It's quite simple really. Even take a LS9 or LSA etc....they'll do 600-650 all day every day on any pump fuel, and even they're a totally fine 9.0:1 or thereabouts.

Too many are fascinated with high compression these days because they have easy access to ethanol, which is great for them. Bares no relevance to pump fuel though. The higher CR's offer almost negligible benefit, for much higher risk.

The lower quality fuel you can get, the more conservative/sensible you need to be with CR. Although in this day and age, with good intercooling etc etc I can see little reason to go below 9:1 unless there were some very specific needs.

 

If 408ci Twin turbo uses 15 psi.
How much hp is it?So if use 93 octane.
Is it better to use 9.5:1 compression?

 

Depends on the build, the turbos, charge cooling, heads, cam, tuning.....

You're asking a massively open ended question with a thousand possible answers....or more.

If pump gas was the only fuel you can ever use, or use most often, then 9-9.5:1 as has already been said is a sensible target assuming the rest of the build is done sensibly and not tuned by an idiot.
But really...that applies everywhere.

 

He's asked the same question a million times along with "which head should I use".

 

IMO the best he can afford that suits his needs lol

 

When you can't afford more $$$$ in parts, turn the boost up

 

I agree, keep compression as high as you can get away with on the fuel you plan to use.

On the other end of the spectrum, I don't see what people are whining about with low compression engines being dogs. If the turbo and hot side are sized correctly, with realistic power goals, it doesn't really matter. You'll be into boost quickly enough that the loss in power isn't noticed. Its roughly a 4% NA loss in power per full compression point. This is easily counter balanced with 1 additional pound of boost. Most DIY home build guys are killing themselves hot-side wise too. If your larger than 2" OD piping and not making over 900 hp... your piping i too large! If your planning on more power than that you're a race car IMO and shouldn't be looking at pump gas options anyway.

My dished piston 5.3 with LS9 gaskets and 317 heads calculated out to like 8.2:1 (though most claim 8.5:1) Either way It made 26lbs on the trans brake pretty dang easy, could basically pick my HP level with the boost. So who cares about NA performance if you're in boost before the wheels turn? Personally I'd say error on the side of caution and go lower when in doubt and building a pump gas motor. On a mild 6.0 your talking a loss of 15-20 hp dropping a full point from "stock". It's not a game changer. And for every point of compression you drop you can usually run an additional 4-5psi of boost. Pretty easy choice IMO.

 

You hit 26 psi on the trans brake? Did it hook and change the rotation of the earth?

 

Planning on lq9 with 4l80e in a 1966 f100. Want to make 600-700 horsepower with a single turbo most likely a 7875 turbo. Forged connecting rods, forged pistons dished 9.7cc for 9.0:1 compression, waiting on cam recommendation from comp cam, depending on lift I’ll need roller rockers or trunnions upgrade, 80lbs injectors, PAC 1218, Holley terminator x max, probably meth kit, planning to run on pump gas 93 octane. Probably more I’m missing. Want reliable not questionable lol.

 

For only 700hp, you do not need a silly cam, you do not need a lot of lift etc etc.

A simple mild cam upgrade at or under 600thou lift will be more than capable.

Just look at some of the tests RH has shown recently on Youtube.

 

Agree on the don't need a big cam , you've probably read the praises of the triple 12 from Jarret /isky , I'm running a shelf cam motion 214/214 with 7875 on a 6L and hitting you're power goal - on Gen 3 lq4 internals. I get the make it reliable part and if you are OK with spending, go for it. I used to think that the aggressive lobes and high lift cams were the way to go ( sbc ,BBC ) but for long life in a DD it's not the way to go. Cam motion advertises a gentle lobe design I'm using 1218s and stock everything else -no issues. From what I've read in here the 7875 on a 6L runs out of steam by 6000 rpm so don't cam for more. Good choice on the 9:1 , you won't be sorry. Gen 4 rods will be plenty if you decide to keep them. 600whp in that truck will be plenty , unless you have great chassis and tires

 

I’ll see what they recommend for camis it’s to big I’ll either do the 214/214 ish cam or around there. Would meth injection be beneficial since just pump gas? And would a bigger turbo be better for later on more power if the 7875 just works for the 6.0?

