redbanditz28

Right now, its up in the air between the iron block 370 engine or the 402 LS2 Aluminum block engine. I don't have the budget for a 408 iron block. So its strictly these 2 right now. I am looking to run the STS rear mount at high boost eventually once i get it. I've heard pros and cons to both. The iron block weighs more but has strong walls for forced induction. The 402 weighs less and makes more power as is with forced induction and would require less boost and may be more street friendly. Right now, i am leaning towards the 402 LS2 engine. I like the idea of having a newer generation engine and i have heard that the LS2s are plenty strong and capable of handling a lot of power. So Chime in if you could. Maybe i may be missing something about my options that you could bring to my attention. Bare in mind, this will be a new engine. The only thing i am using off my existing LS1 of my Z28 is the windage trays. Both will obviously be forged and have low compression. And i will be keeping my Edelbrock Tubular exhaust system headers for use of the turbo.

01FireTurd

iTrader: (2)

sounds like your idea on the 402 seems the best to me

StoleIt

iTrader: (3)

Ls7 ftw

redbanditz28

Ehh, LS7 would be nice, but not affordable. Plus it will be nice to beat the LS7 once i get my setup going. I just want to get out there and beat some mid life crisis guys in Z06 vettes and vipers. I know there will always be some one faster, but i just want to beat a good amount of them where i live. So, Does anyone else have an opinion on the engine choices? 370 iron block or 402 Alum LS2 engine?

300bhp/ton

I think this is a no brainer the 402ci offers more HP for less weight.

Although I'm not sure how you say you can't afford a 408ci? When I last looked the aluminium 042ci was more money than a cast iron 408ci. I think there are cast iron 402ci's as well, maybe that's what you've been looking at.

But I guess it depends on what your final goals are. Many people have produced well over 1000bhp with the aluminium block so I don't think it's likely to be an issue unless you plan on running the pro stock class.

But a bigger engine will require more air which means bigger turbo. I know the STS can be made to work fairly well, but a 402ci may be a tad too much unless you want massive turbo lag.

I really want to like the STS setup, but each time I look at it I always end up at the same conclusion

-Procharger
-Or FMIC front mount turbo

redbanditz28

I am not too worried about the lag though for the 402. The way i see it, if cartek (the people i plan on going to) can make the 427 vettes amongst other 400 +ci engines get real good tracks times like 8's and 9's, i am confident as well as they are, that the lag won't be an issue with their tuning. They seem to know how to tweak the system really well. But i do have a question while we are on the subject. In theory, being i would be running at a higher displacement with the 402 vs the 370, wouldn't what ever amount of extra torque i would have with the 402 make up for any lag to begin with if tuned right?

BayAreaSS

iTrader: (10)

Another vote for the 402 Ls2

300bhp/ton

ok this is where it gets complex.

First off there are many definitions and not everyone agrees what is what.

This is my take on it.

You have two aspects to worry about with a turbo.

1st

Spool up time - this is where in the rpms the turbo will kick in. This is a known quantity as to produce boost the turbo must be spun at 'x' rpm and requires 'y' exhaust gas flow.

So for example lets say 2500rpm before boost is seen and 3200rpm before maximum boost is seen.

This means below this rpm threshold you will be totally n/a.

When you launch the engine will not build boost without load on the engine, simply revving it will not work. There such things as 2 steps which can help a lot. Another approach is to spay a shot of nitrous out of the hole.

IMO spool up time is the lesser of 2 evils, mainly because it's fairly consistent and a known quantity and can be over come in various methods.

2nd

LAG - now this is where the turbo enters a stall period and then generates a lag time before being able to produce boost again.

At the drag strip this is not really an issue, but for street use and circuit use it is a major problem. An example being if you lift off the throttle at 4000rpm (this could be for corning or to measure speed or any number of reasons). You then re-apply the throttle at 3800rpm.

Now according to the known spool up time (seeing full boost by 3200rpm) we should be well in the turbo rpm range. However when you lifted off of the throttle several things happened, first there was a massive reduction in exhaust gas flow and depending on whether you are running a dump valve (BOV) there would have been trapped high pressure air between the turbo and throttle body.

All this contributes to “stalling” the turbo, i.e. it stops spinning and producing boost. When you re-apply the throttle the exhaust gas flow will increase, this spins the turbo once more and when up to speed will start making boost. But the time period before boost is produced again is LAG.

