Engine Torque Strap 2

[TomOstrowski]

I have a 360cid smallblock Chevy making approximately 400 lb. ft. of flywheel torque in an '84 C-10 1/2-ton pickup. I plan to add a torque strap consisting of grade-8 bolts(to the frame and cylinder head), small sections of chain and a turnbuckle. My question is this- how do I go about determining a minimum tensile(?) strength rating for the parts of this torque strap in relation to the torque(twisting/canting of the engine upon the motor mounts) of the engine itself? Should I choose parts rated at, say, 500 lbs. based upon the engine's 400 lb. ft. of power and a 100 lb. safety margin? Or...?

Great forum, gents, and I give thanks in advance for any replies.

 

[TomOstrowski]

I should probably look into the shear strength of the mounting hardware(the grade-8 bolts) also, correct? Comments?

 

[GregLocock]

You are reacting the torque in the driveshaft, typically 3-6 times the engine torque.

Cheers

Greg Locock

 

[rmw]

Greg,

In the lowest gear ratios, especially if the ratio was real high, wouldn't this torque be much higher than 3-6 times?

I have seen tractor trailer drivers completely lift at least one side of the front end of the vehicle off the ground in "granny" gear.

Aren't the engine/transmission mounts and, hence the bracket that TomO... is trying to design having to do this?

3-6 seemed low to me. Just a question to add to TomO..'s question.

rmw

 

[rmw]

Greg,

Further to the above, when I was a high school kid, I had a
"souped up" car which had had a 6 cylinder engine removed, and a powerful V-8 engine installed, (Corvette, baby) and the person who modified the car did a poor job of the engine/transmission mounting system. I mean by this that he put no rear engine mounts on, nor any on the transmission (tranny rested on a crossmember only, broke a lot of bellhousings, too,) and only had a crossmember type mount on the front of the engine.

This set up was bad about shearing off the bolts in the front crossmember in the torque loaded side, at which time, upon accelerating in the lower gears, the gearshift lever would come right over into my drivers seat area if I got on it too hard before I could get it into a shop, and have new bolts installed. It was pretty dramatic. Radiator fan would get into whatever was nearby, etc.

Hence, the reason for my question above.

rmw

 

[Metalguy]

Hmmm, how could the gear shifter move to the left-or do you live in England? Engines turn CCW from the drivers seat, and the torque reaction is CW.

 

[TomOstrowski]

Is the consensus now that I should use components rated at around 2500-3000 lbs.(4-7x engine torque)? I have a limited amount of room to work with(about 8 inches- the distance from the cylinder head to the frame) and when I looked at turnbuckles and related parts at Home Depot I was disappointed to find that even their largest forged pieces(which were entirely too large for my application due to length) were rated at only 300 lbs. I can imagine a turnbuckle rated at 2500 lbs. would be much too large to fit into the room I have to work with.

Resources found online regarding automobile engine torque straps is limited. I've mostly found references to the standard chain setup, nylon belts and some complicated tubing/heim end setups. I would like to keep this project as simple and as cost-effective as possible for obvious reasons. The chain method seems to be the most common but I've come across the mention of chain stretch which I had planned to avoid by using chain strong enough and utilizing the turnbuckle to apply some preload and a means of taking up slack should the chain indeed stretch. I've ruled out nylon straps due to mounting difficulties and the tube/heim end style is just too expensive.

If part size for the strengths required is indeed going to be a problem with my original idea I'm now thinking maybe I will look into a length of curved flat steel bent for mounting tabs at the ends. This may prove to be the best approach as far as simplicity and strength now that I think about it, with an additional benefit of shock absorption due to the curve of the piece(quite a few folks mention a rough ride due to the solid-mount effect of traditional torque straps- namely the chain style). I understand that if I choose to use a section of flat steel my area of strength concern would now be just the mounting hardware as long as I have a piece of flat steel strong enough for the job.

I appreciate the input, folks, and welcome any further comments/insight.

 

[Metalguy]

You need to measure the distance from the crank centerline to your engine attachment point. The longer it is the less strength you need. Figure 3,000 lb for 1 ft, 6,000 for 6", etc.

 

[GregLocock]

In a rear wheel drive car the maximum torque seen at the engine and gearbox is engine torque*first gear ratio*torque converter or clutch factor.

