fuel octane

[Fired Up]

Is there a rule of thumb formula for determining what octane fuel you should run based on your compression ratio?

[Fiat48]

I don't know of any "rule of thumb". I did a little searching and was amazed at how many fuels are offered. I suspect a lot of this is marketing.
http://www.vpracingfuels.com/vp_01_fuels.html
http://www.smithtex.com/racing/fuelcomp.html
Anyway at the last site one thing said I know is correct. A higher octane fuel (and thus a slower burn) is not always better. In PGF I ran 15 to 1 compression and used VP C12. When C14 came out everyone rushed to it as I did thinking we would pick up power. But C 14 actually slowed us down even after tuning.
Also at another race the VP truck did not show and all that was available was Trick racing gas. Though many guys used Trick we struggled badly at that race getting a tune up. So you don't want to jump around using different fuels.
Reason #98 why I run alcohol.

[Fired Up]

Thanks Bob, good to hear from you this year. I was hoping (expecting) you to be one of the voices of experience to chime in. You are right with the variety of octanes today. Will the higher octane (slower burn) cause plugs to load up prematurely? Or is a load up condition more of a fuel volume problem? Broad question to a specicific problem..I know. Assume all fuel systems are adjusted and flowing well.
Reason I ask is when warming the motor the plugs really load up on the trailer. Make a pass and the insulator cleans up, first couple of threads are dark though. I can't let the motor idle too long or she starts to foul the plugs. My thinking was a fuel swap I made last year. Running at 13:1 compression with 110 octane. Will a 105 octane improve my symptoms and not be in danger of detonation??

[Fiat48]

Thanks Bob, good to hear from you this year. I was hoping (expecting) you to be one of the voices of experience to chime in. You are right with the variety of octanes today. Will the higher octane (slower burn) cause plugs to load up prematurely? Or is a load up condition more of a fuel volume problem? Broad question to a specicific problem..I know. Assume all fuel systems are adjusted and flowing well.
Reason I ask is when warming the motor the plugs really load up on the trailer. Make a pass and the insulator cleans up, first couple of threads are dark though. I can't let the motor idle too long or she starts to foul the plugs. My thinking was a fuel swap I made last year. Running at 13:1 compression with 110 octane. Will a 105 octane improve my symptoms and not be in danger of detonation??
I would think the idle circuit plus a cold motor aggravates the plug fouling. Plus you are probably jetted rich as most guys are. You should use a hotter plug for warmups....say a 7 or even 6 heat range..projected tip. For racing go to a 8 or 9 heat range and keep the projected tip. I think with your setup a projected tip plug will clear no problem.
I don't think the octane is gonna make a difference much on warmup. We always have to err on the high octane side to stay away from detonation.
If it were me I think I would go to C12, use an NGK Projected tip plug in a 8 heat range to start. To save time I would do a single cylinder pyrometer with a memory (such as Computech has) as an exhaust temp reading compared with what I was reading on the plug. I would imagine that when you get pyro readings of close to 1400 degrees much of the symptoms will go away.

[Fired Up]

Using BP6ES NGK's now. As I recall Brodix recomended these for a starting point for my heads and compression range. Thinking lower than a 6 for warmups...?

[Fiat48]

Too hot for racing and are warm up only. If you are loading those....look to those idle circuits. They need restricting. Can be done with a small strand of electrical wire.
Do you find that presently the idle mixture screw are screwed way in.....and/or does it seem like turning them in or out does not change much in the idle department?

[steelcomp]

Another thing to check is to make sure the (idle) butterflys aren't open past the transfer slots.
Fuel octane has more to do with dynamic cylinder pressure than static compression ratio. Most likely a higher static CR will result in higher dynamic CP, but that's still an assumption. Cam profile and timing, RPM range, chamber/dome effeciency, etc. can all play a part.

[Moneypitt]

Scott, Bob, what about the vent holes in the butterflies. About 1/16th to start, I just can't remember how far from the edge to drill them..........Bob, he said the mixture screws were out 2 1/2 turns to make it idle without cracking the speed screw. Which way does that 4500 richen/lean?......Ray

[DMOORE]

Another thing to check is to make sure the (idle) butterflys aren't open past the transfer slots.
Fuel octane has more to do with dynamic cylinder pressure than static compression ratio. Most likely a higher static CR will result in higher dynamic CP, but that's still an assumption. Cam profile and timing, RPM range, chamber/dome effeciency, etc. can all play a part.
That's so true. a lot of people forget that they are different. I actually knew a gentleman who was running a 12.5-1 SBC in a street car on 91 octane. It all had to do with his cam. He manufactured his own, and was very successful in SCCA as well with it. The cylinder pressure, due to overlap and such, was kept in check.
Darrell.