There are several ways to help minimise this.

1. A well matched turbo. Essentially the turbo you are using needs to be well matched with the displacement of the engine and the HP required. A large turbo will take more exhaust gas flow to produce boost and will thusly have a larger LAG time (as well as probably having a longer spool up time also). A too small turbo will be inefficient to supply enough intake air flow for the displacement of engine, this will reduce HP and because the turbo is working harder means it is less efficient (more heat) so even more loss of HP.
2. Dump valve (BOV). The high pressure air that becomes trapped between the turbo and throttle body, this aides in stalling the turbo, a dump valve simply releases this trapped air the moment the throttle plate is closed. Most people have a “to atmosphere” dump valve as it makes the “psssstth” sound. Although competition cars usually run recalculating dump valves, this takes the high pressure air and feeds it back into the intake system on the other side of the turbo, this means it can have a secondary benefit of aiding intake air flow just after the turbo has exited a LAG period.

Sadly these methods do only minimise LAG and will not eradicate it.

There are Anti Lag Systems ALS which get around this, most often seen in Rally cars. The “Bang Bang” is a common setup, this essentially forces fuel down the exhaust when the throttle is closed. The fuel then ignites, this generates high velocity exhaust gas flow which keeps the turbo spinning, the boost at the time is unwanted and is vented, but as soon as the throttle is opened there is maximum boost available. The down side to this is it is very destructive and very un-economical in terms of fuel usage.

There are more modern systems which use an EGR style setup, but I’m not familiar with their workings.

There is one other aspect to LAG and that’s turbo location. To get the gist of these you need to think back to physics lessons.

Remember looking at matter density and particles. In a solid the particles are packed close together where as in a gas they are spaced further apart. And hence a gas can be compressed more easily and to a greater extent.

Well the further the turbo is away from the intake manifold the more air needs to be compressed before it reaches the engine, this means it take more time.

Imagine a front mount turbo, lets say in total it’s 2 ½ feet away from the TB this means it has to compress 2 ½ feet or air before positive boost will be seen in the intake manifold (the most important place to see positive boost). With an STS setup the turbo is up to 10-15 feet away from the intake manifold. This means a lot more air to compress which means more time and LAG.

The addition of an intercooler on either setup makes the distance even greater.

Again for drag use I think the STS works perfectly fine, but for street use and in the twisties I can see LAG be a major issue, for me at least.

You would notice it as coming off the throttle and then back on only the engine doesn’t pick up immediately but lags before offering up it’s full potential of HP and acceleration. On high boost setups it can be rather survive when the turbo kicks in and can catch a driver napping causing them to spin or loose control. Or just make it a real handful.



From here it gets even more complex.

This is with respect to the question “will the extra displacement offset the LAG”.

The answer is some what convoluted, but in short – no, not really.

The reason it’s convoluted depends on setup and use. High CR engines are great for n/a applications but bad for turbo applications. This means to run a successful turbo setup you will generally need to lower the CR of the engine, this means worse n/a or off boost performance but more boost potential and better on boost performance.

Examples:

Assume you keep the stockish CR of 10.1:1 this means you can only run a small amount of boost (6psi may be a little higher). This is due to detonation, you can run higher octane fuel but you will still be limited on how much boost you can tune for.

This means two things.

1. That the off boost n/a performance will be very good
2. That running only low boost means you won’t make such big on boost HP numbers.

This all adds up to only having a comparatively minimal difference between on and off boost performance. So the shock of coming “on boost” will not be that extreme. So yes a large or in fact any displacement engine will have enough torque to cover much of the LAG and spool up time.

But the down side is you are limited with such a setup and you are not making the most of what a turbo can offer.

To get more HP you need to up the boost, but due to detonation you will need to lower the CR of the engine. Running somewhere around 8.5:1 will allow 14+psi of boost on the same fuel as a stock CR engine on 6psi of boost. And you will make more power.

But at the cost that off boost performance is now comparatively sluggish. And more importantly there is a much bigger percentage increase in HP when the turbo kicks in.

This means there will be more of an extreme difference between on boost and off boost performance. And it’s physically impossible to counter this with large displacement because it is a percentage difference which occurs, not a set HP/lb ft amount.

Non Ls1 examples of numbers for illustration.

High CR low boost setup - 4000rpm makes 300hp off boost and only 20% higher on boost. So during the LAG period when you go from n/a off boost performance to on boost performance you would only gain 60hp (300bhp to 360bhp). This would be noticeable but not drastic.

