high compression and boost

[BigTeddy254]

Here's the deal with compression and boost, to clear up some blatant misinformation in earlier posts. Statements have been made that too much compression causes detonation. This is utter nonsense. Boost and compression are exactly the same thing, with one exception that I'll cite later. Let's say you have a 10:1 compression motor with 5 Lbs of manifold pressure (boost). Your effective compression is 13.4:1 Now let's drop the static CR to 9:1 You can now run 7.2 PSI for the exact same effective compression of 13.4:1. The exact same number of air / fuel molecules are being elevated to the same pressure in the combustion chamber.

There are still two elements in the detonation equation which are lacking. Those are temperature and spark advance. Here's where the intercooler makes the big difference. Any compression of air raises the temperature. It doesn't matter if its a turbo, roots, twin screw, or simply the normal compression of a piston during the compression stroke. Compress X volume of air to Y times atmospheric pressure and you will raise the temperature by the same amount. The difference with compressing the air outside of the engine is that it can be cooled before being induced into the combustion chamber. This cannot be done in the normal process of squeezing the air with a piston in the motor. A small amount of heat may be lost into the surrounding metal, but it's not significant compared to passing it through an intercooler. That's it in a nutshell. You can get an intake charge that is overall cooler using external compression with an intercooler.
We still haven't addressed timing, where myth still abounds. The purpose of spark advance is to cause peak cylinder pressure to occur at around 14-16 degrees ATDC. This is geometric, determined by the mechanical lever advantage of piston, pushrod, and crank. It never changes, but spark varies with RPM, naturally. Unfortunately, decades of hot rodding inefficient piston / valve / head designs led to the belief that more timing produces more power always. More timing produces more power only until peak pressure occurs at the optimum spot. Further advance lowers power. Modern cylinder head design produces much faster combustion than in previous years, so far less spark advance is needed. Add elevated temperatures due to a blower or turbo, and the combustion occurs even faster, requiring less timing. Frequently, forced induction setups are detonation limited, that is detonation occurs before timing brings the peak cylinder pressure to the 14-16 degree range. The only cure for this is to drop the temperature, or add octane. It's informative what you can accomplish with certain combinations. I have an Australian friend who used to develop "tunes" for General Motors in Australia before branching out on his own. He runs a 383 LS1 stroker with 10.8:1 static compression with 15 PSI of boost in a daily driver that runs 10.22 in the 1/4 and gets better fuel economy that a stock 346 going for groceries. Naturally, he could get more power lowering the compression, and the car would be a pig of a DD, but once you've got your 3900 Lb grocery getter into the low 10's, how much faster does it need to be?

okay im brand new to the site but ive been thinking of a turbo ls1 build. i had a 00 ss t65 blah blah good car 345 stock hp, any ways i got a few things to ask since u seem to know whats up so check it out, this is my idea for a lasting engine unlike soo many ppl pushin high c/r and blowin em, i dont wanna stroke the heck out of it because to me thats not going to last as long and b more money butt what about a stock bore a crank a tiddly bit bigger than stock with arp fasteners 6:1 comp, ported polished heads(need help with head idea), with a mild roller cam, beast push rods, nice roller rockers of course. i wonder this because when u race who launches at under 2grand? no one ive ever seen anyway. so my turbo idea would b a single bigger turbo and not a smaller turbo i say this because the baby turbo has less lag but more heat and its harder for it to push higher boost cuz it works harder. ok now a bigger turbo can push the same psi as a little one with way less effort and b cooled easier ok (learned this from a friend that had one of the 1st 900 hp evo8 in tx if not the 1st, he explained to me the same as i did to u) well okay common sense it seems like to me and he swears by running the exact same boost as the smaller turbo the bigger one was better. ok now this goes into play with using a bigger turbo than a lil t20 somethin or w/e some one els previously posted. ok so this could run off premium 93 oct because the molecular strand is shorter(---) like that, meaning better combustion normal unleaded has a longer strand of molecules(------------) like that. i would intercool it i wouldnt have a problem making a custom fit for intercooler and id want it to y off the headers, i think a 4-5 inch stainless pipe to run the turbo then branch bak out to 3 1/2 or 3/34 stainless out the back could handle the flow until it branched back into duels and wouldnt have to travel 300 feet before it hit the engine like rear mounts. im having trouble explaining this kinda but the gist of it is to have a fast wrap nice balanced higher boost engine. i think just the stock stroke is long in a 350 anyway so that should be plenty enough to have it spooled by 2 2.5 grand at launch and i always got best track times in my nat asperated ss shifting at 5.2 ish rather than wrapping it out to more than 6 so i just kinda need some input. the moral of this is i wanna break a turbo before i destroy a block. i mean if a supra can run a t50plus no prob then so could a extra 2 more pistons lol,, riight? so im hoping for big boost ill prob run close to stock gears not tryin to drag everyday im just a firm believer in 1 snail instead of 2.. make any sense? input plz

[BigTeddy254]

Like I said: It is all a compromise, and there are a ton of variables to factor in when trying to consider the best setup for you.

As far as turbos here are my 3 humble opinions:

1) I would say that it would be easiest to do a STS kit, but you will have to settle for the odd single outlet exhaust look. (which most people hate... unless you like that "sleeper" look) Bonus: no IC needed for low boost apps.

