Turbo Engine Planning. 400, 406, 415, or 420. Discuss

[Scott Liggett]

None of ours are boosted. Still have not seen any data to support nor any of the experts I talked so far say stay away from siamese blocks.

[Bob Holmes]

Given the marginal increase in cubic inches that you are contemplating, I'd want the thickest cylinder bore wall possible. Turning the screw a little bit will give you far more horsepower and torque than an increase in cubic inches.

6 to 7 psi isn't much. I'm not sure I'd drop the compression ratio too much or it'll be soggy off throttle. Especially if you are going to use a set of up-to-date aluminum aftermarket heads.

600 hp turbos when you are only looking for 300 horsepower per is going to leave you with the potential for quite a bit of lag. Are you kidding us and yourself about what your true goals are. The desire for 1 more psi, and then 1 more, and then 1 more...can be addictive.

You're going to need some big wastegates.

[yellomalibu]

While I know very little about turbo engines, general rules for me include never boring a perfectly good bore. The extra 6 inches of displacement isn't worth sacrificing one (more) rebuild. Also, the thinner the cylinder walls, the less likely they are to stay round under pressure.

So, of the choices listed, I would vote for the straight up 400.

I believe the siamesed cylinders in the middle of a 400 are a valid concern. That is where I blew the head gasket on my (non turbo) 406... on both sides... right in the middle in the thin area. Two problems are the exhaust valves are right next to each other there AND the siamesed cylinders possibly don't cool as well. I did not, however, have my heads drilled for steam holes, which may or may not have been a large factor.

Rather than go down in cubic inches from your 383 to a 350, consider keeping it a 383?

...or ring the deck/ heads and use solid copper head gaskets?

[Beagle]

anybody want to talk about squish/quench in a boosted power plant? Big bore and long rod (piston dwell time / detonation threshold) with forced induction? Port sizing and low engine speed port velocity?

[SuperBuickGuy]

None of ours are boosted. Still have not seen any data to support nor any of the experts I talked so far say stay away from siamese blocks.

you asked for opinions, but frankly, you should listen to your experts not smucks like me on here. I'll enjoy watching your build.

[squirrel]

looking at fast turbo cars on drag week....lots of LS motors down to the mid 8s, lots of big blocks faster than that. not many traditional small blocks. I wonder how the port layout (IEIEIEIE vs EIIEEIIE) plays into this observation? I don't know, just curious.

[Loren]

Keeping in mind I'm no engine expert and am just adding thoughts in here for...discussion...

In spite of the fact that I've had engines w/ siamese bores that have done OK (a 130,000 mile '73 Chevy Vega ---which must have set the record for early Vega durability--- and my 496" Challenger, how's that for a spread) I still have issues with them. Here's a section of cylinder wall getting a 1500 degree blast on one suface of it but has a tank of 200 degree coolant on the other side,and a half-inch next to it as part of the same casting is a section just getting the blast on both sides. Cooling there is only what can soak out into intake charge for two of the four cycles, and across the iron out to the cooled areas. It's a design compromise, for cubic inches only...the smart guys who tout them may also need to show me one they've put 50K of regular use on, not just ones they've sold that are making big power this year, before I'm really that sold on it.

Also, still pretty leery of most aftermarket heads and blocks. GM has had enough problems building castings over the years, how is a little aftermarket shop going to improve on that? Not saying they can't...I just want to know exactly what they're doing better, and see some product that has proven out. My experience w/ aftermarket items is that sometimes the money behind the creation of it cares about different things than what the customer is hoping to get. I am NOT looking to put anybody down or cast negative light, just show me you can do it or prove it out somehow.

Re: the above comment about siamese-bore issues while not having steam holes...yeah, those steam holes have got to be there. The siamese portion of those 45-degree banked cylinders creates a wall of pockets where vapor can accumulate and block water contact, if it's not let through the temps can go through the roof there (literally), that's a killer.

Re: Compression with a turbo motor: You can compress a particular mixture (not talking amount yet) of fuel and air up to a certain pressure and have it not detonate, that is a given. The total volume of compressed mixture under a 62cc head (that may have happened to make 10:1 static compression) is less than the total volume under a 72cc head (that may have happened to make 8.5:1 compression). On a naturally-aspirated motor we already know the higher-static compression motor is the winner, but when you can tweak volumetric efficiency w/ something like turbos, you can easily fill those 72 cc's to the same ultimate pressure as you could the 62. 72 is 1.16 times 62, so it stands to reason that if a turbo motor with a given short block w/ 62cc heads has the boost pumped up to max before detonation is making 600 hp, then the same combo w/ 72cc heads pumped up to max (which will be with a little higher boost pressure, easily done) will be 600 x 1.16, or 696 hp. With a side benefit that with a lower static compression you can run crappy gas that day if you want. Of course there are other considerations, but this is what they mean by "letting the turbo make the power", not the static CR amount...just something to consider when choosing compression ratio in a boosted motor.

