3-4% up to like 11ish then the gains drop down even less. I think the big drawback is waiting on the line for an eternity to get the turbo spooled up. Or stomping the gas from a roll and not seeing boost till 4-5k. Most don’t want to fart with nitrous or super loose converters.

I ran an 8.6:1 5.3. Dished gen3 motor with 317 heads. Went 8.9x @ 153 before I lost the auto zone timing chain on the 2-step on the starting line (or so I’m guessing). Ran 25ish lbs on the small S475. Saw less than 1mph gains over 23lbs or so. So I was pushing it harder than I should have. Was also running the LS9 cam so it made very little power under 5k. This was with a 3.10 gear and 28” tire, so I was really limiting myself RPM wise too. Amazing it ran as well as it did.

It drove very much like my mini van out of boost. (which I didn’t mind) Rolling in high gear I’d see positive manifold pressure around 4k. I ran a 17 stator PTC converter and could build 20+lbs on the brake really easily. So at the track it literally made no difference. It liked slightly richer mixtures and more timing. And took about 5 more lbs to run the same times as the 10:1 alum 5.3 (799 heads) I ran the following year.

I have an alum 5.3 I’m working on now. Has a 4.8 crank and the short gen4 6.098” rods. Piston is .177 in the hole at TDC. Would be around 7.5:1 with a milled truck head and LS9 gasket. No idea what I’ll put it in at this point. But I’m sure it will need help getting the turbo(s) lit. Should be super stout with a short stroke and short rod and take a ton of boost at relatively high RPM. Would also have a huge tuning window, esp on e85.

 

I would cry if it didn't see positive pressure until 4k lol...but it obviously ran really good at the track so it must have worked.

 

No, what I said was a lot of people who pay people to tune their cars are sold tunes that are predominately done in SD. Why? My guess is:
1. It is a faster turn around for the tuner to punch out a SD tune that can trim itself +/-50% into spec than it is to create a blended tune; and
2. Cost. Customers are usually budget limited so the additional costs for the tuners time and associated parts to tune in a MAF is not seen as a value to them.

And for that reason I am certain there are many tuners who have limited experience in effectively running a blended (MAF/VE) tune. They just tell you SD is better because it is better for them to get you tuned and out the door happy (and none the wiser). It is that last 10% of refining a tune that is the hardest and most involved part.

I am in the process of building a turbo SBE 4.8L and it very well may see a blow through card style MAF too. But, I am running a hobby here, not a business.

 

Can’t expect a lot of power down low with the a 8.6:1 5.3 and LS9 cam for sure (ls1 intake too). But I had the opposite issue with my 9.5:1 370. I had so much power/boost early on that I had to do all sorts of tricks to reduce power to get it down the track. Retarding timing and ramping in boost over time from the brake release point. So there’s defiantly a “happy place”. I can’t see the purpose of a setup that instantly makes boost either, as it’s not useable. 8.5-8.75ish would be a good target leaving a lot of cushion room in the tune. But it isn’t easily targeted with gen4 SBE parts.

 

I was talking with a gentleman on here regarding this same topic on a boosted 5.3.
He was running a SBE LM7 w/ dished pistons and stock 317 heads.
We're both running the same T7875 turbo and spec'd the same LJMS Stage 2 cam, while all the digging I've done suggested a 5.3 w/ flat tops and 317 would yield about 9.5:1 comp. and a total boost limit of about 11-12 lbs. on 93 only while his combo with its 8.5:1 comp. was able to push upwards of 16-17 lbs. and made about 670whp while on 93 only as well.
He did mention that it seemed a little slow to build boost with his T56 trans but an opinion on spool time is just that, an opinion lol.

 

This is right on point, my tiny TC7868 would build boost so quickly it was like a light switch and while it was fun initially it became annoying.
I'm loving the larger T7875 that builds boost in a much more linear fashion which for a street car with actual street tires makes pulls a lot more fun.
I think the sweet spot for my setup will be the 5.3 w/ flat tops I'm putting together which with LS9 gaskets should net me about 9.3-9.4:1 compression on pump only.

 

Thats pretty interesting, I wish you guys had some logs or something so we could see how quick it was to get into boost.

 

And that itself makes the test pointless with respect to a pump fuel build, although throughout they dont specifically say what fuel is being used, but that statement at the end does imply race fuel.

Who in their right mind would build a very low CR engine like that for low-moderate boost and run race fuel ? 3 wrongs still dont make a right.

 

Yup, I agree, the lack of spec on fuel used and the same boost level doesn't make a ton of sense. But purely for curiosity's sake seeing how much power just a change in CR makes on a boosted setup is interesting. That's why I would have liked to see if they could have turned the boost up another 11psi or more to make up the HP gap.

