[Lasse H]

For a year or so I've been thinking about a new engine for my car. Finally this project has advanced a little. Soon I've got a 1.9l block with 95.5mm pistons (by AE), new piston rings, bearings, etc. The bad thing is, somebody had cut a 0.75mm slice from a piston. Now the the slice has to be taken also from the block and all pistons will be made equally high to get the same compress ratio for every cylinder.



The head I'm first going to use is a 20S head (42mm/37mm valves), maybe slightly lowered for more compression. Intake manifold ported, and on it a Weber DGV 32/36. Injected CIH exhaust manifold and custom made downpipe (from manifold to the first muffler). Rest of the exhaust system is stock. Slightly lightened flywheel. Pertronix Ignitor.




I'd like the idle *not* to sound exactly like a rally/race engine... Car should also be nice to drive with lower rpm but still give a nice performance, and fair fuel economy and reliability.





What would be a good camshaft and compression ratio combination for this kind of engine?

Is a Enem Y12 (288 degrees, 11.1mm lift) with a 9.5 CR be a bad guess? Are there cheap and good alternatives for standard springs?
Are the standard jets for DGV enough?

I've got a set of older forged conrods with old 93mm pistons, but currently the 95.5mm pistons are with newer (cast) rods. Would it be worth of effort to change the conrods? Are the older ones also lighter?

What size head gasket I should use? Are gaskets with 95.5mm holes even available.


Lasse

 

[RallyBob]

I'm not familiar with Enem's camshafts, so perhaps you should speak to them directly to ask about idle quality and rpm range. I would be concerned if you lowered (milled) the head. The valve notches in the pistons will be shallower now since they are machined, and lowering the head more will bring the valves closer to the pistons. With a modified camshaft, the exhaust valves will be very close unless you correct camshaft timing.

I think Piper cams or Kent cams has low cost replacement valve springs to fit that head.

You will probably need to change jets on your Weber carburetor for best performance. With raised compression the idle jets will remain nearly standard, maybe one size larger. Main jets will have to be two or three sizes larger.

The early con rods are much stronger. If you are going to rev above 7000 normally, you should use the forged rods. They weigh about 625 grams, the cast rods are about 640-645 grams.

You can use a standard head gasket even with 96 mm pistons

Bob

 

[Lasse H]

Tuning CIH for street

Thanks for advice!
I'm also a bit worried about the valve clearance with pistons. So I will use an adjustable camshaft sprocket (machined from the original) and the valve notches will be machined a bit deeper. Maybe 1,0mm deeper to help recovering from lowering the block and provide some extra safety. Notches for exhaust valves will be widened. But how much? About 3mm towards cylinder wall? I'll try to make some measurements to know more precisely.
I hope that would give enough valve clearance for lift of about 11.7mm even if head is lowered 1mm for more compression.

Because the con rods have to be removed before machining valve notches I can as well replace cast rods with forged rods if they are lighter also. Probably I won't be revving beyond 7000 anyway, so this just for sure.

In that (really interesting!) intake porting article was a mention about boring second barrel of a DGV 32/36. What kind of result can be achieved that way? The idea sounds very good, almost stock outlook is a nice thing. Would that be enough for 140hp? How big bore would be recommended? All information is welcome, of course.

Btw. what kind of rear muffler is usually used with over 130hp GTs? I've seen some replacements in German GT parts catalogs, but those are terribly expensive and have usually chrome more than enough.

Lasse

 

[RallyBob]

I have machined the 32/36 second venturi from 27 mm to 31 mm. Going larger risks breaking through casting. This will require much larger idle jets on second venturi...about 4 or 5 sizes larger. Also main jet will need about 4 to 6 sizes larger ( I used 135 main jet on primary side and 175 main jet on secondary side with this carburetor modification on standard 1.9 litre). This will work well for street use, not quite as good as 38 DGAS throttle response, but very similar power. No problem with 140 hp if the remainder of the engine is well-tuned. Glad you liked my intake manifold article!

For a custom GT muffler system, use better front muffler, and use only resonated rear tips. I like to use 2" (51 mm) main pipe and muffler, then 2.5" (63.5 mm) pipe over axle, then split pipe into two 2" pipes out back of car. Use resonated tips like Ansa, or Simons (Jetex). I also like the Simons mufflers....good sound! If this is to loud for road use in Finland, then a smaller round muffler can be used in the main pipe also. Most GT rear mufflers are bad for power, the turns inside the muffler are very sharp.

Bob

 

[RallyBob]

Here is a photo of a stock 32/36 Weber's secondary venturi
(27 mm), and also of another one that I modified by boring to 31 mm.

Bob

 

[GT_Pheel]

How is that done?

Bob,

Are you using a mill to open up the secondary or something else? Would you be willing to write up a proceedure or how-to on this? What kind of max HP rating would this give on a tuned engine, the 140 you mentioned earlier or would it be higher. What would this do to a stock engine? Would this hurt more than help? What other mods would you recomend to go along with this? Sorry for the 20 questions bit.

You are a true wealth of knowledge. I read that same article on intakes and planned to use your info to modify mine. Have you thought of writing any kind of tech manual or performance guide of sorts. Most of us here benefit from your experience, but you may be able to benefit as well. Since manuals are not readily available it would make since to combine a factory type manual with a performance and modification section. I bought a similar type of manual for my Mazda a few years back and prefered it to the factory version because it would run through not only how to do a job but also what to watch out for, tricks from experience, and exactly what was required to do it. I am aware that this would be a huge project, but it may be possible to have multiple people involved. I'm getting off topic though. Thanks for your time and sharing all that you have on the forum. The things you have posted have given me a great deal of help since I'm still pretty new to the Opel scene. I'm goin' on two months now and loving my GT more every day.


Phil

 

[RallyBob]

The venturi is milled out but the final work is done by hand with a die grinder to get the appropriate radius, then blended with "cross-buffs". There is enough airflow to feed about 160 hp, but getting there with this small a carburetor requires the rest of the engine to be built to racing specifications...not a street unit. It would be far easier to modify the engine to a lesser spec and run a larger carburetor/carburetors.
Yes, I've even installed one on a bone-stock engine and it ran very well, just required quite a lot of re-jetting.

As far as writing a tech manual, isn't this basically what I've been doing all along here? No seriously, I considered it years ago when I was still in the Opel business, but when I sat down and figured the time involved and especially the photography, the documentation of the dyno time, the flow bench work, the suspension work, the braking work, the rollcage/chassis work, the transmission/driveline/rear axle.....it would be truly overwhelming and would take years to do. Not to mention, by the time I was done, a lot of the info would be "old news", as I'm constantly experimenting with new ideas (the roller camshaft is a perfect example, I should be getting all of mine in about a week)

Bob