1mm Skim

[ALEX 98 1 Cars]

Just found this old email from Dave Baker (Pumaracing)

 

Inlet guides are ok. Exhausts have enough wear (2 to 3 thou) to warrant changing. The bottom end blow up damage from bits of broken piston ring in chamber 2 will mostly skim out if the CR is raised and everything else (stud threads, cam bearings) looks fine. I suggest while it's being skimmed I do a couple of extra passes and take a good chunk off to raise CR to 10:1 which will make the std cam run nicer and improve power as well as idle quality. I've already got it half ported and as it doesn't need seat inserts I'll be getting it out of the way soon.

 

Basic ported head

Exhaust guides

1mm extra skim to remove damage and raise CR

Fit and shim up cam

 

At the time I had work done by him I knew very little about engine tuning, but looking at this email It's left me with a couple of questions.

Recently I've learnt that 1mm of skim is sh*tloads. Thats about 40 thou of an inch!

I took a 106 TU head to be skimmed the other day and they skimmed it by 25 thou (0.6mm) and said thats as much as they would dare to take off and thats a lot apparently. Most heads only need about 5 thou (0.127mm).

I dont have any problems with the head in fact I'm sure Daves done a good job.

I have a smooth even power curve all the way from 2000 - 6000 RPM. It feels faster than any 205 I've been in. But still not had time for a rolling road session.

It does Pink a bit though (mainly when hot) but the dizzy has 160,000 miles on it so that could be the cause (bob weight springs relaxed /stuck).

Has anyone else had their head skimmed by as much as 1mm?

Do you get Pinking issues using the standard injection system?

What extra thickness of gaskets are available. (I cant for the life of me remember what I fitted)

[Dom9 2]

1mm is fairly standard on the GTi engine, some of us have had a fair bit more taken off. It has quite a lot of meat on the face and you shouldn't be in danger of damaging anything, DB knows what he is doing with regards to CR.

 

It shouldn't really pink on 10:1 although it is probably worth running on SUL or Optimax and tuning the dizzy for the engine spec.

 

It would be interesting to get the car rolling roaded and see what your spec produces as it sounds like it is a fairly basic spec but might provide some nice gains.

 

There are a few gasket thicknesses available... Give Matt at QEP a call to see what he recommends and has in stock as you can vary the CR a little by using different gaskets.

[petert 586]

You can take up to 2mm without any problems on an XU9 8V head. Yes it might ping in the 2000-3000 region if you don't alter the dizzy springs or fit mapped ignition. There's too much advance down low.

 

If it was mine, I'd go a bit further. They go a lot better at 10.5:1 than 10:1.

Edited August 30, 2006 by petert

[ALEX 98 1 Cars]

Thanks for the reassurance Dom .

I built the engine using a brand new set of pistons & liners.

It would be good to see it on the rollers as everything is standard bar Pumaracings porting

I bought a do-a-upper house since and I'm still at it 1 year on, so I've not had time for it.

Not had any problems other than the pinking, but the old engine pinked anyway (which had a skimmed head) reason for bringing this up.

I refurbed an old dizzy once, which didn't make much difference,

Retarding the timing of the dizzy just makes it run sluggish and it still pinks then.

Using Optimax, super or 4 star (LRP) doesn't make a noticeable difference from using unleaded.

With 4 star it sounds quieter after a couple of tanks of it, which I mainly use, but it's not always available.

The AFM is Brand new too, it only made it idle a bit smoother tho.

Think it's worth investing in a new dizzy, AFAIK £105 from Auotfive.

[petert 586]

Just rebuilding the dizzy doesn't fix the problem, nor does retarding the base timing. The engine still needs 31 degrees of total advance to make maximum power. You need to fit stronger springs in the dizzy so the advance rate is slower, but you still get a total of 31 degrees. I'll dig up a graph which compares a dyno proven mapped ECU curve to the std. dizzy curve.

[ALEX 98 1 Cars]

Just rebuilding the dizzy doesn't fix the problem, nor does retarding the base timing. The engine still needs 31 degrees of total advance to make maximum power. You need to fit stronger springs in the dizzy so the advance rate is slower, but you still get a total of 31 degrees. I'll dig up a graph which compares a dyno proven mapped ECU curve to the std. dizzy curve.

 

 

That would be cool .

Where do you get these springs from?

[PumaRacing 2]

If you keep running it with it pinking then eventually you'll blow a head gasket or worse. Pinking is just a slightly nicer sounding word than detonation but that's what's actually happening and it blows engines up.

 

It's probably running a bit weak because of the extra head flow and wear and tear on injectors etc. The air flap or fuel pressure can usually be tweaked to compensate. A cleaned and flow checked set of injectors might be nice too.

 

My website mentions the dangers of saving a ha'porth of tar by not getting modified engines set up properly on the rolling road. Best sort yours while it's still in one piece.

