Highest advised compression to run with turbo?

[Gixxer Squid]

What is the highest/safest CR I can run with a turbo engine? I would like to build something in the 9.1 to 1 or 9.5 to 1 range. Current 8.1 is forcing me to run to high a boost to get power numbers I want which keeps blowing engines.

 

I have intercooler, water/meth kit for keeping combustion temps down.

 

Suggestions and reasons against requested?

 

James

[Pyro]

What is the highest/safest CR I can run with a turbo engine? I would like to build something in the 9.1 to 1 or 9.5 to 1 range. Current 8.1 is forcing me to run to high a boost to get power numbers I want which keeps blowing engines.

 

I have intercooler, water/meth kit for keeping combustion temps down.

 

Suggestions and reasons against requested?

 

James

 

depends a lot on cam timing events and ignition timing control.

 

what cam are you using? and does your efi system have ignition timing control?

[FlatBlack]

What is the highest/safest CR I can run with a turbo engine? I would like to build something in the 9.1 to 1 or 9.5 to 1 range. Current 8.1 is forcing me to run to high a boost to get power numbers I want which keeps blowing engines.

 

I have intercooler, water/meth kit for keeping combustion temps down.

 

Suggestions and reasons against requested?

 

James

 

I'm pretty sure the norm is to lower compression to safely run more boost. The 8.1 CR might have been too high. If your timing was off much, combined with a higher compression ratio [not to mention the detonation prone L6] this could be your problem.

 

Had you been running the meth kit before you toasted this motor? When did it start injecting? Details on your old setup might help you diagnose what went wrong

[Tony D]

What power numbers do you want? That would be a good start instead of working backwards.

 

You are going to get a lot more HP from the boost than you are from the 1 point bump in compression...including before 'boost threshold' when improved breathing will net more gains than a higher static compression ratio.

 

What are the uses for the engine, and ultimate horsepower goal. People blow stuff up on at STOCK ZXT Boost Levels through timing and fueling errors despite massive modifications to 'combat heat' (or whatever) at supposedly much higher boost levels.

 

If the subsystems aren't tuned correctly, the basic motor needs to be conservatively built in order to withstand the abuses while someone learns the tuning curve or subsystem dynamic.

 

After all that, you can start playing with static compression ratios, but likely it's easier once you're tuned to simply add more boost and fuel. More productive, cheaper, and ultimately doesn't do anything to compromize longevity across the board like a higher compression ratio would. 8.0 is a good number, 7.5 is even better.

 

Boost doesn't blow engines, detonation does. Period.

 

If something went boom with intercooling, methanol, and an 8:1 CR, something is wrong. ADDING compression is the LAST thing you want to do at this point!

[Gixxer Squid]

I'm pretty sure the norm is to lower compression to safely run more boost. The 8.1 CR might have been too high. If your timing was off much, combined with a higher compression ratio [not to mention the detonation prone L6] this could be your problem.

 

Had you been running the meth kit before you toasted this motor? When did it start injecting? Details on your old setup might help you diagnose what went wrong

 

 

Ok, lets reprase, how low is to low on compression? Engine management system is Electromotive tech3R.

 

Meth comes on at 6psi partial graduating to full spray at 15 psi and above. I don't know what the timing was set out, obviously its pretty darn critical even though you have other bells and whistles like in Bo's case.

[Gixxer Squid]

What power numbers do you want? That would be a good start instead of working backwards.

 

You are going to get a lot more HP from the boost than you are from the 1 point bump in compression...including before 'boost threshold' when improved breathing will net more gains than a higher static compression ratio.

 

What are the uses for the engine, and ultimate horsepower goal. People blow stuff up on at STOCK ZXT Boost Levels through timing and fueling errors despite massive modifications to 'combat heat' (or whatever) at supposedly much higher boost levels.

 

If the subsystems aren't tuned correctly, the basic motor needs to be conservatively built in order to withstand the abuses while someone learns the tuning curve or subsystem dynamic.

 

After all that, you can start playing with static compression ratios, but likely it's easier once you're tuned to simply add more boost and fuel. More productive, cheaper, and ultimately doesn't do anything to compromize longevity across the board like a higher compression ratio would. 8.0 is a good number, 7.5 is even better.

 

Boost doesn't blow engines, detonation does. Period.

