Should I build for 87 or 91 oct. fuel?
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From: Alabama
Car: 92 Camaro
Engine: 305
Transmission: T5
Should I build for 87 or 91 oct. fuel?
I'm getting together a new engine for my newly purchased Camaro, it has about 9.5 compression and I was planning on running preminum fuel in it, then I just read a post on another fourm and after reading that I was wondering if I should get different pistons to lower my compression to 8.5 so it could run well on 87 oct. Just how much power is to be gained from a point or two of compression anyway? Here is the post I was refering too. Anyone know anything about this?
I have been doing some research on fuels, their make-up and how they react with parts in the engine. I have come across some great info that I would like to share for both the performance use and everyday driver use.
I ran across a private report that was done for several OEM manufacturers on the use of premium fuels in their engines. I would offer it up for reprint but it is copyrighted and owned by a consultant that I work with some time, so I can't reprint it here...or anywhere for that matter. But I can offer some abreviated insight to it's content. So, you "couchguys" get the chips and beer out.
As some of you might know already, not only do we use premium fuels in our hotrods but alot of "performance" OEMS recommend premium fuels as well. Most of the OEM recommendations comes from good, bad and ugly assertations. First, there is the assumption that fuel is frequently aged and not up to snuff. The recommendation of premium will help drivability in this case because age degrades octane performance. They don't want the car to be blamed for poor performance so to speak. The second is that you REALLY need it for the ultimate driving experience.
But life is not ALL good with premium fuels. If you remember some of my dissertations on fuels of the past, you have learned (or most of you already know) that they contain much higher percentages of aeromatics such as xylene and toluene, which in fact are the very chemical concoctions that raise the octane performance of the fuel. This is good.....buuuuuuut... aeromatics burn much differently than other hydrocarbon compounds and leave a ton of soot (yes..that's carbon) all over the place! Have you ever seen the tailpipes of your car covered with soot, yet the plugs look to be right on.....questioning your carbs (or fuel injection) mixture? My 32 roadster was notorious for this and my wife's Acura as well. The pipes out the back where jet black yet the tune was perfect. This can ruin your plug reading skills too. Black soot all over the base rings of otherwise perfectly good looking plugs could be fuel related....not jetting.
The answer is that aeromatics leave this stuff all over the place and especially on cool surfaces. So, running cooler thermostats (which we know is a no-no anywho) exacerbates this problem...especially in the heads and valves.
So what am I saying here? Don't use premium unless you HAVE to...and don't even ask about racing gas. Loaded with aeromatics!! It is far more harmfull than useful. This also is really important our newer daily drivers because these aeromatic ladened fuels will plug your EGR systems to boot and other things. How many of you out there have taken your egrs out only to find a snowdrift of carbon blocking the ports and or tubes.
Just some information to share with my buds.......anybody have any experience with this that could be related?
I ran across a private report that was done for several OEM manufacturers on the use of premium fuels in their engines. I would offer it up for reprint but it is copyrighted and owned by a consultant that I work with some time, so I can't reprint it here...or anywhere for that matter. But I can offer some abreviated insight to it's content. So, you "couchguys" get the chips and beer out.
As some of you might know already, not only do we use premium fuels in our hotrods but alot of "performance" OEMS recommend premium fuels as well. Most of the OEM recommendations comes from good, bad and ugly assertations. First, there is the assumption that fuel is frequently aged and not up to snuff. The recommendation of premium will help drivability in this case because age degrades octane performance. They don't want the car to be blamed for poor performance so to speak. The second is that you REALLY need it for the ultimate driving experience.
