High compression low boost questions?

[Jayyyrah]

1978 280z n42 block and n47 head on the car, anybody tested high compression with low boost? searched a little through the forum and didn't really find anything relevant to this question, Was alot more information on how to get the high comp rather than high comp + low boost.

 

Ideally i'd like 300hp, I'm between doing this or just swapping for a VQ35. Kind of don't want to go this route because of the hassle of fabricating motor and tranny mounts.

 

Anybody got some good info on this? thanks.

[OldAndyAndTheSea]

It's all about the tune. If you're using the OEM engine management, you will most certainly detonate (whether or not this will destroy the motor entirely is questionable/probable)

 

But with a standalone ECU, like megasquirt, haltech, etc.... You could certainly tune a high compression motor for a turbo application.

 

A bit of advice: LIVE in the FAQ section here. Especially the turbo/supercharger FAQ.

 

All the info is there. You just need to do a little digging.

[dexter72]

Im not familiar with what compression ratio that combo will be, You will have to find the info for that. There are engine combo calculators that have been posted on Hybrids before, so search for them. Just put your combo in and it tells you the ratio.

 

If you found posts about n/a turbo engines, or flat top piston turbo engines, those are the high compression turbo engines. If not, do a search for those.

 

You will need a Stand Alone engine management system for your setup.

[new2Zs]

why not just get lower compression pistons? you could use the time to throw in new rings and clean up the cylinders as well.  Its not simple to just "slap on a turbo"  Its a fair amount of work, so if you are going to put the work in, you might as well get the benefits.  For longevity of the motor and power, it would be better to do a low compression boosted engine vrs high compression boosted. 

[str8pipez]

Search, FAQ, Google. Lots of people doing this.

[BluDestiny]

There are multiple people on here who have just "slapped on a turbo" to their NA engines, me being one of them, and it worked decently, could probably have done better with a real tune, but I was pushing stock boost, like 7psi. 

 

I recall a thread where someone was doing this and using a 8:1 fmu and was at like 10 or 12psi. By no means is this the right way, but it will work. 

 

Assuming your engine is stock, you should have 8.8 or 8.3 compression, which is not "high" compression at all, considering there are cars that come with 10+ stock from the factory. 

[bahelion89]

to truly get the most out of a high comp turbo motor its all about the TUNE AND FUEL. I have some expericence with smaller high comp turbo motors and you need to feed it properly or it will blow. unless your willing to do a total overhaul go with a different motor.

[Tony D]

Define "High Compression"

[DAW]

I would find a F54 shortblock with flat top pistons and install a P90 head on it. This should be about 8.8:1 static c.r. and provide good off-boost response.

 

Use a small turbo but with an external wastegate that opens at low boost and has a reliable sensitive valve.

You will have the benefit of high quench by using the flat tops and the closed-chamber head. Knock control sensor circuit must function well. 

 

There will be a lot of bypass going on that you want to flow well and external wastegate would be better than internal for that.

 

I have an N42/LD28 hybrid N/A engine which is 10:1. It has instant off-idle response and torque but really needs an aftermarket cam. The cranking pressure is over 200 psi. The LD pistons are flat-top (except for the little cloverleaf divit which has some diesel engineer's name that I can't remember). That divit is located where the gas head spark plug tip is located and it likely helps initiate flame travel with either glow plug or spark plug.

 

I also have a F54 flat-top block and N42 combination which is also 10:1 c.r. Using the same stock cam on both engines, they have different power band characteristics. The same high cranking pressure in both and same displacement but different rod/stroke ratios. This engine is triple side-drafts and desperately needs a slalom cam but my solo class rules required stock cam only.

 

I've thought about converting a P90 to use on the LD block to run a low-pressure turbo. Even though the bore/stroke are different LD28: 84.5/83; L28E(T): 86/79; the displacement and static c.r. works out to the same 8.8:1 discussed above.

 

I don't know if I'm up to modifying another gas head to fit the LD block. Not enough spare time, but I wish I had used a P90 head in the first place for N/A baseline tune and then go with low-pressure turbo.

 

With the benefit of modern engine mgmt systems, a lot of factory cars are running higher static c.r.s and limiting boost accordingly. Volvo has offered choices between low-pressure turbo engines for drivability but less hp; and a high-pressure version with lower static c.r. and more overall hp at full boost.

