wndrllama103 New Reader
3/3/21 8:48 p.m.

Hi all, 

I've been (re)building this mid-engine mk1 rabbit project for a while now. Right now, I'm running a single-turbo B5-era 2.7T into a FWD 01E. I'm in the process of rebuilding the engine, so while it's out I'm looking to fix more stuff.

One of the big problems I have is poor tunnel design. For some dumb reason, I made it too small and worst of all, in the wrong location (it exits right in front of the crank). Not sure wtf I was thinking, but too late to fix that now.

My coolant system is 25-30' of 1-1/4" 0.060 wall aluminum tubing, joined together with a bunch of silicone adapters, all pushing from an OEM water pump. It's a mess, looks like crap, is a real tight fit (picture below) and will easily be an engine-out scenario if the right silicone adapter fails. All that being said, the system worked great. It never overheated.

I'm looking to replace this all with a couple big custom pieces of stainless, bolted to the chassis. This a is no-brainer.

The question I have is that I'm also considering dropping down to a 1" OD line. That extra 1/4" will really make things a lot easier, but I don't know if it will be too much of an added restriction for the pump. Does anyone have any experience in this department?

wndrllama103 New Reader
3/3/21 8:49 p.m.

Here are some pics, just for fun





kb58 SuperDork
3/3/21 9:09 p.m.

I'll be very impressed if you get an answer from someone who's done exactly the same...

Without that input, you're asking a question with no black-and-white answer. It'll work "some" amount less efficiently than what you have now, and decreases your cooling system's capacity by another "amount". Unless you know the cooling system's capacity, it's all guess work. A middle ground is to try it with the smaller tubes, but plumb the new system so that it can return to larger tubing if need be, and plan ahead for improving the pressure drop across the radiator as well.

wndrllama103 New Reader
3/3/21 9:30 p.m.

kb58, I'm not looking for someone who's done exactly the same as much as someone who's someone who's done something similar. 'cause you're right. I'm guessing no one else has done this.

I'm not that concerned about capacity, as I've added a bunch compared to the stock setup in a B5.

As far as testing it out, it's an expensive and time consuming test. So, I'd like to get it right the first time :)

Honsch Reader
3/3/21 9:41 p.m.

I'm not familiar with the 2.7 cooling system, but when we moved the radiator to the back of the car in our VW Fox wagon (2.0l ABA) we also used 1.24 tubing.

It did not work very well.

Later we increased airflow to the rad box several times, upgraded the fans, and moved up to 1.75 tubing for the coolant.

It works fine now.


Trent PowerDork
3/3/21 9:49 p.m.

When I first relocated the radiator from the rear of my fiat to the front (rear engined car) I did so with 1.25" tubing. The car took over 30 minutes to fully come up to temperature.  The data logs showed the temp coming up, the thermostat opening and the coolant temp dropped from 180 to 75. Over and over. 

I ended up reducing the line size to the front to 7/8" tubing and it stabilized much quicker. It reduced the total coolant capacity by well over a gallon. I still had no overheating problems beating the hell out of the thing for hours at a time on 100° days.

wndrllama103 New Reader
3/3/21 10:02 p.m.

Trent, it does take a while for the whole system to come up to temp. That being said, the engine is able get up to temp in a decent amount of time.

I am a little concerned about "fixing what's not broke" - so if I were to air on the side of caution, I'd keep it at 1.25". I really don't want to have to do this twice.

That being said, 1" would be way easier to deal with. I'm torn.

matthewmcl (Forum Supporter)
matthewmcl (Forum Supporter) HalfDork
3/3/21 10:27 p.m.

I had a front engine rear radiator car. I did not get all the issues fixed, but running a Davies Craig water pump at the radiator in addition to the regular pump on the motor helped quite a bit.

bgkast (Forum Supporter)
bgkast (Forum Supporter) GRM+ Memberand PowerDork
3/3/21 10:59 p.m.

I'm currently pondering the same thing in my mid engine build (kb58's Midlana). Stock my engine used 1.25 inch or so radiator hoses.  My original plan was to use 1.5 inch aluminum tube down the tunnel, with hoses to the radiator and the engine. The trick will be finding hoses that are the right shape. I want to avoid a bunch of joints for the reason you mentioned. If I can't find the right shaped hose I'm considering using 1.5 inch flexible hose, one piece from the engine all the way to the radiator. I'm also considering running an electric pump up by the radiator to help the stock water pump.

For your application I would be hesitant to drop down the size of the plumbing to the radiator. What does your tunnel look like, is it a straight shot? What size is the stock inlet and outlet on your engine?

Honsch Reader
3/4/21 2:35 a.m.

Oh, I forgot to mention we have a swirl pot in front of the rad to make sure no airlocks happen in the system.

Keeping both high points bled is very important if you want good coolant flow.

APEowner GRM+ Memberand Dork
3/4/21 5:19 a.m.

Going from 1 1/4" diameter down to 1" is a 36% reduction in area.  I didn't do the math but over the lenght you're talking about that's a significant increase in pressure drop.  I wouldn't risk making that big a change in a system that's working.


Bkast: if you go with flex make sure its something that won't suck closed on the return side. 

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
3/4/21 6:17 a.m.

I use 1.25" as a min on the drag cars I commonly build. I find this size is marginal even with short duty and would likely default to larger on such a long run. If it's working, don't mess with it.

obsolete Reader
3/4/21 11:55 a.m.

Awesome project. Thanks for posting pictures.

I think an electric helper pump up front is a great idea. It certainly can't hurt; there is no such thing as too much coolant flow. I would connect the pump inlet at the lower radiator hose to help push the coolant back toward the engine. If the system worked great with 1.25" tubing before, this might allow you get away with 1", but as others have stated, there's no guarantee...

wndrllama103 New Reader
3/9/21 10:53 p.m.

Sounds like I'm gonna leave it as 1.25"

Thanks for your input!

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