head gasket size and compression

[trunks_phil]

so i just had my motor rb25det rebuilt and i put a 1.5mm headgasket and drove it from about 800 k and did a compression test and its 135 psi across instead of the usual 150 psi. my car seems slower then normal too. any one know if this is the reason why my car feels a little bit slower with my old tune ? compression test was done when it was warm.

[Daryl @ RightDrive]

A thicker head gasket will lower compression. Is there any particular reason why you used a 1.5mm head gasket? That is pretty thick for an RB25.

[trunks_phil]

thought i bought a smaller one but looked at my order and it was a 1.5 instead of 1.2

[Dragon Humper]

That does two things, lowers the compression and increases the detination risk. You would think that the lower compression would make it better but with pump gas it doesn't. The quench area gets totally opened up and makes the motor more detonation prone. People should be using the gasket that gives you .040"-.045" of space between the quench pad on the head to the piston. That will give you the best detonation resistance.



Jon.

[99_SI]

What was your logic in using a thicker head gasket on what I'm assuming is a stock engine/turbo setup?


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[trunks_phil]

motor has all the bottom built, with a gt35R 555cc injectors. i guess imma have to pull the motor out in the spring and redo the head gasket .

[99_SI]

head gasket size and compression

Wouldn't you need a retune to support those mods?


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[99_SI]

You don't have to pull the motor out. I did one on my buddy's L28 280z in his driveway


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[trunks_phil]

car was tune for the same set up but with the smaller headgasket . so it was tune for 150 compression across . not my motor has a compression of 135 across . everything boost fine just . just not all the power is there

[99_SI]

From my time here Skym looks like the "knowledge database". My guess you would have to retune.

[trunks_phil]

yea im just waiting for him to come in and comment haha

[99_SI]

My coworkers, former engine machine shop employee, opinion : "Yea, you increased volumetric efficiency of the engine but have you done anything to increase the flow potential of the cylinder head?"...the way I see it only time you would want to reduce compression is if your increasing air flow I.e multi angle valve job, increasing valve diameter, lift, duration...


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[Skym]

yea im just waiting for him to come in and comment haha

I see.

From my time here Skym looks like the "knowledge database". My guess you would have to retune.

I don't think I know that much, just some things I have learnt. There's people that know way more than I do.

From what I understand, with lower compression you lose power, torque off boost, but can run more boost to gain more hp (they ran into high 7's with compression on the Group-A RB20DET's when running over 20psi, around 1.7bar from memory, that's down from the stock 8.6:1 with stock cams, etc). Also the heads were fully modified with custom cams, etc on Group-A engines.

From what I have learnt, the rule of thumb on RB engines, is if running over 20psi (1.5 bar) you would look into dropping compression (forged pistons, thicker headgasket, etc or in some cases forged pistons with stock headgasket thickness) to gain reliability when thrashing car. Also run better gas (E85, etc). Below 20psi stock compression or close to that compression is used.

Tomei mentions rough compression figures with bigger Tomei pistons, stock RB25DET pistons and different thickness headgasket's -

http://www.tomei-p.co.jp/_2003web-catalogue/e105_RB25_gasket.html

The compression formula is here -

http://www.tomei-p.co.jp/_2003web-catalogue/e102_gasket.html

Piston manufacturer mentions compression with their pistons, so check their website or catalogue. I don't remember seeing headgasket thickness for that compression, but it might be mentioned there.

Also how to reset the camtiming to suit a rebuilt engine -

http://www.capella.co.nz/sites/kelford.co.nz/sole-purpose-of-degreeing-your-cam

They say in above link that camtiming can be retarded if skimmed head, block (rebuild). Retarding intake camtiming can cause engine to gain hp, torque in topend, but engine takes longer to really move car and lose hp, torque at lower rpm. If you feel that happening when driving car, then you'll probably know camtiming is out or it's headgasket, tune related.

Also with bigger turbo you would fit cams to suit (with ECU tune, camtiming changes via adjustable camgears, as it helps to produce more exhaust gas at lower rpm which helps to spool turbo earlier in rpm).

To get the correct cams for application you tell the engine builder to send engine, etc specs to head specialist and they make the cams to suit your setup.

This is what head specialist wants from engine rebuilder -

https://www.capella.co.nz/sites/kelford.co.nz/design-shop/

[frankydodo]

Hey skym if you change cam and camgear ( set a 0 deg) on a rb20 , is it necessery to reset cam timing with degree wheel and dial gauge. Because some say you can drop in cam and gear but other say you need to , but i think they were rebuilding their engine?

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[Skym]

It's best to use a degree wheel and dial gauge, etc to set them up properly.

As far as I know, at factory the stock camgears should have etch marks on the edges and they match etch marks on backing plate, so fairly easy to set camgears in correct spot (if engine is factory, not rebuilt). The adjustable camgears at 0 should be the same as stock camgears when comparing them, so marking where the etch marks are on aftermarket cam gears with twink should get them roughly where they need to be and should match stock camgears. But I would do it properly with degree wheel, dial gauge, etc to make 100% sure, as don't want piston hitting valves.

This is the hp gains (click on thumbnail) from adjusting exhaust camtiming, then intake camtiming on a RB20DET -

http://www.uniqueautosports.com.au/service-centre/dyno-centre/skyline-r32-gts-t.aspx?back=1195

ECU tune is usually adjusted to suit (bigger hp gains with tune to suit), but I don't think it's done on that dyno chart. Note the improved boost response (reaches full boost 300-400rpm earlier).