How High Lift Can A Std Engine Take?

[Henrik P 0]

Hello. I am looking for camshafts to my XU9JAZ and i think this one from CatCams sounds like a good cam

 

4900366

 

INTAKE EXHAUST

 

 

clearance [cl] 0.30mm 0.30mm

duration [0.1mm+cl] 282° 282°

duration [1.0mm+cl] 248° 248°

valve lift 12.00mm 12.00mm

peak angle 110° 110°

lift at TDC 3.15mm 3.10mm

 

Since my plan was to raise the comprission ratio to about 10.5:1 the head needs to be skimmed minimum 1 mm. If i do that will the valves stil go free of the pistons with the new cam?

[Sandy 191]

Valve to piston clearance is generous in the standard engine, up to about 6mm lift at TDC is possible. The issues are more valve springs keeping it all in check and improving the fuel and ignition arrangement to cope.

[Saveit 0]

When you say that 6mm LTDC is possible, how do you mean that then? Cause i am going to skim my head 1.4mm and i am also going to use a Kent PT27 cam which provides 4.31mm LTDC. If i take 6mm and minus this by 1.4 and 4.31 i end up with 0.29mm. I am a bit worried about this? And also, if i were going to use a PeterT cam pulley advanced by 5 degrees i could end up with the valves hitting the pistons? Or is there something i have misunderstood?

[Henrik P 0]

Valve to piston clearance is generous in the standard engine, up to about 6mm lift at TDC is possible. The issues are more valve springs keeping it all in check and improving the fuel and ignition arrangement to cope.

 

So you think there is more then enough room?

Do you think i need other springs and what about a adjustable pulley? Is that money in the garbage ore is that money well spend?

[tony perks 1]

When you say that 6mm LTDC is possible, how do you mean that then? Cause i am going to skim my head 1.4mm and i am also going to use a Kent PT27 cam which provides 4.31mm LTDC. If i take 6mm and minus this by 1.4 and 4.31 i end up with 0.29mm. I am a bit worried about this? And also, if i were going to use a PeterT cam pulley advanced by 5 degrees i could end up with the valves hitting the pistons? Or is there something i have misunderstood?

 

 

Dont forget to add the thickness of the head gasket, and how far down the bores your pistons sit, and remember the 6mm he has quoted is at tdc, there are not too many XU cams that accelerate the valves off the seat quick enough to let them catch up with the pistons, why use peter t pulley just time the cam to give the 4.31mm lift at tdc, or time full lift at 106 degs.

[petert 588]

why use peter t pulley just time the cam to give the 4.31mm lift at tdc, or time full lift at 106 degs.

 

That's right, why use one. I can provide an advanced pulley that will take into account head skimming, but it's impossible to guess where the cam has to be without measuring/timing.

 

I think the point Sandy is trying to make, is that's it's almost impossible for the valves to hit a piston at TDC (or just before/after), on an 8V. Unlike a 16V, where it's relatively easy to get it wrong and bend valves.

 

12mm of lift is quite a lot and well outside the range of the standard springs. They will spring bind. Keep the lift under 11.6mm if you want to use standard springs.

[Sandy 191]

What I should always say too, is do your own measurements and dry build it to check after. It's unlikely in this case, but the engine you're looking at might have a variation on what the rest of us are familiar with!

[Saveit 0]

why use peter t pulley just time the cam to give the 4.31mm lift at tdc, or time full lift at 106 degs.

 

Because i cant do it without a PeterT pulley? I dont have the knowledge nor the tools to use a fully adjustable pulley and as i understand this is the only other way to adjut cam timing?

[Mattsav 1]

You need to check & measure for every engine you build. Manufacturers like nothing more than chaning things seeminly at random (but usually emissions or cost related).

 

You can just fit a cam and hope that a 5 deg offset dowel is going to put the timing anywhere near close. At least check it once you've fitted it or you'll be wondering why the engine's not making any power!

 

And piston/valve clearance is not going to be an issue with that cam unless you had a huge amount skimmed off the head (inwhich case the CR will be through the roof!)

[Saveit 0]

I dont have a dial indicator nor any of the required knowledge to measure og adjust clearence or cam timing. That is why i wanted PeterT to make me a non-adjustable advanced pulley. I was told that for every 1mm you skim off the head you need to advance the timing by approx. 4 degrees (crank degrees i guess?). Now i am going to skim 1.4mm off the head and furthermore i am going to use a quite aggressive cam, so 4-5 degrees cant be that much off? At least it must be closer to the optimal setting than the standard pulley? Remember it is only used for very low budget racing so almost everything made to these engines is DIY.

[James_R 3]

Sorry to dig up an older thread.

 

But I am currently running this cam on a 1760cc bottom end 10.5:1 and std valve springs, it's not coil bound, but I am having valve control issues as I approach 7000rpm, and would like to go further.

 

Does anyone know of single valve springs which I could upgrade to which will give me a better maximum rpm ceiling, as I really dont' want to go to the expense of double valves if I can avoid it (all the machining costs!!)

 

Many Thanks

 

James