how much hp do you gain boring out a 2.8 to 3.1? i ask because i bought a 86 se with a 2.8 or i thought it was a 2.8. i got it from a guy who was flipping it and had it for 2days so he knows little about her. after i got her i did some looking around on here and notice people talking about doing the 3.1 bore..... thats when i noticed on the trunk lid it has stickers of all the performance mods the owner put on her. they all match up never thinking anything about the huge 3.1 L sticker in the middle lol the guy i bought it from said it was a guy who worked for pontiac and it had a rebuilt engine with only 40,000 on it. so it may be true that its bored out. no way to tell without tearing her down right?

which brings me to if it is how much HP do you gain? i just bought a 87 gt 5sp with a blown engine (its rebuildable but ill pass) im just not sure if i wasnt to sell the 86 se whole and running to pay for the engine swap for the 87 or just drop the runing engine i have into her.??

thanks for your time and thoughts !!!!

According to the rule of thumb, it would be 15 HP, but it really depends on the rest of the engine config.

You are going from 170 cubic inch displacement to 189.17 cubic inches. Its about an 11% gain in size. My guess would be 10-15 HP depending on your mods. It will make a difference.

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

thanks Blacktree i did not know that im a newbie at this stuff! ok just got to decide if i should drop that one in the gt or put a 3800 in her i would love to do that just hope i can make that happen on my limited skills. i know i could do a even swap lol

The key phrase is "depending on your mods". Just an increase in displacement will not give you much at all. Not even worth the expense.

The reason is the intake. The intake barely supports 173 ci let alone more. I had an iron head 3.1 and it was rated at 140 hp. Exactly the same rating as the Fiero.

It increase displacement you should do it as part of an overall mod. Like adding the Trueleo intake, or the carb conversion.

Arn

quote Originally posted by ten11crash: how much hp do you gain boring out a 2.8 to 3.1?....the guy i bought it from said it was a guy who worked for pontiac and it had a rebuilt engine with only 40,000 on it. so it may be true that its bored out.

Whoa!

If the gentleman who supposedly converted this 2.8L into a 3.1L did indeed work for Pontiac, he presumably would know that GM's 3.1L engine came about not from boring out the 2.8L engine, but from stroking it (i.e., substituting a crankshaft with a longer "throw" in order to increase the engine's displacement to 3.1L).

Boring a 2.8L out enough to create a 3.1L runs the risk of making the engine's cylinder walls dangerously thin, a potential problem GM --- or anybody else --- wouldn't face when stroking a 2.8L engine in order to transform it into a 3.1L engine.

project34,,,,,,, the gut i got the car from was just a tow truck driver who picked it up days before thats just the info he told me was that its a rebuilt 2.8 with 40,000 mi on it... after i got the car home is when i seen the 3.1 lt sticker on the back of the deck lid. i have no idea if it is a 3.1 or not just assume from the sticker and the fact i was told it was rebuilt. plus the fact i dont know much of anything about that stuff. there is know whay to know unless i take the engine apart im assuming right? thanks

About imposible to tell what is in the engine ..Keep the runner for at least 3 months before you do any thing,, this way you will get to know the car..as you drive the car ,,and read this forum you will very shortly have a lot of fiero knowledge,,Plus you will get Ideas of what you want to do,, Many on the forum make an engine swap seem like a keg party followed bya panty raid on the loose girls sorority..
If you drive the car you will learn the good and bad of the car..Maybe you could drop a letter to the original owner who did the work on the engine,if any info is available ,,maybe you will luck out and he is a forum member ..
It is always better to start with a good running car it saves much frustration,,search the archives for swap info about engines and transmissions,, this is a very active forum and I wish I had known about it when I got my 2nd fiero....If you have limited skills do not jump into an engine swap.. learn first ,, this forum is an everyday lesson ,, you will learn a lot about fiero problems and the solutions..
Do you have 4 jack stands ? do you have a hi lift jack? do you have a place to do the swap? can you have a car in the garage or location for 2 to 6 weeks?? have you ever pulled an engine ?? there is/are plenty of experienced Fiero people in your area..