But life is not ALL good with premium fuels. If you remember some of my dissertations on fuels of the past, you have learned (or most of you already know) that they contain much higher percentages of aeromatics such as xylene and toluene, which in fact are the very chemical concoctions that raise the octane performance of the fuel. This is good.....buuuuuuut... aeromatics burn much differently than other hydrocarbon compounds and leave a ton of soot (yes..that's carbon) all over the place! Have you ever seen the tailpipes of your car covered with soot, yet the plugs look to be right on.....questioning your carbs (or fuel injection) mixture? My 32 roadster was notorious for this and my wife's Acura as well. The pipes out the back where jet black yet the tune was perfect. This can ruin your plug reading skills too. Black soot all over the base rings of otherwise perfectly good looking plugs could be fuel related....not jetting.
The answer is that aeromatics leave this stuff all over the place and especially on cool surfaces. So, running cooler thermostats (which we know is a no-no anywho) exacerbates this problem...especially in the heads and valves.
So what am I saying here? Don't use premium unless you HAVE to...and don't even ask about racing gas. Loaded with aeromatics!! It is far more harmfull than useful. This also is really important our newer daily drivers because these aeromatic ladened fuels will plug your EGR systems to boot and other things. How many of you out there have taken your egrs out only to find a snowdrift of carbon blocking the ports and or tubes.
Just some information to share with my buds.......anybody have any experience with this that could be related?
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From: Cypress,Tx
Car: 89 Camaro RS
Engine: 350 carbed now
Transmission: World Class T5
Axle/Gears: Peg Leg w/ 3.73's
It really depends on what your cam specs are, type of heads, ignition timing etc. I would think a drop from 9:5:1 to 8:5:1 compression would take away at least 10hp or so. Look at the Lg4 engine compared to the L69 the only difference is compression and a slightly larger cam and the L69 has 40 or so more hp depending on the year model of the Lg4. I personally wouldnt run any less than 9:1 compression on any kind of performence motor.
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Joined: Jan 2004
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From: So. Ohio
Car: 88 Camaro
Engine: L98 350
Transmission: 700r4
I'd definately go for the 9.5 comp. With good vortec style heads, tight quench, and correct fuel mixture you could drive normally with mid grade gas, premium for some serious driving.
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From: West Texas
Car: '82 Camaro
Engine: 305
Transmission: wc T5
Axle/Gears: 3.23 posi
It also depends on what you want your car to do. If you want to cruise, low compression is just fine; if you want to be competitive you have to go for high compression. I built a big block for my pickup with stock compression, about 8.5, but used a wide lobe seperation, mid range cam, cleaned up the stock heads, Edelbrock Performer (not RPM) and 750 double pumper and got an excellent bumpy idle, unbelieveable mid range torque, hardly any money spent on aftermarket parts, and when I got to the pump bought the cheapest gas--a really nice bonus on a tow vehicle. It also went from about 10 mpg to 12 mpg--20% better--and double the power of stock.
My Austin Healey has an 11/1 327 first built in 1972--but it doesn't see much time on the street.
My Austin Healey has an 11/1 327 first built in 1972--but it doesn't see much time on the street.
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From: Cincinnati, OH
Car: 1990 GTA Black/Black lthr
Engine: 305 TPI stock
Transmission: T-5
Axle/Gears: 3.42
I think it totally depends on what you are going to do with it. Is it a daily driver driven over 50 miles a day? That extra .20 a gallon is going to add up in a hurry. Or is it a weekend warrior, where winning is the only thing? Run it on racing fuel then.
Also, I think you could switch to thicker head gaskets to lower the commpression some if you want. I'd build for 9.5:1 with a thin gasket, and if things get bad, switch to a thicker gasket.
Also, I think you could switch to thicker head gaskets to lower the commpression some if you want. I'd build for 9.5:1 with a thin gasket, and if things get bad, switch to a thicker gasket.
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From: Alabama
Car: 92 Camaro
Engine: 305
Transmission: T5
The engine in question is a 383 with a Tremec 5-speed and 3.42 rear gears. It's going to be a daily driver and have a 700cfm 4 bbl TBI Commander 950 EFI on top of a Vortec Air-gap RPM, E-Tec 170cc aluminum heads, I'm shooting for a quench distance of .035 so I would never want to switch to a thicker head gasket, It's not going to be raced, I just want to have fun with it and I'm hoping to get more than the 14 MPG I get with my current daily driven truck. I know I could put in a big cam to reduce the DCR and run 87 but that's going to kill MPG also. So I guess my question is can I run a small cam that will pull 16-18" of vacuum with 9.5 compression?