If something went boom with intercooling, methanol, and an 8:1 CR, something is wrong. ADDING compression is the LAST thing you want to do at this point!

 

Numbers? 400+ at wheels, a reliable weekend and track "whore" car.

 

TWO valve seats fell out of head, #4 intake first, fixed car, drove it two weeks, put it on dyno to tune it since it was running 8-9 AFR's in boost, tuner has it in the 12.2-12.6 range based on dyno sheet I was given. A and #6 intake retainer decided to fall out. Upgraded steel retainers too so SOMETHING is F'ing things up. I am not a mechanic and shop is doing the work so I am out $$$$$$$$$$$ money and car F'd again.

 

I am relying on the tuner to get the timing right. Would the tuner who data logged this have the "timing stats" on the laptop??? I have not asked for them?!

 

Thanks,

 

J

[TimZ]

I am relying on the tuner to get the timing right. Would the tuner who data logged this have the "timing stats" on the laptop??? I have not asked for them?!

 

The tuner has the timing and fuel curves. If he doesn't you need to find a new tuner.

 

Just curious - what firmware are you running in the Tec3r? Also, do you have a wideband?

[Gixxer Squid]

I will ask the tuner for the data, I am sure he has it. I am unsure of firmware being used and YES I have a wideband.I have owned the car for 1 1/2 years and it been in the shop all but two of these months. I bought someone elses mess and promises of what was SUPPOSED to be under the hood....won't bore u with details but evryone reading has certainly been where I am which is trying to fix the bugs. May just start all over rather than throwing good money after bad...

[TimZ]

I will ask the tuner for the data, I am sure he has it. I am unsure of firmware being used and YES I have a wideband.I have owned the car for 1 1/2 years and it been in the shop all but two of these months. I bought someone elses mess and promises of what was SUPPOSED to be under the hood....won't bore u with details but evryone reading has certainly been where I am which is trying to fix the bugs. May just start all over rather than throwing good money after bad...

 

Easiest way to tell on the firmware is what version of Wintec runs with it - should be 3.x.x - what are the x's?

 

What wideband are you using, and do you have it interfaced with the TEC's analog input (EGO input or A/D4) so you can log it synced with everything else?

[gregkring]

Shouldn't we be worried about the dropped valve seats and lost retainers? I seriously doubt the tuning, compression ratio, or software version is causing those issues. Depending on your setup 400 HP to the wheels is a pretty lofty goal. Don't get mislead by the handful of guys here who have been able to achieve those numbers. It does not come cheap,easy, or by accident. I prefer 9 to 1 and less boost personally, but a 8 to 8.5 motor is a very good compromise. Get second opinions on the tuning and find out what is wrong with the motor mechanically before you spend money to change compression rations in your quest. Dropping valve seats is very bad.

[Gixxer Squid]

Still waiting for the tuner to send me the info. Tim, you ask some great questions but I hate to admit that I am a bit ignorant about the logging and syncing because I am relying on someone else to do it. Like I said, the car has been in the shop a little over a year and I have not had enough time to really do my homework like I should. The wideband was just installed fairly recently by the tuner so I am unsure of how he did it but its under the passenger seat and hard to see who makes it without removing the seat.

 

Greg,

 

You have the best point, car has dropped 2 seats two different times. Talking with Rebello about starting all over with a 3.0 or 3.2 stroker and scrapping the piece of crap in there all together. What do you guys think? I am kinda in way to deep $$ to support the inline 6 at this time otherwise another engine build might suffice. I want to keep the orginality so to speak of the inline 6 though.

 

 

BRAAP has told me that going to 9 to 1 would technically only yield an extremely small potential gain in HP and it would be better to go even lower to 7.5 to 1. Obviously I have more choices if I start with Rebello from scratch, I just want this one to be RIGHT!!!

James

[Tony D]

I would agree that 9:1 will net you nothing appreciable, and likely will only aggravate the horsepower problems with inability to tune for anything on pump gas.

Dropping to 7.5 will not cost you that much 'off boost' drivability if the car is flowed and cammed properly...you will always have 2-3psi of boost on hand no matter what rpm you are at, so even below boost threshold the 7.5 will act like a higher compression motor because of the boost down there that is always available.

