Octane requirement...

[Waffles]

Alright, here's what got me wondering. The mod motor Mike Bell is building for me will be right around 12:1 compression. The stroker kit came from Livernois, and their people said it'll be no problem on 93 octane. Cool. Then, I'm chit chatting with a guy at the track who's going on and on about how I'll need race gas. Well, I don't think that's true, but I wanna know how to tell for myself.

I've had various things explained to me, about how boost effects octane requirements as well. I was told it was calculated off the effective compression. Okay, so the effective compression of my Supra is 8.5:1 static compression @ 13psi = about 16:1 effective compression. Wow, I must really need race gas then, except I don't. It runs fine on 89. Also, if I throw 5psi to my 12:1 motor I'd also have about 16:1 effective compression. Will that also run on 89 octane? I guess it doesn't depend on the effective compression then.

Anyway, someone please take me to school.

[Mista Bone]

All in the tuning.

[Waffles]

It can't ALL be in the tuning. I mean, can you run 87 octane at 14:1 static compression "with a tune" for it? I doubt it.

[Jeff88coupe]

You may get away with running it on 93 pump gas....but I'd also look at maybe running it on E85. There are guys running 15:1 on E85.

[Mista Bone]

It can't ALL be in the tuning. I mean, can you run 87 octane at 14:1 static compression "with a tune" for it? I doubt it.

12.5:1 on 87 octane
14.0:1 on 93/94 octane

If it doesn't preignite (diesel) then it is tuneable.

Timing might not be what you think or make the desired power, but it can be tuned. Almost like tuning a nitro motor, need very little timing advance at times.

If you want more timing and power, then you need more octane.

I would suggest talking to a tuner at MD about this, he understands it more than me.......he tunes Honda's and Ford's :)

About fuel mileage as well, ask Mad Matt Johnson on his very noticeable improvement in his Honda running a copy of the chip I used to get 53 mpg AND 121 whp from a motor that was 85 whp stock using simple bolt-ons and a cam swap.

The beauty of EFI and not jetting a carb.


Sorry KenB/DaveB if this causes more work load :)

[Waffles]

I know you guys know this stuff from your past experiences with it. That's great, but does it really just come down to an experience of trial and error to know what you can do and what you can't? It seems like there must be some formula, or rules, or guidelines... something that I can actually figure out for myself without having to count on other people's different experiences.



If it doesn't preignite (diesel) then it is tuneable.


So, what compression does pre-ignition start with what octane? How does boost effect that?

[Waffles]

Oh, Bones, we finally got that B16A swapped into the EG Hatch. It was actually a pretty easy swap. The guy who owns the car thinks it was one of the worst experiences ever lol. In his mind, if an engine that doesn't belong in a particular car doesn't just bolt right up, it's the end of the world. lol I'm thinking, "Hey, we didn't need to fab anything ourselves... very smooth!"

[cstreu1026]

I know you guys know this stuff from your past experiences with it. That's great, but does it really just come down to an experience of trial and error to know what you can do and what you can't? It seems like there must be some formula, or rules, or guidelines... something that I can actually figure out for myself without having to count on other people's different experiences.



So, what compression does pre-ignition start with what octane? How does boost effect that?

I don't think there is any one set number. There should be a lot of factors that influence it including the cam specs and combustion chamber design.

[Waffles]

So, you build a motor with an educated guess of what it needs and you could be $5 per gallon off?

I was toying with the idea of putting a Kenne Bell on my Cobra instead of going with spray. The thing is, I don't want to limit my ability to drive it because fuel would cost me $7/gallon. I wouldn't mind going E85 because I'd just buy a drum of it at a time to solve having no close E85 stations.

Then, because of the explanations I was getting about that, it brought up questions about my Supra. I'm limited on the Supra by the stock ECU cutting spark at 13psi. Bigger injectors, supporting fuel system, and an air flow meter from a v8 Lexus tricks the computer past 13 to 19 (although the stock turbo will only do 16.) As I look at that, I wonder why what effects one car doesn't effect the other so much.

