Opel GT Forum banner
1 - 15 of 15 Posts

·
Opeler
Joined
·
3 Posts
Discussion Starter · #1 ·
Can anyone help with some information on building a competition engine (Rally). The spec is: Big valve head 41mm exhaust and 46mm inlet, Bore is 95.5mm, stroke 85mm (“Vauxhall frontera” crank). which gives a cc of 609 cc per pot, 2434 cc in total (“Vauxhall frontera” crank). The block and pistons have been skimmed by 1.35mm. By my measurements this leaves 5 cc volume at tdc (piston indent and valve pockets). Gasket volume 8cc (estimate). The combustion chamber is presently 53cc this I believe will give me 10.2:1 compression ratio. The cam does not have any identification markings but by my measurements it has 300 degrees duration (solid lifters). I have built the engine up to take some measurements and the piston to valve clearance is minimal when the cam is set at 102 degrees full lift (inlet)

What I would like to know is:

What valve to piston clearance is needed?
What is the minimum (safe) thickness of the piston crown that needs to be left after pocketing the pistons?
The head only has single springs, does any one run double springs? (I am concerned about over revving the engine due to missed gears.)

I would be grateful for any help!
 

·
Super Moderator
Joined
·
14,870 Posts
What valve to piston clearance is needed?

You should strive for a minimum of .080" (2 mm) for the inlet valves, and .100" (2.5 mm) for the exhaust valves.

What is the minimum (safe) thickness of the piston crown that needs to be left after pocketing the pistons?

For forged aluminium pistons, the general consensus is a minimum of .090" to .100", however for cast pistons this should be increased to perhaps .150" thick at the thinnest point.

The head only has single springs, does any one run double springs? (I am concerned about over revving the engine due to missed gears.)

As you should be! Standard valve springs will not allow many rpms before valve float becomes an issue, particularly with larger valves which are normally heavier. Do you know the rated tension of the existing springs? Do you know to what rpms you wish to safely rev the engine to?

HTH,
Bob
 

·
Opeler
Joined
·
3 Posts
Discussion Starter · #3 ·
Hi Bob

Thanks for taking the time to answer.... I would estimate that 7000 rpm would be the maximum rpm but possibly 7500 if I get carried away (A rev limiter is fitted). I do not know the spring rates all I know is that the cam and springs are from a quick 2 litre engine that I had previous, it was reputed to be built to "works spec" and it did seem to give a fair bit of power. The engine unfortunately expired on an event after it over revved over a brow (mid air) that I was not aware of until it was to late, loss of power and lots of smoke! The original engine had "mahle" intruder/dome pistons, one of which was smashed to bits together with the head and a bit of block damage. I would be confident that the springs are good enough to handle 7500 rpm but I am more concerned about missed gears as The ZF box I am running has been easy to miss select (3rd to 2nd instead of 3rd to 5th) as my dear brother will testify, possibly the start of my troubles? I have not seen double springs advertised anywhere but a local head specialist said that they could fit some bmw internal springs which would improve the "harmonics" but I do not really want to lift the seat pressure etc unless they are really necessary.
 

·
Member
Joined
·
1,881 Posts
With the stock 9:1 flat top pistons how much can the valve reliefs be cut to allow additional clearance for larger sbc valves in the 1.9 L? Can the process be done safely with the pistons still in the block? Best proceedure to follow when doing this?
 

·
Old Opeler
Joined
·
5,564 Posts
Hi Bob

(A rev limiter is fitted). I do not know the spring rates all I know is that the cam and springs are from a quick 2 litre engine that I had previous, it was reputed to be built to "works spec" and it did seem to give a fair bit of power. The engine unfortunately expired on an event after it over revved
Rev limiters are real good ideas - they save motors every day.
However - why on earth are you using a second-hand set of springs that have already been on an over-reved motor? They are "toast" !

