Im building a super low compression 5.3, getting a 8-71 blower, im fully forged (minus crank) with wiseco pistons, scat h beam, 6.1 in length, arp everything, mains, heads, cam bolts..etc. I have ported 317 I will be using, that will put me at 8.3:1 compression. I'm looking for lower than 8.0:1 compression (you probably think I'm crazy but that's what I'm doing to run pump gas)

What can I do to lower it further? Thicker head gasket? If so where can I get one that will handle tons of boost.

Thanks

 

Cometic makes the gaskets you're looking for.

But I still don't understand why you need to go that low on the compression.

People run the factory hypereutectic pistons up to 15PSI and some over 20 PSI by just re gaping the rings. Most factory set ups are 9.5:1 some are even in the 10:1 range.

 

Seriously?

I have 10:1 compression and run 24 PSI on pump gas with bone stock internals. Once you go so low, you're not fending off detonation, you're just giving up power.

 

I agree why so low on the compression? I have plenty of customers making some pretty big power on pump gas and over 10.1:1 compression. I can understand wanting to give yourself some room with the tune, but in my opinion you are looking to go way too low on compression.

 

thrice

you will have better fuel economy and more torque out of boost by letting it sip methanol in boost with a higher CR

 

Ok, so what do you guys think about just using a ls9 gasket and keep the 8.3:1 cr? That's as low as I really want to go, I know it's going to be laggy, and I could give a crap about gas mileage, i just want a reliable (I know 1000hp+ will never be reliable) and safe engine, something I can drive everyday and fill up 93 at any gas station, money is no issue here at all, I'm willing to do anything to get it to live for a while, ive dumped 7 grand into the motor along with another 4k for supercharger kit and another 4k for actual blower so I'm not trying to cheap out on anything, I'm willing to run under 8.0:1 because I heared with a roots blower that's what's needed, but what ever yout guys recommend, ill do

 

A roots style blower is going to mask any deficit of torque due to compression ratio when you nail the pedal. Honestly I am more concerned about the seals of the engine, the pcv system, the bearing clearance and whether or not you are running an accumulator.

You can keep the CR low and use 93, sure, since you do not care about fuel economy that fits the bill just fine for the roots style blower. But IMO one of the most important things on that engine is how the oil seals are treated, and how you handle crankcase pressure and oil flow. Really with so much $ in the motor you will want an oil pressure data logger, accumulator, and a serious vacuum pump with a nice loose set of rings.

+ methanol injection and frequent oil changes for longevity recommended

 

I am definatly going with a data logger, really loose rings, and every thing else you mentioned but I'm not too keen to run meth injection, i know I should but the biggest thing about this motor is that I want to run just pump gas, although I might consider meth

 

93 is ok, until you hit that special temperature where it starts to explode violently. If you have a built engine, you probably have one hell of a head gasket going, which means you will not know about these violent explosions until it puts a hole somewhere else. The meth just helps you keep the temps down and octane up, moving you away from that scenario. You can also use water injection (100% distilled is fine) if you do not like the idea of meth. It will reduce power and the temperature, making 93 safer. You should be able to get more back (by increasing boost if there is compressor left) using the water injection overall, than you lose spraying it.

The thing to remember is that the combustion chamber temps (of the piston/head/cyl/etc) changes as you run the engine. If you go WOT from 1st, 2nd, 3rd, etc... by the time you reach final gear/top speed everything is much higher temp than it was when you started. Now if you come to stop and do it all over again... without cooling off first... everything will get even hotter. You might hit a steady state if you have the necessary equipment (coolers and blankets) or it might just keep going up and up in temp until it explodes violently. That is why it is nice to have the logging (IAT and EGT for example) and the water (drop that EGT) for when you want to keep your foot in it, without worrying about the temps escalating.


example
run 1:
1st through 4th, we race and I use no water, EGT ends at 1290*F and IAT at 175*F on 93
run 2 a couple minutes later:
1st through 4th, we race and I use no water, EGT ends at 1440*F and IAT at 202*F on 93

now I am getting worried, so

run 3 with 100% water:
1st through 4th, we race and I start using the water and raise the boost slightly, EGT ends at 1220*F and IAT at 200*F on 93

See IAT stays high, thats fine. That is not what water is for. But by looking at the EGT I can see that the temp is within a very safe range and I would keep running the engine without much concern, as long as oil temps (and other logs) are also in check. Otherwise, I would not have attempted that run#3 without the water, judging by the first 2 runs increments.

 

Ah yes makes sense, I apppreciate the time you took to explain it to me, thank you

 

Nothing wring with cushy low compression engine IMO. Just need to run enough boost to make it worthwhile. Could put a stock 4.8 crank in what you got if you want compression lower. I was going to run a 4.8 crank on a spare 5.3 for fun just to prove it's possible and not a total turd, regardless what people say... Looked like It would have been around 7.6:1.

I ran a 7.8:1 383 for years. Ran 13.6's sucking through a blower that wasn't turning. Threw the belt back on and went 12.20's on 8lbs with a little P600b blower (Non-intercooled 87 octane no water/meth). Daily drove it for 2 years in a 4000lb Gbody.

 

in the 90's I used to mess with the olds big blocks with CR's in the mid 8's
made okay torque on 87 octane and a tame cam and a Q-jet.

I think the trick is in camming it right to maximize dynamic compression.

 

Hmmm 4.8 crank, i completly forgot about that. I might go with a forged crank with same specs, so if I throw the blower on another motor, i can boost the hell out of this thing. Thank you for the reply, im going to start searching for a 4.8 crank

 

No need to go below 8.5 to 1 with a roots on an LS engine on 93, It would be better to invest in an intercooler setup and not have to worry about IATs. I currently have two blower motors, One is a 408ci still being put back together that made over 900hp with a 10-71 on E85 and the other is 6.0 with LSA supercharger, Hasn't been on the dyno yet but should be making around 500-550hp to the tires on 8lbs and 93 octane and make around 550-600+ with 12lbs of boost and E85.
The problem with the 8-71 and 8.0 compression is the fact that you have to spin the blower harder to make the same power that you could make with 9.0 to 1 compression with less boost and it will be more responsive on the street at lower rpms. The harder you spin a roots blower the more heat it puts into the intake. The ideal street setup would be more compression and an intercooler or no intercooler and E85 if available.

 

ah great, thanks for the info, ill leave it at 8.3, my factory crank is already balanced so that would save money not going to a 4.8 crank. thanks again

 

Yea I wouldn't go any lower. Im running a bone stock 4.8 other than cam with my 871 and it's fun as hell.