[Rick93coupe]

You want to build a 12.0:1 motor and put a Kenne Bell on it? Unless your talking about another car, its E85 or race gas for that combo.

[mustangrfast88]

yeah man it is gonna be a e85 or race gas motor

[Waffles]

E85 is fine, I'm down with that. So, how much boost can I run with E85 and 12:1?

[ibstrokin]

No one can give you exact answers, there are too many vaiables. Generally, low comp,high boost setups are capable of more power at a given octane level,and it's easier to change boost than it is comp.

[Mista Bone]

Oh, Bones, we finally got that B16A swapped into the EG Hatch. It was actually a pretty easy swap. The guy who owns the car thinks it was one of the worst experiences ever lol. In his mind, if an engine that doesn't belong in a particular car doesn't just bolt right up, it's the end of the world. lol I'm thinking, "Hey, we didn't need to fab anything ourselves... very smooth!"

5 hour job, how can it be that hard?????

But then again the Hybrid Honda forum dates back 14 years :)

[Rick93coupe]

Generally, low comp,high boost setups are capable of more power at a given octane level,and it's easier to change boost than it is comp.

Exactly. Isn't 1 point of compression worth something like 4% of a power increase? Keeping the compression down and adding a couple of pounds of boost will make way more power and be way more flexible on the fuel you can run. If it were me, I'd opt for swapping the pistons out for a set with a decent size dish if it's going to be boosted.

[JASON408]

I don't think there is any one set number. There should be a lot of factors that influence it including the cam specs and combustion chamber design.

This is a very true !!!! the cam has alot to do with cylinder pressure as does the chamber . Is real easy to say most cars with alm heads can handle 11 to 1 on 93octane. this covers 85 % of the cars out there . Now heres where it get confusing . Depending on cam timing and overlap you can have a 14 to 1 motor that runs on pump gas and a 10 to 1 motor that needs race gas . There is alot that goes in to it now they both will run on pump gas but when you start hamering on it you if your tune up is not right .You getting to rebuild it soon . Now the timing might need to be so low on the 14 to 1 motor that it will not run good . So to make power it needs race gas . Nos and boost work alot of the same when it comes to comp ratios alot of nos motors are 12.5 to 14 to 1 now they will make alot of power at 16 to one but your tuning window becomes very small to much timing its not so forgiving . Turbos smae way 8.5 to 1 big window 11 to 1 alot smaller . Best bet is to get a good engine builder and listen to what he has to say

[Waffles]

Well, my question to the forum was not really about my specific setup. It was the general question I wanted an understanding of. The likelihood of me boosting that motor is nil, was mostly a fleeting thought that left me asking the questions of why or why not. I just want to understand the reasons.

I'm not going to complete the motor, of course, without a plan that Mike is comfortable with and is confident that it can be tuned. Just searching for the knowledge is all....

[Mista Bone]

Exactly. Isn't 1 point of compression worth something like 4% of a power increase? Keeping the compression down and adding a couple of pounds of boost will make way more power and be way more flexible on the fuel you can run. If it were me, I'd opt for swapping the pistons out for a set with a decent size dish if it's going to be boosted.

More about 3% per point until about 13:1 then the returns diminish.

Small bore Hondas, 3"-3.25" bores normally see close to 5% gains per point of compression.

[NXcoupe]