 

Sure, an 8.5:1 will give you 'more' off boost...but it's marginal. And if you are having timing issues then lower compression solves a lot of that criticality when under higher boost pressures.

 

Hell, I'm finding detonation at 10psi on my wife's ride now (115 Ambient) running 87 octane pump gas. She's carping about having to run 91... So it's back to the drawing board to pull more timing. Soon my kid will be able to drive the car, and I can trim the curves while underway.

 

And that one is 7.5:1! But hey, what can I say, 87 octane fuel is cheap! LOL

 

Good Luck!

[jgkurz]

Still waiting for the tuner to send me the info. Tim, you ask some great questions but I hate to admit that I am a bit ignorant about the logging and syncing because I am relying on someone else to do it. Like I said, the car has been in the shop a little over a year and I have not had enough time to really do my homework like I should. The wideband was just installed fairly recently by the tuner so I am unsure of how he did it but its under the passenger seat and hard to see who makes it without removing the seat.

 

Greg,

 

You have the best point, car has dropped 2 seats two different times. Talking with Rebello about starting all over with a 3.0 or 3.2 stroker and scrapping the piece of crap in there all together. What do you guys think? I am kinda in way to deep $$ to support the inline 6 at this time otherwise another engine build might suffice. I want to keep the orginality so to speak of the inline 6 though.

 

 

BRAAP has told me that going to 9 to 1 would technically only yield an extremely small potential gain in HP and it would be better to go even lower to 7.5 to 1. Obviously I have more choices if I start with Rebello from scratch, I just want this one to be RIGHT!!!

James

 

I personally run 7.6:1 and have had good luck with my engine. I wouldn't run more than 8:1. In reference your your 3.0L or 3.2L comment, you might consider NOT over-boring past 88mm (I'm at 87.5mm/3.0L). Thick cylinder walls in a turbo engine have significant advantages like minimizing wall flex under boost and better heat transfer to the cooling system. I built my engine for reliability as well as performance so sacrificing a bit of cubic inch was acceptable to me.

[jgkurz]

Hell, I'm finding detonation at 10psi on my wife's ride now (115 Ambient) running 87 octane pump gas. She's carping about having to run 91... So it's back to the drawing board to pull more timing. Soon my kid will be able to drive the car, and I can trim the curves while underway.

 

87 octane.... ouch. You may not be able to pull enough timing to remove the ping unless you run premium. 20 deg btdc or below on boost just kills an L28's ability to rev worth a darn.

[Gixxer Squid]

Thanks for all the comments. I have an appointment this Friday with Dave Rebello. Going to scrap the new engine, less than 1k on it and start all over from scratch. Too many people had there hands in this thing before I got it and I would like to drive it more than 2 weeks before it goes back to the shop for 6 months.

 

He wants to do the 3.2 stroker, I will talk to him about the cylander walls etc.

 

Hopefully I will be back on the road before winter hits, LOL

 

james

[Tony D]

87 octane.... ouch. You may not be able to pull enough timing to remove the ping unless you run premium. 20 deg btdc or below on boost just kills an L28's ability to rev worth a darn.

 

Yeah, it seems O.K. at 18 right now. It really is extreme heat, No Intercooler and running with air filter inlet temperatures going to Palm Springs in the 150F+ range! Remember, it is the WIFE's car...she rarely runs it over 5500 anyway. If it was mine...it would be a different story. Then again, I won't carp about running 91 all the time, either!

[Six_Shooter]

All this talk of extremly low SCRs makes me cringe.

 

I have never ran anthing lower than 8.8:1 on a boosted engine, a couple as high as 9.6:1, one of my next builds I plan to push it to around 10:1 (next one will be around 9.5/6:1).

 

SCR is just a guide line.

 

There is one post so far that I've seen in this thread has some good insight, cam events, and a few other variables can really change what the actual cylinder pressure is.

 

Another thing to consider is that on some engines, raising the SCR, with no other changes has reduced the tendacy to "knock" or pre-detonate. Some say this is due to the higher quench that is acheived with a higher SCR engine.

 

People are absolutly astonished that I am running a turbo on my 8.8:1 L28, and wonder how it doesn't detonate. Especially running no IC (plan to soon enough) and pump gas (91 octane). All this talk of lowering the SCR IMO should be left in the '60s where engine management was literally decades behind where we are now. Back then there were only carbs, or in some cases mechanical injection, with points ignition maybe a few "transistorized" ignitions coming along later, all of this gave very poor control over mixture and spark timing, so the only option at the time was lower the SCR, in an attampt to reduce pre-ignition. It usually worked.

