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Discussion Starter #1 (Edited)
I have a set of big bore (52mm versus 48mm stock) Dinan TBs on my engine, which is supercharged. I am thinking about swapping them over to a NA build that I am doing and installing stock TBs on my engine on the theory that the Dinan TBs make little or no difference in the FI context. Discuss. Thanks.

--Peter
 

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Trying to stir the F ing pot???

:nono:

I would guess that they make no material difference until you are making well over 700 fwhp and then then the difference wouldn't be worth the cost.

What you guys need to do is ditch the ITBs altogether, but that's another a discussion, one that focuses on real benefits ;)

Remember Peter, everything and everyone is great :D
 

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I would agree with Rob. The advantage of the larger throttles will be negligible in a FI applications. They will improve the engine's response time but so little that one would never notice. The improvement will be better utilized in a NA application.
 

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If you give me a pulley ratio and compression ratio, I can run the numbers through Engine Analyzer Pro.
 
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I agree with everyone else. The difference you're going to get from these throttle bodies is negligible at best, unless you're putting down an insane amount of power.
 

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Discussion Starter #6 (Edited)
I appreciate all the replies, and this would be an easy decision if it were a question of whether to acquire the larger TBs. But it is not; they already are on the engine. The question is will I lose anything material in the way of power if I change over to the stock TBs. Probably a question of what I think is material.

Toobs1234, info on the way to you soon!

--Peter
 

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Per my understanding in a forced induction car, regarding everything between the blower and the engine - bigger is better. I'd imagine with F/I you'd be better off with the larger TB's as it will more quickly fill the piston chamber. At higher RPMs I'd imagine the larger TBs would allow for higher peak output.

Is this not true? I'm far from an expert, I've studied a lot of fluid dynamics so have a decent understanding of N/A intake systems, but am very much still learning regarding F/I. The above makes sense to me regarding what I know with N/A systems and fluid motion and trying to apply it to F/I, is my above statement not correct or is it just a matter of it being an insignificant difference? I'd personally imagine in F/I you'd be more likely to notice a bored TB versus N/A. If not - why is that?
 

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My guess is that you will want to keep them on your FI setup. The reason being is that with a larger bore, the acoustic resistance of the intake is reduced (resistance to air velocity) at the expense of a lower acoustic inductance (resistance to a change in air velocity). Short velocity stacks make the same tradeoff as a larger bore: lower resistance at the expense of lower inductance. A high acoustic inductance keeps air moving through the intake and into the cylinder even after passing bottom dead center and also results in a pressure spike on intake valve close that can be usefully tuned acoustically to provide a pressure spike at intake valve open. Inductance seems to be very important for tuning the acoustics of naturally aspirated engines, but should be less useful in a forced induction setup where the supercharger is providing a pressure differential that will swamp acoustical resonances. A low resistance to air flow on the other hand should be more important in a FI setup than a NA setup since you are moving more air. I'm assuming here that for your race car setup, you mainly care about max hp and not low end torque.

Caveat: I may be totally full of ****e. I used to be really into acoustics (loudspeakers) before I lost hearing in one of my ears a few years back and moved on to motors. The two areas have a surprisingly large overlap. However, I still have a lot to learn about engines.
 
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Get a .005" smaller pulley and negate any theoretical benefit. hiha

You will ge a much larger benefit opening up the intake to the supercharger itself ;)
 

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No empirical data. Unfortunately, i swapped in the bigger TB's when I swapped motors.

From dyno runs and seat of the pants, I am not sure more than a handful of extra ponies, if that. But the car does seem to feel quicker off boost, and boost spools up quicker and I can see full boost around 4-4.5k rpm. With stock TB's, I wouldn't expect full boost until around 6000 rpm, since boost is a function of rpm on our setup.

I am guessing that at full boost ( 8ish psi) the stock TB's are not all that restrictive with the curved air horns. Short stacks would probably increase top end just a bit. Note that Dinan kept the bigger TB's when going from S2 to S3, so it certainly isn't hurting top end.