Kent Cams and Piper both have springs specifically for high-performance CIH Opel/Vauxhall motors ... get in touch with either of them and go with their recommendations. Critical components like valve springs need frequent changing on hi-rev motors as the heat generated by their rapid flexing quickly reduces their original spring rate and free length. If any spring has a free length 2mm shorter than when it was new - replace them all. You will be surprised how much 'fresher' the motor will feel with new valve springs of the correct tension for the cam you are using.
 

·
1450 Seeker...
Joined
·
613 Posts
With the stock 9:1 flat top pistons how much can the valve reliefs be cut to allow additional clearance for larger sbc valves in the 1.9 L? Can the process be done safely with the pistons still in the block? Best proceedure to follow when doing this?
The short answer is "Yes, it can be done". I believe Car Craft or one of the other Hot Rod magazines did a write up on how to notch pistons in the block using the valve guides with the head on the block as a jig, using a kit from Isky cams. They would then pull the head off, apply some Play-doh to the top of the piston, reinstall the valves to the head, and the head to the block, then pull the head off, cut the play-doh in half and mic it... they performed this procedure hundreds of times until the notches were adequate... times eight, for each pot.

This is an enormously time consuming procedure. Then you have to worry about all those little metal fillings that have been shaved off the top of the piston are now floating around in the cylinder. The only way to fully clean the fillings would be to pull the pistons, which is what you were trying to avoid doing in the first place. If you are interested in doing this yourself, pull the motor out of the car and go through the above procedure, then be prepared to pull the pistons and fully clean the cylinder walls, pistons and head.

We've talked about valve reliefs and the best way to cut them on this site before, but I wasn't able to find that thread.

I found this article as well, Security Clearance: Measuring Piston-To-Valve Clearance The Right Way National Dragster - Find Articles

It's two pages, the link to the second page is at the bottom. This is a very detailed method, but again be prepared for a lot of off and on of various parts.

There are also two types of valve clearance and they are related to what happens when the timing belt or chain breaks. An "interference" motor means that if the timing chain/belt breaks, a valve will hit a piston. A "non-interference" motor means that if the timing chain/belt breaks, a valve will not hit a piston.

A non-interference motor will be less likely to cause damage if a valve floats, but is still possible because in a valve float scenario at extreme RPM's, the valve can actually extend beyond maximum lift due to the inertia the valve itself carries.

I hear that a broken timing chain in a CIH is about as rare as a finding a 16v cross-flow head for a reasonable price. So when determining valve clearance, it's not worth considering a broken chain scenario so you should only worry about clearances as the motor will turn in its normal life (0.080" intake, 0.100" exhaust, more if you are using aluminum rods).

What's the downside of creating reliefs that are too large? Other than losing points on the static compression ratio, you also lose volumetric efficiency. Air is easily compressed and expanded, so the smaller the air pocket between the valve and the piston, the greater a vacuum is created across the valve opening as the piston starts to move away from TDC. So why not create a piston that would create an almost perfect interface to the valves? Now we are talking about the effects of scavenging. I still have a lot to learn on the topic of scavenging, but the smaller that air pocket, the more narrow the range that scavenging can occur.
 

·
'NO,......O P E L..G T!'
Joined
·
585 Posts
Can anyone help with some information on building a competition engine (Rally). The spec is: Big valve head 41mm exhaust and 46mm inlet, Bore is 95.5mm, stroke 85mm (“Vauxhall frontera” crank). which gives a cc of 609 cc per pot, 2434 cc in total (“Vauxhall frontera” crank). The block and pistons have been skimmed by 1.35mm. By my measurements this leaves 5 cc volume at tdc (piston indent and valve pockets). Gasket volume 8cc (estimate). The combustion chamber is presently 53cc this I believe will give me 10.2:1 compression ratio. The cam does not have any identification markings but by my measurements it has 300 degrees duration (solid lifters). I have built the engine up to take some measurements and the piston to valve clearance is minimal when the cam is set at 102 degrees full lift (inlet)

What I would like to know is:

What valve to piston clearance is needed?
What is the minimum (safe) thickness of the piston crown that needs to be left after pocketing the pistons?
The head only has single springs, does any one run double springs? (I am concerned about over revving the engine due to missed gears.)

I would be grateful for any help!

