Safe Rev Limit For Hydraulic Lifters

[jonand 0]

I have been unfortunate enough to experience valve dropping due (I believe) to overevving my high compression QEP hybrid Mi/S16. My engine datalog shows it to have happened at 7700 rpm (although the limiter was set at 7400). Matt from QEP always suggested that 7500 was limit with the engine as the valve to piston clearances were very small.

 

I am now faced with a rebuild, and am considering my options. There seems to be varying opinions about safe revs before solid lifters are required, and I accept that the build spec will have a fundamental impact on the consequences of any hydraulic pumping up of the lifters - the question is at what revs would you expect pumping up to be an issue, and is it more to do with the valve springs ability to resist valve bounce ?

 

Any comments welcome.

 

Thanks

Jon

[Sandy 191]

I treat 7500rpm as the limit for hydraulics in the XU. What springs and cams do you have?

[petert 588]

Matt from QEP always suggested that 7500 was limit with the engine as the valve to piston clearances were very small.

 

I also concur that 7500 is the absolute limit for std. valves and lifters. The valve to piston clearance also effects this. A close limit for std. inlet would be 0.080" and 0.100" for the exhaust, measured 6 deg. after and before TDC respectively. It's possible to run 0.060"/0.080" with solid lifters and lighter components.

 

Change to solids and you could run to 7500 all day, safely.

[Miles 331 1 Cars]

Not sure on the Iron block but my old Alloy one run to 8500rpm without any problems, Well bar a big end bolt snapping and ending it after a yr, but I wasn;t expecting it to make as much power as it did

[jonand 0]

Thanks for replies guys.

Sandy - it uses catcams 4900528's, not sure about springs - maybe Matt could advise ?

The engine is used for oval racing and is mostly between 5000 and 7500 with odd 8000 moments. Would it be feasable to machine the pistons to increase valve clearance, but still be possible to maintain the 12.5:1 compression ratio. The thinking here is that the engine should be fine for the majority of its useage, but if it gets momentarily overevved to 8000ish and lifter pumping is induced, it should still have enough valve to piston clearance. This would avoid the need for expensive solid lifter conversion and the associated new cams.

 

Cheers

Jon

[DrSarty 90 1 Cars]

I'm not a guru, but surely machining the piston top face to provide pockets for valve clearance (at any revs) cannot maintain the same CR, unless you had new, adjusted gudgeon pin heights to take up the volume you've just created in the combustion chamber with the pockets.

[guffe 2]

I treat 7500rpm as the limit for hydraulics in the XU. What springs and cams do you have?

 

Is this a rule of thumb for every XU? Or just Mi/S16? I would be interrested how 7JP4 hydraulics with std springs would cope with 10J4RS cams, could they get to 8k?

[James_R 3]

I'm not a guru, but surely machining the piston top face to provide pockets for valve clearance (at any revs) cannot maintain the same CR, unless you had new, adjusted gudgeon pin heights to take up the volume you've just created in the combustion chamber with the pockets.

 

Or

 

Deck the block

 

Skim the head

 

Use a thinne h/g (but then accoutn for the head being closer to the pistons in the poocket depth)

[jonand 0]

Yes, sorry didn't make it very clear. I know the squish ht was very small (approx 1.2mm from memory) using a std thickness gasket (1.4mm ?) - so may not be much room for decking block further. Not sure how much the piston pockets would need relieving for safe clearance - i.e. how much the lifters are likely to pump up. From PeterT's info it looks like 0.10" would be the starting point + an allowance of ? I'm assuming that the pump up would be the equivalent of the height of any valve bounce (as the lifter takes up the induced clearance). I'm also assuming that valve springs of a higher rating than currently fitted should alleviate the problem of valve bounce.

 

Thanks Again for your comments

 

Jon

[welshpug 1,661]

may be something for sandy or Pete to answer, but would changing to Solid tappets require any other changes at all?

 

Should be fine with the current cams I would have thought? its only with more extreme cam profiles that solids are absolutely necessary (as well as sustained high RPM)

[jonand 0]

may be something for sandy or Pete to answer, but would changing to Solid tappets require any other changes at all?

 

Should be fine with the current cams I would have thought? its only with more extreme cam profiles that solids are absolutely necessary (as well as sustained high RPM)

 

 

You may be right, but evrything I've read when I searched this issue suggests that new cam profiles would be required - with less aggressive ramps. I assume if I used the existing catcams with solid lifters, the lifters and / or cams may not last long.

[welshpug 1,661]

hmm, I would have thought the opposite!

[jonand 0]

hmm, I would have thought the opposite!

 

My understanding is that the hydraulic profiles can have a more aggressive opening ramp because they are already in contact with the lifter, whereas the solid profile has to take up the clearance first before applying lift i.e. a less aggressive opening ramp to avoid 'smashing into' the lifter.

[B1ack_Mi16 67]

My understanding is that the hydraulic profiles can have a more aggressive opening ramp because they are already in contact with the lifter, whereas the solid profile has to take up the clearance first before applying lift i.e. a less aggressive opening ramp to avoid 'smashing into' the lifter.

 

Jonand is correct afaik.

[Mandic 0]

Jup, correct.

 

BTW, what about XU10J4RS? It has lighter lifters and valves, surely they push red line up for 200-300 RPM?

 

Cheers

 

Ziga

[Sandy 191]

The solid cams are generally more aggressive beyond the initial contact.

 

You ought to restrict oil flow to the head will solid lifters, because less volume is required.

[rich_gti6 0]

I've ran upto 8,400 RPM on hydraulic lifters on my 540 BHP XU10 J4RS.

 

The piston valve reliefs are deeper than OE, so should I suffer a little hydraulic pump up or if the pistons catch up with the valves there is reduced risk of interference.

 

 

This is 1,8 BAR of boost, which obviously has a negative effect on the valve-springs ability to close the valve quickly.

 

On a N/A engine it should be safer in theory, with no boost holding the valves open.

 

IMO you're definitely OK for 8,000 RPM all day long though.

 

Rich