Turbo....

-
RustyRatRod

RustyRatRod

I was born on a Monday. Not last Monday.
FABO Gold Member
Joined
Jun 7, 2010
Messages
104,805
Reaction score
102,367
Location
Georgia
I know a good bit of theory with turbos. Forced induction. Not difficult to understand. But I have a question for some of yall who've tried or run turbos on slants now. What's a "safe" static compression ratio? Is 9.5 too much? 9:1? I've not ruled out making my current build a turbo build. Keep in mind this will be with the factory prototype closed chamber head that will have some quench. That will be "however thick" the head gasket is. I have head gaskets ranging from about .018" to about .045" compressed thickness. Compression with the thin one will come in at about 9.4. I know some will depend on the camshaft too and how it can limit cylinder pressure. Thank you drive through.
 
I have had the same thoughts, with mine. So watching to see what they say here.
I don't have the same luck finding special heads as RRR did, but my engine is in a heavier vehicle than his.
I kept a slant 6 for power just to be different than the average Mopar guy, but "arrgh.... More power" certainly won't hurt. Also have to include curiosity about how much extra stress "rebuilder" grade pistons can take, etc. In my case even if I can "only" go to ~6 psi or so, that's better than none..... Or is it? There are high limits as to how much is too much, sure/ but at the low side, what's the least boost, to justify being "Worth it" in noticeable power (and maybe mileage too) improvement?
 
mine is going together right now, the current block was milled .120 ( yes 1/8" ) we will see how it works. I think someone is running a long rod / wisco ( 10 to 1 ) set up with a turbo on e85.
 
Rusty, it’s really application and fuel dependent. Pick the octane level you are comfortable running, decide how much power you want to make, and how much boost it will take to get there. Then build to suit. You can run 14:1 with 30psi on methonal but 10:1 and 10psi will not last on pump gas. I’d say for a street car run as low of a static compression number as possible to keep the engine docile and easily tunable out of boost, but enough compression to keep some off boost torque. Typically that will end up around 9.5:1, a good compromise for a street car. The slant has a limitation on cylinder head sealing so careful attention must be placed on machining the deck and head and IMO studs and MLS gaskets are a must.
 
TT5.9mag said:
Rusty, it’s really application and fuel dependent. Pick the octane level you are comfortable running, decide how much power you want to make, and how much boost it will take to get there. Then build to suit. You can run 14:1 with 30psi on methonal but 10:1 and 10psi will not last on pump gas. I’d say for a street car run as low of a static compression number as possible to keep the engine docile and easily tunable out of boost, but enough compression to keep some off boost torque. Typically that will end up around 9.5:1, a good compromise for a street car. The slant has a limitation on cylinder head sealing so careful attention must be placed on machining the deck and head and IMO studs and MLS gaskets are a must.
Click to expand...

I can have as little as 9:1 and as much as 9.5:1 depending on my head gasket choices. I am using the stock cast 2.2 dished turbo pistons, so those will be the limiting factor for boost. They do have a nice wide band around their perimeter that will be at zero deck and that ring is almost the dead perfect width to match up just right with the quench pad on this closed chamber head, so I'll still have just as much quench as I would with a total flat top, just with less static compression. That's why I swapped to the dished pistons. Static compression was going to be about the same as the engine I currently run at 10.2 and I did not want a repeat of that. Not that it was hard to tune it to run on pump gas, but pump premium is my only choice now. Not to keep it from detonating. It doesn't detonate on 87 even with 175 PSI cranking pressure......it just runs SO much better on premium. I guess the safest thing to do is just go ahead and run the thin head gasket, have quench at about .018-.020 and just run the engine N/A and be did wif all dat. It will be the most cost effective, too.
 
But that would be much less fun. Run the turbo. Once you do I promise you’ll not want a rowdy high compression NA engine ever again.
 
Yea that cast piston is a big question mark for me. Hopefully they have really thick crowns. I’d run it at 9:1 and probably limit boost to about 12psi with an intercooler and 8 or so without. That will run on 91 octane. Those pistons will not tolerate any detonation at all so be careful tuning. Start low and sneak up on it.
 