2) Traditional single turbo setup. Probably better for bigger turbos... higher HP output, probably better spool up time and less worries about water splashing in the intake. But more time and expense to install... and you will have to shell out extra cash for a nice big cool looking FMIC.

3) Custom Twin turbo setup. I never seen it on a LS1 Camaro... but have seen some killer setups for corvettes. Very expensive and exotic. Im sure someone has done it before, that would be a PITA to try to make fit... but boy it would be cool.

Lastly...

Dont rule out a supercharger. Do your homework. I hear the Procharger kit is one of the best. It will cost a bit more than the sts turbo kit.... but then again... with a SC you can have true duels or much more flexibility with headers and exhaust!

(for the record.... I have NEVER heard anyone ever say anything good about the Powedyne SC kit, so be weary of it.)

or u can go buck wild and have supercharger and fat snail of a turbo. supercharger gives ya insta power in return spools the turbo faster and when mr turbo kicks in supercharger cools turbo boost what what?? and really u could make it crazy with 2 bigger than normal rear mounts n den! if u wanna just drive around put some cut out switches so u dont always have to run turbos and jesus christ i just thought of how nasty that would sound. good idea?!!? =)

[35th-ANV-SS]

This thread is 6 years old and the person you are quoting to ask a question of hasn't logged in since 2010

[BigTeddy254]

I just signed up to the site yesterday i was just enjoyin bsin. dont kill my buzz i dont care how old it is lol

[35th-ANV-SS]

OK - by all means continue to BS with yourself then

[BigTeddy254]

Merry christmas! Nice car

[Chris Arnold]

im having trouble explaining this kinda but the gist of it is to have a fast wrap nice balanced higher boost engine. i think just the stock stroke is long in a 350 anyway so that should be plenty enough to have it spooled by 2 2.5 grand at launch and i always got best track times in my nat asperated ss shifting at 5.2 ish rather than wrapping it out to more than 6 so i just kinda need some input. the moral of this is i wanna break a turbo before i destroy a block. i mean if a supra can run a t50plus no prob then so could a extra 2 more pistons lol,, riight? so im hoping for big boost ill prob run close to stock gears not tryin to drag everyday im just a firm believer in 1 snail instead of 2.. make any sense? input plz

Teddy, I might be one of the only previous posters in this thread to respond to this because I honestly can't even remember it, anymore. LOL

Alright, to quickly address the quoted portion above, think of sizing the turbo as a balancing act. You're trying to balance response with efficiency. Generally, the smaller the turbo, the more quickly, in terms of engine RPM, it spools. The larger the turbo, the more efficient they are at moving higher volumes of air.

In my opinion, the choice between single or twin turbos comes down to a choice between packaging, complexity, and cost. It's conceptually true that being able to mount twins closer to the exhaust ports provides some advantage to twins, but in practice a well sized single can spool and make power very similar to well sized twins. More practically, a single often creates less clutter in the engine bay, and it costs less.

Unfortunately, it's not as simple as, "if a supra can run a t50plus no prob then so could a extra 2 more pistons." I believe there would be a LOT of problems trying to run a similarly sized turbo on such a large engine. First, airflow would likely be extremely limited by the exhaust-side turbine. This would create enormous back pressure in the exhaust that would create an extreme limitation on the power output of the engine. Depending on the power output you're looking for, for a 350, you'll probably end up in the 76-90mm range with a single turbo. I'd guess you'll find yourself in the 60-70 with twins.

Compound charging (super with turbo) has been tried, and in the examples I've seen, it's always worked out less efficient than using a single method.

Good luck!

[35th-ANV-SS]

One of reasons I went with a supercharger (Procharger) over a turbo - less complicated and easier to install.

[Firebirdjones]

Prochargers have really come into their own recently. Just look at the heads up classes, especially the small tire classes, they have cought up and are starting to put a hurt on the turbo cars now. I especially like their new electronically controlled unit that adjusts the boost, and can be switched on the fly.

I'm looking at this unit for a 502 build I've had in the back of my mind.

I've always prefered the Prochargers for the very reasons mentioned above. Easier and cheaper to package.

[RayzS10]

Just to interject something into a dead thread, I went to the Garrett web site; 6.0 ltr., 1000 h.p., single turbo, a/f ratio at 11.5 tp 1, BSFC at .46 and a 2v at .080 VE, a/a intercooler, and 1lb pressure drop = .070 effectiveness, my area "BARO" = 14.71 at 68' F., 21 lbs boost at 4k = 658h.p. at 58.06 corrected airflow, (lbs/min), and the same boost at 6000 is 1001 h.p. at 88.26 lbs/min. They recommended the GT4708 turbo.
I would be using a 10-1 comp engine and LS3 heads, with a cam with 114' lobe centers, but would need 110 octane at that boost, even with an intercooler. I could get by with 9.2-1 and not have to use race gas, pump gas at 93 and less boost, 15lbs, but still get 550 at 4k and 875 at 6k with a smaller turbo.
Just my 2 cents.....

[dian]

what makes you think, you need 110 octane? is that ron? doing e85 would be the solution.