Modifying engine mounts with £15 of silicone......

gizlaroc · 25 Mar 2025

After saying to @Nicholasrage in his 90k mile car overhaul thread found here...

Thread '997.2 Major Service Cost'

19 Mar 2025

New here, picked up my first 911 about 5 months ago, (997.2 3.6) really shocked at my 90K miles service cost at 7 grand.

Wondering what everyone else’s was at this point? I was expecting a reasonable high bill but seems very high. I could post the report if anyone’s interested. Thanks

...that I would be very careful going with RS semi solid engine mounts, and linking to a couple of videos of one of the guys on Rennlist who used some 3M Windo-Weld to fill the gap on his oem mounts, I thought I would try it myself.

I had bought some new OEM mounts from Type911 after getting my 997 as I thought mine were original, they may well be, but still feel great and have not drropped, the new ones were 65a shore hardness, but they felt no firmer than the ones I took out. Now, of course they shouldn't be, but I presumed my one would be tired at this age and 70k miles.

So I decided to put some silicone into the gaps around the rubber.

This is the video...

Typically Windo-Weld is only available to the windscreen trade in the UK. So I spent a few hours looking for something else with a shore hardness of around 60a. (Windo-Weld is actually around 75a, he says 60a in the video).

I had bought some Sikaflex 555 for my transmission mount on the 996, but couldn't find it anywhere. Went to order some more and it was either a week wait and £32 or nearly £50 if I wanted it now, and I always want things now.

So looked around and realised that CT1 has the right properties in black, 55a, 300% tear, and will take plenty of strain.

Oh and I think it was £13 from Toolstation and always in stock.

Anyway, filled the mounts last night and already today they feel solid, very solid, in fact, wonder if they might be too solid now, but ho hum, got to try these things and being a a durometer of 55A they should still be softer than the 80a of a semi solid mount.

I will try and fit them tomorrow evening and report back. Fingers crossed.

911UK · 25 Mar 2025

job's a good 'em

Nicholasrage · 26 Mar 2025

It’s an interesting solution, how long would it take to dry though? I don’t have spares so the engine would be jacked up whilst they are drying for a day or two.

gizlaroc · 26 Mar 2025

I replied in your thread, what I put was...

I filled them on Monday night 5pm and last night they were rock solid.

These were not worn as such, just wanted to take some of the flex out of the oem 65a mounts without going solid.

If this is too firm, I will leave them in and put some softer silicone in the new ones I just fitted last week. Probably around 35-40a hardness.

I want to try and tighten up gear changes a bit and stop the movement you can feel when cornering, but, I don't want to loose the the way the mount absorbs all the vibrations and noise from the engine and gearbox away from the body you can get with solid mounts.

Someone said last night that the RS semi solid mounts have got a bit softer recently, so maybe ignore everything I said and get them. Haha

For me they were just too noisy and harsh when I tried them, but if they have softened them a bit, they could be a good move.

gizlaroc · 26 Mar 2025

So, quickly swapped the mounts over, got both sides done in 20 minutes this time, can't believe I put off swapping them the first time for months. Haha

Anyway, the verdict is in.

Superb!!

No extra vibration or noise at all, there is absolute no movement of the car now when pulling away or braking, even coming on and off the brake/gas at lights with the oem mounts you could feel the engine move a bit, which is turn is making the clutch feel far tighter and the bite point much easier.

The gear shifts as expected have got much sharper, nicely slot into place now with a bit of a snap. Really nice.

Going round tight corners though is where you can really feel the improvements, that change of weight, the slight delay as weight at the rear caught up with the change of direction has gone, it just feels like the turn in is much sharper, and you know what the back end is doing more than before.

One thing I will mention is the rear end feels firmer, like you have a slightly firmer rear damper. Not as firm as turning PASM to sport, if sport was 100% and with them off was 0% this feels like around 20-30% firmer at the rear. Not a bad thing, but worth mentioning.

Great upgrade. I will update again in case anything changes,

Mac68 · 26 Mar 2025

I’ve done this to a set of spare mounts but haven’t got around to fitting them. I’m off to Europe shortly and might have a crack at fitting them before I go based on your feedback

Hooner · 26 Mar 2025

I did this on mine but only left it to dry for 2 days, and when i got back from the test drive i had black gloop all over the tail pipes! epic fail.

gizlaroc · 26 Mar 2025

What did you use?

Please don't say CT1.

That is my concern, how well it stays in.

Hooner · 26 Mar 2025

OB1 black silicone

gizlaroc · 26 Mar 2025

Here is the Porsche mount cut in two...

Very clever design.

The mount has channels to allow the fluid to flow from the bottom section to the bottom section, so there at slower speeds of movement it will flow easier, but higher speeds of movement it will stiffen up. Much like a shock absorber.

drmark · 27 Mar 2025

gizlaroc said:
Here is the Porsche mount cut in two...

View attachment 115721

Very clever design.

The mount has channels to allow the fluid to flow from the bottom section to the bottom section, so there at slower speeds of movement it will flow easier, but higher speeds of movement it will stiffen up. Much like a shock absorber.

Well, it was clever until you filled it with silicone

gizlaroc · 27 Mar 2025

Haha, good point.

The reason I wanted to do this was it will keep the essence of the design but just firm things up slightly, unlike the semi solid mounts which are for all intents solid but rather than billet use a solid poly insert.

If we think of it like suspension springs, you have some after market progressive springs, which can be very compliant and stiffen a bit as you press on, then you have something like an Ohlins spring which is super firm and probably too much for the street and then you have springs from Eibach and H&R that are pretty soft for the first inch or so of travel but then pretty firm after that.

These sort of feel like the ethos of the Eibach/H&R type spring, going over speed bumps and rough bumpy roads you still feel the mount absorbs that shock nicely, same with engine pulses, but it has massively reduced the lateral movement you get with quick shifts in direction.

I suppose some of the Porsche RS and GT mounts just use a thicker oil inside them, or have smaller holes for the fluid to flow through.

I wonder if the active mounts close the holes or whether they have a magnetic type fluid in them that thickens when they are turned on?

Bigbaddog997 · 27 Mar 2025

gizlaroc said:
Here is the Porsche mount cut in two...

View attachment 115721

Very clever design.

The mount has channels to allow the fluid to flow from the bottom section to the bottom section, so there at slower speeds of movement it will flow easier, but higher speeds of movement it will stiffen up. Much like a shock absorber.

I'd always wondered what was inside the OEM style mounts, so thank you Giz. Very interesting.

jl-c · 28 Mar 2025

I have active ones so am not going to mess with them

gizlaroc · 29 Mar 2025

I would imagine the active ones would be a little firmer than these ones, or around the same anyway.

Bigbaddog, I grabbed that photo from someone else, just thought it was worth showing as I found it a very clever design.

I went out in it again today and the car feels so much better. There are no negative effects at all.

I'm surprised Powerflex have not released an insert for the standard mounts, I think they would sell well. Far better to drop some inserts in than wait 48-72 hours for silicone to dry.

@David Power what do you reckon?