Modding the Nissan DIG-S to make DIG-T-R power

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yldouright

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I had hoped to end up with a DIG-T-R but failed so I'm looking at building one up from the DIG-S. What are the challenges I'm likely to run into?
 
The DIG-S is a 3 cylinder supercharger the DIG-T is a 4 cylinder turbo so there are quite a few differences.

Are you just looking at matching the power from the DIG-T? You can probably get overdrive pulleys for the superchargers that will get you close.

Turboing the DIG-S will not be easy, the high compression and low air flow are not great for driving a turbo which is probably why Nissan opted for a supercharger in the first place.
 
@obi_wayne
The DIG-S has two versions, one with 12:1 and one with 13:1 compression but when you extend the bore to ~4", the compression drops to 9.5 on the 12:1 engine. I'm looking to push 40lbs of boost on the 3 cylinder.
 
yldouright said:
@obi_wayne
The DIG-S has two versions, one with 12:1 and one with 13:1 compression but when you extend the bore to ~4", the compression drops to 9.5 on the 12:1 engine. I'm looking to push 40lbs of boost on the 3 cylinder.
Click to expand...

That doesn't make sense to me at all..V(

IF you increase the bore size then you will be increasing the swept volume of the cylinders so you will be compressing more air into the same combustion chamber in the head so your CR will be higher not lower PROVIDED that the piston crown design and shape and dish volume of the larger pistons remains the same as the existing ones.:)
 
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I think you'll have lag issues, the 3 cylinder will not flow much exhaust until it hits higher rpm ranges so the turbo will not spin up and you'll have no low down power. You could supplement the supercharger with a small turbo and create a twincharge setup perhaps? This might give you the best of both worlds, you could even set the supercharger to come up at low rpm and the turbo to spool up at mid rpm where the boost is sufficient to make a difference.

I'm guessing you will be using low compression pistons after the rebore or a different crank profile?
 
IMO the most practical way to lower the CR is to install forged pistons that have a bowl or dish in the crown that will increase the total combustion chamber volume @ TDC.
 
TCJBOLDIE said:
IF you increase the bore size then you will be increasing the swept volume of the cylinders so you will be compressing more air into the same combustion chamber in the head so your CR will be higher not lower
Click to expand...
Increasing the bore increases the volume of the cylinder for the same stroke so the CR is lowered. The "bowl crown" pistons are also increasing the cylinder volume for the same stroke and that also lowers the CR.

@obi_wayne
Yes, I considered that but don't know of any elegant way to disengage the supercharger when on the turbo. At 7500rpm plus on the clock, no supercharger is safe. I can live with a pokey motor down low. I've had split personality cars before and I like them. This one should make about 80whp at 2000rpm and 280ft/lbs of torque at 5250rpm by my reckoning. At 3,000lbs on the curb with a driver, it'll put around just fine.
The turbo(s) should come on steadily from about 4000rpm but before I start looking that far down the road can I even mate the DIG-S to a T5 transmission? Flow can be improved with head and valve work if necessary so I'm not too concerned about that yet. First challenges are mating the engine to the transmission and finding the parts that will hold together at 7500rpm with 2.5 bars of boost. The injectors will need to be 120lbs/hr or better but how much pump and pressure will be enough for the 3 cylinders?
 
yldouright said:
Increasing the bore increases the volume of the cylinder for the same stroke so the CR is lowered. The "bowl crown" pistons are also increasing the cylinder volume for the same stroke and that also lowers the CR.
Click to expand...

So can you tell me by what type of logic or math tells you that having to compress a larger volume of air into the same space actually lowers the CR ????

Bowel in crown pistons are made to lower OR increase the CR to the desired level.

Just to be sure I have contacted a race engine builder acquaintance who confirms my thinking/reasoning.:)
 
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TCJBOLDIE said:
So can you tell me by what type of logic or math tells you that having to compress a larger volume of air into the same space actually lowers the CR ????
Click to expand...
It's not the same space when you increase the bore. The increased diameter increases the volume displaced. I don't know how I can state it any more plainly. If your race engine builder agrees with you I don't want his engines :)
 
His engines win races so I guess he just may know a thing or 2 about engines.

Have also discussed this with one of the local drag record holders engine builder who agrees with my thinking as well.

FYI the piston crown shape has no bearing on the manufactures quoted engine capacity BUT it does govern the CR.

Hopefully you will come to realize that you are incorrect eventually.
 
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I need to apologize publicly to @TCJBOLDIE and eat the crow that comes along with that. Yes, when you bore a cylinder you increase the amount of air and if you don't proportionally address the heads, you are in fact pushing more air into the same vertical space so compression does increase, however, if you maintain the proportions of the chamber, addressing the heads, there will be no change in compression. In my example, I thought I clearly expressed my intentions of using a bigger head space but upon re-reading my posts, I recognized that I hadn't. That omission makes me look like a dumbass, deservedly, so kudos to TCJ for holding me to what was written. I should have known better than to challenge men from the land of the Barra :)

Concession made so I'd like to get back on topic. How do I mate the DIG-S to the Tremec T5?
 
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Spoken like a true gentleman, a rare thing nowadays |B

Down under the Barra is the new 2JZ :)
 
Sorry I've not heard of the two being put together I will ask around though. Are the ratios right in the T5? You'll need closer ratios down low to match the gearing and power band, at the top end you could probably widen the ratios if you're raising the red line as the power is going to be at the top end.

What car is this going into?
 
The T5WC I'm planning on using has a 3.73 final gear. It's set up for overdrive to save fuel. First gear is pretty short so a puny torque down low shouldn't hurt me much. Its top is ~142mph@5800rpm with 225/50-16 rubber. If I use the same wheels and don't improve the aero, the top will be in the 175mph range @7500rpm. The chassis should be able to handle that when I'm through with it. Depending on the compound, 50 profile 16's should be enough rubber for a 3,000lb load and 280ft/lbs of torque. The fox body Ford is one option but I'm looking at everything as a potential host.
 
That sounds like a solid plan. What are you using the car for when this goes in? Something small and light with a fairly wide footprint would work well IMO.
 
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