Learn me engine mount bushings

As I explore engine options for my Miata, one of my sticking points is that most of these swap kits use hard polyurethane bushings. I get that they're a nice, cheap option that helps deal with clearance issues and that a lot of their customers want something stiff, but I've had cars with poly mounts or poly inserts and IME it absolutely ruins them for the street. For instance, the KMiata site says:

Note: The engine mounts included in this kit are performance-oriented and designed to limit engine movement. The additional stiffness may result in a slight increase in NVH (noise, vibration, harshness), but can withstand years of hard driving as a result.

Slight increase my eye. I've got a friend telling my to suck it up and find a suitable rubber bushing replacement somewhere, but 1) I worry that they're not going to be strong enough in a lot of cases, and 2) how do people find generic cylindrical sleaved rubber bushings in random sizes?

Just bushings vs. meaty mounts are indeed quite harsh. I'd recommend a modified factory mount for mixed use.

You say solid poly mounts or poly inserts are too tough so maybe try rubber inserts or partial poly reinforcement.

I'm now running factory mounts with solid cylinders of urethane in them. The vibration is a bit more than my old ones which were filled by myself with 3M window-weld urethane (over the stock rubber) but they're not too bad for regular street use IMO. I don't get "lawnmower hands" or vibrating eyeballs and bolts aren't backing themselves off. It's similar to the vibration you'd feel on a modern ATV or I4 motorcycle. It feels quite smooth at mid/high revs.

My experience is with Hasport swap mounts for a Honda, which are the same size as the "beefy" OEM mounts, but without all of the holes and made of polyurethane instead of rubber, similar to what it sounds like you're running now:

I'd even ordered the softer of the two durometers, my wife rode in the car once, told me it felt broken, and refused to ride in it ever again.

In my Integra I briefly tried the energy suspension inserts, which retain the beefy OEM mounts but fill the gaps with polyurethane, not unlike your window weld solution, and I took them out at the first opportunity because it was awful:

Looking at something like the K mount kit, there's not a lot of room there for a soft rubber bushing:

I know people will insist that it's not bad but I swear the emperor has no clothes, I don't know how people tolerate it. And I know compromises must be made for performance at a budget but I want to eat my cake and have it to.

To me, rough is a Radical SR3 (which uses thin bushings). It was starting to give me lawnmower ass (although I had to take out all the seat padding to fit) and a little eyeball vibration at low revs.

If you want to have your cake and eat it too, the best options are to fit engine dampers (little shock absorbers between the engine and chassis) or retrofit active engine mounts (Don't ask me how!)

Are the Mazda competition mounts still available? They work well for me.

Edit: Looks like they are, http://949racing.com/mazda-motor-mount.aspx

Only $35.75 through Mazda Motorsports.

Edit 2: I missed the part about the swap. Disregard me.

Yeah, the only swap I found that uses OEM mounts is the AJ30 Duratec kit out of the UK, but that's $$$ and not yet available.

It's easier to pick a poly bushing out of a catalog that fits nicely in the tubing on hand than to put the work into controlling NVH. Sometimes you have to restrict engine movement if you've got very tight clearances, but good engineering can often work around that. There's a reason we used OE C6 Corvette mounts in the ND V8, and why that car feels more like a factory job than most.

Now, these generic poly bushings are usually originally intended as replacements for something. Energy Suspension didn't just randomly make up the dimensions. I'd call your desired engine swap supplier, ask what the ES part number is for replacement bushings, then call up Energy and ask them what the original application was. Chances are it's something like leaf spring bushings for a 1960-1980 GM car. Put them in and inspect them frequently, then you'll know if they're strong enough. Remember that OE engineers are generally really good at what they do.

BTW, if you do go with the KMiata swap and are planning on running the car hard, reinforce the rear control arm mounting points. If there's room, put a cross brace between them. It's where all the braking loads go and the V8R/KMiata subframes have a weakness there.

Find an outlet with pictures and you may be able to find what you want. When I stuck a GM V6 in an MG I ended up using Jaguar rubber mounts (they are bolted to the frame brackets which I hadn't yet welded to the frame).

not knowing what the swap mounts look like this may be a moot point, and it may just be a dumb idea all together but could you drill holes in the poly mounts as a way to soften them up? I have seen motor mounts like these used as a 'soft' version compared to a solid poly mount:

although i imagine these mounts were poured/cast/whatever you call it when making poly mounts with the holes already there instead of being drilled out. but i wouldnt think drilling the holes would affect durability that much.

... or i could be completely wrong

Like Keith said they're probably leaf spring bushings for something old, I'd hazard a guess that the OE rubber variant will be plenty strong considering what they put up with holding a rear axle in a heavy V8 car.

BTW, polyurethane comes in different durometers, so they don't have to be rock hard.

Drilling holes in otherwise solid urethane mounts could be worth a shot...although at that point, you're almost recreating the factory mounts.

But there are no factory mounts for a Honda engine in a Miata, that's the original problem

Keith Tanner wrote: BTW, if you do go with the KMiata swap and are planning on running the car hard, reinforce the rear control arm mounting points. If there's room, put a cross brace between them. It's where all the braking loads go and the V8R/KMiata subframes have a weakness there.

Keith, do you have a picture of what this would look like? I had never heard of this before, but now I'm curious on how to do this.

Drilling holes will result in poly bushing failure, ie they will split.

I am in the suck it up camp, throw the es bushings back on and call it good

Bump from the past, after blowing my 2nd BP in as many years I'm actually doing this K swap thing.  The poly bushings that come with the kit are made by Hasport, they said that all of their bushings are made to their custom specs and there's no OEM equivalent.  So I've started searching for stuff that looks close and just ordering it to see.  Here's what I've got so far:

The bushing on the right is a diff mount bushing for an Evo IX, unfortunately it slides right through the bushing carrier and it's very, soft, like I can move it by hand and I'm sure it wouldn't last 100 miles.  The one in the middle though, that's the rear bushing from the front lower control arm of an early 2000s Elantra.  It's very, very close.  Incredibly stiff, but unlike the poly bushings I think I can drill this out some if I need to soften it up.  I'd need a washer on either end to fill the gap in the crossmember.  But then there's the fit inside the bushing carrier.

I can *just* get the chamfer on the bushing to sit in the carrier.  I have no idea if a press could push it in without destroying anything.  Should I see if I can have a machine shop mill off like half a mil from the bushing itself?  Or is that not smart with a rubber bushing?  Or should I just try and press it in?  I feel like I've come close but I'm out of my element now.

Powdercoat has a thickness, you may be able to get enough clearance by sanding that off. I'd like to see a good press fit. Make sure you press around the outer edge of the bushing instead of the center.

Also, how the heck are you blowing up serial BPs? They're tough little bastards.

In reply to Keith Tanner :

First one was a simple low coolant situation (friend was driving, didn't catch it) due to a leaky radiator, somehow it messed up the head pretty bad so I opted to replace the whole thing.  Second one was a junkyard motor, current assumption is that it just wasn't healthy but I haven't cracked it open to see yet.  It was full of stop-leak when I bought it and I never opened the long block to inspect it, spun a bearing so possibly previous damage?  Had oil in it, no warning on track, just started knocking and then seized.

Ultimately it comes down to me not being as thorough in maintenance/installation as I should be, but also some bad luck.

I'll sand off the coating and see what I'm left with, thanks for the tip.

A press fit is pretty dang tight.