Firstly, HI from another UK member. Can I make a recommendation....coming from someone who has a built up 2.4/2.5 motor? Have you heard of the forum Turbosport Entrance ? If not it may be an idea to log onto there and put your question out there, ideally to a member called Graham Bahr as he did the majority of work on my engine....and I'm very happy with the results:)
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
The short answer is "Yes, it can be done".
I agree, the short answer is... anything is possible... If I were to attempt this feat, I would pretend I were operating on a patient and take every precaution to prevent ANY aluminum bits from entering the engine. If you've ever seen surgery on TV, It would appear they cut on the person through a film or membrane covering that has been stretched over the persons skin. Thinking of this, I would tape everything off with Duct Tape except for just the notch area to be enlarged. I would do both 1 & 4, then painstakenly clean everything, remove the tape, clean again, rotate the 2 & 3 to TDC. While 1 & 4 are down in the bore, I'd carefully inspect their bores for any particles that might have slipped by... Now repeat the process for 2 & 3.

Now with all of that said, It just might be easier to remove the pistons.....
 

·
'NO,......O P E L..G T!'
Joined
·
585 Posts
Can anyone help with some information on building a competition engine (Rally). The spec is: Big valve head 41mm exhaust and 46mm inlet, Bore is 95.5mm, stroke 85mm (“Vauxhall frontera” crank). which gives a cc of 609 cc per pot, 2434 cc in total (“Vauxhall frontera” crank). The block and pistons have been skimmed by 1.35mm. By my measurements this leaves 5 cc volume at tdc (piston indent and valve pockets). Gasket volume 8cc (estimate). The combustion chamber is presently 53cc this I believe will give me 10.2:1 compression ratio. The cam does not have any identification markings but by my measurements it has 300 degrees duration (solid lifters). I have built the engine up to take some measurements and the piston to valve clearance is minimal when the cam is set at 102 degrees full lift (inlet)

What I would like to know is:

What valve to piston clearance is needed?
What is the minimum (safe) thickness of the piston crown that needs to be left after pocketing the pistons?
The head only has single springs, does any one run double springs? (I am concerned about over revving the engine due to missed gears.)

I would be grateful for any help!

Firstly, HI from another UK member. Can I make a recommendation....coming from someone who has a built up 2.4/2.5 motor? Have you heard of the forum Turbosport Entrance ? If not it may be an idea to log onto there and put your question out there, ideally to a member called Graham Bahr as he did the majority of work on my engine....and I'm very happy with the results:)
 

·
Super Moderator
Joined
·
14,870 Posts
Just an FYI, this thread was started nearly three years ago!
 

·
Opeler
Joined
·
87 Posts
1.9 flat top valve reliefs

1.9 flat top valve reliefs is the 2nd question?big valves?

I have done this exact thing with engine in car and head removed.
think I posted how but not sure as chemo befuddlemuddles the brain.

almost 100% of the fillings can be trapped in grease between piston and bore and wiped out.duct tape over bores not currently working on.oiling cylinders and rotating engine and repeating repeatedly makes sure all filings are removed--at least to the point it is no worse than metal generated in new engine with a normal hone finish during first few 100 revolutions.

if post cant be found i will post all details if any intrest.

David
 

·
Registered
Joined
·
2,733 Posts
Funny this should pop up again.
Some of you know that this is right on topic of what I'm doing to one of my engines lately.
It is very tedious and time consuming. The Isky tool we discussed before and elsewhere works well.
Personally I wouldn't want to do it on an engine that has its pistons and rings installed, more specifically after its final assembly.
I don't want even the tiniest bit of aluminum shaving left behind!
David, how are you doing? You hint that you are in chemotherapy. I hope this is working and you are gonna be okay.
 

·
Registered
Joined
·
2 Posts
using some grease it is possible to trap and retrieve almost if not all the aluminium filings if you were doing the job with the engine still in the car, if its on an engine stand though it would be silly not to remove the pistons,

incidently though being so soft a few aluminium filings wont do any harm anyway
 
1 - 15 of 15 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top