If you are(on other cars) a convicted, card carrying ,motor gearhead ignore my previous advise,,

[This message has been edited by uhlanstan (edited 11-24-2008).]

quote Originally posted by uhlanstan: About imposible to tell what is in the engine ..Keep the runner for at least 3 months before you do any thing,, this way you will get to know the car..as you drive the car ,,and read this forum you will very shortly have a lot of fiero knowledge,,Plus you will get Ideas of what you want to do,, Many on the forum make an engine swap seem like a keg party followed bya panty raid on the loose girls sorority.. If you drive the car you will learn the good and bad of the car..Maybe you could drop a letter to the original owner who did the work on the engine,if any info is available ,,maybe you will luck out and he is a forum member .. It is always better to start with a good running car it saves much frustration,,search the archives for swap info about engines and transmissions,, this is a very active forum and I wish I had known about it when I got my 2nd fiero....If you have limited skills do not jump into an engine swap.. learn first ,, this forum is an everyday lesson ,, you will learn a lot about fiero problems and the solutions.. Do you have 4 jack stands ? do you have a hi lift jack? do you have a place to do the swap? can you have a car in the garage or location for 2 to 6 weeks?? have you ever pulled an engine ?? there is/are plenty of experienced Fiero people in your area.. If you are(on other cars) a convicted, card carrying ,motor gearhead ignore my previous advise,,

ya thanks i understand what your saying, yes i have done a few swaps never a fiero tho. lol i have all the tools and a nice 500 sq ft garage and a few good fiends who know way more than me haha i was really just wondering if the engine i have is a 3.1 would it be worth keeping and putting it in the GT. the gt needs and will get a motor just not sure which one it will get thanks for the advice from all of you.

quote Originally posted by project34: Whoa! If the gentleman who supposedly converted this 2.8L into a 3.1L did indeed work for Pontiac, he presumably would know that GM's 3.1L engine came about not from boring out the 2.8L engine, but from stroking it (i.e., substituting a crankshaft with a longer "throw" in order to increase the engine's displacement to 3.1L). Boring a 2.8L out enough to create a 3.1L runs the risk of making the engine's cylinder walls dangerously thin, a potential problem GM --- or anybody else --- wouldn't face when stroking a 2.8L engine in order to transform it into a 3.1L engine.

Correct you are. Boring the 2.8L .030" will give you about 2.9L. Beyond this the wall thickness puts you in jeopardy. The crank must be changed to the one used in the 3.1L and you can bore to get about 3.2L. I believe that the question more correctly should have been, how much horsepower can you gain with an increase in displacemnt from 2.8L to 3.1L?

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

I'd swap a 3.4 V6 out of a 93-95 Camaro / Firebird. It will have the same look as the 2.8 fiero engine. Just hafta re-locate the starter holes to the other side, and everything will mount the same. These engines had a crank sensor but just block it with a bolt.

At least you'll have a bigger displacement, and no need to screw around with boreing or changing cranks. If I was in need to replace my 2.8l, that's what I would do...

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fierogt28

88 GT, loaded, 5-speed

quote Originally posted by fierogt28: I'd swap a 3.4 V6 out of a 93-95 Camaro / Firebird. It will have the same look as the 2.8 fiero engine. Just hafta re-locate the starter holes to the other side, and everything will mount the same. These engines had a crank sensor but just block it with a bolt. At least you'll have a bigger displacement, and no need to screw around with boreing or changing cranks. If I was in need to replace my 2.8l, that's what I would do...

I agree. The 3.4L push-rod is a great swap. That is what i did, and I love it.

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1986 Pontiac Fiero SE 3.4L | The Fiero Blog (Since April 2000)
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My Portfolio site | JustinChristie.ca

quote Originally posted by ten11crash: project34,,,,,,, the gut i got the car from was just a tow truck driver who picked it up days before thats just the info he told me was that its a rebuilt 2.8 with 40,000 mi on it... after i got the car home is when i seen the 3.1 lt sticker on the back of the deck lid. i have no idea if it is a 3.1 or not just assume from the sticker and the fact i was told it was rebuilt. plus the fact i dont know much of anything about that stuff. there is know whay to know unless i take the engine apart im assuming right?

If it's actually a 3.1L engine that was created from the 2.8L engine, I'd think that almost certainly would have been created by using a 3.1L/3.4L crankshaft. (The 3.1L/3.4L "crankshafts" are actually both the same crank.)

Maybe someone else on this forum knows of an easy way to detect the existence of such a stroker crank in a 2.8L block without taking the engine apart, but to answer your question, I don't.

you would not gain that much HP, BUT, you would gain TORQUE. (longer the stroke, =more torque) Hp is a bi product of torque. A dyno measures Torque, then a formula is used to calculate Horse power. thats what you "feel".

quote Originally posted by project34: If it's actually a 3.1L engine that was created from the 2.8L engine, I'd think that almost certainly would have been created by using a 3.1L/3.4L crankshaft. (The 3.1L/3.4L "crankshafts" are actually both the same crank.) Maybe someone else on this forum knows of an easy way to detect the existence of such a stroker crank in a 2.8L block without taking the engine apart, but to answer your question, I don't.

2.8 has flat top pistons , 3.1 has a dish .
I guess he should be able to "feel" if theres a dish using a stick throug a plug hole when piston is on top