RB30 build, the importance of measuring!

[Stealth-Z]

It has taken me over six months to get the lower end of my RB30 built. Unfortunetly it has run into a slight snag. Now I am not a professional engine builder. My education on calculating compression ratio and machining tolorances has been accumulated over the last few days.After assembling and measuring, my compression ratio comes out to around 7 to 1! Way to low for what I was expecting. This is using custom Wiseco pistons I got from this thread here. http://forums.hybridz.org/show...isecoNow let me emphasize "THIS IS NOT AKWIKZ's FAULT NOR DO I HOLD HIM IN ILL REGUARD". He was going by what Wiseco told him. Plus he has a set of these too and is in the same position as myself.



After installing the pistons, they were 0.022 inches in the bore. This measurement gave me a basis to start calculating my compression ratio. The bore measures 3.406 inches, the stroke 3.346 inches, head gasket diameter is about 3.45 inches, head gasket crushed is 0.047, and the RB26 head measures out at 64cc'sUsing Wiseco's calculator http://www.wiseco.com/Calculat...s.xls Comp Ratio Calc For 4-Stroke Only The ratio comes out to 6.783:1

Okay, so lets juggle some number by doing some decking of the block and shaving on the head.In order to achieve a good quench of 0.040 inches, I can deck the block a total of 0.029 inches. This will raise the piston out of the bore by 0.007 inches. That changes the CR to 7.089:1 Still to low. Quench is good though.

Next comes the head. In order to get the CR up to around 8.5:1 requires the cylinder head to be around 48cc's. That is way to much shaving on the head for me! Plus taking almost 0.030 off the block is somewhat excessive to me as well.

Always measure and double check. Nothing worse than getting something together and it not perform like expected. Now it is back to the drawing board for pistons. A dome is definetly needed to get a higher CR. These pistons would most likely work great for a drag racer running very high boost.

[Shocker]

aww bummer man... time for some new pistons..... :/

[Chaos the Xile]

Big bummer I know you have been wroking HARD on this ever since your block arrived!

[Carl H]

on a side note it would handle 40psi no problem...but you are right it is too low for a street car.it would be an absolute dog to drive you would pretty much have to be on boost all the time to go anywhere.hope you can resolve it man, would be cool to see a running 30det stateside.

[l0nestar]

Man, that is harsh! Can you get Wiesco to replace them?? seeing as how they gave you the wrong dimensions in the first place. (I assume they will say that they made them to order, but when they gave you the wrong dimensions, it is kinda hard :-\ )

[Darius]

That sucks so bad man. I would test out Wiseco's customer service first and see if they can at least cut you a deal. Even if you decked the block, it would be one slow, harsh-running mofo off boost. Plus with a large volume pent-roof combustion chamber like that, it would most likely detonate like hiroshima at high boost settings.

What is the measurement from the centerline of the wrist pin to the top of the piston? I'm assuming you had to give them that information and even that appears to be off a bit (0.022 in). I'm not up on the RB30 hybrid swap, but fill me in on why you couldn't just use their standard 86.5mm pistons for an RB25/26? I'm assuming it has to do with the wrist pin location on the piston.

[Stealth-Z]

I have emailed Wiseco about this but have yet to get a response. Plus it has been over 6 months since they were ordered. I would not expect them to be supportive.To much time was spent getting the crank collar installed, balancing the whole thing, and finally to the point of dropping in a few pistons for measurement. Supposedly these pistons were worked out by Wiseco. As it turns out, these are off the shelf items in their catalog.http://www.wiseco.com/PDFs/Cat...a.pdf

Their RB25DET pistons have a compression Ht. of 1.240. So these would have had even a lower deck height than the ones I have. My goal was to have a quench area of 0.040 ~0.050. The ones I have are 1.260. If the valve releifs were not taking up almost 12cc's it would be a different outcome. I am not running high lift cams.

As I said in the first post, the guy I purchased these from has a set himself. He was going by information provided by Wiseco. The only way I could have found out for sure was to measure everything out. At the time I did not know much about percision measuring and custom piston design.

The R32 Nismo turbo's, running at 16psi max, I recently had rebuild would definetly would not have worked to well. In a way I am pissed but in another I am thankful the motor was not just assembled and run as is with out checking.

[Darius]

If they are the 0.020 oversized SR20 pistons, you shouldn't have a hard time reselling them if Wiseco doesn't take yours back.

The 0.020 oversized RB25 pistons that they list have a dome volume of +14cc vs. the SR20 of -12cc. That is a 26cc swing and will make a huge difference in compression. If you could mill the block to achieve an effective quench area, that could be a good option. I dunno, you'll have to run the math on that.

[Carl H]

come to think of it the top of that piston looks awful strange for a dohc rb piston.

[Stealth-Z]

If they are the 0.020 oversized SR20 pistons, you shouldn't have a hard time reselling them if Wiseco doesn't take yours back.

The 0.020 oversized RB25 pistons that they list have a dome volume of +14cc vs. the SR20 of -12cc. That is a 26cc swing and will make a huge difference in compression. If you could mill the block to achieve an effective quench area, that could be a good option. I dunno, you'll have to run the math on that.

Good eye catching the SR20 specs. The pistons I have are part number K577M865. The specs sure look simular to the SR20 part K556M865. Cool if an SR guy could use them. They make expensive paper weights for me!

