CR vs. Twin Plug vs. Octane

1colbrz · 12-10-2006, 10:37 AM

I've read alot of post's regarding twin plugging high compression race/street engines. The information about flame travel and bisecting the combustion chamber with a high dome piston makes perfect sense. One thing that comes up occasionally in these post's is reference to the grade of fuel.

Here is my question:

Can a 2.7 with MFI and 11.0:1 CR run on race fuel (110-115 octane) without twin plugging? If so, would it have a performance disadvantage and how much (torque/hp)?

Intended use is track only: SCCA, POC, and PCA racing.

Your thoughts?

BURN-BROS · 12-10-2006, 11:19 AM

Gerry,

Twin plugging will allow you to achieve peak cylinder pressure. Race fuel will help,allowing for more advance to be utilized, helping the single plug configuration to get closer to peak pressure. Generally, 10 to 15 horsepower is gained throughout the rev range over single plug set-ups. Race gas is mandatory for 11/1.

Vintage911Racer · 10:26 AM

I have heard contradictory info on this. I could be wrong. But from what I have been told, twin plugging will not give you that much of HP increase.

For example if you take two identical motors with 10.5:1 CR, twin plug one and single plug on the other. The HP increase will be minimal and the performance gains are hardly noticable.

Now if you were taking a super high CR motor, that is wound super tight, and you are looking to get every ounce of HP out of it, then the twin plug is the way to go. A single plug set up would not be able to handle the high CR such as 13:1

Is this correct?

BURN-BROS · 12-13-2006, 05:32 PM

Mark,

The difference is noticeable. The Hemi design requires the most advance of all the current combustion chamber types. My personal findings on a 3.2 S.S. with single plugs was that at higher rpms the car "wanted" in between 40-42 degrees total on pump gas. I decided to limit it to 36 as the safest limit this particular build could take considering the circumstances. This leaves quite a bit of power on the table. Race fuel does help with peak cylinder pressure but I am not convinced that you can get all thats available. I have heard of conflicting ideology on the matter but have not heard the specifics. I'd love to hear about it though.

tadd · 12-13-2006, 05:47 PM

Don't forget work and really important for our motors, heat. The closer to TDC you light off, the more work goes into pushing the piston down and the less time you have the charge 'compressed hot' where it can transfer energy to the heads/piston. We are not running diesels... yet...

tadd

Vintage911Racer · 08:04 AM

On a 3.2 yes, but is the differnece the same on lets say a SS 2.5?

I have talked to drivers who have twin plug and they do not feel much of a difference on a 2.0 or 2.5L motors. these guys used to run single plug, went to twin plug and the only difference they claim to feel was a smoother transition through mid range. Lap times were no better wiht our without. Unless you are running a real high strung, high compression motor the requires twin plug it would seem to be a very pricey upgrade for such little gain on the track.

Again, this is all here say... so dont bash me... I do also know that I have run side by side with guys who have twin plug and tehy do not pull me down the straights.

Vintage911Racer · 12-13-2006, 07:37 PM

Oh yeah, Tadd. You cant post any more. You total post count is up to 267. that is my race car numebr.. so please stop. that is a lucky number.

armandodiaz · armandodiaz's Garage

It was my understanding that twin plugging will let you run higher compression with minimal octane. With that understanding, I would think 11.0:1 would allow premium pump gas (93 octane).

garibaldi · 12-14-2006, 05:30 PM

Two cents worth...and not a penny more, since Im not going to write a book about the theroy of flame front, cr, and the like....twin plugging in itself doesnt yeild more hp, it allows, under the proper circumstances, other factors to aid in producing more hp through efficiency. Basically, it gives you a better shot of lighting off the mixture. But, and a big but, its a zit on the ass of the gnat thats lying on the ass of the beetle, that lying on the ass of the bird, thats perched on the ass of the rhino, lying on the edge of the stream that feeds the lake that dumps out into the waterfall that goes to the river that feeds the atlantic ocean, that sits on the earth, thats part of the solar system.....get me drift. In a nutshell, at RPM, sh%t is flying around in there so fast, you really think any of it matters? just my opinion, on paper, in theory, it all makes sense, in physics, and in the books, and even when you draw it out in slow motion in you head,........ the air comes in , the valve closes, the piston comes up, the spark goes off and so on, but in reality, I think that God takes over after 3,000 RPM and does his thing.

BURN-BROS · 12-14-2006, 07:16 PM

Mark,

Race fuel does tend to equalize out the difference between the two(single/twin). The 3.2 vs 2.5 would react similarly the big difference should be compression ratio. I still believe that there is a noticeable difference between equally built engines( cams, compression, port size, displacement, etc...) Comparing your engine with twin-plugs at the track isn't quite apple and orange but there are too many variable between them such as dyno time/state of tune.
The difference between advance curves leads me to believe that you can get 100% of the possible cylinder pressure with twin plug and single plug around 90 to 95% with race fuel. This is my gut feeling. IMHO I agree with the choices made to Porsche's engine development program by their engineers were well quantified.
The modification is pricey no doubt. If you are at that 95% level I guess it would be hard to justify. If there are competitors that are still too quick I would look at it seriously....but I am a bit biased.

Vintage911Racer · 12-14-2006, 08:30 PM

I Hear you Aaron.

Thanks for the info. I guess when I start getting my A$$ handed to me by twin plug cars then.............how does the old saying go. If you cant beat them..........join them... LOL..

talk to you later.

1colbrz · 12-15-2006, 06:12 PM

I really like Garibaldi's response. Still the question calls: Can an 11:1 CR 2.7L with single plugs and race fuel run hard all day on the track without detonating itself to death? Has anyone done it successfully?