Ok something different here. How feasible would a turbo be with E85, whether straight or in some sort of blend with regular gas? Since E85 has more octane
 
volaredon said:
Ok something different here. How feasible would a turbo be with E85, whether straight or in some sort of blend with regular gas? Since E85 has more octane
Click to expand...
Feasible? It’s entirely feasible. Lots of people make big big power on e85. Myself included. My v6 turbo Honda in my falcon has a flex fuel sensor and I can run any blend of e85 to 91. It takes a good ecu with multiple fuel maps but it’s like race fuel at the pump. Two problems, not enough gas stations carry it yet, and your fuel system has to flow lots and lots of fuel. With a carburetor on e85 it’s a little more tricky. You need high flow needles and seats with e85 calibrated circuitry in the metering blocks and a big fuel pump and all the rubber in the system needs to be compatible. Totally doable. But it’s a pain in the *** to switch back and forth.
 
Last edited:
Keep it under 9 psi boost. As I have researched, 9 psi is the limit a stock distributor timing profile can take, what a boost referenced fuel pump can generate and not have to use an intercooler. Intercooler is a plus on any turbo but the higher boost levels really elevate the charge temperature. For a slant and crappy gas, 9:1 would be your best bet..you wanna keep it together long enough to drive it. Your detonation will dictate how far you dare take your compression or boost level. compression is alot harder to adjust.
 
AJ/FormS said:
If you use the Wallace Calculator and input a boost psi. it will spit out a number for effective boost compression ratio, reflecting static c.r., cam timing, altitude, at the boost number that you input.
Can't say if it will be useful to you, or not.

Wallace Racing: Dynamic Compression Ratio Calculator
Click to expand...
That’s a good calculator. I’ve used it. I wish there was a factor for temperature and volumetric efficiency though.
 
There's no way I'd do it without an intercooler.....but you know what my biggest hangup is? So far I've not seen but maybe ONE turbo slant 6 that didn't look like *** with all the hoses, lines and whatnot running everywhere. That's the ONE thing that'll keep me N/A unless I can find some REAL NEAT installations to mimic. .....and I mean REAL DAMN NEAT.
 
Last edited:
Yeah using that calculator my effective boost dynamic is like 12.2:1 using a 60 degree intake closing and 10PSI boost. I just don't think that'll work.
 
It can work. And here’s why. That is a generated number and it assumes a lot of the variables. The engine is not at peak boost at the very second the timing is all in and at the torque peak, which is where cylinder pressure is the highest, very often or for very long. Which is why that calculator is not the end all be all. @RustyRatRod I run a big block Chevy in my dually at 9.2:1 on 12 psi intercooled on California’s piss 91 octane. And it makes big power. Would it make more with 110? Sure but then it wouldn’t be a daily driver. You just have to be smart about the timing, when to pull it back and when to bring it in. Efi has made this so easy with 3D timing tables that it’s no wonder all these engines make gobs of power. The turbo part of the build is very easy, it’s only nuts and bolts. It’s all the supporting hardware that takes some thinking and planning.
 
TT5.9mag said:
It can work. And here’s why. That is a generated number and it assumes a lot of the variables. The engine is not at peak boost at the very second the timing is all in and at the torque peak, which is where cylinder pressure is the highest, very often or for very long. Which is why that calculator is not the end all be all. @RustyRatRod I run a big block Chevy in my dually at 9.2:1 on 12 psi intercooled on California’s piss 91 octane. And it makes big power. Would it make more with 110? Sure but then it wouldn’t be a daily driver. You just have to be smart about the timing, when to pull it back and when to bring it in. Efi has made this so easy with 3D timing tables that it’s no wonder all these engines make gobs of power. The turbo part of the build is very easy, it’s only nuts and bolts. It’s all the supporting hardware that takes some thinking and planning.
Click to expand...
Plus, I'll have the quench of the closed chamber head. Something to think about.
 
