4.1 v/6 question


May 25, 2001
can a 4.1 be made into a turbo motor i have a complete 4.1 and a complete spare 3.8 and was wondering if it is possible anybody have any ideas????????
I wouldn't do that

My idea is not to do it.....4.1's can be and have been turned into turbo engines...but they are weaker. There's only 14bolts on that oil pan for a reason....not enough material there to allow for more. There's really just not enough material to handle much stress on the block. If you're talking about a relatively stock build, then I don't see why not, but I had a few discussions with my machinist about it and I'm convinced it's not the way to go. You have to machine out the area for the oil return, but not really anything else. I've heard and read on the boards about guys with them that have done ok so far, but I've heard my fair share of horror stories as well. Post this in the general tech area and I'm sure the debate will begin...We've heard it all. From 4.1's being modded to whether or not there's a difference between the NA3.8's and the Turbo 3.8's....I like to think that maybe there's something different despite the reasoning from the metal content debate and stick with production turbo engines. HAve yet to lose one to an actual failure of a hard part(maybe I'm just not going fast enough though ;) )To each his own,...I guess.
Yes...It is possible...It just costs more...go here for more info...

I've known Phil(who wrote the article) for a few years now...He has had to build several 4.1L motors to get a combo that worked and held together...but as he said in the article, quote"First, if you have a GOOD 3.8 block from an 86-87, then stop thinking 4.1 and build your 3.8! The advantages to running a 4.1 are definitely
apparent, but hardly worth the price of custom pistons if it can be avoided, IMHO. "

Really, about the only thing you would really want out of that 4.1L motor is the crank(rods are the same, too), if it's still the original crank and hadn't been replaced yet...But tons cheaper to build the 86/7 3.8L block...
If you doubt the 4.1, look at the hotair 3.8 compared to the the 4.1L I doubt you'll see much difference. I had a custom set of forged pistons made for $450. Hypers for the 4.1 are only $250. Other than that the cost is the same for building the 3.8L. I ran my hotair 3.8 that was strong enough to pull two Viper GTS's on the top end for two years so I don't think strength is an issue. We'll see this year. I have a combo that should go into the 10s with the 4.1. Lee Thompson has also been in the bottom 11s for a long time now with the 4.1. 14 bolts don't mean anything. My dad's stage II is a 14 bolt block.
Nah nah man....14bolts does mean something on a production block. Let's not go misleading someone by throwing in Stg 2 facts...there's too many differences between a Stage block and a production block to go comparing. More webbing, lack of water jackets between the cylinders,...I can go on and on. I have no doubt that you have gone that fast with a 4.1 and applaud you for going a different route with your engines, but you did crack a cylinder didn't you...? What I'm getting at is that the later 20bolt rails are 20bolts because they are stronger due to a slightly improved design. Ever looked at the two next to each other? Do it sometime and you'll see. They didn't just add them cause they felt like it or just didn't think of it before. They could add them cause they have more strength to begin with. From what I understand(and if I'm wrong, I need a new machinist..) the earlier ones just aren't strong enough to have that many holes in em. Correct me if I'm wrong.......Anyone? I have to agree that for most of us, building a 4.1 is not cost effective for what you get. As for the custom pistons....I thought I was the only one with the Wiseco hookup ;) Are they Federal Moguls or somethin? They tried to sell me some of those pistons they designed for use in the blown alchohol chevys....pin bore is in the wrong place...needs to be further down. While it may make no real difference in even a mild 10sec car, it does to me so I'm sticking with a true turbo piston. If you're getting complete sets of pistons for 450(that include rings, pins, and locks to float em)...hook it up man!! I can get custom order Wiseco's with rings, pins, and locks to float em for that, but my machinist is a little cautious when it comes to selling stuff in groups. I'd have to put up the money for the first order(probably about 10sets or so) to convince him and I don't have that kind of money laying around. Anyway, I guess it's like I said before, to each his own. Do what you want but figure your cost first and then decide. If you aren't gonna try and cram a bunch of boost in and go run 10's, a 4.1 will probably be fine. For me, a 3.8 is the logical choice. I like having an original production block that came in my car in it, and not something else. That's me though. I like em stock :cool:
I like to push the limits. So I am going to build one up this year and push my luck. Maybe it will hold together. I think it is worth a try.
4.1 block