 

With the controls we have now, EFI, Distributeless Ignition Systems, meth/propane injection, and many other controls that are very finite, allow for very close control of the intake charge and exactly what is happening in the cylinder, which now allows us to run previously unheard of combinations, being daily abused and living very well in those conditions. EFI also allows much more flexible options in control, such as pulling timing at a certain RPM, but then adding back in a few RPM later, if it's necessary to do so. There's just so much that EFI can do for any engine, especially when it comes to forced induction set-ups that I don't think the real potential has been realized yet.

 

Just one example is a Honda Engine, that was over 10:1 SCR (10.8:1 IIRC), ran 18 PSIG of boost and was the shops daily beater/delivery pick-up vehicle. Yes I know that's a honda, and the mechanicals are different than an L-series, but some of what worked on that engine can be applied to other engines. IIRC it was built by a guy known as "The Old One" who has been working with Hondas and Turbos for a very long time. It's been a while since I read the article so I might have the name wrong.

 

My own Datsun has an N/A L28 that has from specs I've read 8.8:1 SCR, like I said, I run no IC currently, and run 91 octane pump gas. I am currently running 10 PSIG of boost, which ramps to 14 PSIG above 70 MPH. I haven't had much time above 70 MPH to get that mapped out completly yet. I might see around 2* of KR (Knock Retard), if the ambiant temp is warm or I had just been on a hard blast, since my air filter is currently living beside the exhaust manifold, also something I plan to change. My MAT sensor is reporting temps of around 130* F on a hard blast, where even cruising I see around 100* (Which is why I will be moving the filter, just haven't had the time yet).

 

I don't see the valve seats dropping due too low of SCR, I see that as more of an installation problem. The heat of running the engine, be it in boost conditions or not, likely caused the two dis-similar metals to expand at different rates and what caused the seats to fall out. They also likely fell out at high RPM, where the valves will be opening and closing rather often, and could also cause the seats to start to resonate, and with the heat there can dislodge something like a valve seat.

[Gixxer Squid]

All this talk of extremly low SCRs makes me cringe.

 

I have never ran anthing lower than 8.8:1 on a boosted engine, a couple as high as 9.6:1, one of my next builds I plan to push it to around 10:1 (next one will be around 9.5/6:1).

 

SCR is just a guide line.

 

There is one post so far that I've seen in this thread has some good insight, cam events, and a few other variables can really change what the actual cylinder pressure is.

 

Another thing to consider is that on some engines, raising the SCR, with no other changes has reduced the tendacy to "knock" or pre-detonate. Some say this is due to the higher quench that is acheived with a higher SCR engine.

 

People are absolutly astonished that I am running a turbo on my 8.8:1 L28, and wonder how it doesn't detonate. Especially running no IC (plan to soon enough) and pump gas (91 octane). All this talk of lowering the SCR IMO should be left in the '60s where engine management was literally decades behind where we are now. Back then there were only carbs, or in some cases mechanical injection, with points ignition maybe a few "transistorized" ignitions coming along later, all of this gave very poor control over mixture and spark timing, so the only option at the time was lower the SCR, in an attampt to reduce pre-ignition. It usually worked.

 

With the controls we have now, EFI, Distributeless Ignition Systems, meth/propane injection, and many other controls that are very finite, allow for very close control of the intake charge and exactly what is happening in the cylinder, which now allows us to run previously unheard of combinations, being daily abused and living very well in those conditions. EFI also allows much more flexible options in control, such as pulling timing at a certain RPM, but then adding back in a few RPM later, if it's necessary to do so. There's just so much that EFI can do for any engine, especially when it comes to forced induction set-ups that I don't think the real potential has been realized yet.

 

Just one example is a Honda Engine, that was over 10:1 SCR (10.8:1 IIRC), ran 18 PSIG of boost and was the shops daily beater/delivery pick-up vehicle. Yes I know that's a honda, and the mechanicals are different than an L-series, but some of what worked on that engine can be applied to other engines. IIRC it was built by a guy known as "The Old One" who has been working with Hondas and Turbos for a very long time. It's been a while since I read the article so I might have the name wrong.

