super_kev

iTrader: (5)

I couldn't think of a good place to present this idea, and since it has to deal with how our cars were built and where the stress loads are, I'm posting this here. Feel free to give me the dumb idea award of the month, but I haven't seen anyone do it, and I am determined to make it work.

I am currently installing a LS1/T56 combo in my 3rd gen. I am not able to pass the exhaust along the sides of the transmission tunnel and over the transmission mount, as there is not enough room. My only option is to pass the exhaust under the transmission mount, or cut and modify the subframe in front of the mount. I do not want to do the former, as it will absolutely kill my ground clearance.



When I cut my donor 4th gen in half to take to the dump (the car was twisted, otherwise I would have saved it), the subframes were pretty thick; I'd say around 1/2". The thirdgen subframes do not feel as thick as those on the 4th gen, but I do not want to cut into them and cause damage or miss-alignment before I weld in and re-enforce the piece of 3x3 .120" square tubing that the exhaust will pass through.


The tubing



Right now, the car has the LS1/T56 installed and the interior is gutted. I have welded in custom SFCs & 6 pt. cage, and I have also seam welded the interior seams every two inches for 1", from the firewall to above the gas tank. The car is supported in front by jacks on the k-member, so that helps take the majority of the engine and front end weight. If I cut and modify the subframes one at a time, I am concerned that the area of the car behind the firewall will loose it's supporting strength, the gap will open (think upside-down V) and the car will sag a bit (or worse).


Looking rearward: Marks of where I am going to cut, and the firewall seam (dark streak)

I am posting this to get input from those of you who have worked on these cars extensively in this area. I don't mind the hard work of doing this (cutting through the subframe will be no small job). Should I support the car somewhere besides the k-member? Should I remove the engine and transmission to get rid of more weight on the body? Is the tubing thickness ok (3x3, .120")? I feel that the modification will be good and strong after it's done, as I will re-enforce the two sides of the cut subframe with 1/8" plate which will also be butted up and welded to the tube. But I do not have any knowledge of where the loads are placed on these cars, and what will mess things up.

Old SStroker

Thoughts:

The area you are going through takes heavy bending loads. Any vertical structure you remove should be replaced with an equivalent amount of steel in the vertical.

Look at the exhaust pass-thru on the rear crossmenber of C3 Corvettes. If you can use a thick wall round tube thru the subframe it will carry the loads better than a rectangular one. You could use a 4 inch tube and ovalize it a little. If it bulged out the bottom a bit that wouldn't be bad. Weld everything completely.

I suggest completely unloading the subframe during the modification.

Assure you have clearance around the exhaust to the subframe or the NVH will drive you batty.

super_kev

iTrader: (5)

NVH = ?

Here's what I was thinking about doing (just posting this to help visualize).


The orange shows the area where the plates would be welded. Roughly 6" long, sides and bottom (including over the square tube on the bottom, so a double layer). The yellow is the 3x3 tube.


And here is how the subframe is laid out inside. I felt around and looked with a flashlight, and it is nowhere close to the 4th gen setup. IIRC, the 4th gen has around 4-5 layers of metal sandwiched to make a thick unit. This one has just two layers, and on the inner side (blue), the piece from the rear is separated about 1/4" inch from the front piece for an air gap for the whole length of the rear piece being in the front piece. However, both pieces are joined/welded at the top and bottom. The outer side is a 2-layer sandwich of both front and back sub-frames. So now I know that it's not as thick as I thought it was, so that makes me think that there is not as much of a load there as on the 4th gens.

As far as using round tube instead of square; I was going to use square due to it being easier to cut a square notch vs. trying to trace and follow a round/oval one. But I can see that a round tube would be stronger than a square one. Any ideas on how I would cut a round hole of that size?

Old SStroker

NVH = Noise, Vibration, Harshness

I'd try a hole saw. Drill a pilot hole (usually 1/4 in.) to guide the saw. Start with a hole saw a tad smaller than the tube you are inserting and use a die grinder and 1 inch or so wheel to finish the holes to size. You could use a plasma cutter, but I wouldn't do that with the subframe installed.

Your plating of the subframe looks good.

FASTFATBOY

iTrader: (23)

I don't know if I would do that, if it was me. There have been more than a few do that swap, 9 Ball on this board is one who comes to mind, also Year One. Dr Gas makes a notched peice of ex tubing to clear deals like that, they also make oval flat type tubing that will help....I would think twice before I cut that subframe. You will essentially make it two peices, even if you box the notch...think of taking a box tube frame connector and doing that to it, how much the twist resistance would be taken out of it. The car would probably handle weird with diiferent load carrying ability from side to side.


Just my .02

David

super_kev

iTrader: (5)

I thought about the flattened exhaust tubing and even having some custom made, but I don't have much space in that area for two wide flat oval tubes, and they still take away ground clearance. As for the subframe twisting more after the notch and weld, would it really? Here we would have a round tube with 1/8" or 3/16" wall, 1/8" plate bracing all around, as well as a continuous weld. The stock sheet metal subframe is spot welded and is at least half as thick as the plate and tubing would be. I would worry a little if it were the same size thickness that was going back in, but it's not...