i ran one all season few years ago, it had cheap pistons, esp mild cam, ported heads, with o rings, pt 70 turbo, all the work was done by my self, i ran 10.50 all the time 22 lbs boost, never hurt it, i sold it to my best friend, he ran 11.16 at 122 mph, hot air car, never had a problem, i wouldnt put no more than 25 lbs boost in it, the deck is not as thick as a turbo block, some people say they run hotter because they have siamese cyl, so does all 4.1 stage 2 blocks, just my 2 cents,:D
I only cracked a cylinder because I'm an idiot for getting drunk and letting a friend drive my car with no wastegate. 30+psi on 87 octane doesn't work too well. There were also two bent rods but at least the crank stayed put.

I got a great hookup on forged pistons. They're $450. I sent in a stock 4.1 piston and a forged TRW 3.8 turbo piston to basically copy. I got them with 1/16, 1/16, and 3/16 ringlands and 8.5 compression. They are for the stock pin and come with spiral locks so the stock rod will have to be honed a little to make it full floating. I had a set of hypers never used that I paid $230 for and got scared and sold them for $150.

I've looked at the hotair blocks next to the 109 blocks and the 109 next to the stage II. There's no doubt the 109 is a nicer block but as far as absolute strength of the bottom end I don't think there's a difference. I've said it before, Ken Duttweiler told me there's no difference in strength. My car trapped at 110mph on extremely low boost. I ran it around for years at 24psi, in fact for about a whole year it ran on nothing but av gas or race gas. It never saw pump gas. I beat on it everyday and never had a mechanical failure.

So where you gettin the pistons man? Are they TRW? I thought they got bought out and weren't being made anymore....I know availability went way down for a while and now I see folks putting them in again... I prefer the Wiseco's or BRC's over anything else for my engines, but that's just me.
I forget the name of the company that's doing it, I'm getting the deal through a local machine shop. Mine got shipped out Friday so hopefully I'll get them early next week. This is the first set they've ever made for the 4.1L so I want to get mine and look them over and measure them before I recommend them to anyone. I'm taking them over to a guy who did a lot of the Stage motors to get his opinion also. I need to get the info on them since there's a few people interested.

They really don't realize how much people will pay for those pistons and I don't want them to find out. When I was told the price, the first thing the guy said was I'm not going to like the price and when he told me it was all I could do not to smile.
TRW got bought out by Federal Mogul/SpeedPro...From what I have heard, can't remember who I heard it from, but it's not a permanent shutdown...just temporary until they get a new production facility built...Probably won't be until mid-summer until they are up and running again...Probably be a whole lot longer than that until they re-start producing the Buick 3.8L turbo(or any Buick) pistons again...They most likely will catch up on all the OEM, and aftermarket Ch*vy/F*rd stuff before they move on to do all the small market stuff...

I just received my set of TRW/SpeedPro forged pistons this past week...I don't know how many places I've called to find them and still had to pay about $70 more than the other set I bought 2 yrs ago...Probably got one of the last sets out there...
20 bolts means nothing at all

The 20 bolt pain rail is meaningless as far as strength goes. The problem with the 4.1 block is that the head bolt bosses are tapped flush with the deck surface. They're so close to the bore that they cause a stress riser which makes the deck crack to the cylinder. The only reason the 14 bolt 3.8 block doesn't crack is that the bolt bosses are farther from the cylinder.

The solution for the cracking is to use ARP _STUDS_, and do not over torque them. If they are properly installed, the deck is flat and square, and so is head, you should not have a problem if you do not over torque them.

