I recently got my 2003 Z06 retuned to overwrite the 20-year-old mystery tune that was on it. The only performance mods done to the car was a Vararam CAI (which I replaced), the original BTR stage 4 camshaft, and Kooks 1 7/8 long tube headers. The car was running wild rich, I'm talking -30% at times and causing a repeated check engine light to pop, unsurprisingly. Anyhow, with this new tune, the fuel trims are fixed, and the general drivability is much improved. However, he noticed that there were strange occurrences with the throttle position despite what he was commanding and leaned on the idea that perhaps my IAC was bad--well, it's a 2003 with DBW and it doesn't have an IAC.

I went down the rabbit hole to try and find a replacement throttle body or even a reman one and I've gotta say, I don't like what I see. It's either used throttle bodies with 150k miles on them going for $250+ or ones without mileage listed going for $400+. With that being said, I started to investigate just getting an aftermarket throttle body but then I discovered that a lot of them out there do not play nice with the C5 electronics, despite pig tails to convert the plug. So, here's my thing, I don't want an LS2 silver blade throttle body because those are also, in my opinion, too expensive for what they are. If I'm going to spend hundreds of dollars on something like a throttle body, I'd rather it be new and aftermarket.

So, here's the TL;DR:

Which 102/103mm throttle bodies work just fine with the C5's DBW electronics?

As a tidbit of information, according to Nick Williams, his throttle body was never intended to work for C5's so, that kind of throws that option for a loop. I know that FAST at least used to make one but, I was hoping that someone with experience knew. And yes, if I'm going to swap out my throttle body, I will get an intake manifold that can use a 102/103mm throttle body along with it. There are long term plans for the car that will utilize that manifold and throttle body more effectively as time goes along.

 

C6 aftermarket throttle body with a harness adapter to connect to the C5 harness.

 

Copy that but, will it play nice with the electronics? I'm tracking that even with said adapter that tuners can have a lot of trouble getting it to actually behave.

 

I had a Nick Williams that was defective. To his credit, he did replace it.

After I gave up trying to make it work - and before I realized that it was simply defective - I got a Katech 103 that Katech said was made for the C5. It does works, but only just barely.

The problem stems from two conflicting issues... first, the TAC module (which cannot be tuned) enforces an 18% limit for idle throttle. Second, the Katech 103 blade needs to rotate quite a bit before it flows any air at all. Combined, that means that for a cold start in the winter the PCM calls for about 22% blade angle, the TAC caps that at the 18% limit, and at 18%, the throttle body just barely flows enough air for my engine to idle. Once it's warmed up it's fine at 14-15%, but cold starts in winter are sketchy.

Just to be super clear about this: 18% is what the throttle position sensor reports, but the throttle opening area is much smaller than it would be with a stock throttle body when a stock throttle body sensor reports 18%.

In other words: sensor position, blade angle, and opening area are three very different things, and the relationship between those things is quite different in the Katech 103 versus the stock throttle body.

It was possible to get it working, just barely, but it required editing the tables that define the relationship between throttle sensor and throttle opening area. Those tables are not available in HP Tuners. EFI Live might have them, but I suspect not. I do know that they are available in Tuner Pro if you use the 12587603 XDF.

When I asked Katech about the tiny opening area at the 18% limit, all they could say was "it requires tuning" over and over like a broken record. They have no idea what's normal, they have no idea whether mine is normal or defective. But they are no longer pretending to sell a 103 throttle body that works with a C5, so I assume that mine is just normal. It's stupid. If they had half a clue they could calibrate the damn things to work fine. But to them, their own throttle body is just a black box that behaves in mysterious ways.

Prior to that, I had a silver-blade LS2 throttle body. That required minor tuning, but it wasn't nearly the headache that I had with the Katech. These are the one to get, but they are out of production, and the cost can be kind of absurd.

I have heard that the gold-blade (LS3) throttle bodies work fine if you limit them to 95% in the tune. They still open fully - but their sensors report 95% when the blade is one fully. I had to do the same thing for the Katech to stop it from going into REP at full throttle, and it solved the problem, so I have no reason to doubt what I've heard about the gold blade throttle bodies. But I have to admit that I haven't tried one myself.

The gold-blade version available new for about $150ish last I checked, but that was a couple years ago.

 

I had the katech "c5" 103 on mine. Once we limited it to 93-95% opening it was fine. As NSFW said, any of them realistically will work, but if your tuner doesn't know how... Then you are going to play hell until you find someone that does. I don't think we even changed any settings on mine for the first startup going from the ls2 to the 103. Only thing that needed the big change was the 95% max rotation vs pedal. Otherwise every time I stabbed it it would go into reduced power.

 

I appreciate the feedback, fellas. It seems super strange to me that this is such a niche issue that folks haven't addressed properly and instead have to do these workarounds. And now that the new BTR canted valve heads were released yesterday, I'm now contemplating a full top end refresh with this throttle body swap in mind.

