a Lamey 8 mil stoker on 200 psi, Msd enhancer, RRP pipe, 148 pump 23 deg timing…I run 91 pump gas in it, it’s a surf rec boat. I don’t hold it pinned, usually cruise 30 mph max, sometimes I’ll take it up to 40 mph briefly… I should be running higher octane as well… you should too, especially since you have a lay down pipe and Msd total loss..

SS 100 and pump 93 mixed 50/50 would be around 96.5 ?

The higher the Stoichiometric ratio the leaner the tune needs to be….

Since you have Msd T/L…. I’d pull the back cover off the brain and get your dip switch positions and plot your timing curve next… that’s critical to know

Fuel Selector | Sunoco Race Fuels

www.sunocoracefuels.com

 

[Storbeck]

Unless you have something unusual going on 93 octane will be fine. Pump gas will mean that your tuning needs to be decent. If you're holding it pinned for long periods of time with a high pump load (basically racing) then maybe not, but most people don't do that with an 850 Dasa. If you have something odd about your setup with super high ignition advance or some problem that would cause the motor to burn down, nobody on this forum knows about it so they can't help.

All the stuff above about stoiciometric ratios, for me...I'm pretty skeptical. Stale race gas tends to have lousy volatility (it's ability to evaporate) so it doesn't burn, it acts lean because the fuel that is there doesn't thoroughly burn. Pump gas has regulations for volatility that it has to meet. I've always suspected this is the cause for most of the stories about people trying race gas and seeing poor performance.

 

[Quinc]

After reading @Harbor post I looked up 100LL. 14.7

 

[Schmidty721]

Unless you have something unusual going on 93 octane will be fine. Pump gas will mean that your tuning needs to be decent. If you're holding it pinned for long periods of time with a high pump load (basically racing) then maybe not, but most people don't do that with an 850 Dasa. If you have something odd about your setup with super high ignition advance or some problem that would cause the motor to burn down, nobody on this forum knows about it so they can't help.

All the stuff above about stoiciometric ratios, for me...I'm pretty skeptical. Stale race gas tends to have lousy volatility (it's ability to evaporate) so it doesn't burn, it acts lean because the fuel that is there doesn't thoroughly burn. Pump gas has regulations for volatility that it has to meet. I've always suspected this is the cause for most of the stories about people trying race gas and seeing poor performance.

This. RVP in race fuel is all over the place, even within a sealed 5 gallon jug. Spec fuel for a series I tune motorcycles for is VP C-10 which has a RVP 4.02 psi. Testing new, un opened jugs typically results in actual RVP of somewhere between 1.5-2.5. As race fuel loses RVP, or goes "flat", it requires a richer mixture and more ignition advance to get full power out of it. Basically, you have to create much more heat to get the fuel to burn. Two strokes are extremely sensitive to this. Peak power can vary by as much as a full point AFR change between a proper fuel which has 4 psi RVP and a completely flat fuel.

So much more to fuel than octane rating, or even the stoich spec. Really the only way to do it proper is to adjust out a 55 gal drum to a spec you like and then keep it under pressure with nitrogen to protect the fuel. Pump gas is much more stable as long as your not switching between summer and winter blends.

 

[catfish4767]

So much more to fuel than octane rating, or even the stoich spec. Really the only way to do it proper is to adjust out a 55 gal drum to a spec you like and then keep it under pressure with nitrogen to protect the fuel. Pump gas is much more stable as long as your not switching between summer and winter blends.

So I could get by mixing 93 no ethanol pump with say 110 and be ok or would you suggest a different method?

I talked to the original owner of the motor said he was told by Kyle at DASA that "with the stroke the compression ratio is too high and needs 110 regardless". He said he initially saw 185 PSI and ran 91 octane but blew the motor (as mentioned in OP). What makes me scratch my head is hearing some of the scenarios above with 898cc/1000cc/1100cc motors running pump gas or the mixture. I see where some have mentioned it all depends on the overall set up of the ski. I don't know the first thing about timing so I will have to read up on that. Im sure some are reading this and thinking this motor is over my head, which is probably is, but no one around me has anything other than 550s/SJs that I know of. Just trying to get myself squared away and not totally fudge it all up. Thanks for the help for those who have chimed in

 

[Schmidty721]

So I could get by mixing 93 no ethanol pump with say 110 and be ok or would you suggest a different method?

I talked to the original owner of the motor said he was told by Kyle at DASA that "with the stroke the compression ratio is too high and needs 110 regardless". He said he initially saw 185 PSI and ran 91 octane but blew the motor (as mentioned in OP). What makes me scratch my head is hearing some of the scenarios above with 898cc/1000cc/1100cc motors running pump gas or the mixture. I see where some have mentioned it all depends on the overall set up of the ski. I don't know the first thing about timing so I will have to read up on that. Im sure some are reading this and thinking this motor is over my head, which is probably is, but no one around me has anything other than 550s/SJs that I know of. Just trying to get myself squared away and not totally fudge it all up. Thanks for the help for those who have chimed in

Sorry, I can't speak to your specific setup. My advise would be to pick a fuel or combination of fuel and stay with it. I've been out of the ski game for a while now but still browse here because I enjoy the content. I do ,however, have experience in engine tuning and fuels.

