Skimming The Head On A Rebuild

[driversdomainuk 8]

Another one!

 

As I am planning to fit a 288 degree cat cam I am going to increase the CR to 11.5:1...

 

Ok right, I am shorly going to have the engine rebuilt, new rings, liners, bearings etc....I do not want to fit the cam until the rebuilt engine has had time to run in, as the other garage who is doing the cam has a rolling road so I want to see the before and after figures.

 

I am trying to save money and so during the rebuild I might ask the garage doing the rebuild, to skim the head to reach this 11.5:1 CR in preparation for the new cam (so I dont have to ask the other garage to do the same, as during the rebuild I am sure they will have to take the head off).

 

I just need to know, is is safe to run this increased CR, for around 1,000 miles or so in a rebuilt engine which still has the old original cam in..?

 

Thanks

Edited September 26, 2006 by driversdomainuk

[smckeown 1]

Dave Baker came up with a neat soluution fot my head recently, by running a thicker H/G initially to latwr allow the Cr to be dropped by fitting a thinner one. He had to perform head work to ensure both cams fitted though.

 

Have you investigated block decking vs. head skimming as a method of increasing C/R ?

 

By getting before and fater figures you are spending more money, where later you mention you are trying to save money

 

There are plenty of people that can advise here, but I think they would need more info on your engine forst, like is it a 1.6 or 1.9 etc ? what pistons are you using during the rebuild ??

 

11.5 is rather high also...

[driversdomainuk 8]

Dave Baker came up with a neat soluution fot my head recently, by running a thicker H/G initially to latwr allow the Cr to be dropped by fitting a thinner one. He had to perform head work to ensure both cams fitted though.

 

Have you investigated block decking vs. head skimming as a method of increasing C/R ?

 

By getting before and fater figures you are spending more money, where later you mention you are trying to save money

 

There are plenty of people that can advise here, but I think they would need more info on your engine forst, like is it a 1.6 or 1.9 etc ? what pistons are you using during the rebuild ??

 

11.5 is rather high also...

 

I am running a 1.9 on twin 45s. I am using standard pistons and liners. I know 11.5 is high but have been told this is required to get the best out the cam.

[petert 586]

As the chamber is small, you can't enough CC reduction by head skimming a 16V head. You'll also hit the inlet seats. More importantly, you need to fix the squish height at the same time. A quick fix is to use a thinner gasket. I like the copper ones made locally here in Oz. A 0.7mm gasket will give you 11:1, which is about what you need. A 0.5mm gasket will give 11.25:1. A more permanent fix is to skim the block/liners as Sean suggested. Either way, you'll need to verify piston/valve clearances. With a big cam you'll loose some compression from the increased pocket depth required. So you tend to chase your tail a bit. If you know what the lift is @ TDC I can give you an indication of the extra depth required.

 

Of course if it's an 8V, valve/piston clearances aren't an issue.

Edited September 26, 2006 by petert

[smckeown 1]

what's the goal with the car / engine ? As I have discovered recently, with cams sometimes less is more

[driversdomainuk 8]

what's the goal with the car / engine ? As I have discovered recently, with cams sometimes less is more

 

 

My goal is to make 150bhp if you are talking figures - for what they are worth. I use the car for hillclimbs only. To be honest if it is going to involve all this f***ing around I may just go for a fast road cam or something - afterall the most inportant thing quite often in motorsport is that nut behind the wheel....

 

i.e. the driver

[smckeown 1]

well if it's for hill climbs then I think you need to think about this a little more. As you want max mid range grunt not top end power. So have a research of blackscoobies posts, he had problems initially with wrong cam choice. As with my experience, i lost shed loads of mid range with a wrong cam choice.

 

also your not going to get 150 bhp (and more to the point lots of mid ranbge torque) without some effort, it's not bolt on and forget.

[driversdomainuk 8]

also your not going to get 150 bhp (and more to the point lots of mid ranbge torque) without some effort, it's not bolt on and forget.

 

 

I think 150 bhp with carbs and the right cam is realistic..?

 

Not sure what you mean about bolt on and forget - I am not a machanic and so cant just chop and change things if it does not work - I simply want the engine to be reliable and quick,well set up.

 

The cam I am after is a rally cam and so is very well suited to hillclimbs as opposed to opting for a full race jobbie.