First gear ratio is typically about 3, torque converter is typically 2, and clutch factor for a manual is 1 (gentle driver)-2(me). Hence my estimate of 3-6. It is not usual to change gear at the max torque of the engine so there is an additional safety factor in there.

I'd use leather, nylon webbing (forklift hoist type stuff) or a stainless steel cable (yacht rigging), at the top /and/ bottom.

All of those could be custom made (the cable in particular seems to me to be the easiest solution) and so do not need to be adjustable. Any chandlers could knock one up and deliver it to you. I'd leave half an inch of slack in it. 1/8" wire has a breaking strain of 1600 lbf, typically.

Just looking through a marine supply catalogue I see that a 4 ton "Eye and Eye" stainless steel turnbuckle is 15 US dollars, I hardly think that any other solution will be cheaper and more satisfactory. Use the same size thread for your mounting hardware.



Cheers

Greg Locock

 

[patprimmer]

Tom

I think Greg has it right, but here are my thoughts on some of the details.

Your engine makes about 400 lb ft of torque and a light truck will have about 3:1 to maybe 3.7:1 low gear ratio and a max of 2:1 in the clutch. Also an auto is more likely to be toward the taller geared low gear, but it is more likely to gain a little more torque through the converter, so call it 3.5 times 2 for safety.

A SBC has a deck height of 9.025", so if you attach to the head at 2 or 3" above the deck, you have 11 to 12" above the crank centreline, so call it 400 + 10% X 7 or 3080 lbs force.

If you leave some slack, you will get a dynamic effect, but you will also get considerable help from the original rubber mounts.

You can use polyester strapping from a sling, or maybe old seat belt webbing, a stainless steel cable from a yacht rigging shop or some chain and shackles.

Cable or strapping has the advantage that it yields a bit, and so absorbs shock loads, but strapping can weaken or melt if close to the exhaust, or it can be cut by sharp edges.

You also need to consider the attachment points.

The only viable place on the SBC heads are going to be the bosses for mounting ancillaries to the front end face of the head. On all SBC heads since the early seventies, there has been a 3/8" UNC threaded hole just below the tappet cover gasket on the exhaust side, with provision to put two more (sometimes already drilled and tapped) just above the deck on the inlet and exhaust sides. At these points the head has been milled flat, and the material is a lot thicker so as to accommodate a decent length of thread.

You will need a substantial steel plate with the mounting holes well clear of the edges, and an extra hole for the shackle. You might need to shorten any spacers used on the mounts of ancillaries mounted at the same location.

You can buy load rated shackles at various lengths from your ships chandlers.

You will need a similarly secure attachment point to the chassis, or maybe if there are no obstructions, you can go around the chassis rail and back to the head, provided the chassis wall is thick enough so as not to crush.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bub579]

Early 70's full size chevys had a dealer installed steel cable on drivers side only, no throttle return with broken mount. It had no preload but not much slack either.I think the bracket attached outboard front of the exhaust manifold, cable down to frame. Bub

 

[rmw]

Metalguy, a good observation, but I don't live in England. I do, however, still remember that gearshift, with the chrome "8" ball handle getting over onto my (right) leg, and trying to pin it when my engine mount was broken. Maybe it was when I was in reverse, which probably had a deeper reduction than first gear.

I would not have cared about the gearshift lever going towards the passenger, and at that age, and station in life, might have not minded "chasing" it over there, depending upon who my passenger might have been, if you get my drift.

rmw

 

[TomOstrowski]

The heads I have are World's Sportsman IIs which feature tapped ancillary holes on both the front and rear faces of the heads. Luckily this allows me to use the rear of the head as an attachment point rather than having to work around the power steering unit located at the front. After considering the use of bar steel I've determined it's probably easiest and cheapest to go the route Greg suggests, steel cable properly rated, and finding a suitable way to secure it.

Thanks again for all the help. After browsing this forum for some time prior to taking the plunge and becoming a member I knew this would be the place to get the best input as far as any questions I may have.