I don’t even care really where the quench is. The mixture is so uniform and dense at 10 or 12 psi it’ll make very little difference. Especially when blowing through a carburetor.
 
How much difference is there if all else is the same except running the carb as a draw thru vs as a blow thru?
I know about brass vs plastic floats, but I'm talking besides that. If (and that's a BIG if) I were to go turbo on my build I was looking at draw thru.
 
volaredon said:
How much difference is there if all else is the same except running the carb as a draw thru vs as a blow thru?
I know about brass vs plastic floats, but I'm talking besides that. If (and that's a BIG if) I were to go turbo on my build I was looking at draw thru.
Click to expand...
The biggest problem with draw thru, is you cannot run a charge cooler. You don't want a big radiator filled with fuel/air mix, waiting to explode if you have a intake backfire. You would have to rely on water/meth injection.
 
I am with you on the clean part. I have tried to keep mine tidy as I go. My problem right now is intercooler room, dont want it hanging down below the front bumper and not enough room behind the grill.

slant 8-1.2.jpg


slant 8-1.jpg
 
Just pull the 400 out from underneath the workbench and pluck a 727 onto the back of it...
EDIT: DUMMY...:D..
 
Last edited:
@vitamindart that looks really nice.
@volaredon draw through is yesterday’s technology. The turbos must have seals to hold vacuum and most don’t anymore. Blow through carburetor technology has improved a lot and boost referenced power valves have changed the game. IMO draw through turbocharging should not be in the conversation anymore. And as @Charrlie_S said with gas mixed in to the air charge so early on it brings dangerous puddling issues and wet flow issues with it.
 
RustyRatRod said:
There's no way I'd do it without an intercooler.....but you know what my biggest hangup is? So far I've not seen but maybe ONE turbo slant 6 that didn't look like *** with all the hoses, lines and whatnot running everywhere. That's the ONE thing that'll keep me N/A unless I can find tome REAL NEAT installations to mimic. .....and I mean REAL DAMN NEAT.
Click to expand...

Have you considered a remote set-up? those can be pretty neat and tidy.
With your engine already having a higher compression-ratio,you sorta have the bottom-end covered and you can use the longer pressurized return-pipe as a form of charge cooler, or hide it in a Lake-pipe.
Or how about a 2-stage system?
One thing that might be in your slanty's favor, is the actual Force on the top of that 3.4xx" piston when the ignition fires. Whatever psi is there, the actual force would only be 85% of what it would be on a 4incher. I wonder how you could exploit that, with the super-long stroke?
 
It seems to me that the more volume in the chamber, the more horsepower you can make. So the lower the compression the better. You want to maximize the unit air charge.
 
AJ/FormS said:
Have you considered a remote set-up? those can be pretty neat and tidy.
With your engine already having a higher compression-ratio,you sorta have the bottom-end covered and you can use the longer pressurized return-pipe as a form of charge cooler, or hide it in a Lake-pipe.
Or how about a 2-stage system?
One thing that might be in your slanty's favor, is the actual Force on the top of that 3.4xx" piston when the ignition fires. Whatever psi is there, the actual force would only be 85% of what it would be on a 4incher. I wonder how you could exploit that, with the super-long stroke?
Click to expand...
I just want to clear something up that I’ve read a few times. I run a remote mount on my dually, under the bed back where the muffler was originally. I run 3” aluminum tube from the turbo back to the intake. At first with no intercooler. I’ve seen a few company’s claim (and AJ said it here) that the long aluminum tube acts as an intercooler and my experience was the exact opposite. Cruising around off boost the temps would hover about 30 degrees above ambient and on boost I saw as high as 260 deg AIT. I installed a water to air unit and now cruising I see 5 degrees above ambient and on boost (12psi) I have never seen more than 140 degrees and thats usually when I’m towing up a grade staying in boost for a while and the water tank gets heat soaked. The tube acting as an intercooler just doesn’t work. @AJ/FormS what do you mean by a “2 stage system”? Can you explain that a little please.
 
You must log in or register to reply here.
-

Back
Top