[Stealth-Z]

come to think of it the top of that piston looks awful strange for a dohc rb piston.

That was my first thought upon receiving them. But when I asked a few guys in Australia about them, the conjecture was put them in and measure. Between decking the block, head gasket thickness, and head shaving, I would be able to get anywhere from 7 to 9.3CR. I just do not feel comfortable cutting that much material off the block & head. I think it is just the depth of the valve cut releifs that screws things up.

[Chaos the Xile]

come to think of it the top of that piston looks awful strange for a dohc rb piston.

Very true the pistons look like a set from a 305 Block with trickflow heads. Not a Rb motor...

[Darius]

On the bright side, it looks like you will no longer have an "interference" motor. You could break timing belt after timing belt with no ill effects. I'm sorry man, just trying to lighten the mood. Any new info?

[Stealth-Z]

On the bright side, it looks like you will no longer have an "interference" motor. You could break timing belt after timing belt with no ill effects. I'm sorry man, just trying to lighten the mood. Any new info?

That was my first thought after measuring.

Wiseco has not responded. Not that I expected them to anyway. Next plan is to get everything back from the machine shop and start working on a design for pistons. The guy that rebuilt my R32 Nismo Turbo's has an account with JE. He has offered to help with the design specs. Now that I have some measurements, I can design the piston versus machine the block and head for the right CR.There is no way I am building a street motor with any less than 8.3:1. Anyone that has ever driven a low compression setup on the street knows how much of a slug they are.

[Stealth-Z]

This is Wiseco's reply a few moments ago. I included all the numbers in the first post.

Correct. That part with your combination would be 7:1.Best Regards,

Wiseco Piston Co. Inc.

[Carl H]

when you say low compression motor, the vw beetle comes to mind...that and mg engine blocks, hooray engine inferno!

[Darius]

hahaha to interpret what wiseco said, "Yep! You're on your own! Thanks for calling! mmmbye."

It's good that you have a connection to help you out with this. It's a lot tougher when you're trying to do all the work and research yourself. Have you looked at any of CP's piston specs?

[Sil240]

This is Wiseco's reply a few moments ago. I included all the numbers in the first post.

Correct. That part with your combination would be 7:1.Best Regards,

Wiseco Piston Co. Inc.

What did you put in the e-mail????

[Stealth-Z]

What did you put in the e-mail????

I stated the way the pistons were sold to me as being 9:1 and included the part number on the box and the measurements.The guy I purchased them from was misinformed by the Wiseco representative. He is now in the position as me.

Six months has passed. Neither of us have any writen documentation about the discussion with the Wiseco representative. So basically unless I want to build a motor for the drag strip, I am out of luck.

[Shocker]

that sucks man, get into contact with someone from the AU, they have loads of rb30's down there, I'm certain you can get pistons you need as well.

[Sil240]

Man that sucks.

Well.... sell them to the guy a couple thread down. He's going to be running 38-40psi. LOL

Can you get your $$ back from the guy you bought it from??

[Stealth-Z]

Man that sucks.

Well.... sell them to the guy a couple thread down. He's going to be running 38-40psi. LOL

Can you get your $$ back from the guy you bought it from??

I will not ask him for the money back. After all, it has been six months. He feels bad about it as well. We are both out based on receiving bad information. Not his fault. Niether of use are machinist's. But I can say I have learned alot in the past few days!

[Eikon]

The amount of technical information that is being shared and discussed in this thread is outstanding. Thanks for sharing so much info in such great detail with us Stealth.

From reading the above.. it sounds like you will be able to recoup some of your costs by selling the pistons to SR owners if that's the route you choose to take.

Keep up the good work. You are going to have a real beast on your hands when you get it done.

[OpeLok]

I can summarize this thread in a one word statement.

f.uck.

[themadscientist]

Good god those valve reliefs are rediculous!

[Stealth-Z]

Now the block is at home on the engine stand. You can see how the pistons look inside the bore.

I seem to be collecting Wiseco's. The one on the right is an 87mm RB26. Wiseco must have figured anyone building the RB30 DOHC was going to use some very high lift cams and lots of boost.



From the side view you can see some differences as well. The skirts and ring lands are different heights.



Now I get to spend some time designing something to match up with the increased stroke of the RB30. I still have the factory RB30 SOHC pistons and rods so at least I have some height and skirt measurements.

[themadscientist]

My plan for my longstroke RB is to have custom-length rods made so I can use off-the-shelf RB26 pistons. You might want to try that instead of playing another round of piston roulette.

[Stealth-Z]

The Wiseco RB26 pistons I have are trashed. A few years ago someone here was selling a disassembled RB26 from a botched rebuild. He claimed the pistons were "mint". They all had major nicks and scratches to deep to be usable. Looks like they were tossed across the room and bounced off the walls. For rods I am using these from Spool Imports. http://www.spoolimports.com/co...S.asp

[Sil240]

Is is possible to have those Wiseco's polished???

Or are there bad gouges?Can't really tell from the pic but they don't look horrible.

[Stealth-Z]

Is is possible to have those Wiseco's polished???

Or are there bad gouges?Can't really tell from the pic but they don't look horrible.

They do look much better in photo's. But there are deep gouges in the side skirts. Everyone I show them to says the same thing. "You are not going to use these, right"? Besides, I only have 4 left. Been giving them away as paper wieght / ash trays. There were very few parts off that RB26 buy I even considered reselling to someone else.