[Fiat48]

Guys you got me...been away from carbs too long and I dont remember much about carbs as they were short time for me.
Yes if the butterflys are open too far and getting into the transfer port then problems occur....seems the guys with big cams drilled holes in the butterflys so the throttle plates could be closed to solve that.
So more info is needed for sure.

[steelcomp]

Ray:Scott, Bob, what about the vent holes in the butterflies. About 1/16th to start, I just can't remember how far from the edge to drill them..........Bob, he said the mixture screws were out 2 1/2 turns to make it idle without cracking the speed screw. Which way does that 4500 richen/lean?......Ray
Not sure it needs them, just wanted to mention it in case no one checked. 4500 can idle off all four, and should. I treat a 4500 like two seperate two-barrels...I do the exact same thing to both ends, especially with 1:1 linkage. Even with "soft" progressive, I adjust idle the same on all four. If idling only off one end, then the butterflys may be too far open.

[Moneypitt]

Ah, the plot thickens.....But is "in" lean or "out" lean. Does the screw restrict air or fuel? Sam, are you idling off all 4 corners????....Ray

[Fired Up]

Ray, define"idling off all 4 corners". Do you mean are all 4 adjusted equally..yes, about 2 1/2 turns out. Would you ever close 2 down and just adjust your idle mixture with 2 remaining?:jawdrop:
And one other thing...there are small holes drilled into the butterflys already...carb came that way.
I understand the transfer slots should not be exposed, but when i want my idle at 2000 and start to turn the idle screw guess what...transfer slots start to expose....can't do one without the other happening.

[Moneypitt]

Ray, define"idling off all 4 corners". Do you mean are all 4 adjusted equally..yes, about 2 1/2 turns out. Would you ever close 2 down and just adjust your idle mixture with 2 remaining?:jawdrop:
And one other thing...there are small holes drilled into the butterflys already...carb came that way.
No, all 4........How big are the holes, and was it new when you got it? Are the butterflies cracked at all? I don't have any sure fire cures, just picking at what comes to mind....You can't read plugs from an idle as everything changes so drasticly at WOT. Power valve (s)?? Back in the day, a good running drag car may require the return road in low gear to prevent loading up. If the 89 boat sat and idled low for a short time I believe it would also load up, but idling at 15-1600 seems to keep it clean........Then again, I've never studied the plugs after an extended idle......Alot of overlap will create a low vacuum impulse at idle. I think I would crack the butterflies and let it idle on the main jets, closing the fuel idle ports down low, or off. With the butterflies cracked the idle slots shouldn't work...Temperatue and atomization could also be playing hell with the idle, liquid fuel doesn't burn very well, and cold intake runners don't add alot to the vaporization..... As long as the motor responds at the hit???.........One of these days I want to see this sucker run, maybe then I would have some more ideas..........ray

[steelcomp]

Are you using both pri and sec butterfly openings for idle? (you said when I turn my idle screw...you should be adjusting both of them equally)If your idle seems rich, why don't you just run the AFR screws in a little? 2 1/2 turns at all four corners sounds like a lot to me.
I gotta hit the hay...good luck

[Fired Up]

I do adjust both pri and sec butterflys equally. Ray...come out to bako this weekend....good night all..continue tomorrow.

[Moneypitt]

I do adjust both pri and sec butterflys equally. Ray...come out to bako this weekend....good night all..continue tomorrow.
Sam, Bako last weekend, Vegas this weekend, good thing I don't have a dog or I might get bit as a stranger.........Also still trying to get to BHC.......Ray

[BDMar]

Are you using both pri and sec butterfly openings for idle? (you said when I turn my idle screw...you should be adjusting both of them equally)If your idle seems rich, why don't you just run the AFR screws in a little? 2 1/2 turns at all four corners sounds like a lot to me.
I gotta hit the hay...good luck
I agree. We run a lot of 4500's and they always end up around 3/4 to 1 turn out on the secondary side and around 1 1/2 on the primary side. 2 1/2 on all four corners seems like a lot of fuel at idle.

[Fiat48]

Yes....I wanna say 1 to 1 1/2 turns out is what I remember.