 

[Metalguy]

rmw,
Yes, I read you loud and clear! Love to be that age again. I had a friend who rigged up (I am not making this up, as Dave Barry would say) an old Ford Model T coil with some wires under the right half of the front seat-no buckets seats back in the 1950's. One shot, with him explaining some BS to the girl about a weird ignition problem, and she was right next to him.

He also made a "flamethrower" with a spark plug in the tailpipe. Rev up engine, pull choke all the way (remember those manual choke cables?), and pump the gas pedal too!

Another friend had a 1952 Ford 6, with a glasspack muffler. He developed a technique thru trial and error (many tests!) whereby he would turn the ignition off while coasting in high gear. Something like -key off at 42 MPH, don't touch gas pedal, key on at 30. The resulting explosion was the loudest and deepest I've ever heard a car make (well, at least a non-fueler anyway). It was so loud and deep it was ridiculous. He liked to do it under bridges, especially when a group of girls were walking. They would jump so much! This was back before you'd catch a hail of lead for doing something like that.

Once we did it near a group of stores, and a cop in a pizza place (where else, no donut shops around) ran out and actually commandeered a passing car. He had the driver follow us and we stopped at a light, not even knowing he was there. He ran up and told us to pull over, yelling "OK, which one of you guys thru the rock". The concussion of the blast made him thing someone had hit the front window with a rock. He searched the car for more rocks, and gave up upon finding none. Ah, the good ol' days!

 

[rmw]

Metalguy,

Been there and done most of all that. The rod I referred to above was a '53 Chevy two door convertable, with a '57 283 corvette power pack, etc., added later in it's life.

Funny, Sirius Satellite Radio had "the british invasion" this weekend on the '60's stream, so between listening to a lot of that, and this post, it has been a real trip into memoryland.

I guess the point of all this in a technical forum for engineers, where TomO was actually trying to get real technical advice, is that in surviving our youth, we are able to relate our knowledge and educations to his technical problem in a real world way, as practising engineers, with a little "grease" under our fingernails.

TomO, I saw your last post, and hope that you are successful in your endeavor to "chain" this engine down, and if not, come back for another dose of our memories. Just do it right, so that no one gets hurt.

rmw

 

[turbododge]

It does not take all that much to hold the torque, from what I have seen. Even the big hemis were held with rubber only mounts in the old days. I run a 600ft-# torque twin turbo 340 Chrysler with about a 9 to 1 low gear overall ratio, manual trans. The torque strap is a piece of .09 X .75 cold rolled steel bar about 9" long. It mounts to the head on drivers side with a grade 5, .250 diameter hex head cap screw. The other end is rigged to a cut off shock absorber end attached to the fender well (to get the isolation), with a .375 hex head grade 5. It has been in place 15 years without a trace of engine movement problems or failed motor mounts. Schumacher makes a nice torque strap that many folks are using, and even a cable, or bolt loosely through the drivers side motor mount is enough for most setups, unless you are running very deep gears and slicks.

 

[Metalguy]

Remember that the axle ratio has little to do with engine straps. If anything, a lower gear (higher number) back there would slightly lessen the load on the strap.

 

[evelrod]

Not to be totally agreeable with all the suggestions put forward so far, but---
Having done the 'cable', the 'seat belt', the Heim/rose jointed bar, etc. on my drag cars of the late 50's, all I can say is that all systems do actually do the job---for a time! Every restraining strap/cable/bar eventually failed catastrophically under load with a variety of very exciting events following!
The last time I had the 'torque monster' to deal with was in a turbocharged Ford and I solved the problem by using a Koni gas shock mounted at first horizontally and later vertically via a small bellcrank. Both systems seem to do the job ok. Time will tell.

Rod

 

[ejit]

5/32" wire cable, complete loop with ends spliced, use largest bolt that goes to the driver's side head. Leave some slack or it will "hum" under even light load.
Weighs nothing, costs nothing, small, lasts 10 years, quiet, adjustable in minute increments.
More strength: double loop.

 

[truck]

Several thoughts -
1. I would work really hard at finding a method to tie to the block rather than the head; head attachments are highly stressed and adding odd and variable loads (which will occur at the same time as the highest loads on the attachment system) is not in the best interest of the system.
2. Consider an added roll stop to acheive the same purpose if possible. An added compressive element would help remove some of the bad behavioral characteristics from the torque strap.

Truck