My post was more on the fact that different race fuels, or even the same race fuel batch to batch can vary more than just octane rating. A "fresh" fuel will run and tune completely different than a "flat" fuel. Hopefully someone can chime in with a definitive answer that is running a similar setup to yours.

 

[DylanS]

So I could get by mixing 93 no ethanol pump with say 110 and be ok or would you suggest a different method?

I talked to the original owner of the motor said he was told by Kyle at DASA that "with the stroke the compression ratio is too high and needs 110 regardless". He said he initially saw 185 PSI and ran 91 octane but blew the motor (as mentioned in OP). What makes me scratch my head is hearing some of the scenarios above with 898cc/1000cc/1100cc motors running pump gas or the mixture. I see where some have mentioned it all depends on the overall set up of the ski. I don't know the first thing about timing so I will have to read up on that. Im sure some are reading this and thinking this motor is over my head, which is probably is, but no one around me has anything other than 550s/SJs that I know of. Just trying to get myself squared away and not totally fudge it all up. Thanks for the help for those who have chimed in

The stroke, while it CAN affect compression if not adjusted for, does not change the fact that you still have 185psi. The stroker crank DOES cause longer dwell times at TDC (amount of time piston is at top dead center). Interestingly enough longer dwell times at TDC actually help combat detonation because your combustion area is kept significantly smaller for a longer period of time. The general consensus from what I’ve researched is that longer dwell times at TDC allow you to get away with running a less than ideal octane to some extent.
It will be impossible for anyone to give you the exact optimal octane to run for your specific setup but you will be able to be “safe” and close by listening to Kyle or any of the other experienced guys on this thread.
If you aren’t doing WOT pulls across the lake you really aren’t at a huge risk of blowing your motor up as long as you’re in the general ballpark.
Imo you’d be wasting money running 110 strait.
I don’t think the motor is over your head, I think you have to start somewhere and you’re getting a lot of awesome info. Keep researching and reading what other guys are running on similar setups to get yourself a number to start at or listen to what Dasa tells you and start experimenting from there.

 

[Storbeck]

If you were able to provide every last detail about this motor it would be a little bit dicey getting advice on this topic on the internet.

It is way more dicey getting advice from the internet when none of those internet people know anything about the motor that you're asking about.

850cc's from a 4mm stroker is an odd combination. Usually when I think 850cc dasa it's a stock stroke 89mm bore. I can't think of any common bore stroke combination that comes out to 850 with a +4mm stroke. So already we are dealing with kinda sketchy info.

 

[catfish4767]

850cc's from a 4mm stroker is an odd combination. Usually when I think 850cc dasa it's a stock stroke 89mm bore. I can't think of any common bore stroke combination that comes out to 850 with a +4mm stroke. So already we are dealing with kinda sketchy info.

I think this is whats throwing me for a loop. I cant find anyone with the same set up or a similar set up so who tf knows whats actually there. I think I may just get a pail of 110, mix a couple gallons with 93 and go from there.

 

[Storbeck]

I think this is whats throwing me for a loop. I cant find anyone with the same set up or a similar set up so who tf knows whats actually there. I think I may just get a pail of 110, mix a couple gallons with 93 and go from there.

You said DASA, but is it a billet motor or cast? If it's cast it could be some odd combination of bore and stroke in an old race motor that has a lot of exhaust duration, in which case 185psi of cranking pressure could be really high compression ratio.

Or is it a billet 850 that somebody then ADDED a 4mm stroker crankshaft and now it is more like 900cc's. Depending on how that was done it could be a very odd setup or not.

Do you even know that is is a 4mm stroker, or doe you even know that is is something more than stock stroke?

Do you actually know that it is running 185psi of cranking pressure? Compression gauges can be finicky and inconsistent. Did you measure yourself? With what type of gauge?

 

[Sparkplug]

Aviation fuel absolutely without a shadow of a doubt burns different than 89-93 octane. I chased tuning issues for a solid week before I realized it was the gas.

Not sure about race fuel. But 100ll absolutely will not run properly on an engine tuned around 89-93.

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[catfish4767]

You said DASA, but is it a billet motor or cast? If it's cast it could be some odd combination of bore and stroke in an old race motor that has a lot of exhaust duration, in which case 185psi of cranking pressure could be really high compression ratio.

Or is it a billet 850 that somebody then ADDED a 4mm stroker crankshaft and now it is more like 900cc's. Depending on how that was done it could be a very odd setup or not.

Do you even know that is is a 4mm stroker, or doe you even know that is is something more than stock stroke?

Do you actually know that it is running 185psi of cranking pressure? Compression gauges can be finicky and inconsistent. Did you measure yourself? With what type of gauge?

It is a billet motor. My guess is its a 850 that a 4mm stroker was added by Kyle at DASA. Original owner is supposed to track down the paperwork and send to me. Cant confirm if its 4mm or not but original owner says it is. I used a compression gauge that I have. Equus brand. Feel free to recommend a different one if thats garbage. Read 185psi for me.

 

[Harbor]

You can measure your crank stroke with a dial caliper thru the spark plug hole and determine your crank stroke. Use the depth measuring portion of the dial caliper and measure the piston travel from BTDC to TDC