ThirdGenLS1

iTrader: (11)

FYI, theres a guy on Thirdgen.org that makes a tranny crosse member that allows you to run 3 inch pipeing under it. I would go search over there, i think his name is drew on teh board.

super_kev

iTrader: (5)

Yes I know, I've seen it. The problem is that Drew's design requires the torque arm to be on the transmission, while the Spohn unit I am using relocates it to the x-member.

11SECDWS6

iTrader: (3)

I would not cut into the subframe period. Your likley to get very undesirable results as the whole dynamics of the car is likely designed around the subframe.

Is there anyway you can move the engine forward a few inches to increase clearance between the tranny and the frame? Its hard to tell from your sketch as there are no clearance dimensions. Also, could you possibly have a custom tranny crossmember fabricated?

I would try thinking outside the box and exhaust all other options (no pun intended) before cutting the subframe.

tuske427

iTrader: (1)

Having performed the LS1/ T56 in a 3rd gen myself and on top of that my car is lowered 1" I will definitely agree that the exhaust ground clearance is horrible. I have to mind any speed bumps I might take. Sometimes I scrape on curbs/ driveway entrances, etc.. Some things to consider are this:

1- do you have to use the Spohn torque arm? Any reason you want to stick with this one aside from having it already?

2- I like your ideas for the frame modification. The 4th gen cars were definitely reinforced in the front subframe area as you have mentioned. (Books I've read reference this, too) Some good points were brought up about possibly upsetting the chassis stiffness. While this may add more work- if you're set on using the Spohn unit and adding a hole to your subframe- what about matching it on both sides? Even if you don't use it- at least your frame will be symmetrical in loads, stiffness, etc. It'll at least keep it balanced.

some aftermarket frames have the tubular holes cut in them for exhaust clearance, so doing this should not be that big an isssue.

more food for thought.

Good luck- and if you do pursue this project- please post many pics along the way!

Thanks,

-BV

MrDude_1

iTrader: (4)

uhh,
this may be my smartass obviousness talking.. but rather then do the large fabrication work required to hackup and re-enforce the car there.....


.... why dont you just weld on a Tq arm mount to a "drews style" crossmember, and re-enforce that??

Old SStroker

Like any frame, stiffness in torsion and bending has a large effect on the "dynamics" of the vehicle.

If one was to modify the subframe and wanted to assure structure wasn't compromised, you could rescue another subframe from a recycling yard and do some bending and torsion tests on it while it was anchored on it's mounts on the garage floor. Check at various points using dial indicators. After recording the lb-ft/degree torsional stiffness and bending stiffness, you might:

1) Mig weld all the seams you can find then remeasure torsional and bending stiffness. It may very well increase, which is a good thing.

2) Cut your holes for the pass-thru, measure again. Add the internal pass-thru tubes and remeasure. Note where the deflections are more than the original subframe, and start adding plates there.

3) You should be able to at least duplicate the original subframe stiffness and probably improve it even with the pass-thru.

From an engineering viewpoint, with appropriate testing, modifying the subframe is neither unsafe nor will it degrade the vehicle dynamics.


Moving the engine forward would be a monumental task, and it definitely would screw up the vehicle dynamics. We don't really want more front (less rear) weight percentage which you'd get from moving the ~600 lbs even a few inches forward.

How about zoomies thru the hood. Sorry, 11SEC. I'm not necessarily picking on you, but your post was an easy place to inject these ideas.

My $.02

11SECDWS6

iTrader: (3)

True, but who has the resources and equipment for doing such testing (properly) besides general motors?

I mean, we are talking about Joe blow here (no offense) basically redesigning a structural member of a car that GM has spent millions of dollars designing, analyzing, testing, etc.

Could you actually sleep at night knowing that you recommended to someone that they make a modification to their car that could compromise their safety?

Would you be willing to suffer the repercussions of a lawsuit if something happened because you said in writing that it would be safe to cut up a subframe?

If you could, then your obviously not licensed professional engineer.

ThirdGenLS1

iTrader: (11)

Thats exactly what i was gonna try doing, that or sell my spohn for a BMR set up.

But i agree with most, i think theres other ways to do it other then cutting the sub frame.

Justin

Old SStroker

Evidently you are a licensed PE. Sorry if I'm stepping on your toes.

Perhaps you haven't done a lot of vehicle frame testing in your engineering career, but one doesn't need FEA or a 4-6 post shaker rig to measure torsional stiffness and bending stiffness. It can be done mechanically with dial indicators, appropriate weights (sacks of readi-mix concrete or sandbags work well), some knowledge and a little math the way it was done PC (pre-computer).

I suggested a method of analyzing and verifying modifications that super-kev, who, from his posts is not your average Joe, could make which would not compromise safety. IMO, kev might be more confident in his mods if he verified the subframe was stronger than originally produced by GM.