Vintage911Racer · 12-16-2006, 02:34 PM

If a 2.5 can do it. Why not a 2.7??? I know of people running that in a 2.5 without problems.

Bobboloo · Bobboloo's Garage

.

garibaldi · 12-17-2006, 06:58 PM

Yes you can run that combo, but your up in the high end of the zone. Theres a parabolic curve to the sensitivity of the whole process once you get up around and above 11:1. Sorry cant wow you with the scientific aspects of it, I just know that there is becasue thats the way it goes. Im sure one of the bookworms up here will chime in a rattle off a chapter or two for us

The cr and the displacement isnt the issue, as much as the efficiency and accuracy of controlling the whoel damn operation from start to finish. If you can keep a tight reign on the timing, the ignition, and combustion, the mixture, and all the other factors that are getting sucked in and blown out with every revolution then yes, it will work, and if the fuel has the proper octane , then it will take the higher CR, but your getting right near the fine line. All it takes is one sneeze, one hiccup and boom! Ive seen big blocks shake off a catastrophic failure like that a couple of times, and you pull it, put in a new rod and piston, lick the cylinder and go back out and keep racing, but a 911 motor.......Well, it sort of like accidently dropping your moring cup of coffee in its mug, they just sort of shatter. I worked with an old racer that had a car at Daytona, up on the banks it missed something, who knows what. The car came in , the driver said he thought he through a belt, since he heard something and felt a shake, and when they pooped the hood there was a intake manifold off to one side, and the headers, but nothing in between was really left. I built a 2.0 4 cyl with 1100 ccs worht of staged injectors per cyl, ran 9500 RPM, AEM engine managemetn with complex maps, sensors out the ass on the motor, and also was topped off with 39 psi of boost from a single turbo. It hiccupped at Willow springs and threw no 2 and 3 rods and pistons straight out of the block, wedged one into the firewall, the other tore through the radiator, , bent the crank and spit the head loose which was clamped down with 16 1.2 inch ARP head studs adapted from a Hemi application. So I guess the point is......it doesnt take much.

Its not like all that stuff really matters anyway, how tight do you think a distributor is controlling the spark at that high RPM, hell even the MSD stops shelling out "Multiple sparks" at a low RPM, There just isnt enough time to dish out what the name implies up in high revs. And then theres blow by, so the cr changes, and then cam profiles, overlap, so that affects dynamic compression, and that joke of a bicycle chain that runs the cams, you should see that thing bouncing around at RPM, and tell me how spot on your cam timing is at speed, and you sent all that time with the dial indictor chasing that last little fraction of a degree. It all goes out the window in my honest opion, its all mental masturbation for a good portion of it....and like I said, God....well word around the campfire is that he is a racer at heart

SO simply, yes, you can run the combo, it can work, you need to do it properly, and as best as you can, keep spares, dont think youll get 100k miles out of it, and at the end of the day.....who cares, its a freakin motor, have fun and kick ass with it, its just a chunk of metal.

garibaldi · 12-17-2006, 07:01 PM

oh and one more thing, verify everything, document it, and keep records, that way when and if it goes, you can take a look and maybe back off here or there. Its not a new combo, the cr is getting up there, but you should be fine. Pour the motor and know exactly what it is though, I never had a set of p/c s come out to what they claimed to be

ttweed · ttweed's Garage

Quote:

Originally posted by 1colbrz
Can an 11:1 CR 2.7L with single plugs and race fuel run hard all day on the track without detonating itself to death? Has anyone done it successfully?

Read this article by Steve Weiner. Detonation depends on a lot of factors, including timing advance, fuel quality, and ambient temps. I would say that a good quality 100 octane unleaded race fuel would be sufficient to allow an engine such as you spec above to live well on the track, given adequate oil cooling, as long as ignition advance is not pushed too far. Twin-plugging will allow it to run with 10 degrees less advance and increase the safety margin, reduce heat generation, and provide some marginal (3-4%) performance increase, according to Steve.

Relevant quotes from it:

"Unless you have a static compression ratio over 12:1, these unleaded racing fuels should work just fine, especially with twin-ignition."

Twin-plugging "...reduces the need for as much ignition advance to start and finish the combustion process when the piston reaches Top Dead Center. Since the spark event is starting closer to TDC in the compression cycle, there is less pressure from the beginning of ignition that is pushing 'back' down on the piston crown as the combustion event progresses. This lessens the 'negative' work done by the expanding gasses and allows all of the pressure building in the cylinder to push the piston in the correct direction, making the engine more efficient."

"By installing two spark plugs per cylinder, you will increase the acceleration of the ignition sequence. This can reduce the required advance by 10 degrees or more thus lowering cylinder head temperatures. In terms of power, twin-ignition will add some 3-4% or more depending upon compression ratio, over a single ignition system. RPM can increase as much as 700 RPM at top speeds. If high compression ratios are to be used, twin-ignition allows all of the power benefit to be gained from the increase. Twin-plug equipped 911 and 930's run much crisper and cleaner with lower cylinder head temperatures and improved throttle response. Plus, a twin-plug 911 is much less prone to plug fouling with today's fuel."

1colbrz · 12-28-2006, 04:36 PM

Thanks Guys,
Great replies. Looks like a high CR single plug engine would work but more stressed than a twin plug unit.

Drawbacks are cost and performance points (added by sanctioning body).

Pluses are a cooler running less stressed engine with marginally more hp/torque.

I think I'll have the heads prepped for twin plugs. That leaves the option open. Don't know if I'll install the twin ignition or not.

Thanks and Happy New Year