 

Trouble is with what the buggers charge these days a rolling road session is no longer a ha'porth. 15 years ago 50 quid would cover a carb rejet and a timing swing. Now it's £80 or more an hour at many places and half the time they don't do it properly anyway.

[petert 586]

Where do you get these springs from?

 

A dyno operator with grey hair? Although I might be able to help you out. Here's the graph.

[ALEX 98 1 Cars]

The engine has covered almost 30,000 miles since I've fitted it. Most of them motorway miles.

The engine doesn't pink at 4000rpm + (70MPH in 5th). In fact the graph posted by Petert makes a lot of sense as between 2000 & 3500 RPM is where it's worse, It pinks at 4000 when going up hill.

The injectors are from a 70,000 mile engine, which I had cleaned & tested, The AFM is Brand new as mentioned.

I have a second-hand Fuel pressure booster (FSA power boost valve) which I fitted once but it pissed fuel out of it everywhere, I took it apart cleaned the gaskets put it back together but forgot about it since....Hmmm, might try fitting that again then.

I took the car to the rollers in Burnley once with my old engine to solve the pinking, He retarded the timing to what he thought should cure it and told me to go for a spin.

It felt slow as ever. We put it back on the rollers and It was pulling 60BHP.

He advanced the timing back to where it was and sold me some expensive Octane booster which did nothing.

I'm reluctant to go back as like Dave says £80/h isn't cheap.

The only reason I want to take it to the rollers with the new engine is to see what the output is.

With the new engine fitted I had a Crypton tuner to set it up, He set the advance to the book setting and the mixture to as high as he could to pass an MOT.

The timing setting had too much advance IMO as it pinked more than usual but it did feel slightly quicker.

I've since bought a Gas tester and retarded the timing to come to a compromise between pinking & power, adjusting the mixture to suit.

 

 

A dyno operator with grey hair? Although I might be able to help you out. Here's the graph.

 

If the graph is true It would seem that stronger springs could do the trick.

I'll be happy to buy a pair off you if you have some.

 

Alex.

Edited August 31, 2006 by ALEX

[benlilly 0]

Hi Alex,

 

Have a look here:

http://www.guy-croft.com/viewtopic.php?t=268

 

It covers some of the fueling issues covered above.

 

Cheers,

 

Ben

[Rob_the_Sparky 9]

Your findings are very similar to mine (50thou skim). I've got an SBC dizzy (presumably stronger springs) and it has helped control the mid-range but still have to drive carefully to avoid pinking. Much fiddling has got an acceptable loss of power versus mid-range pinking if not cautious with the right foot but not the best set-up.

 

Soon (well soonish) will fit an ECU to get it to run right...oh and fit some TBs while I'm at it! All of which is in my garage, just need a manifold and some time to dedicate to it.

 

Rob

[ALEX 98 1 Cars]

Your findings are very similar to mine (50thou skim). I've got an SBC dizzy (presumably stronger springs) and it has helped control the mid-range but still have to drive carefully to avoid pinking. Much fiddling has got an acceptable loss of power versus mid-range pinking if not cautious with the right foot but not the best set-up.

 

Soon (well soonish) will fit an ECU to get it to run right...oh and fit some TBs while I'm at it! All of which is in my garage, just need a manifold and some time to dedicate to it.

 

Rob

 

How much did you pay for the SBC dizzy?

If Petert doesn't get back to me about the springs I might be investing in one.

Good luck with the ECU.

I fancy doing it myself at one point, but I bet it's a bit pricey

Edited September 5, 2006 by ALEX

[boombang 2]

For the performance dizzy, try:

 

http://www.h-h-ignitionsolutions.co.uk/

 

Time and time again hear good things and will be using them for mine.

[ALEX 98 1 Cars]

Hi Alex,

 

Have a look here:

http://www.guy-croft.com/viewtopic.php?t=268

 

It covers some of the fueling issues covered above.

 

Cheers,

 

Ben

 

For the performance dizzy, try:

 

http://www.h-h-ignitionsolutions.co.uk/

 

Time and time again hear good things and will be using them for mine.

 

Thanks guys!

Edited September 5, 2006 by ALEX

[Rob_the_Sparky 9]

How much did you pay for the SBC dizzy?

If Petert doesn't get back to me about the springs I might be investing in one.

Good luck with the ECU.

I fancy doing it myself at one point, but I bet it's a bit pricey

 

 

TBH I can't remember now, I got off NickR (who I got the head from) and as said it was an improvement but not a true solution. H-H are probably worth a try...

 

Rob

[phatgti 0]

I skimmed a 1.6 head to give a CR of 10.8:1, i skimmed it 52 thou to get a chamber cc of 28.

 

No problems apart from the obvious det issues.

 

I skimmed a 1.6 head to give a CR of 10.8:1, i skimmed it 52 thou to get a chamber cc of 28.

 

No problems apart from the obvious det issues.