piston to valve clearance

pete917 · 07-19-2006, 02:16 PM

I am doing one of Waynes top engine picks. The spec is as follows:

- base engine 3.2 euro spec carrera
- 3.5ltr Mahle Piston and Barrel upgrade maintining high 10:3:1 C/R
- twin spark;
porsche GTII Turbo cam profile.
- original 3.2c conrods

What I couldnt see in the book was Piston to valve clearances.

Can anyone advise me of the minimum acceptable clearance that I should not go near.

Many thanks
Regards
Peter

shbop · 07-19-2006, 03:59 PM

1.5mm exhaust, 1mm intake. I think this ought to keep you safe. I'm sure others will chime-in.

Eagledriver · 07-19-2006, 06:13 PM

Porsche mentions this in one place in the shop manual and it specifies .8mm for the intake valve. It says nothing about the exhaust valve. This was for the 2 litre 911S engine. The numbers Jon mentions are what I've seen from others as a bare minimum. I'd use these numbers. Be sure you know how to measure this correctly. Some of the methods I've seen are not accurate.

-Andy

shbop · 07-19-2006, 09:52 PM

If I remember right, every full turn of the adjuster screw is 1mm, or .040". In theory if you were to take out the slack out of your intake adjuster, and then turn it, one full turn, it would push the valve 1mm towards the piston. I remember hearing that if you could get 1 1/2 turns on the intake and 2 turns on the exhaust, there would be enough clearance. I know this is down and dirty, but if you have your motor assembled--- ?

pete917 · 07-20-2006, 12:24 AM

Thanks Guys,

This something I dont want to get wrong so I really appreciate the advice. I am building the engine as we speak, so I have the opportunity to measure the clearance in the most accurate way. I had also heard of turning the adjustment sscrews. I have also heard of using plastocine!

Any other views of measuring the clearance?

Regards
Pete

Eagledriver · 07-20-2006, 07:27 PM

Ok,

Here's the thing about measuring clearance. The best way would be to assemble the engine with some clay or putty or something similar and do all the timing/adjusting torquing and then turn the engine over. Some people do this and it is best but very time consuming. Other people check the clearance by turning in the adjusters. This can work but is done wrong most of the time IMHO.

The piston and valve don't make their closest approach at TDC on the overlap stroke! The piston moves very little before and after TDC but the valves move alot. Therefore the closest approach for the exhaust valve is before TDC and the closest approach for the intake valve is after TDC. How much before and after? That's the problem. We don't know. You'd have to do several measurements at different crank angles to find out.

This is what I do: I put the crank about 7 degrees BTDC and check the exhaust valve clearance. This seem to be about the closest approach on the engines I've built. I do the same for the intake at 7 degrees ATDC. Realize this is not as accurate as the clay method and give yourself a margine for error. I like to see 1.5mm minimum at these angles.

You can do a last check by turning the adjuster screws in 1mm and turning the engine over carefully feeling for binding.

-Andy

pete917 · 07-21-2006, 01:46 AM

Thanks Andy,

Although time consuming I have decided to go for the plastocine/modelling clay approach. This way I can be absolutely certain that the clearances are adequate.

I do agree with the TDC issue. It would be easy to assume thats where all the action is, but it makes absolute sense to have a window BTDC and ATDC. Somene else hhas said that they goes as much as 15 degrees before and after TDC.

The 1.5mm minimum clearance aappears to be a popular chopice also.

Many thanks again
Regards
Peter

davidppp · 07-21-2006, 03:39 AM

Hello there.

Is this to be raced..ie overrevved (!)?

If so, 2mm ex valve clearance is not too much..IMHO..

Kind regards
David

pete917 · 07-21-2006, 04:11 AM

Given the engine spec I am sure it will rev higher than standard. I will be tacking it to the track for Driving days? How can I work out the required minimum clearance.
Best
Peter

camgrinder · 07-21-2006, 08:06 AM

Peter, I sent you a PM.

davidppp · 07-22-2006, 12:48 AM

Hello, Pete.

Sure, the spec is rev-happy.

But that is not what I meant at all...

Driving in a race Will result in unintended over revs..both from missed or botched shifts, and from concentrating on the race..

The engine spec needs to take full account of this IMHO..better springs and retainers, like those Camgrinder sells, more exh clearance..f

Kind regards
David

pete917 · 07-22-2006, 02:15 AM

Thanks David,
I have installed EBS Titanium Racing Springs and Retainers so hopefully that dshould help me out in this regard. The GTII Turbo profile is by Camgrinder, and he is recommending a minimum clearance of 1.2 mm intake and 2 mm on the exhaust side.
Best regards
Peter

911pcars · 07-25-2006, 11:01 PM

Another method is to temporarily replace the valve springs on one cylinder with light weight springs. Use springs with just enough tension to hold the valve train together. After rotating the crank to the desired position, it's just a matter of pushing the valve open by hand to see if there's any interference. A dial indicator on the valve retainer will record the distance before any contact.

Sherwood

pete917 · 02-03-2007, 04:43 AM

In the end I used a dial test indicator with a piston at TDC and then measure the drop until the valve touched the top of the piston. With POrsche GTII Turbo cam profiles I was pleased to still have 3mm free. Thats as big as a barn door....some I should have any piston to valve collisions!

Thjanks to everyone for their help.

:-)