Re: BBC vs. SBC handling. My mom's (bless her soul) '72 Chevelle 350, stock, handled like crap while my all-iron '70 396 El Camino was like a sports car. Just, the El C. had all the mods, stiffer/bigger/whatever. So there... That said, any time you can get away with the mouse motor you sure do reap the benefits...whenever we all hop in the 454 truck I just look at the gas guage and cringe, my former 350 beater was soo much better and made the same power.

[Scott Liggett]

Loren, you are right. I do want to build a fairly mild engine naturally aspirated, then pump up the power with mild boost levels. I am not after internet hero status of 1000 hp. To me, it's useless on a car you want to drive everyday. Day to day driving, boost numbers will stay low, the mild engine will move the car, the EFI I am planning will help with drivability; then I put my foot into the wood I will have the power.

The other reason for not going overkill on the engine HP is the cost involved in beefing up everything behind it.

[Beagle]

looking at fast turbo cars on drag week....lots of LS motors down to the mid 8s, lots of big blocks faster than that. not many traditional small blocks. I wonder how the port layout (IEIEIEIE vs EIIEEIIE) plays into this observation? I don't know, just curious.

Valve angle... why mess with 23* heads when you're getting serious?

[Huskinhano]

Scott, I'm no expert but to pass along some comments.

There were a lot of Mustangs making 600 hp on 5.0 short blocks, while they were ticking time bombs they did make it on a small displacement.

I understand the trend today is small chambers with big piston dishes. But while a custom piston with dish would be no problem with a NASCAR budget we're reality based.

There are the turbo camps that believe in high CR, big cams and low boost. After all cylinder pressure ir cylinder pressure no matter how it's made. An argument for, is you're not adding as much heat on the intake with lower boost.

Jim made a great comment about the 5.3 with a turbo. Used 5.3's are dirt common and cheap and we've all seen then on youtube kicking butt




Hard to argue with that!

[Huskinhano]

[BigAL]

Valve angle... why mess with 23* heads when you're getting serious?

All depends on who's heads you are using. AFR 23* heads are no joke. I made 1.36 hp per cubic inch with a 10.5:1 355 small block, using a mild hydraulic roller cam with 236/248 .050 duration and .520/.540 lift on a 113 LSA. If Scott goes with the 4.155 bore and the 3.875 stoke he'll have 420 cubic inches, if he was to build it like I built my 355 he could make the same 1.36 hp per cubic inch and that would yield 571hp. Now saying that dropping the compression for the turbos would knock about 40hp off that number so he should make about 530hp if he built his motor using the same AFR heads I'm using which are the 210cc. So he would have to make about 7psi of boost to reach his 700hp goal. As for the 400 block and boost, Nelson does it all day long and doesn't seem to have a problem. I'd just make sure the use a good MLS head gasket.

[Scott Liggett]

Scott, I'm no expert but to pass along some comments. There were a lot of Mustangs making 600 hp on 5.0 short blocks, while they were ticking time bombs they did make it on a small displacement. I understand the trend today is small chambers with big piston dishes. But while a custom piston with dish would be no problem with a NASCAR budget we're reality based. There are the turbo camps that believe in high CR, big cams and low boost. After all cylinder pressure ir cylinder pressure no matter how it's made. An argument for, is you're not adding as much heat on the intake with lower boost. Jim made a great comment about the 5.3 with a turbo. Used 5.3's are dirt common and cheap and we've all seen then on youtube kicking butt Hard to argue with that!

Part of not getting a junkyard LS and sticking a turbo on it is because who I work for. Nof only do I want to build something different. (Same reason I do not own a Camaro) I also want to promote some of the really cool parts we build. I am not going the easy, cheap route. Any schmuck can do that. It is not in my personality to do so. I follow my own path. Always have.

[Bob Holmes]

anybody want to talk about squish/quench in a boosted power plant? Big bore and long rod (piston dwell time / detonation threshold) with forced induction? Port sizing and low engine speed port velocity?

You're just trying to stir the shit. LOL