Thread: Need opinion on strength of this modified car part

If it was me, I'd be making the hole bigger and making the pipe a sleeve fit rather than flanged as this would allow me to still remove it.

As long as the re-welding was done properly, I reckon it'd still be as strong.

If you were to dramatically change the profile as in the second option, I'd be keeping a very close eye on it as you are changing the torsional construction of the cross member

Originally Posted by johnjohnmulley

Hundreds of people around Australia have done this so it's not like I'm a test dummy.

Doesn't make it OK. Modifications to your chassis usually require engineering signoff and licensing approval. Check your state regs. If you are in a serious accident and they find a non-engineered modification to be a contributing factor you will be left holding the can as all insurance companies wave goodbye.

Burnsy:
I will get it mod-plated after it's done.
I wasn't implying that it's ok to do it this way, that's why im asking because I think the opposite. But, I was implying that I'm not making up my own thoughts about the topic, rather I'm using what hundreds of people have done so you don't think its a random design i mocked up 30 minutes ago...



DJ-
So you think the first option is stronger?
If my calculations are correct, the stress at the edge of the hole is 4.3 times larger if the hole is expanded. This worries me... the fact that hundreds have done it this way..

Originally Posted by johnjohnmulley

Burnsy:
I will get it mod-plated after it's done.
I wasn't implying that it's ok to do it this way, that's why im asking because I think the opposite. But, I was implying that I'm not making up my own thoughts about the topic, rather I'm using what hundreds of people have done so you don't think its a random design i mocked up 30 minutes ago...

John, good to hear you are getting it plated. My advice is still to talk to whoever you are going to get to modplate it. Some engineers are finicky and want it done via their design/specifications even if the other way is equally as strong. Spent long enough mucking around with custom cars to know that you can't asume anything or make any decisions without talinking to whoevre is signing off on it first.

Good luck with it, what is the car?

Datsun 1600
Had some problems with the previous owners diff setup, so I'm resorting to a better way...

Just need a hand with some fabrication sometimes thats all.... anyone who has the tools and wants a bit of money would be much appreciated, especially a tig welder for stainless exhaust!!!

Keep the opinions coming please

Originally Posted by johnjohnmulley

Burnsy:
DJ-
So you think the first option is stronger?
If my calculations are correct, the stress at the edge of the hole is 4.3 times larger if the hole is expanded. This worries me... the fact that hundreds have done it this way..

"The fact that hundreds have done it this way"

Is that as in made the hole bigger as in the first group of images (First Option) or in the second where they've filled in the hole and scalloped a piece out of the bottom with a new cross member also being welded in where the diff housing goes through (Second Option)?

If you're going with the second option, how have you applied the testing forces or torsional forces or twist?

Usually in the case of cross members which have a plate across the bottom and top, it relies on both top and bottom pieces being one piece, as in a full length pressed plate that hasn't been welded together, you may get away with one that has been riveted but not one that has been welded as it is no longer torsional or have any flexibility.

Depending on the ground clearance, why not go the filled and scalloped way but add a removable plate to go under the pipe. You could have 4 captive nuts in the subframe on either side of the scallop and make a formed plate to bolt between the 2 sides. That should retain a lot of the strength and still be easy to remove the pipe.

bollie7

I don't have the data to run FEA, but my experience designing car chassis and suspension systems tells me neither method is ideal, but if I had to choose, I would enlarge the hole to accommodate the exhaust. Removing the lower skin will drastically reduce the strength of the member.

However, if you do that, then no doubt you'll be forced to relocate the near side top link (as in the fourth picture) which is diabolical! That will screw with the suspension and the car's handling.

Another issue I noticed is the exhaust hanger strap. The way it's oriented, it allows for slight axial movement of the exhaust, which in a transversely mounted engine set-up is alright, but the fact you've got a rear diff and prop shaft tells me you're running an in-line engine and there's no allowance for radial movement of the exhaust. That will eventually lead to fractures in the system – most likely at or close to the header flange(s).

Has the owner of the car in the pictures had the car engineered? I know my engineer wouldn't entertain those modifications.