 

How did he manage 8.5:1? Custom pistons?

 

Unless he had no intercooler, there is no way 9.5@1 CR will limit him to 12psi. People have ran more than that on 10.5:1

Pretty much every engine I've used I've kept to 9:1 or close to it.

I can easily run 20-25psi on 93 alone. I have meth too but generally as traction is always non existent, I only use it when racing or for mile type events, which I've ran just shy of 30psi for those

As for lazy...how can any 6.0 or thereabouts V8 be lazy ?

 

Laziness is going to be in the tune, turbo, and kit design mostly. Start dipping into the 7’s or low 8’s compression wise though and it will be a “turd” off boost even with a 6.0 IMO.

 

According to a compression calculator I got off this forum a 5.3 w/ the dished factory pistons and the 71cc 317 heads gets you down in the mid 8.5 range.
Hence why I'm adding flat tops to my 5.3 to get me into the 9.3 range because for a dedicated street car with a manual trans I'd rather have a little more compression for out of boost driving.
I'm not sure if the engine would be a dog as most say but I can't afford to find out the hard way lol.

 

Different strokes for different folks I guess.
The guy who is tuning my car has been tuning LS engines forever so when he advises against running more than 12 lbs. on straight 93 pump with a decent A2A I'm inclined to listen.
I'm not trying to argue, just offering another perspective and of course I realize the combination also determines these types of factors.

 

sbcgenII, I'm only quoting you for context. This is not a direct reply to your post.

I used to really like the magazine A-B test articles. Cam A to cam B, heads A to heads B, manifod A to manifod B.... Compression ratio A to compression ratio B.

At first thought, these type of tests seemed to be "chissled in stone", concrete evidence that either A or B was superior in whatever way.

As time went on, I learned more and more, and realized that simple A to B comparisons sell magazines. People, in general, (myself included) want stuff to be uncomplicated, easy to understand, and easy to choose. A to B does that.

However, one change to a complete combination, changes the whole combination into something entirely different.

If one combination is maximized upon, through a process of calculations, a true A to B should have both comparitors/competitors combinations maximized through the same calculative process, so that we're not comparing a perfect combination to a lesser rendition of a perfect combination.

Just my opinion that, the results of simply lowering compression without also maximizing upon the resultant combination could be highly misleading to the general population.

Low static compression, generally requires an earlier intake valve closing to retain most of it's low end torque, which typically reduces the point in the rpm range it will make peak torque and HP. Which would dictate a smaller intake runner head to maximize port velocity and cylinder filling...

This is just looking at the engine. Yes, there are many combinations of things you could do for vehicle setup to maximize certain engine performance curves to offset a particular engine's nuances.

Lower compression and put a high rpm cam in... that's what causes the symptoms, which in turn leads everyone to say low compression is lazy down low... and it absolutely would be. Dynamic compression might only be 5-6:1... a complete lazy turd... until the turbo spools. Wouldn't be so bad with a screw charger, but would still be lacking port velocity in the lower region and come on strong up top.

Kind of a long winded way of saying, I wish someone would start doing A to B comparisons, based on fairly static, complete combination formulas and parameters.

Unfortunately, that may just be too much time and money to be worth it.

 

I'll be curious to hear if the lack of quench causes a more detonation prone condition, even with the low compression. I've read that it does/can, but it will be interesting to hear how this turns out for you. I read it on the internet, so it must be true. LOL

 

Thats what i always thought, i am a huge fan of tight quench but theres some threads on YB discussing it and everyone basically said in a turbo motor it doesnt matter, or it doesnt matter enough to be worth worrying about. Im still not sure how i feel about running big dishes that far in the hole.

 

From what I've read there's a difference between a poor quench area that promotes detonation and a complete lack of quench. There's a go/no-go zone for detonation prone quench. I believe .050-.080 is the area to steer clear of depending on the CC size/shape. Either way .177 in the hole would eliminates the quench effect all together. It would sure make for a lazy burn, until you threw 2-3 bar of boost at it. Also I'm talking about flat tops .177 in the hole, not dishes.

 

Gotta give it up to the guys willing to go outside of popular belief and see if the earth really does fall off just past the horizon.

I too, see the importance of quench induced combustion chamber turbulence... but who knows? Maybe a little extra timing to make up for lack of burn rate along with additional air mass in the cylinder will produce a situation where the gains outweigh the losses before detonation becomes an issue?

Curious to hear the outcome.

 

Please keep us apprised.