 

I have a built LS7 in my 2000 C5. I talked to Nick on the phone about getting one for my car. He talked me right out of it. He said that the 90mm TB on my LS7 intake was fine. He actually recommended a ported 90mm for my car. He strongly advised me to forget about the 102/103 TB. I have to believe that the vast majority of C5 owners are better off passing on the large TB. He told me I might be leaving 10-15hp on the table on a 427, and it wasn't worth it. I mean, since he sells them, and could've sold me one, I think he would've done so if it would've been beneficial. He sounds like a pretty straight shooter.....

 

I appreciate that feedback but, I have a pretty solid plan for the car that I really don't want to deviate from. That plan will include a larger throttle body--102/103mm to be as exact as possible. I'll have to speak with my tuner to see what he is comfortable with getting to work but, I won't go with a 90mm. I haven't broached that topic with him as I used someone local to tune the vehicle just to get it to behave correctly but, this person won't be the one to tune it going forward. I'll need someone with much more niche experience than the typical LS tuner can offer.

 

Looking at the combo in your 1st post, a 90MM isnt going to be a limitation on power amd a 102/103 isnt going to make that much on that combo. I spent time on this in 2024 looking for a larger TB and the Katech DBW was only option but Katech sold the last one in November of 24. I almost bought it but in reading the comments about its compatability, I'm glad I didnt. How much power are you expecting to make and are you upgrading the stock MAF as well?
The silver blade LS2 then the gold blade LS3 TB's are your best options unless you can find a used TPIS 90MM. An aftermarket ECU would help make a DBW 102/103 work but the question remians, why do you need that much TB when the 90MM seems like it would be more than enough for your combo?

 

You will never see the difference between a 90 and 102 on your combo. Obviously, your car, your choice. But if Nick said a 102 wasn't beneficial on a built 427........

 

Some of it comes down to your tuner as well. Even the silver blade requires the right setup.


I am swapping from a FAST 92/90Tb setup to a FAST 102/103 setup. if the 103 TB doesn't work then it is back to the ported 90.

 

While I'm not going to get into every detail of the build, my intention is to reach around 600whp N/A. I don't think I'll get there via the FAST manifold or even using cathedral ports (who knows with these new heads but, I'm not holding my breath) but that's less of a concern than the throttle body itself--given the weirdness around the electronics.

 

Speedmaster calls their 102mm DBW throttle body an "LS2" throttle body, so maybe theirs could work?

It's kinda baffling to me that nobody has found a way to serve this market. Granted, Gen-3 LS isn't a huge market, but it's not tiny, and a throttle body could be super easy to tune for if somebody would just make one that has a sensor output curve that's similar to stock at low throttle openings.

Another option might be to use dual silver-blade / gold-blade throttle bodies, and some creative plumbing, and the Torque Rush X2 wiring harness.

 

 

I agree that it seems odd that no one has a direct fix for this. However, back when the C5 was developed, I don't think they envisioned people making big power on them and felt that 90mm throttle bodies were more than enough--which is even evidenced by this thread. Granted, I didn't say what my plans were for the engine but, I did ask about 102/103mm throttle body setup and not a 90. I think that in today's standards, 600whp N/A is a fairly modest number. 550, I think, is my floor. I hope that the combo doesn't make that little power but, you never know, right? But 600 is more ideal for the setup and I don't think that a 90mm is going to help me on that journey.

I reached out to Speedmaster and asked about the compatibility of their throttle body on a C5 so, hopefully I will get a response soon.

@vette4fl I appreciate the video link. I'll check that out this evening.

 

If there was a market, someone would make it. But you would end up with an expensive throttle body that maybe a couple people would buy. Everyone else is going to use whats already available for less money since this issue is easily solved in the tune.

 

The car in the video is making 700-ish rwhp. The creator is a cf member if you wish to contact him. @Mr. Black

 

Very interesting.

About 6 months ago or so I had sent Katech a long winded but clear and concise email about wanting to try one of their throttle bodies on my C5 that us running a P01 411 PCM and oem TAC module.

Never heard a peep out of them.

No way was I going to spend 700 dollars on an experiment.

My car with the PD blower is highly sensitive to intake restriction. I'm confident with proper blower snout porting and a 102mm TB that would work I could pick up upwards of 100 wheel horsepower.

Without switching the car to Holley computer I have all but given up and just enjoy the car as is.

If you can trust online inventories it looks like many prominent online stores like Texas Speed still have the Katech throttle bodies but I don't know for sure?

 

I have one of these in my garage for the new engine build. It looks beautiful, and costs less than other market offerings. I have not tried it yet though as Im still building the engine....

https://summitracing.com/parts/sum-227110

I currently have a gold blade LS3/7 TB on my car and my tuner didnt have any issues with that. I dont expect this one to be a problem either.

 

intake manifold and heads will be a restriction long before a 90MM TB. I make 655 with a TPIS 90 and its still not the restriction