The question has to be asked; why increase the exhaust size? Have the maximum revs of your engine increased substantially or is this really just a fashion accessory - honestly? If you genuinely need that size of exhaust, then I doubt if the car would be road legal. The number of other modifications required (body seaming, full cage with door bars, massive brakes to handle the additional power, radical steering and suspension geometry etc.) would give any road authority engineer a fit.

dump it out the side...
but i agree one hundread percent with woodwould. i am building a hot rod with 650 hp..its not easy getting that through rego...but i will.
good luck with it.

DJ-
Hundreds have done it by enlargeing the hole,
Only a few that I know of have done it under the crossmember.

Bollie-
Good idea, I have a picture of a similar setup but it was not bolted on like you said, still welded (I don't know the idea behind this setup though looks pointless). However, the ground clearance would definitely be an issue then.

woodwould:
Thanks for the informative reply

What do you mean by the "near side top link"? What part of the actual car are you talking about.

I see what you mean by the exhaust hanger mount. Mine allows radial movement and isn't set up like that. Maybe because his hole hasn't been drilled out larger, as you can see he has probably tried to make it solid mounted (stupid) because every time he drives it will constantly bang in the crossmember.

Thirdly,
The size of exhaust that can fit through the standard hole (about 1.5") is extremely restrictive for the engine in my car. My current exhaust is 2.5" and for example, if i WAS to go back to an exhaust that could fit through I'd have to find a stock datsun exhaust then get it custom modified to fit my engine and would cost me a lot more than cutting out a bigger hole to use my current exhaust. Most people run a 3" system, I'm not saying I will be doing the same, and I'm not saying thats what everyone should do, but for my purpose with my 2.5" I need to make my exhaust fit.

And I wish I had a FEA program and knew how to use it...

Thanks again the for replys

After speaking to the Datsun people, the hole through crossmember seems the right way to go. Apparently the guy who put it under the crossmember in the pics above, only did it because he changed diff's and mounting points (as you can see in the pic) and the hole wouldn't have even fit there. It's easier/cheaper to do a bigger hoel through crossmember, and the cops won't get angry because my exhaust wont be hanging below the crossmeber thus making it probably too low. Also, wont need any major bends to get the exhaust back up over the driveshaft.

Originally Posted by glen boulton

dump it out the side...
but i agree one hundread percent with woodwould. i am building a hot rod with 650 hp..its not easy getting that through rego...but i will.
good luck with it.

Yes thought of that, but i think the only legal way is exiting the RIGHT side of the car. That means I'd have to go under the tailshaft and introduce a new problem of exhaust being WAY too low. My exhaust exits engine on left hand side.

good luck with your hot -rod. sounds nice.

I agree with you Wouldwould. This seems like an huge amount of effort to go to in order to put an unnecessarily large exhaust on a 4 cylinder car. Too big an exhaust not only makes your cars note sound hollow and dull, but can reduce torque too.

I personally am into V8s, and in particular yankee stuff. There are many examples of people achieving 500 or 600hp out of 400 cube small blocks with a 2.5inch system. These blokes would have used bigger if there was benefit in doing so.

I found this on an Isuzu "Exhaust Performance Basics" page. It agrees with everything I have ever read on the topic of exhaust selection.

For non-turbo engines under 2.4 liter in displacement and turbocharged engines under 2.2 liter displacement and producing less than 400 HP, 2 1/2 inch mandrel bent exhaust pipe size is optimum for high performance

Just stick the exhaust out the side of the car...Straight through no muffler like this car http://www.autocult.com.au/img/galle...l/RNEM734J.jpg

mine is a turbocharged engine under 2.2L producing less than 400HP and my exhaust is 2.5" mandrel bent. That means my exhaust is optimum.

I agree with what you're saying, large exhaust on non-turbo cars are really stupid, as they require a certain amount of back pressure. However turbo cars usually the bigger the better. not always, and to some degree that is wrong, but usually.

Anyway, as I have the optimum exhaust and the hole is smaller than my exhaust, I still need to drill it out. It looks like the preferred option from both forums I'm asking this question on, so I'll take it to an engineer to look at.