I would think you would see a bigger difference on an NA car. So while bigger TB's may help FI a little, it will help NA more. Just a SWAG.

Regards,
Jerry
 

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Computer says 28hp (with the Dinan throttle bodies). That's with stock heads and cams and stock length stacks. Should be more of a difference with your VAC heads and high-lift cams. You can get a similar increase in HP with shorter stacks, and stock diameter, but shorter stacks have a negative impact on lower rpm torque.
 

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No empirical data. Unfortunately, i swapped in the bigger TB's when I swapped motors.

From dyno runs and seat of the pants, I am not sure more than a handful of extra ponies, if that. But the car does seem to feel quicker off boost, and boost spools up quicker and I can see full boost around 4-4.5k rpm. With stock TB's, I wouldn't expect full boost until around 6000 rpm, since boost is a function of rpm on our setup.

I am guessing that at full boost ( 8ish psi) the stock TB's are not all that restrictive with the curved air horns. Short stacks would probably increase top end just a bit. Note that Dinan kept the bigger TB's when going from S2 to S3, so it certainly isn't hurting top end.

I would think you would see a bigger difference on an NA car. So while bigger TB's may help FI a little, it will help NA more. Just a SWAG.

Regards,
Jerry
Boost "spools" quicker?

That comment made zero sense jerry. You see "full boost" at 4-4.5k rpm, then you say yourself manifold pressure is a function of rpm...You can't have both. I didn't know you guys were running centrifugal blowers with negative parabolic relations now.
 

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Boost "spools" quicker?

That comment made zero sense jerry. You see "full boost" at 4-4.5k rpm, then you say yourself manifold pressure is a function of rpm...You can't have both. I didn't know you guys were running centrifugal blowers with negative parabolic relations now.
Boost comes in sooner ( I believe more a function of the 92mm pulley). Full boost comes up at a lower rpm than before. Best I can do without an engineering degree!:sad2::)

Regards,
Jerry
 

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Boost comes in sooner ( I believe more a function of the 92mm pulley). Full boost comes up at a lower rpm than before. Best I can do without an engineering degree!:sad2::)

Regards,
Jerry
It's not easy with the way a centrifugal compressor is designed for pressure to peak anywhere but redline. The only way peak pressure is seen at a lower rpm is if you have an undersized blower which becomes choked. The V3 on your car will ruin your stock engine before that point is reached. The other way is to spin the compressor at a greater speed so that you see 6psi at 5k rpm, but then run a diverter valve which releases all the additional pressure. This is for a centrifugal compressor setup of course, positive displacement ends up being different.

The pressure coming on sooner is true because the compressor is spinning at a faster speed all across the range. That's why you see the graph shift slightly to the left, very minimal though if its 1 pulley size.

-R
 

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This is an interesting thread. I'd like to see any proof on either side, since some seem to say it will help, others say it wont, and both sides seem to have valid reasoning for their beliefs.
 

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I am glad my passing inquiry stimulated so much intelligent discussion (other than Raza's posts, of course).

--Peter
It's pretty simple if you break it down. The restriction is not the area in a forced induction setup, it's density. In a N/A application you're increasing the area which decreases boundary layer effects of the volume of air that comes in contact with the throttle body. You slightly increase the velocity of airflow by decreasing the amount of air introduced to the boundary layer. So in the mass flow equation (Mdot = Rho*Velocity*area), you've increased the velocity and area slightly, yielding more mass flow in the system.

Whereas the compressible flow situation, you're drastically increasing the rho(density). The velocity and area are small factors in the equation now and don't have such a large impact on the mass flow going in the cylinder. Which is why the benefit of the oversized throttle body is very negligible.

This is a very crude and dumbed down version, but it gives an idea of what's going on. As to why it doesn't make much of a difference in the forced induction setup. Your limiting factors with compressible flow in that throttle body is when the flow goes sonic. That never happens so the area is not a concern so much, answering your initial question.

-R

PS: Sorry my discussions aren't intelligent enough for you peter.
 
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