Compared to:

Low CR high boost setup - 4000rpm makes only 210hp off boost (n/a) but gains 95% more on boost. So 210hp to 410hp and an increase of 200hp would make for a far more noticeable and extreme change.

So in this instance no matter the ci of the engine to difference between off and on boost will be dramatic to say the least and will consequently feel more laggy.

(Remember these figures are only for illustration).

Now I’m not trying to sway you away from an STS setup at all, because, as you said they are proven to work on the track. But do look at the overall picture and what your aims are.

A 402ci is going to require a pretty hefty turbo and if you plan on high boost you’ll probably need some form of intercooler to keep the intake air temps down. There are such things as meth/snow kits which can help with this however.

From my understanding of how turbo systems work, the STS is best suited to more of a moderate setup relying on good off boost performance and lower boost. This will offer the best driveability and minimal LAG.


Hope this helped

boostedbuford

Very good info....... Sticky??

logo_craig

^^^


+1 for a sticky! very very informative

Sharpe

iTrader: (7)

:clap: :clap: :clap:

staringback05

iTrader: (8)

if he dont have money for a 408 he sure as hell aint gonna have the money for a 427.....

id go with the 402 with a forged rotating assembly, the alumn block is stronger then people give it credit for, its the rotating assembly that decides it wants to fail on a stressed basis

the 370 is a strong motor but your also giving up a good bit of weight that will set the car off

that big of a difference in cubes is also important, at least it would be for me in a deciding factor

ive only once heard of someone splitting a block and that was with a major shot on a already high horsepowered car..

redbanditz28

Okay, i am not sure why i put what ever i put about a 408. In general, i am not interested at all in the 408. I am not even sure to myself why i mentioned it. So lets forget about it. I think my state of mind is that aluminum vs Iron engine is what gets rid of the 408 engine option. I figure its one of those things where if i decide to go with the bigger cube engine, i might as well fo with the aluminum engine, but if i am going with a smaller cube engine, i want to stick wit hthe 370 iron block engine. So what is the difference in weight between the 402 alum vs the 370 iron engine?

In terms of the forced induction thing, i guess i am little biased against superchargers cause literally everyone i know that has one always complains about throwing belts and messing things up and these are smart people too, not beginners, these are professional mechanics that have been tinkering with this stuff for decades. And they have had their share of problems with all systems like ATI and vortech. And the few people i know with turbo V8s like vettes and gtos have yet to have any problems and they say the turbos work very well and they have made impressive numbers. So when you are in my shoes, you can't help but thin positive of turbos and negative with worries about belt slippage with superchargers. But i realize there are a lot of factors at play. I mean i am not making it up with all the things i just mentioned, so you can say they don't know what they are doing or they are using something wrong, and that may or may not be true, but i can only tell you what they tell me and thats all i can go on. When i visit cartek and talk to the guys there who own the 9 sec rear mount turbos with 427 engines, the 402 seems like the right choice to me. I don't really hear as much about the supercharged cars there though. Not to say that they aren't running just as good times or better, i just hear that the few customers who do use superchargers have belt slippage issues at times and that throws me off. But i realize turbochargers have their faults too. But i like the idea of NOT having a belt going very fast and breaking off into the rest of the engine bay. Thats my stance. But i am not knocking superchargers. Hell, i was even considering it myself at one point. So i don't want any arguments spawning out of this post.

Ell Ess Won

The guy from England gets it done!

402.

redbanditz28

300bhp/ton thanks for the thorough post. That was a good one. Lots of info. For me its also a matter of whats available out there. I mean i really would try a front mount system, but it seems like no one offers one to install as is and most who make one want to install them inhouse. I mean if you guys can point me in the direction of a person who manufactures a complete front mount system that can make some nice numbers, i am all ears.

300bhp/ton

Have a look in th FI forum.

TTI, Ohio Forced Inductions make some front mounts.

GMR Speed used to sell a front mount, and might still if you speak to them.

Plus there are lots of custom one off's.

I'm not saying the STS is no good, but I do feel the potential (real world) is some what limited. For 90-100% track use I can see how it could work. But for a serious street setup I do feel it's limited.

There's a good thread/poll concerning STS vs Procharger. It's quite evident from that thread that the STS is not favoured, and there's a lot of good info as to why.