 

We've already discussed the meth injection in your lm7 thread , I'm a fan. Bigger turbo ? Really gonna depend on your goals . A few members on here will tell you that for a mostly street car/truck a 6L 7875 is a really fun set up, unless you're going for dyno bragging rights. Going to make as much power as you can probably use @ 400ish $, small , light and proven it's hard to beat. Could it be more efficient and make more power with a bigger turbo especially turbine - absolutely !!!! Doesn't make it a bad ComBo though

 

I <3 meth/WWF after actually trying it out.

 

Ok well thanks for the help guys. One last question what plug type is the injectors on the lq9 ev1 or ev6 or another type?

 

Don't know off hand but you should be able to easily find pics online or on the sloppy site

 

Spun... a little... Heres a 24.5 lb launch... Wheels up 1.33 60'. Nothing to write home about. Almost needed to leave on that kinda boost to get the thing moving. Had a 3.10 gear with a 29" slick. 8.2:1 compression and an LS9 cam. It basically needed a little help till 5k or so. Then it came on hard.

 

LOL that makes a little more sense. That's a heck of a low SLR and compression, but that kind of boost will get it up and going haha.

 

So let's say his setup is:

Weekend warrior car that runs the occasional 1/4 mile 9.x sec pass
408ci gen 3 or 4 iron block bored 30 over built for boost with 823 stock heads and LS3 intake
Hydraulic roller, aftermarket valves, springs, etc
9.5:1 compression
Tuned on 93 for street at 110 for track
Twin 6266 T4's with 0.81 A/R, housings 2.5" discharge
Air to air cooling, griffin, all that
Shifts at 6500, launching off transbrake at 4500 rpms
Say it makes 1050 rwhp on 18-20 lbs on the race gas, and the heads never push water.
Cam and stall are dialed in for good spool on the street, sacrificing some performance at the track
Holley Dominator with a tuner that knows what they are doing.

Just throwing a 408 TT combo out there.

If this is the situation, what would be the benefit of running E85 and higher compression?

And if it's a street car, couldn't he just run a S488 ETR billet T4 with 1.25 A/R or precision 7675 with a T4 1.27 AR and spool just as fast as the twin setup above while still keeping the AC?
Not sure if the exhaust side is big enough on these?

 

Sounds like a lot of overbuilding and speculation to me. Money put in places it isn't needed for a relatively easy goal. Also are we talking a 9.99? 9.0? What kind of weight?

First off, Why in the world would you bore and stroke the crap out of a stock block for cubic inches that aren't needed to get to your goal? You weaken the block and spend a mint on a rotating assy. Then add a baby turbo to big cubes? It will be a torque monster...you want less TQ down low, and big HP up top... esp for a street car IMO.

If the goal is a weekend warrior street strip car that will run 9's. Run an Alum gen 4 5.3 and don't touch the bottom end. (gen4 6.0 is fine too but again...whey spend the extra $) If you have the money for a 408 stroker, drop a set of decent heads on the 5.3 and go to town. I'd keep it at 9:1 or under for pump gas personally. You shouldn't need 1000+HP to get there either, unless you weight 5000lbs.

As a quick example my cam only notch mustang with full interior is trapping 134 on 12lbs. I run water/meth and no intercooler. It has 241 heads (68cc) I'm on 19lbs now and it feels like a 140+ trap car easy. Twin 64/62's cheap china GT35 clones. Spool up is crazy fast. And i'm running a 2.73 gear, so not exactly optimal for track duty. Toss an intercooler on it and gear it properly and it would be a 9 sec car easy. I do run e60 because its cheap and handy. But i'm pretty dang confident I could run the same boost with pump gas. Just an example of how a bunch of mismatched low compression junk could still hit your goals. A well thought out 5.3/6.0 could easily do it. I have to pull a TON of power on my janky setup to get it to plant the tires on the street. Last thing I'd want is another 100-150 ft lbs NA at low RPM to deal with.


Just a thought...