Rb 26 comp ratio

[cory]

Ok I'm having trouble trying to figure out what comp ratio I will be running if I don't cut the head. I'm Tryin to keep the c/r close to stock or even above a small bit.
I have:
wiesco pistons 86.50
1.2 mm -87 mm tomei head gasket
No machine work has been done only to bore the block. From what I understand with a 1.1 mm hg with wiesco pistons it will be around 7.9. I'm trying to figure out how much I will have to cut the head to get the c/r back to stock. Should I go with a oem hg? Or will that not be sufficent as I'm gonna be running the tomei arms and looking for bigger numbers? I've done a bit of searching but haven't come up with a clear answer yet

[cory]

this is the specs the site give me the machine shop already has the pistons

[speedfreak]

I would check the cc of your cylinder head first before doing anything.
I did work to mine and when I recheck all my combustion cc's
It was not 70cc like weisco states.
So check your cylinder head first.

[cory]

Thanks. Trying to get that done now... The head isn't near me as I had it sent ours be done. What is the oem cc's? Or do they vary the head hasnt been touched other than valves reseated but that was very minimal

[Skym]

From what I understand, need to know the formula's and that's where a engine builder can help.

OEM headgasket has a frayed thick, thin seal and would fail at higher boost levels and should be around 1.2mm. A good metal headgasket has a thick solid seal and is designed for higher boost levels.

This link has the formula for working out compression -

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

This is gasket thickness with Tomei 86.5 pistons (not the pistons you have, but get an idea of how headgasket thickness affects compression) -

http://www.tomei-p.co.jp/_2003web-catalogue/e103-104_RB26_gasket.html

Probably mentioning things you already know. I would think if you have the pistons, skimming the head more to reduce the dome volume / head cc's on the list above (supposedly not all cylinders have same dome volume at factory and need to be evened out on high performance engines) would raise compression if can't adjust piston crown height or thinner headgasket won't work. Also skimming deck of block to reduce piston crown to deck volume. This would need to be the engine builders decision (who should know the formula's and can measure the domes, etc) once you know what compression you want.

From what I understand, from what I have seen, head specialist or engine builder uses a piece of clear perspex and a tube that goes into middle of piece of perspex to fill each dome with water (with valves in place) and on hose it has marks that indicate how big the dome volume is. They then workout how much head needs to be skimmed to even the domes out or reduce dome volume enough to get the compression you need.

Also once you have skimmed head, block, need to degree your cams (adjustable camgears) -

http://www.camshaftshop.co.nz/index.php/Camshaft-Information/sole-purpose-of-degreeing-your-cam.html

[cortexx]

Just to let you know my head CC'd at 64cc , i am using 87mm wiseco pistons and with a fatory stroke my engine currently sits at 8.1to1 compression , the head will be skimmed to make it 8.8to1 compression using a 1.2 mm 87mm cosworth gasket. I also used supertech +1mm valves which were dished and this added more cc to the head chamber assembly and had to be taken into account.

[cory]

Great lots of info there... Some I already knew helps quite a bit just gotta figure
Out the cc's now. Does anyone know how much can be takin off the head?
Cortexx how how much are you taking off? Are the cam gears a necessity right away?

[cortexx]

I would do what i did , swallow your pride and visit a GOOD engineering shop and have them do the math and calculate everything for you . I got the tomei adjustable cam gears for the simple reason that although the poncam 'bs are great for oem setup , skimming the head and machining will make your cam timing slightly inaccurate (I think it retards them slightly) plus I also intend to advance the Exhaust side to increase pressure and help drop spool time on my turbos .

[cory]

Yea talking back and forth to the shop couple times we got it narrowed down to. Me having to assemble the block to find the piston deck height and they are going to calculate the rest.. Aiming for a c/r of 8.4 to 8.6 to 1. Only thing that is really throwing us off is the piston. Wiesco doesn't give much of a description on what c/r should be with there pistons if everything else is stock. Starting to second guess my choice maybe should have went with Mahle or tomei pistons

[cortexx]

I was going to go Mahle but you have to slightly notch the skirt to get them to miss the squirters , my engine guy made a point that if they are made for the RB26 they should fit without further machining. Mahle said to remove the oil squirters and block off the oil galleries , not what we thought was a suitable solution . Mike my engine builder said although the wiseco are not cooling channel pistons , the squirters still contribute to cooling the pistons from underneath and that this in itself is a benefit that should remain no matter how small .

The Wiseco are sold as a set of replacement pistons designed to work with 8.5:1 compression , the actual adjustment of the ratio (which you will need to do if you rebored ) will be the head planing , you machinist should be familiar with this.

If you know your exact head cc and you measure the stroke of the pistons in the bore , and have the bore diameter you can actually work it out relatively accurately . you may need some pie though

[cory]

Thanks for the help ... For now the head is on hold untill the block is assembled then we can figure the rest out only thing the need now the the piston to deck height.

The weather been so good lately it's been itching me to get it together. Just Trying not to rush it cause don't want any problems

[cory]

According to Nissan the oem head gasket is 1.2 mm so the only thing the could throw it off is the pistons

[cory]

Found out some more info. Wiesco are saying there pistons sits -.006 below the deck Anyone know what the oem is?

[speedfreak]

After I finished the chamber work on my cylinder head but before it was
Milled I measured mine at 65.5cc.

Here's an online calculator which can give you a rough guesstimate
Just use the wisecos piston specs but NOT there chamber spec.
It's way off

http://www.race-cars.net/calculators/compression_calculator.html

I get 8.48:1 ratio with your numbers and a 65cc chamber

[cory]

thanks...i found one on the wiesco site....helps quite a bit being able to see what the actual numbers are