Edited April 27, 2014 by DAW

[Six_Shooter]

why not just get lower compression pistons? you could use the time to throw in new rings and clean up the cylinders as well.  Its not simple to just "slap on a turbo"  Its a fair amount of work, so if you are going to put the work in, you might as well get the benefits.  For longevity of the motor and power, it would be better to do a low compression boosted engine vrs high compression boosted. 

 

 

There are multiple people on here who have just "slapped on a turbo" to their NA engines, me being one of them, and it worked decently, could probably have done better with a real tune, but I was pushing stock boost, like 7psi. 

 

I recall a thread where someone was doing this and using a 8:1 fmu and was at like 10 or 12psi. By no means is this the right way, but it will work. 

 

Assuming your engine is stock, you should have 8.8 or 8.3 compression, which is not "high" compression at all, considering there are cars that come with 10+ stock from the factory. 

 

I'll raise my hand as one of those that "just slapped a turbo on" my car. That was 5 years ago. I only planned to leave it this way for about a year or so, then swap out the engine, but the Turbo L28 has been a lot of fun.

 

Define "High Compression"

 

This is exactly what my first question/statement was when I read the the OP.

 

 

 

My combination starts with an F54 (with flat tops) and a P90 head (stock port). L28ET exhaust manifold, 6.5 diesel turbo (Mid '90s GM pick up), home made IC, N42 intake manifold, DIS, GM (Delco) EFI to control it all. Aeromotove FPR, 30 lbs/hr injectors, 3" exhaust, Walbro 255, etc, etc.

 

I'm currently running a max boost of 17 PSIG, but I have had it as high as 20 and plan to take it back there this summer with some larger injectors, and more tuning. I daily beat on my car too.

 

The 8.8:1 SCR is a "low compression" engine IMO, my next engine (not an L-series) will be at least 9.5:1 if not 10:1 with plenty of boost as well, but it's a different design.

 

The biggest problem I've found is intake temp control. The L-series with it's no cross-flow head tends to heat the intake manifold which can cause the intake temp to rise enough to be a detonation issue. Some creative fuel delivery can help reduce this effect.

 

Keep in mind I've done pretty extensive tuning on this to get it to live with 17 PSIG of boost, and have started over a few times with the tuning over the years. So you need to put time aside to tune it well. Start at low boost, get it tuned well, then move up to where you're comfortable.

 

YMMV.

[m1ghtymaxXx]

Can anyone comment on the durability of stock flattop pistons? I've heard they're quite prone to break ringlands under boost. 

 

I have an F54/P90 Turbo motor, a F54/P79 N/A motor, a N42. My goal is a responsive motor with a decent low end more than a peaky higher horsepower motor, suitable for autocross. 300HP sounds like a reasonable goal, but that may well change once I've achieved. 

 

Anyways I was thinking about mating the P90 to the flattop F54, but I'd like to know if I can expect the pistons play nice in such a build. 

[Six_Shooter]

I've been running my combination for 5 years...

[HowlerMonkey]

I've been running a flattop f54 and p90 using untuned infiniti m30 ecu with it's non-turbo ignition timing scheme.......which probably has too much timing advance for a turbo engine and it has been running fine for at least 5 years.

 

I'm running it in a relatively heavy m30 car with stock 280zx turbo automatic and 3.54 r200 and it puts down high 14 second 1/4 mile at 7 psi boost.

 

I can't run 87 octane in this combo and 93 octane is $4.25 per gallon.

 

Getting the ignition timing curve more turbo friendly will surely help this setup and allow the usage of much more boost.

 

I ran a p79 in place of the p90 for a while and performance was identical though the p90 probably has more available "overhead" at higher boost levels than I am running.

 

I'll be tuning it later this month and report back results.

[Tony D]

Flat Top Pistons tend to break their ring lands when DETONATED, not when under boost!

 

I've run Flat Tops since 1985 at 17-21 psi boost with nary a cracked ring or land yet.

 

Don't detonate, they don't break.

 

Instead of throwing boost at it, work the head and use a proper cam for breathing improvement below boost threshold and you won't NEED the lower compression to get the HP.

 

You can make 300HP at 21 psi, you can make 300HP at 8 psi. One will be FAR easier to tune in regards to spark  blowing out, fueling, etc...