Webers and solid engine mounts
 

I've searched for previous threads but haven't found a good answer to these:

Do solid engine mounts lead to flooding the float bowls?

Has anyone used soft mounting provisions to isolate the carbs from vibrations?

I know what PMO recommends regarding solid mounts for engines.

Hey Paul,

1) Yes,......the frequencies of those vibrations cause mixture variations in the engine. I've never been able to actually observe what happens inside the float chambers so I cannot say precisely what happens, however the use of a flexible engine mount (rubber or urethane) eliminates the problem.

2) Soft-mounts on IDA's do not work due to vibrations. The DCOE ones work in those applications since the carbs were designed for them.

Hope this helps,

Hi Steve,

Yes, thanks a lot! I knew DCOEs used them and wondered why IDAs didn't.

Richard at PMO condemns the use of solid mounts.

Although there is performance loss with carburetors there is a more important issue to deal with.
Solid mounts are generally used to enhance chassis stiffness but that theoretical performance enhancement is greatly diminished when you consider the fatigue factor the excessive vibration /noise creates on the driver.

The vibration is also responsible for incredible amounts of chassis fatigue. Everything shakes and everything breaks.

What is the "problem"? I mean, I understand the principal, but how have you seen it manifesting itself, Steve?

I understand the theory, but not necessarily the application.

So with solid mounts you pretty much get a continuous ultrasonic cleaning of the carbs!


Sent from my iPhone

Pmo
 

We have seen the results of solid mounting while on the dyno which is solid mounted, and with the windows in the FB on the PMO, during a full RPM pull on a race engine the fuel levels look very good until the upper range where the fuel looks like it explodes into vapor and while not showing an issue on the data being the last few moments of a 10 second pull, at a lap at Daytona may be a much different result. The emulsion tubes and their holes by design uncover as the fuel level drops to become bleeds. So with solid mounts it will change alot of things and chances are make tuning a mystery, we like solids on the gearbox end for shifting and softer mounts for compliance on the other.

Mike Bruns JBRacing.com

The solid mounts in question are on a 550 spyder replica. The motor is a 2.4 liter 6cyl. The motor is flipped like in a 914 and the rear mounts are completely solid using a 901 trans. I wonder if it would be easy to just replace the mounts with stock 914 units?

Not sure on the front mounts yet as I have not been able to see in there... should know soon though. I wonder how much just replacing the rears would help.

Hi Marco,

Indeed, I have and one observes the AFR's spiking very lean as the fuel in the float chambers foam up.

In order to make a discernible difference you would want to isolate the entire engine lump. Any solid contact will transfer the harmonic vibration.

the mounts in question
 

Here are the mounts. The fronts do have rubber plates on them. The rears are solid. Not sure what the benefit/reason is to having solid mounts in the rear but I wonder if the factory 914 mounts would work better...? Here are the pictures. Let me know what you guys think.

Regards,
Bryan

http://forums.pelicanparts.com/uploa...1320001149.jpg


http://forums.pelicanparts.com/uploa...1320001166.jpg


http://forums.pelicanparts.com/uploa...1320001184.jpg

Hi Brian
I recognize some of that work :)
The rear mounts as well as the front mounts on that 550 are both flexible I believe.
Isn't that black square made of hard rubber?
You should be fine with that set up.

Henry, I think that black mounts are metal... I will double check. I actually spoke to you about this car a few weeks ago. This was Ed Muscat's.. you did some work on this for him after he purchased it. Still trying to find out who built the motor. Apparently, it was up in the bay area somewhere.

Regards,
Bryan

I have often seen the compromise suggestion of solid mounts for the transmission and rubber (or something not solid) mounts for the engine.

I have never understood how this is going to help with vibration, although it should help with engine/transmission twisting. Like Henry, I'd expect the solid pair to do a fine job of transmitting the vibration, given that the engine and transmission are locked tightly together, so as to act basically as one housing.

I ran my carbureted 2.7 race motor with solids all around for years, and a 2.8 replacement similarly for a while. Never had an indication of dangerous high RPM leanness, and I spun it to 8,000 before I got a torque curve and realized I was better off shifting at 7,600. But my one dyno test did not include the AFR, and the Webers don't have that convenient sight glass.

Doesn't WEVO make semi-solids to deal with this issue? I'm using EFI now so not in the market, but others might want to consider that.

I contacted Richard Parr (the designer and manufacturer of the much desired PMO carbs) to get his take on solid mounts. Here is that exchange:

Well, if one had the tools to make a solid engine mount I suppose someone could make an isolation mount for just the carbs. Basically rubber intake spacers.

Isolating the carbs could cure the fuel issue but solid mounts are a disaster for the entire vehicle and the driver as well. Throw the solid mounts in the trash and everything about the vehicle gets better.

You were able to isolate these spikes to the vibration transmitted through the solid mounts? That must have drove you nuts... :eek:

One thought ... perhaps you could observe what happens in the float bowl on, say, a set of PMOs with windows. ;)

Thanks for the breakdown, Steve. I always appreciate your input.

It sure did drive us nuts since it took some time to discern what was happening. :)

PMO's make this phenomenon easier to see.

So Mike, Henry, and Steve - what engine and trans mounts do you recommend for early 911's with carbs? Are the same mounts recommended for street and track use?

What about for Motronic engines?

I've been using and recommending Wevo blue mounts to all the local guys for a few years now. Are these a good compromise?

Thanks for the info!