Vacuum Line Issue: Upgrading Throttle Body from SOHC to DOHC

[Dbeltran24]

So I have a DOHC throttle body I planned on installing onto my 99 SOHC. I still need to bore my plenum to match, but thats not my issue.

I noticed that the DOHC throttle body has 1 vacuum line inlet where my SOHC has 2. The vacuum lines in question are labeled P.E.A on the throttle body. The DOHC only has P, and my SOHC only has E. and A.

From what I've read in the vacuum diagrams I think one line on my SOHC is for EGR. My problem is the diagram only shows throttle bodies with either 1 vacuum line or 3. So now I'm confused as to which line does what, since mine has 2.

Could I just T of the vacuum line and connect both vacuums to the same port, or will this have negative effects. I read in another thread that a similar setup caused the EGR to stay open all the time, but that guy had three vac lines which I don't.

[MeTarzan]

U can just T it if you need the vacuum port (Vacuum is Vacuum)...After doing a vacuum reduction on my TT I just capped all 3 of the TB ports off

[stealthee]

You can either T it, or get a later model or Cali spec DOHC TB. My SOHC is Cali-spec so it came with 3 ports on the TB. The DOHC TB I installed was off a fed spec and only had one. I teed it and everything works as it should.

[Dbeltran24]

Thanks for the quick responses. I'll T the vacuum off. One less worry for my rebuild.

Stealthee: With your mod list, did you need a tune? If so any recommendations on what I need? The only difference between your list (in your sig) and mine are

I have
.20 overbore pistons
portmatched intake and exhuast ports on the heads
Ported air passages for the intake on heads
Webcam Reground Cams (from 3sx)

You have
Custom Cat
EGR Delete

Everything else motor related is identical

[stealthee]

I just slapped it on. No real difference noted.

Ironically, I drove to Silver Spring to pick up my DOHC TB, as well as a some newly cleaned NA injectors when I bought them back in the day.

[Dbeltran24]

Ha, I need to update my City. I'm in Burtonsville now. Not much of a difference since its the next town over.

Once I'm done the build I will post up pics and details. I'm probably going to dyno it too, so I can put up results. Not much info out there for us SOHC's.

[dbest671]

I would just bore out the SOHC TB and use the DOHC shaft/plate... Hell, I have a SOHC TB with 3 ports you can have if you want to go that route LOL.

You might have some idle issue with the vacuum if the DOHC came from a pre 96 car.

[stealthee]

I don't have any idle issues. Like I said, my SOHC is Cali-Spec so it really isn't much different from dbeltran's.

Why bore the SOHC TB when a DOHC is right there?

[dbest671]

Is your car a 98/99? They changed the way the vacuum is on the 96+ TBs vs the 91-95 TBs. There are actually three variations of the TT TB also.

If you clean the bore of the TB, you should see some holes on the top (above the plate/butterfly)... the placement, size, and number of holes varies. That is why I said he might have idle issues.

[stealthee]

I looked at the TB off the 98 SOHC engine that went in my car and it looked no different than my stock TB off my 94 Cali-Spec.

[dbest671]

Really? They have the same holes on the inner part of the TB?

[stealthee]

They both had the same vacuum ports on top.

As Rich said though, vacuum is vacuum. All that matters is he is getting vacuum to the proper components. If he tees the single one he will be fine.

[dbest671]

This is what I am talking about... it has to do with the ports on top, which are also routed to small holes inside the TB. It will cause idle issues if they are not the same type set-up.

http://i392.photobucket.com/albums/pp4/hoopb23/Internet%20Firefox%20downloads/TB20SOHC20vs20DOHC20001.jpg

[stealthee]

I have a NA DOHC Fed Spec TB and it only has one hole and there are no idle issues. Vacuum is vacuum.

[Dbeltran24]

I found a diagram of the 99 vr4 vacuum lines, and if its anything like my 99 sohc then the two vacuums I have are for the egr solenoid and egr valve. The missing vacuum is on the plenum and is for the purge canister. I would delete the egr but I don't know how to prevent a CEL.

[dbest671]

If you want the N/A throttle body I have pictured above, you can have it for the cost of shipping. It's a 1st Gen Cali-Spec. But all you get is the part that is pictured, I have already disassembled it for parts LOL. So you would to use your shaft/plate/springs/fiav.

The 99 VR4 TB I have only has two vacuum lines also, and one big hole on the inner part.

[Dbeltran24]

The insides of both did look significantly different. But I don't remember what the inside vacuum hole looked like. I'll compare tomorrow. I think I remember reading a thread about having to adjust the idle when swapping TBs

Would hate to get rid of this one since it's been polished.

I'll snap pics of both.

[dbest671]

You may be able to "T" off the the one vacuum port... I don't know how finicky the 98/99 egr/map sensors are. The 95 TB look identical to the 96/97 except for the holes on the inner part (which has to do with the idle) after reading the manual. I've learned a lot of TBs in the last couple of years, but never really dealt with the SOHC TB... the only reason I had one was because the seller thought it was a DOHC. The only reason I have the cali-spec NA was because somebody used it on a TT and didn't realize it had non-reinforced shaft seals.