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The mkiv Supra Owners Club

Ground Clearance Problems With HKS Exhausts


Guest Ash
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Whilst the Drager and the Hiper sound the dogs, I've been looking into the problem of ground clearance.

 

For instance, Justin's exhaust is almost wrecked and he's only done a few thousand miles since having it fitted.

 

The problem seems to be the pipe in the region of the rear axle. Because it is such a large diameter and the fact that the pipe goes under the axle, clearance drops dramatically at this point.

 

It would make far more sense to split the single pipe into, say, 2 x 2" pipes: at least under the axle. This would give another 2" of much-needed ground clearance. Thus increasing the life of the exhaust quite significantly. Well, at least in Justin's case that is.  

 

I was just wondering if anyone else had the same ground clearance problem.

 

Yours,

J

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Yes I have the Hiper.

Only on the car a few weeks, grounded twice with stock wheels and suspension. Not dented yet, just scratched badly:shocked:

The first time was on a rough road out side a cart track so was expecting it.

The second was on a local speed bump taken quite slowly and not expected.

Saw a Super  D on another Supra running 18's and lowered, the main pipe was flattened badly from many contacts with the ground. The pipe had flattend nicely with no holes, I did not see the owner to ask if it grounded any more, but the flatted section gave about 20mm  extra clearance.

I am glad no one saw me grovelling around under someone else car.

When I fitted the Hiper,  getting clearance from one of the small body bracing struts was a pain. I pushed the pipe, up as far as it would go with hiting the underside of the car. 

Perhaps two pipes would do the trick!

Any one going to try to make one up?

What impact would it have on performance creating more turbulence in the pipes?

 

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No, the stock system has two pipes which separate and then come together at the backbox.

 

What I mean is, rather that having a single 4" pipe going from the manifold outlet to the Hiper backbox, you have 2 x 2" pipes exactly side by side following exactly the same route as the current Hiper 4" pipe.

 

There is plenty of width for this which means, piping as I suggest, the twin-pipe layout will be able to keep the same height as the current 4" pipe... thus giving 2 inches more ground clearance at the critical point of the rear axle.

 

I hear what you say regarding oval pipes but they are mighty difficult to work with, bending wise, than conventional tubular stuff.

 

Yours,

J

 

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Phil, grounded with stock suspension!!!

 

No wonder J's zaust is shagged. I'm going to make up a twin-pipe setup for J's car retaining the stock back-box in order to see how it performs. I'm not sure of the turbulence question. I guess we'll have to suck it and see.

 

FYI... last time I spoke to JF, exhaust-wise, he was talking about an HKS twin turbo setup and twin (non-silenced) side-pipes each exiting either side of the car just in front of the rear wheels.

 

The guy is crazy! I thought I was a total scew-ball but someone has gotta talk some sense into that bloke or he'll kill himself before long.  :)

 

Yours,

J

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Yes  grounded with stock suspension!

The road by the cart track was raised the middle and I went over the speed bump with one set of wheels on the flat effectively lowering the car!  I won't be doing  that again.

 

 

I guess I just forgot about the clearance, I will be more careful in the future. The hiper just sounds fantastic, so I dont want to wreck it just yet.

 

It would be amazing to see a MKIV with twin side exits. Just stick some small bore tubing into the fuel line filter intakes, that should slow him down!

 

 

 

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It would be amazing to see a MKIV with twin side exits. Just stick some small bore tubing into the fuel line filter intakes, that should slow him down!

 

 

You are just as crazy as he is!!!

 

And I thought I was mad... Jesus, for once I feel sane. :)

 

Yours,

J

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The HKS exhausts are supposed to be the ones with decent ground clearance. I had a lot of problems with my Blitz Nur Spec grounding out with stock suspension so Leon raised it right up into the body and I didnt have any further problems.

 

I then had Eibach springs and Billie shocks fitted and luckily enough I still dont have any grounding problems. One thing that did surprise me is that the Eibachs didnt seem to bring the car down that much, they took about 10mm out of the front and almost nothing out of the back, I suppose my stock springs must have been sagging a bit.

 

JB

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Are all Eibach springs the same or are there several options for the MKIV?

 

Justin's are low and as hard as nails. Too hard IMO. To the point where the handling of his car needs some serious looking at. Maybe his shocks are shot or something. Haven't had chance to look into it in any detail, but it is obvious there is a *serious* mismatch between the shocks and springs.

 

Yours,

J    

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Im not sure if there are different Eibach springs. I know that Dean has Eibachs and Billsteins and I have Eibachs and Billies but his car sits about 10mm lower than mine. He has the Billie shocks for a Toyota Supra and I have Toyota Bilsteins and there are certainly differences between the two. I find my car has much better turn in now and is more sure footed through the corners, rough surfaces do make it a mit more prone to breakaway, but then my old springs and shocks did that anyway.

 

The main difference I have found is that the back end is very light under heavy braking, but apart from that I think the Eibachs work pretty well as an all round spring. And the car looks better being lowered that little bit at the front.

 

JB

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Hmm... when we get the car on the road I'll look into this further. But I do know that Justin's springs are as hard as nails. It feels like you are driving on a piece of solid board. Plus, it's terribly skittish on bumpy surfaces. The geometry is okay, I know that. Maybe I might swap them for my old HKS ones because, from memory, they seemed to me a much better match to the Toyota Bilsteins.