I have never recommended anyone modify their vehicle in a way that would compromise their safety. On the contrary, I have dissuaded a number of people from performing modifications of street cars, race cars, motorcycles, atvs and machinery in general that would structurally weaken and/or compromise safety as well as function. I have not always been successful, and in some cases they went ahead with the "dumb" modification and things failed.

Kev's not talking here about welding or torching or removing material from a steering knuckle, or even a suspension part. I have seen some control arm or torque arm or Panhard rod modifications that made me shudder. Think about rear roll oversteer when you blow a slick in the lights on your F-body on which you've redesigned the rear suspension geometry for max acceleration only.

If you have never seen cracks in a well used vehicle frame or sub-frame you just haven't looked very closely. Frames crack in use. You can often find the "fixes" that were made to the design after durability testing. Rewelding a used sub-frame is a good thing, not a bad thing.

Actually, if one modified the subframe as suggested above, you might very well be increasing the safety of the frame. Obviously it depends on the quality of the work and testing.

I did strongly suggest testing any modifications. I really don't think I suggested cutting up the subframe without rewelding and testing it.

Are you threatening a lawsuit in writing above? Perhaps you aren't an ME but rather a JD.

MrDude_1

iTrader: (4)

hmmm.... so you dont want us lowest hole in the relocation brackets then?? LOL...






seriously though, i would just replace the crossmember with a custom one.

i have a list....... this list is the list of things i want to make if i opened my own shop.

one of the things on this list is a cutout crossmember...
options would include the common transmissions, and spohn or BMR torque arm mounts.... theres money to be made there... im just not the guy to do it...

ghettocruiser

iTrader: (17)

Hey all... Im pretty interested in this thread so far... Some good info and opinions. I like the idea of notching the subframe. However, with the small amount of support there already, Ide be hesitant.

The idea that I like the best, is using a crossmember like drews. I questioned the idea of mounting the spohn style torque arm to the drews style crossmember... or even modifying the spohn crossmember to resemble drews. However, I thinkt he position of that torque arm mount puts it right in the way of where the drivers side exhaust would want to pass...

I havent really looked really hard at it, but Ive been laying under the car doing other things lately and I was checking it out. The passenger side looks ok. With the torque arm mounted to the trans, the space below it is more open. If someoen comes up with a way to use drews design with the spohn set up, that would be awsome.

I think I would exhaust all other options before trying to modify the subframe. You are atleast doing a lot to stiffen your car already. Cage, subframe connectors, etc will help so that if you do decide to cut/weld into your subframe, there are other things to strengthen the car.

Definetly keep us posted and take pics if you do anything.

Justin

Old SStroker

Dude,

If you understand what that does to the roll steer you're more knowledgable than most.

I suggest that you try some relatively low-speed hard turns with a drag race suspension setup just to be prepared.

11SECDWS6

iTrader: (3)

Nice way to back-pedal. I appreciate your 55 minute long articulating of your response (I watched you from the public profile window). But I still say that in my opinion, its not worth the effort.

Yes, it can be done safely. But only if it were done right. I'm not sure that using bags of concrete is a very sound and professional way to do the testing. I would tend to think that a frame tester specifically built for that subframe along with strain gauges placed along the subframe would be the most professional way to do it.

Will it be cost effective? Heck no. I've personally spent hundereds of thousands of dollars (of my employers money) testing various products and I can assure you that one test can easily cost 20k to perform. Its not cheap by any means.

My point being is that if it were me, I'd rather dump the money (and save some in the process) in making that LT1 run good or buy a Gen IV f-body thats already designed to accept the LS1. I don't see any point in hacking up a Gen III as it will also likely depreciate the value of the car even futher. But thats just me. At the end of the day, its your car and you can do what you want to with it.

BTW, I'm starting to like the zoomies through the hood. Thanks for the idea.

MrDude_1

iTrader: (4)

id like to think i have a grasp of the basic concept of how it interacts...

plus yes.. i know what you're talking about firsthand.. my daily driver has what most would refer to as a "full on drag suspension"... if i dip the arms down to the lowest hole, with the taller (aka stock height) rear springs on, the LCA angle is pretty extreme.. so if i turn hard enough to get some body roll going on (not hard with my lack of rollbar in the front) the rear will reach a point where it wants to snap around... even more so if i let off the gas (aka transfer weight forward)... and the fact that i have a LARGE rear swaybar, REALLY doesnt help.
if i set everything to the max, then in the event of a true instant blowout of my tire, im either hitting the wall, or snapping the car over hard enough to the side that it will "trip" over itself and roll... same thing you can see lots of footage of online...


11SECDWS6:
i think its almost impossible to decrease the value of a 3rdgen when you put a LS1 in it.... because nomatter what, its still worth atleast the cost of the LS1.... and thats more then most thirdgens are worth.. LOL
i know what you're saying about a true professional test... one where you have to put a number to the results....but i believe hes just referring to a comparison test. with 200lbs, it deflected x amount... do the mod... now it deflects Y amount.... ect...

id jsut skip all that and build it so overkill that theres no question.. but in all honesty, i think that its a poor solution... i wouldnt mod the subframe at all.. id just make a new crossmember.