Well the good engine builder said to not use this kit, but it's what he has to use, and so I make the best of it. We'll make it work. That's not what he's asking. He's asking why he can't put a twin screw or roots blower on a 13 to 1 engine and why octane seems to work for one combo and not another. Answer is, it's timing, and cam profiles. Ignition timing can be manipulated to get just about any combo within reason to live and peform, not well mind you, but propel a vehicle down the road at the very least. But that's not what you are going for. You want good power out of your combo, and I can tell you those pistons will not take a lot of detonation at all. They are a low quality piston from Mahle, not their top end piston. Don't get me wrong, it will work well for what you are doing, but don't assume you have a 5k reciprocating assembly there, it's a good one that is much better than the stock stuff but not indestructable. But you will have to error on the side of caution when tuning it and with that compression and E85, you will be able to get quite a bit of power out of it, probably more than you could with pump gas because the alcohol allows you to take full advantage of the extra compression.
As for the cam timing, opening the exhaust valve earlier and holding the intake valve open longer causes lower cylinder pressures. This is also referred to as tightening up the lsa. The centerline on either the exhaust or the intake or both can be moved around to give you a total lsa that is a smaller angle and produces more overlap.
to answer the orginal question, your supra has about 14 degrees total timing probably at wot with your boost level, maybe less depending on camshaft profile and cylinderhead design. If you want to put a blower on 13 to one, run E85, and run about the same timing or a lot less, you will probably not gain enough power to even compensate for the hp it is taking to spin the blower over. Just hypothetical, but you get the drift. what has been said above is that you can tune a car to use just about any combo, but it doesn't mean it's going to make any power with that combo. What you have is a great combo for a nitro methane top fuel engine, but not a good street blower engine. You can't start a build and then change it in midstream, not that you're doing that, but it's a great example. You are building an excellent mild nitrous shot engine. It would hold up to a decent 200 or so shot with a nice conservative tune and live to tell about it. You would like the feel.
There is no magic to this. It's all just simple math and science. After a while of tuning cars, you get to know what timing works well with what combo and you have a good starting point to fine tune the car in.

[JASON408]

Well the good engine builder said to not use this kit, but it's what he has to use, and so I make the best of it. We'll make it work. That's not what he's asking. He's asking why he can't put a twin screw or roots blower on a 13 to 1 engine and why octane seems to work for one combo and not another. Answer is, it's timing, and cam profiles. Ignition timing can be manipulated to get just about any combo within reason to live and peform, not well mind you, but propel a vehicle down the road at the very least. But that's not what you are going for. You want good power out of your combo, and I can tell you those pistons will not take a lot of detonation at all. They are a low quality piston from Mahle, not their top end piston. Don't get me wrong, it will work well for what you are doing, but don't assume you have a 5k reciprocating assembly there, it's a good one that is much better than the stock stuff but not indestructable. But you will have to error on the side of caution when tuning it and with that compression and E85, you will be able to get quite a bit of power out of it, probably more than you could with pump gas because the alcohol allows you to take full advantage of the extra compression.
As for the cam timing, opening the exhaust valve earlier and holding the intake valve open longer causes lower cylinder pressures. This is also referred to as tightening up the lsa. The centerline on either the exhaust or the intake or both can be moved around to give you a total lsa that is a smaller angle and produces more overlap.
to answer the orginal question, your supra has about 14 degrees total timing probably at wot with your boost level, maybe less depending on camshaft profile and cylinderhead design. If you want to put a blower on 13 to one, run E85, and run about the same timing or a lot less, you will probably not gain enough power to even compensate for the hp it is taking to spin the blower over. Just hypothetical, but you get the drift. what has been said above is that you can tune a car to use just about any combo, but it doesn't mean it's going to make any power with that combo. What you have is a great combo for a nitro methane top fuel engine, but not a good street blower engine. You can't start a build and then change it in midstream, not that you're doing that, but it's a great example. You are building an excellent mild nitrous shot engine. It would hold up to a decent 200 or so shot with a nice conservative tune and live to tell about it. You would like the feel.
There is no magic to this. It's all just simple math and science. After a while of tuning cars, you get to know what timing works well with what combo and you have a good starting point to fine tune the car in.

very well said!!!!