 

Yours,

J

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Quote: from Ash on 7:54 pm on June 14, 2001[br]you have 2 x 2" pipes exactly side by side following exactly the same route as the current Hiper 4" pipe.

 

I don't think 2 2" pipes will give you the same flow as 1 4" pipe

 

the area of a circle is pi * (radius to the power of 2)

 

a 4" pipe would have a radius of 2" and the area the air would flow down is 12.56"

 

a 3" pipe would have a radius of 2" and the area the air would flow down is 7.06" x 2 pipes 14.12"

 

a 2.75" pipe would have a radius of 1.25" and the area the air would flow down is 5.94" " x 2 pipes 11.88"

 

a 2.5" pipe would have a radius of 1.25" and the area the air would flow down is 4.90" " x 2 pipes 9.8"

 

a 2" pipe would have a radius of 1" and the area the air would flow down is 3.14" " x 2 pipes 6.28"

 

Of cause flowrates won't follow these areas exactly but you may have to use 2.5" or possibly bigger pipes to get an equivelent flow to a single 4" pipe?

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Thought I would be more objective and get some real measurements.

 

Running the Hiper on Jap spec car, stock suspension.

 

I have 96mm clearance on level ground at back box elbow, full tank, no driver.

 

I can't take the pipe higher for fear of hitting the body or cross members over rough ground.

 

Does anyone have the clearances on a stock system.

Don't want to put them back on just to measure.

 

Anyone have measurements for other pipes and setups?

 

 

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Rich... thanks for pointing that out.

 

I haven't yet looked into the actual flow issue in any great detail. Plus, the way gasses flow through pipes is quite a complex subject - only about 10% of which I understand.

 

From your calculations it would appear that 2 x 2.5 " pipes would flow to the extent where I very much doubt you'd feel any difference, yet still give a useful 1.5" extra ground clearance.

 

From the way Justin's pipe has been flattened, I estimate that if he'd had an extra inch, I doubt whether the pipe would have made contact. Problem is, the pipe is now rusting away along a large section of its length, thus shortening its service life considerably.  

 

Reference the previous post, yes, it would be mighty useful if I knew the stock clearance. Anyone fancy getting a tape measure out?

 

Yours,

J

 

 

 

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Quote: from Ash on 1:47 pm on June 17, 2001[br]I haven't yet looked into the actual flow issue in any great detail. Plus, the way gasses flow through pipes is quite a complex subject - only about 10% of which I understand.

 

Me too (less than 10%) , but the sectional area gives a bit of an idea (I hope)

 

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As we are dealing with x-sectional area... 1.4 times the diameter doubles the flow. Which gives you the advantage of being able to make big flow increases and yet keep the pipe diameters withing manageable limits.

 

Problem is though, when you start reducing the diameter, potential flow is curtailed rather quickly. As was demonstrated in your post: 2 x 2" pipes flowed only about half the one 4" pipe.

 

So it's obvious then, the best way to increase flow of a stock exhaust is to increase the diameter of the existing pipe rather than add another pipe. So it comes as no surprise why HKS have used such large diameter pipes.

 

I think I'll take a look again to see if the exhaust-brackets can be re-worked in order to get the pipe sitting up more (as Branners said he'd had to do with his).

 

Yours,

J

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Mine came with a 5Zigen fitted which I managed to ground over speed bumps a couple of times. The section under the axle has a flattened bit which has about 8.5-9cm clearance.

Not flattened by grounding, just a small flat oval on the underside of the round pipe. Tailpipe is 13cm diameter, not sure of pipe bore (came fitted). 18" wheels 265x35x18 on the back. I now crab over speed bumps slowly & it's OK. I love the sound of the 5Zigen.

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Checked mine over the weekend as it seems to be getting worse lately.

 

I have Eibachs with standard shocks and 18"s and I've flattened a lovely oval section about 80mm long at the lowest part of the exhaust (HKS Hiper).

 

Some of the gouges are fairly deep and it is only a matter of time before they wear thru.

 

I was thinking of fixing a metal plate over this section with liquid metal or something - armour plating if you will.

 

Any thoughts?

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what pressure do you run in your tyres? Im having problems with the side skirt grounding as I go onto my drive way and I know its because the pressure in all 4 tyres is well down on what it should be. I would recommend 34psi all round which will raise the car slightly but will make the ride a bit harsher.

 

Theres no reason why you cant get a steel plate welded to the exhaust to protect it.

 

JB

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I've got 36psi in them at the moment - I raised them from 34psi at Bentwaters and have left them there.

 

I wonder if the car runs slightly lower in hot weather with the extra heat in the springs and damping oil?

 

My F1's are nearly on the wear-bars at the back as well so that's a few mm!

 

I've definitely got to put a plate on this weekend though or I'll hole my exhaust.

 

 

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Im just glad I got Leon to raise my Blitz system right up tight to the car otherwise it would have been ripped off by now.

 

I was looking at the back of Pete McK's car yesterday and even with though its got the 'changed' Blitz nurspec it still hangs very very low. I wonder if any of the exhausts actually sit tight against the body to avoid grounding out?

 

JB

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There is a curved plate that hangs down behind the diff between the rear wheels and where my hiper goes under this it is grounding.

 

The clearance betweek the Hiper and the plate is only 15mm so it couldn't really go any higher.

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