 

My own Datsun has an N/A L28 that has from specs I've read 8.8:1 SCR, like I said, I run no IC currently, and run 91 octane pump gas. I am currently running 10 PSIG of boost, which ramps to 14 PSIG above 70 MPH. I haven't had much time above 70 MPH to get that mapped out completly yet. I might see around 2* of KR (Knock Retard), if the ambiant temp is warm or I had just been on a hard blast, since my air filter is currently living beside the exhaust manifold, also something I plan to change. My MAT sensor is reporting temps of around 130* F on a hard blast, where even cruising I see around 100* (Which is why I will be moving the filter, just haven't had the time yet).

 

I don't see the valve seats dropping due too low of SCR, I see that as more of an installation problem. The heat of running the engine, be it in boost conditions or not, likely caused the two dis-similar metals to expand at different rates and what caused the seats to fall out. They also likely fell out at high RPM, where the valves will be opening and closing rather often, and could also cause the seats to start to resonate, and with the heat there can dislodge something like a valve seat.

 

Great post. I towed the car to Rebello's and we decided to scrap the entire engine and start all over. Dave is going to build a 3.2 stroker with all the componets I currently have (t3/t4, meth, cam...) He is going to engine dyno tune it start to finish so we will be scrapping the old map as well which I just spent $600.00 for. Tired of throwing away money as I don't have Warrent Buffett's pockets.

 

Going to install an EGT as well and really do this one right. I think the previous engine builder cut the seats to large and as you know, there is not much metal between the intake/exhaust valve seats to begin with and the heat as we suspect, may have cause some expansion and the seats dropped. Both seats dropped under full boost at different times. The previous engine builder had put larger valves in for better flow but may not have considered that he cut them to much with the amount of boost we want to run.

 

I think as six shooter suggests that it really depends on the tune which is critical in running a high boost engine, as KTM may agree as well.

 

I am hoping to have it back in 6 weeks, will post info when I get it.

 

James

[jgkurz]

I have never ran anthing lower than 8.8:1 on a boosted engine, a couple as high as 9.6:1, one of my next builds I plan to push it to around 10:1 (next one will be around 9.5/6:1).

 

Your experience is unique and I'm amazed how well it has worked for you. However, ideal static compression ratio for an application is dependent many factors. Additionally, The L series head is no where near as efficient as a modern 4V/cyl cross flow head and will usually require more lbs/min from a turbo to make equivalent power. If you are a person who's priority is max HP from an L28 you will need more boost and less compression. If you want more power under curve (less peak HP) and better street manners then more compression and less boost may be the way to go. My recommendation of 8:1 is a good compromise but not not ideal for a max effort engine. It just depends on what you want out of your engine. Remember, the power benefits of more compression are no where near the power benefits of high boost. An engine will 7.4:1 compression (stock L28ET) will always permit more boost and more "peak HP" than the exact same engine with 9.5:1. That's just physics.

 

 

Dave is going to build a 3.2 stroker with all the componets I currently have (t3/t4, meth, cam...)

 

So how big will your bore be with a 3.2L? I guess he didn't think thin cylinder walls were an issue???

[Six_Shooter]

In all the years I've seen dyno charts and racing times, I have never once seen a lower SCR engine with more boost create any more power than a higher SCR engine with the same or even a little less amount of boost, as much else being equal as possible. I'm not syaing it hasn't happened, I just haven't seen it. I'd also want to look at other aspects too.

 

What people seem to forget is that "boost pressure" is really just a measurement of restriction in the intake tract, and the largest restriction is (just about) always at the valve itself. Where all that air that people jam through 4" IC pipes, huge intercoolers and larger intake manifolds, ends up hitting the bottle neck of turning and going into the cylinder itself. This restriction at the valve or head port is hat causes the pressure in the itake when the forced induction device (turbo or supercharger) is pushing more air than the engine can actually injest on it's own, this part should be known by anyone using forced induction, and bring it up just to make sure we are all on the same page.

 

Now our goal is not really to make lots of "boost pressure", Boost pressure as read on a boost gauge, referenced from the intake manifold really is irrelevant and can only really be applied to that particular engine or set of engines all prepped the same. What we really care about is cylinder pressure, higher cylinder pressure, with the correct amount of fuel mixed in and proper spark, delivered at the right time to create max cylinder pressure just before the piston passes TDC, is what creates the horse power we long for.