[Fired Up]

To answer a few questions....no power valves......1 small hole in each butterfly, about 1/8 inch (.125). With reference to the plugs I'm using, Bob brought up a good point about heat range. I was beat dead tired at work last night and wrote the incorrect plug on the thread. I'm running a NGK B8ES. For WOT heat range may is okay...discoloration in the middle of the strap...maybe a 6 range would serve better for warm ups, like bob said.
Now , say I turn the idle screws back down a turn, turn and a half.....idle speed drops too much. Trying to stay at 2000. If I turn my IDLE set screws up this will start to expose the transfer slots won't it?

[Fiat48]

Maybe not because you have those .125 holes in the throttle plates. You have to look and see where they are now.
PS: you can do the projected tip plug also which will help plug wise.
2000 rpm seems awful high to idle.

[Moneypitt]

To answer a few questions....no power valves......1 small hole in each butterfly, about 1/8 inch (.125). With reference to the plugs I'm using, Bob brought up a good point about heat range. I was beat dead tired at work last night and wrote the incorrect plug on the thread. I'm running a NGK B8ES. For WOT heat range may is okay...discoloration in the middle of the strap...maybe a 6 range would serve better for warm ups, like bob said.
Now , say I turn the idle screws back down a turn, turn and a half.....idle speed drops too much. Trying to stay at 2000. If I turn my IDLE set screws up this will start to expose the transfer slots won't it?
At 2K you should be well into the main jets, and the idle circuit is doing nothing. Now you're battling the overlap and vacuum signal at a relatively low RPM...........And your scoop is doing nothing on the trailer to add additional air to the mix...........Ray

[BDMar]

At 2K you should be well into the main jets, and the idle circuit is doing nothing. Now you're battling the overlap and vacuum signal at a relatively low RPM...........And your scoop is doing nothing on the trailer to add additional air to the mix...........Ray
You beat me to the punch. Exactly what I was typing as you posted this.:D Makes me wonder what your combination is and exactly what carb(s) you are running....

[Fiat48]

I don't know about you guys but what I always did was make the motor idle as slow as possible (throttle plate wise)....then set the air screws and then set my throttle plates to desired idle speed. With big camshafts I purposely did dist curves to have the advance all in the moment it started for better idle control and leave. I kept the idle low to save the V drive when putting it in gear.
In the rolling start days I actually rigged up a fast idle solenoid (like a Q jet had on it) so after going in gear...my approach speed was always the same.
Just some info.

[steelcomp]

I'm not sure I agree that you're "well into" the main circuit @ 2000, but you're definately at the transition stage. Are you seeing fuel drip from the booster venturi's at idle? That's a sure sign you're into the main circuit. Do these carbs have an intermediate? Have you tried playing with air bleeds? You can delay the main circuit slightly with a change in air bleed. If you have an intermediate, you can do the same there.
Try turning the AFR screws in a little, maybe to 1 3/4 - 2 turns, and then opening the throttle a little to increase idle. Then go a little more, and see what happens. Idealy, you shouldn't be more than 1 1/2 - 1 3/4. If you get into the transfer slots, you'll know it, but if your idle stays clean, my guess is that you'll have better throttle response, and a crisper sounding idle. Last resort, you might have to go a few thou bigger on the holes in the butterflys, but I wouldn't start messing with that yet.
Why so high on the idle?

[Fired Up]

Carb is a #9375 1050, air bleeds are fixed. It has a soft progressive linkage. Timing is locked out at 38. At 1400rpm's when I drop it in gear the idle drops a bit much, at 16-1800 rpm's it stays up, burns the prop a little better and the boat takes a better set as I pass the lights.
Didn't get a chance to pull her out tonight. Thursday morning I will have some time in the AM before I pack for the races and I'll try some of your (plural) suggestions. For starters a set of hotter warm up plugs.
Steelcomp, how will I "know it" when I'm into the slots, aside from seeing them?
Bob, tell me about the single cylinder pyrometer...easy set up or expensive sensors,data recorders ,laptops..etc?

[Fiat48]

http://www.computechracing.com/egtsystems.html
They make a clamp on probe that only requires you to drill a small hole in the header pipe. I make all the blown guys put them on. Generally I do #6 as with a blown motor that seems to be the hottest cylinder. I see the new units can now read 2 cylinders. One has always been good enough for me.
The unit has a memory and will record the highest EGT and the lowest. It's a great tuning aid especially when combined with plug readings.
LOL....think I have had mine a long time? I paid $195 way back when.
On that rpm fall off...put a timing light on the thing and you will probably see the timing drop back....a double hit on your idle. A faster curve can help...if you can....you want it all in when the motor starts...light springs.