The lifter galley is somewhat weaker than a 20 bolt or a 14 bolt 3.8. If you use titanium putty to reinforce the lifter galley that won't be a problem.

The mains need a girdle like the RJC girdle, just like the other two 3.8 blocks.

The 4.1 IS a viable option if you build it right. It costs no more to properly prepare a 4.1 block than a 3.8 block.
What would the proper torque be for an apr stud and nut? I have torqued my bolts down to 100 ft pounds on my 3.8.

the 3.8 i have is allready 30 over and needs to be redone there were peices of turbo in the piston the 4.1 i have has never been touched it is a one owner car that a old lady owned it got t boned at a red light so i got it cheap but if the 3.8 is easier to rebuild what is the 4.1 worth
The one thing you can do with the 4.1L head bolt holes is to have the machine shop drill the top 2 threads out thereby lowering the stress to below the surface of the deck like the 3.8L 109 blocks are...

And with the cost of custom forged pistons and the TRW forged being unavailable for a while, the cost between the custom 3.8L and custom 4.1L forged is about the same...So for now, the cost to build a 4.1L is probably fairly close to doing a 3.8L...The rings for the 4.1L will be cheaper IF when you have the pistons made, have them made to a 4.000 bore which is the same size as a Chevy, so ring choice is a lot greater and cheaper as well(more of them made=less cost)...(The stock 4.1L bore is 3.965, so have the custom piston made .035 oversize to 4.000 and use a standard size 4.000 Chevy ring for turbo or supercharged app)

And before anyone jumps in and says to use an off-the-shelf Chevy forged piston...YOU CAN'T...The pin height, ring locations, etc. are all different between the chevy piston and the custom 4.1L piston...
Wow...all this talk is makin me wanna build one just to see how difficult it really is compared to a 3.8.....although I can't agree that 14 or 20 bolts on the pan rail is meaningless and doesn't matter....why would they go to 20 if it really didn't matter????? Doesn't make sense. Maybe it's just cause with any turbo engine, you cannot avoid blow-by. If you are running a decent bit of boost you will get some even if you run total-seal rings. There's just no way around it. Then again, I guess a factory one wouldn't really have that problem for some time and they probably didn't do it for that reason. Maybe it's just as simple as preventing leaks? Someone enlighten me further please.
Most likely it was just to prevent leaks around the oil pan...If you look at the differences, the bolt holes are closer together on the 20 bolt pan which would give better clamping on the oil pan and less likely to leak...Most of the improvements to the block for strength were in the lifter valley...Compare the 14 and 20 bolt(109 casting) motors and you will see the difference...The 20 bolt 109 blocks were cast with a solid lifter valley with large holes drilled into the casting instead of the holes being cast into the block...The late 85 20 bolt blocks still had the same lifter valley as the 14 bolt blocks, but that was the only difference...

But who really knows for sure why they did what they did...

Needless to say, whatever block you use, if you make enough power OR detonate your motor too much, you will do damage to your motor...
The bores on the 20 bolt and the 14 bolt 3.8 are too thin, and the 4.1 is no different. Thin walls flex, and that causes blowby, even if everything else is right, nothing will compensate for cylinder wall flex.

If you are dead serious, I'll tell you how to get blowby to a minimum. But a bunch of people are going to tell you I don't know what I'm talking about. It ain't gonna be easy, it ain't gonna be cheap, so if you want cheap and easy, just ignore this post.