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From: Pacific Northwest
Car: '85 IROC
Engine: LB9
Transmission: 700 R4
IMO - 8.5:1 CR is unnessecarily low for an F-bod.
In one engine I'm running 8.8:1 CR, with .040 quench clearance (zero deck), a Comp 252 flat tappet cam, and 76cc factory smogger heads.
This is in my 1/2 ton truck, and I have never used anything but 87 octane.
If 8.8:1 CR will work in a truck with obsolete iron heads, a car that's 1000 lb lighter should be good with 9:1 or even more, especially with some nice Edelbrock alum heads.
With a CR that's too low, you'll sacrifice both power and fuel efficiency. If you're doing a fresh build, might as well up the CR some.
I'd probably aim for about 9.2:1 if you want to use a small cam.
In one engine I'm running 8.8:1 CR, with .040 quench clearance (zero deck), a Comp 252 flat tappet cam, and 76cc factory smogger heads.
This is in my 1/2 ton truck, and I have never used anything but 87 octane.
If 8.8:1 CR will work in a truck with obsolete iron heads, a car that's 1000 lb lighter should be good with 9:1 or even more, especially with some nice Edelbrock alum heads.
With a CR that's too low, you'll sacrifice both power and fuel efficiency. If you're doing a fresh build, might as well up the CR some.
I'd probably aim for about 9.2:1 if you want to use a small cam.
Last edited by Streetiron85; Sep 7, 2004 at 11:40 AM.
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From: So. Ohio
Car: 88 Camaro
Engine: L98 350
Transmission: 700r4
Keep in mind that 64cc heads, flat top pistons, and the close quench will put you over 10:1 in the 383 I believe. That's marginal for street driving with mid grade gas, need premium I'm betting. I could be wrong, check your numbers.
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From: West Texas
Car: '82 Camaro
Engine: 305
Transmission: wc T5
Axle/Gears: 3.23 posi
Your cam should really match your compression. With aluminum heads and programmable EFI you can successfully go high or low, all depending on your use of the vehicle. It seems you want good mileage, good vacuum, want to kick heiny now and then but don't need to be competitive at the track. OK. With the 3.42's and the 700R4 you should be able to do it, aiming for power between 1500 and 5000 rpm, cruising at 2000. All that stuff about high octane gas additives is nice, but marginal, not having much to do with your basic choice.
I think you're looking for compression between 8.8 and 9.1 and a mild to midrange cam to match. That way you can cut back on your timing and burn 87 octane for general driving or bump up to 34-36 degrees and run premium on Saturday night. Your 383 will laze along getting good mileage during the week near the bottom of its power range but use that low first gear and excellent midrange long stroke torque from light to light.
Use a standard head gasket, like they say, and if the dependability and mileage isn't to your liking you can put in a thin one later, going to 9.5.
I think you're looking for compression between 8.8 and 9.1 and a mild to midrange cam to match. That way you can cut back on your timing and burn 87 octane for general driving or bump up to 34-36 degrees and run premium on Saturday night. Your 383 will laze along getting good mileage during the week near the bottom of its power range but use that low first gear and excellent midrange long stroke torque from light to light.
Use a standard head gasket, like they say, and if the dependability and mileage isn't to your liking you can put in a thin one later, going to 9.5.
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From: Alabama
Car: 92 Camaro
Engine: 305
Transmission: T5
Tom3, I'm not running flat-tops, they are KB forged -18.5cc reverse dome.
rbjones, where did you get the idea I had a 700R4? Like I said above I have a Tremec 5-speed, IMHO no Camaro should ever have a Auto unless it's drag only.