[Jetboatguru]

Scott,
Flatbottoms leaving from a dead stop have a hard time taking the throttle. They need to have lots of fuel at the hit as the prop hooks and tugs the motor down. 2 1/2 turns out is pretty normal for us and in PHX, it might be 3.

[steelcomp]

Scott,
Flatbottoms leaving from a dead stop have a hard time taking the throttle. They need to have lots of fuel at the hit as the prop hooks and tugs the motor down. 2 1/2 turns out is pretty normal for us and in PHX, it might be 3.
Thanks for that input Tony.
To me, it seems that "hit" should be a function of the acc. pump. As soon as you open the throttle, regardless of rpm, you're off the idle circuit. I understand the need for fuel at the hit, but what you're saying dosen't make sense to me. Now mind you, I have zero prop boat experience, but I've been around enough cars with big ass slicks to know what can load an engine. May be apples and oranges...I don't know.
Just as an observation, I've heard some guys bring their engines up before launch...there's a trick with the acc. pump adjustment for that style of leaving. Here's the deal...If you've got the throttle open 1/4 or so before you leave, you've wasted that much travel of your acc. pump, and lost that much of the pump shot when you mash the gas. That's all I'll say on that.
At 2 1/2 - 3 turns on the AFR screws, you some how should be adding more air to that much more fuel. The engine really dosen't care where it get's it's idle fuel...just as long as it has enough air with it, and it can transition to the main circuit smoothly.

[steelcomp]

Steelcomp, how will I "know it" when I'm into the slots, aside from seeing them?At that point you'll be pulling fuel out of the main well, the AFR screws will become in-effective, the idle quality will go away, and it's likely to be way rich.

[steelcomp]

If I'm not mistaken, that 9375 has an intermediate circuit, which may be adding fuel at that RPM. You might want to eliminate that if you can. Don't know if you want to invest in adjustable air bleeds, but that may help, as well.

[BDMar]

The 9375 is a 3 circuit carb. At 2000 you are in the "transition" stage and I would bet you will see fuel being pulled through the boosters. Read carefully what steelcomp said in his last 4 posts. Your answers are there.

[Fired Up]

Thanks for this valuable input. As I understand there are two 9375 carb's (9375 & 9375-1) one with and one without an intermediate circuit. The one with the circuit has adjustable air bleeds mine lacks. When I get home tomorrow morning I'll try some trailer tuning.

[Pismo]

The new Merc 1200 requires 114 octane!

[BDMar]

Thanks for this valuable input. As I understand there are two 9375 carb's (9375 & 9375-1) one with and one without an intermediate circuit. The one with the circuit has adjustable air bleeds mine lacks. When I get home tomorrow morning I'll try some trailer tuning.
The 9375-1 is the second design 9375. I just spoke with my Holley rep and the 9375 is a 3 circuit carb. Just doesn't have the screw in air bleeds.

[Fired Up]

Thanks for the reference BD.
Well here is todays trailer tuning session:
I put a fresh set of NGK BP7ES plugs in. These have a projected tip and are one step hotter from what I normally use WOT. Next, I determined that my IDLE screws (primary and secondary) were turned in 1 complete turn after contact with the linkage arm. Meaning...when the butterflys begin to move. Also, my AFR screws were 2 1/2 turns out.
Okay, I started the motor and warmed it. I began by turning the AFR screws in 1/2 turn ...rpm's increased..I turned them in another 1/2 turn and the rpm's really increased to 2200 rpms. Too high for me. I then turned both IDLE screws out 1/4 turn.. the rpm's dropped some...I turned both out again another 1/4 turn and it dropped to about 1800...better. I checked the boosters every time I made an adjustment and at no time was there ever fuel flowing or dripping from them.
One characteristic I do notice is that if I hold the throttle at say 4000 rpms for about 5-10 seconds then let the throttle go the idle speed does not drop right down to 1800, but stays around 2000-2400 then slowly tries to drop down.....
I pulled number 7 and number 2 plugs and both insulators were still dark and carbon covered. There was no miss that I could hear though. So.....
All of this today and packed for the races..WHEW!:sleeping:

[Fired Up]

Well we made it back tonight...what a great weekend of safe and fast racing. Great weather and a really good showing of boats. Finally figured out the tuning on the carb. She was on a rail all weekend long. ....11 boats in the 10:00/10:49 bracket. I made it to the trophy dash AND I broke out by 5/100 with the quickest pass I've ever made. Now I'm really off to:sleeping: ...