[NXcoupe]

Thanks Jason, and to correct a misnomer in my post, Mahle makes two different levels of pistons. They make a street/strip performance piston, and then they make their over 1k buck pistons for racing. Your pistons have a different type of aluminum that is better suited to street driving, controlling piston slap on start up and not swelling as quickly. They are also coated and are a very good piston, a step up from say probe or DSS entry level pistons. I was tired and in a bad mood when I wrote that, had a long hard week behind me. Sorry, don't want this to come back on you in future years when you are selling it and someone posts up well mike bell said your pistons SUCK! That's not what I meant and this is my official disclaimer, kind of like the one that 5.0 mustang will do after my email about Matt Johnson's bad ass coupe in real street. Nice if they would read the card he filled out.

[Mista Bone]

Tuning is EVERYTHING, period.

OEM rods (Honda) that were thought to only be good for 200-220 hp have seen 400 hp before getting squigley. With proper rotating assembly there is ALOT more to be had on the motor, but DAMN! Easy to get 600-700 hp out of same motor with sleeves/rods/pistons build, but being conservative and easy on the timing you'd be surprised.

Might I ad, 400 hp Honda on the street is useless due to traction, it might get bite at 80-100 mph but at the track......smoke show or axle breaker till 660 ft.

[A05C]

from pro-systems website


UNDER THE SCOOP
How to Choose an Octane



Remember octane is "resistance to ignition".

Octane is the enemy and is a friend. If you run too low of an octane the engine can auto-ignite the fuel like a diesel.

If you run too high of an octane, the fuel will pass through the engine unburned or will burn on its way out the exhaust and cause issues on that end. Increasing the Octane often requires more ignition timing to get the fuel to burn "in the chamber" and without this increase in timing you end up with the max burn and intended peak pressure to occur at a later crank angle. So you lose everywhere.

Choosing octane is very tough, so you have to look for signs.

Too high of an octane is the toughest condition to diagnose.

If the vehicle gets lean when its cold out, BUT adding jet makes it rich down track as the engine gets heat in it. Drop your octane.

If you are required to run more ignition timing than you think you should or you have to run a hotter plug to get heat on the strap than you think you should. Drop your octane.

If the engine is slower to accelerate than a previously lower octane fuel. Drop your octane.

Too low of an octane is easier to diagnose, the engine detonates and it hates timing. So you keep adding jet to stave off detonation and now it just rolls black smoke immediately when put under a load. Increase your octane and pull the excess fuel back out.

Although there is more info you can add to this (we have a tuning DVD with a big section on this), these are the highlights of what you need to know for proper selection.

Now as we said in another post, with too high of an octane the engine can interpret the mixture as being lean as it has an unburnable mixture displacing a location where burnable fuel should be and there are left over available oxygen molecules looking for a companion. These unburnable molecules are what makes the mixture lean.

Yes the car can lean pop and backfire just like it needs more fuel because the engine does not know the high octane fuel is there as that part of the mixture is not burnable anyway.

But if you add the fuel, you are adding good light ends that will burn, but you are also adding some unwanted heavies that will soot and load up ring packs and give you some serious grief.

Too high of an octane will also compound as it gets cooler out as you go even further away from the "auto ignition" that you were trying to escape with the high octane.

Now the mixture as the engine sees it will be even leaner and more of a problem (cooler fuel..harder to light..cooler air more available oxygen).

So then you really get into trouble.

Watch for these signs and adjust accordingly.

Thanks for reading.

[KenB]

The answer is most people err on the conservative side of compression because to tune it 'correctly' you don't have the time or money or proper equipment. OEMs spend insane amounts of money to come up with what's called a MBT (Maximum Brake Torque) spark table for a given octane of fuel on a given engine combo. This requires sophisticated knock detection, extensive data logging in all conditions and trial and error. No one is going to do this for one engine combo in their own car. So based on experience or recommendations of others, you come up with a combo on the safe side and go from there. Remember that just because the pump says 89 or 93 doesn't mean that's always what you get. Same with E85. So a margin of error always has to be built in.

On newer 2005 Mustangs you do have very good knock sensors to rely upon but even these can become unreliable based on changes to the engine. Things like extra piston noise, extreme cam specs or superchargers can trigger false knock. Or is it real knock? hmmm that's the problem.