First, toss all those trick rings in the trash. Why? Well Winston Cup teams can get any ring they want, and they use Speed Pro rings, with no tricks at all. A good set of Speed Pro File Fit single moly rings with medium tension oil rings will seal a cylinder as tight as anything you'll find. Forget static leakdown tests. Any "gapless" ring will look good there, but you don't build boost and make power sitting still with the engine off, do you? You want a dynamic blowby test under power. Winston Cup teams test that way, they don't "leak' an engine for any reason other than a baseline or a diagnostic test. By the way, call the Speed Pro Tech line, and ask them how to gap your rings. They'll tell you to gap the second ring WIDER than the top ring. DO IT that way, they didn't spend a couple hundred thousand dollars working with the best engine builders in the world for nothing. Take advantage of their work. The top ring seals only under pressure from the top, and any pressure between the top and second ring WILL UNSEAL the top ring. The second ring is intended to seal only when it is under slight crankcase pressure (ie. on the intake stroke, where there is normally negative pressure in a normally aspirated engine), it really only functions in a supercharged engine under part throttle as a compression ring that is used to pull a vacuum on the cylinder. Otherwise, it is just and oil scraper. That's all the second compression ring does under boost, scrape the rest of the oil off of the cylinder wall. Next time you get a set of rings (I mean good ones, not cheap "rebulder" sets) take a close look. On the top ring, you'll note a chamfer on the inside, and a dot or pip mark with instructions that the chamfer and the dot or pip faces the top of the piston. That is because compression pressurizes the ring from the top, via the clearance in the ring groove, to force it out to seal the cylinder. On the second ring, you'll notice the pip or dot is up, but the chamfer faces DOWN, and the ring is actually taped so that the BOTTOM edge is what contacts the cylinder. It is a reverse taper reverse torsion ring designed to seal under pressure from the bottom, and vacuum from the top, and any pressure from the top will unseat it. Rings only work under a pressure differential.

Next, pour the block up to the water pump holes with Hard Blok, and fill the lifter galley with titanium putty. Put an RJC girdle on the bottom, do all your oil system modifications, deburr your block, and have it machined. Start be having it align honed. Then, using a BHJ fixture called a Block Tru, have it paralell decked for height and squareness. Then chamfer and deburr the holes in the deck. You are now ready to install a set of ARP studs. Chase the bolt holes, get a set of flat heads, a set of gaskets identical to those you plan to use, a torque wrench, a bottle of Loctite primer, and a bottle of 271 RED Loctite stud and bearing mount. Use the primer to clean and prime each hole, then use the red Loctite on each stud, tighten it to the proper depth, finger tight only, and quickly install a gasket and head, then torque the head to 25 foot pounds. After the studs, the Hard Blok, and the titanium putty have cured for at least 48 hours, you are ready to take the heads off, keep the gaskets. Torque the mains, girdle and all, then bore the block. Then have the block cleaned and scrubbed to remove all boring residue from the cylinders. Measure each piston individually, and assign it a cylinder. Now, install a head gasket (you did save them like I told you, didn't you?) and a deck plate on each side, and torque them to the final torque ARP suggests. Then follow the Sunnen and Speed Pro suggestions for finsh, with a good set of fresh cleaned and dressed stones, flooding the cylinders with all the hone oil the machine will pump, and cleaning them often. Get a slick finish so the rings will seat without excessive heat and wear. Approach the final clearance size carefully, and avoid loading the stones or burnishing the bores. Yes, cylinders can be finished to +/- .0001, just like rod and main bores.

Now, here's the last part of the machine work. And there will be a lot of people who'll say don't do it. It won't work well on a car used more than a couple thousand miles a year on the street, but it won't hurt it either. Gas port the pistons. It DOES make a big difference, and it WON'T cause excessive wear, unless you use a crappy ring, or have a soft block. It will not be as effective on a 5000 RPM engine as it will on a 6000 RPM or higher engine, but it will make a difference. The reason is that it will compensate for any crankcase pressure by pressurizing the top ring from the top.

One other thing. If you take the trouble to mount an air pump (smog pump) to use to create negative pressure on the crankcase, it WILL seal the engine much tighter.

Of course, you can ignore all of that, and just decide that turbo engines are doomed to have blowby, leakage, and crankcase pressure. After all, it is your car, your money, and your engine.

Oh, by the way, when you pour the block, you'll need a dedicated stand alone oil cooler, at least a four pass unit. Put the RJC adapter for the big filter on it too.