I think I'll just try what I have and tune it with the laptop to run good on 87, I may have to load a different program and run 93 oct if I ever decide to run it at the strip for fun.
rbjones, where did you get the idea I had a 700R4? Like I said above I have a Tremec 5-speed, IMHO no Camaro should ever have a Auto unless it's drag only.
I think I'll just try what I have and tune it with the laptop to run good on 87, I may have to load a different program and run 93 oct if I ever decide to run it at the strip for fun.
Last edited by AsphaltAssalter; Sep 7, 2004 at 05:11 PM.
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From: Pacific Northwest
Car: '85 IROC
Engine: LB9
Transmission: 700 R4
It sounds like your combo is a good one for some major torque.
I think you have the right plan, try it on 87... if it isn't right there's always 93 octane.
I didn't see anywhere that you mentioned your cam specs. It seems to me unless you make the mistake of choosing the wrong cam you'll get a lot better than 14 mpg.
I think you have the right plan, try it on 87... if it isn't right there's always 93 octane.
I didn't see anywhere that you mentioned your cam specs. It seems to me unless you make the mistake of choosing the wrong cam you'll get a lot better than 14 mpg.
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From: West Texas
Car: '82 Camaro
Engine: 305
Transmission: wc T5
Axle/Gears: 3.23 posi
Ooops--sorry--I was reading too fast, saw that 700 and the number 4. We think the same on trannies--just converted mine from a Suzie-Creamcheese auto to a stick and made the car worth driving. I like your going for the lower compression. Our first Camaro was a '69 Z28 brand new--ended up with a 12 to 1 427 and 4.56's on the street, fun for a while but really ruined the car. My brother and I--same team as 35 years ago--are having a blast with this stock 305 and T5, thinking very carefully about each step we take away from mileage and dependability. Heck, it runs so good I might sell my Hondacar.
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From: Cypress,Tx
Car: 89 Camaro RS
Engine: 350 carbed now
Transmission: World Class T5
Axle/Gears: Peg Leg w/ 3.73's
You should easily be able to run 87 octane with 9:5:1 compression on an aluminum head. Hell I know of people with 10:5:1 on 87 octane and aluminum heads without any knocks or pings.
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Car: 1989 Trans Am
Engine: 355 HSR
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It is absolutely ludicris IMHO to build a motor depending on what gas you want to run in it. Maybe that is just my $.02, but who knows.
Do what you want w/ the car, compression etc and w/ the cam you want to run, heads etc, and be happy w/ it. So what if you have to run 93 octane gas in it over 89 or 87... do what you want and be happy...
Do what you want w/ the car, compression etc and w/ the cam you want to run, heads etc, and be happy w/ it. So what if you have to run 93 octane gas in it over 89 or 87... do what you want and be happy...
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From: Alabama
Car: 92 Camaro
Engine: 305
Transmission: T5
Originally posted by Chris89GTA
It is absolutely ludicris IMHO to build a motor depending on what gas you want to run in it. Maybe that is just my $.02, but who knows.
It is absolutely ludicris IMHO to build a motor depending on what gas you want to run in it. Maybe that is just my $.02, but who knows.
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Originally posted by AsphaltAssalter
Isn't that what everyone does??? Otherwise we would all be building 14 to 1 engines and running 110 octane racing fuel in them. I just choose to build and engine that will run on anything I can put in it and not complain about it. It's not that I'm cheap, hell, I bet I make more money than 95% of the people here.
Isn't that what everyone does??? Otherwise we would all be building 14 to 1 engines and running 110 octane racing fuel in them. I just choose to build and engine that will run on anything I can put in it and not complain about it. It's not that I'm cheap, hell, I bet I make more money than 95% of the people here.
That is what I mean... Hell if you are worried about .5 compression to make it to 87 octane... that is what I mean...
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From: West Texas
Car: '82 Camaro
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Yes, a tiny little horse and chaps, must have chaps, and a good high range for those long distance cruises to the mountains of New Mexico.
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