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Couple of weeks ago during a weekend drive at 5k RPM in the 4th gear at WOT the engine stalled and white smoke started to come out of the engine bay. After checking the engine it was found out that the HG was blown in its weakest spot. The head itself and the block has not suffered any damage. My engine has APR head studs and I had standard HG.
My head measured out at 23.6mm i.e. around -0.4mm from OEM spec and that is before surfacing/machining it for reinstalling. I plan to use Cometic HG which should work better with APR head studs. I also found out that my pistons at TDC are sticking out 0.6mm above the block, thus it came out that my effective CR was around 8.5 : 1 and I was running 17 psi for the last two years, including several track days.So my options are to by Cometic 0.075 inch HG which after another machining of my 951 head would help to arrive at 8 : 1 CR. The ceramic liners of exhaust ports visually seem OK but you never know how long will they last. I could by a used 8v 951 head, but would face the same risks. Thus, considering the above, wouldn’t a good used 944 NA 8v head be a feasible option? I read that they are the same except the exhaust ports and exhaust valves? Are there differences between early and late offset cars NA heads (mine is 87)? I also read that you could coat the exhaust ports of NA head to reduce heat transfer? I now that many run 2.7 NA and 16v heads in their turbo engines. Should (can) I do something about the pistons sticking out of the block?
If your 951 head looks good just use it, If there’s no damage now why even worry about how long the ceramic liners will last. Yes you can fit an na head and even apply a SwainTech or other coating on the exhaust ports, but why bother if yours is good.
I was reading that when the ceramic liners crack, the debris can damage the turbos hot side. Another benefit for NA head might be that you could improve the exhaust ports before coating them. I also saw in this blog post (http://dave951.blogspot.com/2016/06/...thickness.html) that it is beneficial for the combustion to have 0.04 inch “squish” size and with my shaved head I need to use quite thick HG to achieve that.
FYI i put a new head on mine with good ceramic liners. when i bent the valves, i checked the liners and one had a chunk missing. dont know where it is now, but wasn't in the X-over and it didn't make it to the turbo. pretty sure it would turn into dust if it hits the turbine wheel as it's brittle.
In our cars there is quite a long whay from the headers to turbo, but at full boost everyting could happen – probably depends how lucky you are. I already have a stock Cometic HG but it seems that I cannot use it as my CR would go up to 8.66 and although I have VEMS and could tune the car accordingly, still it would not be the ideal set-up. Thus was thinking about alternatives. You could buy a used NA head for almost the same price as 5 PC Cometic HG, but of course it would need some work.
Has anybody experiance with machining pistons to reduce the CR? What would be the correct way to do it? That way I could use my existing Cometic HG. I think I did read that Peep did it to its 944S engine when putting on turbo.
Why so concerned about the .6 higher CR? It's pretty inconsequential. Fwiw The dodge hellcat (supercharged v8, 700hp) has 9.5 compression and is still port injected.
use that factory 1.4mm gasket before bothering with cometic.
Why so concerned about the .6 higher CR? It's pretty inconsequential. Fwiw The dodge hellcat (supercharged v8, 700hp) has 9.5 compression and is still port injected.
use that factory 1.4mm gasket before bothering with cometic.
Its closer to 8:7 and probably I would need to reduce the boost level dow from 17 psi to saffely run the car on trackdays with pump gas (93). I did read that due to the reason that APR bolts stretch less than OEM ones, it provides more stress to the HG (as the block and head expands due to heat and compresses the HG more as the APR bolts stretch slightly less) and that might be one of the reasons why mine lasted just 6000 miles (3 years). Therefore I would like to use Cometic HG which I have, but it is standard size. I will talk. In the link I provided it is stated that the ideal distance between piston at TDC and the head surface should be 0.04 inch and I could achieve it ny either using thicker HG or by machining the pitons.
Two other things to think about on top of the CR would be how much retarded timing (raises power band) in the cam do you have now and exhaust fitment seeing how the head sets lower, exhaust systems do best when they're not put together stressed.
It wasnt even 0.04" on the stock 951.
what tuning do you have?
I have VEMS and the tuning was done by one experienced guy who is living from tuning cars equipped with VEMS for more than last 10 years. I asked for a tune that I could use on the track, so I am not concerned with that and it could be tuned also for 8.7 : 1 CR, but it might not be the best way to go as far as I have red about CR vs boost for 8v head. I would need to copy log file to my PC to provide more details about the tune I also once tried the autotune when VEMS specialist was not available, but the results achieved by VEMS Auto-tune and experienced tuner were night and day in my case. Probably will also try to contact Peep and ask his opinion.
Two other things to think about on top of the CR would be ......................... and exhaust fitment seeing how the head sets lower, exhaust systems do best when they're not put together stressed.
I am assuming this is around the #4 cylinder. The engineers at Porsche in their infinite wisdom did not do a very good job of designing the cooling system on these cars. They develop a vapor lock around the #4 cylinder and they fail there. LR developed a bypass around the #4 that relieves the pressure. Before reinstalling the head get one of these.
I am running an NA head on my car. Pull the exhaust valves from the 951 head and place them in the NA head as they are sodium filled to help dissipate heat.
I am assuming this is around the #4 cylinder. The engineers at Porsche in their infinite wisdom did not do a very good job of designing the cooling system on these cars. They develop a vapor lock around the #4 cylinder and they fail there. LR developed a bypass around the #4 that relieves the pressure. Before reinstalling the head get one of these.
I am running an NA head on my car. Pull the exhaust valves from the 951 head and place them in the NA head as they are sodium filled to help dissipate heat.
Could you share for how long do you use NA head, in what conditions (street, track) and what are the engine specs? Does it hold OK and did you coat the exhaust ports?
Could you share for how long do you use NA head, in what conditions (street, track) and what are the engine specs? Does it hold OK and did you coat the exhaust ports?
My car is a dedicated track car, it gave up street status along time ago. It is a 3.1+ liter motor, boost is limited to 17 psi for engine longevity, CR is around 8.3, The exhaust manifold was coated at one time but I run a lot of timing on my ignition so it has hot exhaust temps and the coating went away. The head had the exhaust valves changed, it also has a high performance Jon Milledge cam, stronger valve springs, and the ports and valves were ground to improve fit. I am using a cometic gasket.
05-16-2018, 04:38 PM
944 8v NA head for 951?
I also once tried the autotune when VEMS specialist was not available, but the results achieved by VEMS Auto-tune and experienced tuner were night and day in my case. Probably will also try to contact Peep and ask his opinion.