 

If you were to open up the restriction, in this case the valve/head port, you would see a drop in "boost pressure", for the same power being made, or more specifically same CFM. So now we have less "boost pressure", more CFM, and higher cylinder pressure, that means more horse power. Yes, I am simplifying this analogy.

 

Now what else can be done to increase this cylinder pressure?, that's right, squeeze that air and fuel mixture a bit tighter, by raising the SCR.

 

What most people do wrong when they raise the SCR, or just leave it alone as in the case of adding forced induction to a non-factory forced induction engine is leave or install improper controls. The main controller that we never seem to look at is the nut behind the wheel, this is the biggest contributer to engines blowing, forced induction or not. It is these controls that have the biggest effect on the longevity of an engine, not the SCR, not the cam, not pistons, not the block but the controls. You can have the best parts ever made in an engine, but with poor control of those parts it's never going to perform well. On the flip side, and I've seen this, you can assemble an engine with seemingly junk parts, but with good controls on these junk parts the engine can last for a very long time, and quite probably out perform that "better" engine.

 

My experiance is not unique, it's from planning out what I parts I use and not allowing my right foot to get a head of my tune. For all intents and puproses, my engine is assembled with junk parts, many are stock parts from various applications I just applied many years of my own experiance and a lot more of what I've read and information that has been shared with me by others that tend to break away from "conventional wisdom", and find out what really needs to be done to make something work well.

 

Most "max effort" forced induction engines I see are now well into double digit static compression ratios. I've seen quite a few street driven engines, that have double digit SCRs and live with being daily abused for quite a while. This is not a new idea either, people have been pushing this way for quite a few years, just the majority scoffed at the idea because it's not how it was done before, without realizing what was actually different, being the controls.

 

We also have better control of manufacturing processes, that allow previously considered "weak links" to be made with better precision, and allowing them to be abused more before failing.

 

Now if all you had to control your L28ET, was the stock L28ET electronics box (I have a hard time calling it an ECU, due to the very limited control it actually gives), then yeah, raising compression is not recommended, nor is adding more boost, that's just a problem waiting to explode. Same goes for a newer car, even one that has better controls, lets use a WRX for example, add a boost controller to a stock WRX and crank the boost, it's going to likely live a short life, same goes if you decide to change just the SCR, or just the injectors or what ever part you want to, without tuning for these changes then the controls are not matched to equipment and I'll go back to my previous statement that the controls are what will determine the reliability and longevity of a given engine.

 

A few decades ago, all we really had were carbs and by todays standards poorly designed spark distributing devices that would allow the engines to run and would actually run pretty decently, but maximizing the combination was very difficult and would usually lead to either a good running engine for drivability but poor WOT/top end performance or a good WOT/top end machine with poor drivability and then there were the select few who would have a decent compromise in between, and it is these people that realized that the compromise was a much better way to go. You speak of area under the curve, well I'll take a wider "area under the curve" with less peak power to a large power peak anyday. The reason being, that during acceleration, most engines go through fairly wide ranges of RPM operation, at shifts, changing as much as a few thousand RPM, having a wider flatter curve, means that you will be in accellerating more than waiting to hit the power peak than the guy that has a very peaky power band (provided he doesn't have more gears to keep the RPM tight). If I was driving at one or a very small change in RPM, than the peaky engine would be more desirable. But I know that I use my car, as most do, to regularly to head down to the store to grab some snacks, to work, out cruising, etc, I don't want to have the need to rev it to a few thousand RPM, just to get it near a power peak, I like my broader curves, thanks.

 

I do agree that "ideal" paramaters of an engine are determined by many specific factors to how that engine will be used. I haven't seen anyone ask what the use of the vehicle is, commuter, road racing, drag, auto-x, etc, not that knowning the type of use would help lock down what SCR anyone should have, but it would help determine other factors in how the engine needs to be set up and ultimatly controlled. Just arbitrarily saying to someone that XX.X:1 SCR is what you need is like saying this is the food you want to eat without asking what thier prefences are. You may tell them they want chinese cuisine, when they really want a steak. Get what I'm saying?