[Downshift]

I've been re-reading the article on rod ratios and stroke and everything else in it. MD told me that a 1.75 rod ratio is considered ideal. Stock, my B18C1 has a rod ratio of 1.58, agreed? Okay, to get close to 1.75 I worked out that if I use 134mm rods (I was told these were CTR size rods) and the crank form a B16A/B then I get a rod ratio of 1.73, pretty darn close if you ask me. My question is this: By using a smaller stroke, and shorter rods, won't I be adversely affecting the displacement? I don't know the actual terms to use here, but by using this setup, the pistons will not reach to the highest possible useable point of the cylinder, right? I'm basically creating a B16 inside a B18 block, I want that displacement back! What am I doing wrong?

 

[Jacksont001]

you want LoNGER rods and a shorter stroke. Mike will get into a long ass detailed explaination. It's all good stuff. but until he does... remember if you decrease the stroke by 10mm you need to increase the rod by 5mm to maintain the same piston height

overboring will get you the lost diplacement.

 

[Downshift]

I'm not worried about the sideways displacement, because, I agree, overboring solves that problem. Here's my question, or alt least one of the 17439 questions that I have. A "perfect" rod ratio is 1.75, to get the 1.75 using the stock 137.9mm rods I would need a stroke of 78.8 (this combination gives a rod ratio of exactly 1.75) the problem is, none of the stock cranks are anywhere near that number, the closest was the B16A/B crank with a stroke of 77.4 and a rod ratio of 1.78, a little in the wrong direction. Is it totally cost prohivitive and difficult to get a custom crank cut? Is it worth it? If I use the B16A/B crank with a 134mm (CTR) rod length, I would get a rod ratio of 1.73 BUT this would end up with the piston nowhere near the top of the cylinder, which will affect the compression, the efficiency, and probably make my car turn pink and bark like a dog.

 

[DaBoyNBlu]

if it doesn't reach the top of the cylinder, that's extra dead cylinder space, and you can probably reverse those affects by some high-compression pistons and getting some deck height shaved off.

 

[MichaelDelaney]

displacement = bore x bore x stroke x pi

With a b16a crank,

displacement = 8.1 cm x 8.1 cm x 7.7 cm x 3.14159

= 1587 cubic cm (cc)

yep, you lose displacement if you use that crank.

I think you have the whole approach all ass backwards.

Yes the ideal rod ratio IS 1.75.

The problem is you don't shorten the rod and the stroke at the expense of losing displacement to get it. You increase the rod length and add a deck plate or move the piston pin location up so that you have everything fit with the longer rod inside the block (deck height).

I think where you got confused was that you wanted more midrange. one way to get more midrange is to lower the rod ratio by getting a shorter rod like a CTR rod oN A GSR CRANK. You own a GSR, correct?

There has been some discrepancy in the specs posted for the CTR rod. My source has it listed at 137 mm. Another Japanese website has it listed as 134 mm like the B16A. And a third source has it listed at 144 mm. I will attempt to clear this confusion up. But the concept is to use a shorter rod on your current crank to get a lower rod ratio and therefore more midrange (at the expense of losing some top end power). Yes, you would have to mill the block deck to have the correct deck height if you were to go to a shorter rod.

So if you have a GSR or LS and you are in search of the perfect 1.75 rod ratio so you can spin her to 10K ALL DAY without fear of a piston going through a cylinder wall, then you would need a 153-156 mm rod, move the pin location up, and add a deck plate to the block. You would keep your 1.8L and gain a high powerband location at the expense of some midrange.

 

[MichaelDelaney]

we're all wrong! LOL.

It appears that the CTR rod is the ITR rod!

137.9/ 77.4 = 1.78

sorry about that. my source apologises profusely as well.

Just a simple rule...triple check...I screwed up on the B17a rod length as well before...Joe Pettitt's book had it listed as 137mm and my Helms manual for the B17a had it at that listing...we looked at one and checked with Brian Crower and it is indeed 131.87mm (132 mm).

I must be losing it....

So the CTR is just a destroked ITR to 77.4 mm stroke using a B18C block.

my apologies again for the error.

 

[Downshift]

Well, I'm glad that a stupid question on my part cleared something up for everyone. Now, using the 137.9 rod of the GSR (which, yes, is what I have) and the 77.4 stroke of the B16 yeilds the 1.78. Is that close enough? or is it on the wrong side of 1.75? If I use the stock GSR crank (87.2) and use 153mm rods, How do I figure what size deck plate to use? Where can I find a 153mm rod, or would it have to be custom? Also, what equation (if there is one) do I use to calculate what compression ratio would be best for this beast of a motor?

MD, you were right, I did have everything ass backwards. I was starting with the two knowns (stroke and rod ratio) and calculating what rod length would work. oops.

 

[MichaelDelaney]

this is "somebody's" resleeved B18C1 with a deckplate:

the scale is correct...if you know what the deck height is of a B18C1 you can work out the proportional height of the deck plate (I'll give you a hint...check the article again).

 

[MichaelDelaney]

BTW, despite all the hubbabalooo about the CTR rod length, if you're in the market for 134mm rods that fit on a B18C crank, Toda sells them. They are the rods that are made for their B16A stroker kit.

You have to shave off the block deck though....this IS a one way manoever...if you do it and don't like the fact you aren't making top end power, you can't go back. You are stuck with it. This is why I suggest going the boring out route to get midrange power via more displacement. you don't give up anything on the top end.

 

[MichaelDelaney]

what equation do you use for static CR?:

Compression Ratio = (Volume at BDC) / (Volume at TDC)

= [ Swept Volume - Piston Dome Displacement + Combustion Chamber Volume + Headgasket Volume (incl. piston to deck height) ] / [ Combustion Chamber Volume + Headgasket Volume (incl. piston to deck height) - Piston Dome Displacement ]

 

[Downshift]

That first pic looks like the deck plate and the cylinder walls are not flat. Don''t those have to be lined up so that the piston will travel correctly? I've got the CR equation, but how do I use it to determine the best CR for power? (I didn't see it in any of the articles)

 

[MichaelDelaney]

yeah it's on crooked....LOL

 

[Downshift]

Okay, size of Deck plate:

Rod Length = 153mm
Stroke = 87.2mm

Block Deck Height = (Stroke/2) + Rod Length + Compression Height + Stock Deck Clearance

What is compression height? The headgasket thickness?

What is the Stock Deck Clearance?

 

[MichaelDelaney]

head gasket thickness is the stock 3 layer thickness 0.7 mm. Compression height depends on the piston manufacturer spec for compression or dome height. Stock deck clearance is 0.76 mm in a B18C1. The articles already list the Deck Height with stock pistons...so why are you trying to calculate it?

 

[Downshift]

I'm trying to calculate it because if I'm adding a block riser to the block due to using 153mm rods, I need to find out how tall a riser to use.
Block Deck Height = (Stroke/2) + Rod Length + Compression Height + Stock Deck Clearance
BDH = (78.5/2) + 153 + Compression Height + 0.76
If I can calculate what the BDH for this would be, then all I nned to do is subtract 212.39 from that number and that will tell me the size of the block riser, right?

 

[Downshift]

So...
from JE pistons website I got a compression height of 1.195 (I believe this is in inches, convert to mm for 30.353) for a B18C5 11.5:1 compression 81.5 bore piston. plugging this into the equation:
BDH2 = (78.5/2) + 153 + 30.353 + 0.76 = 223.363
223.363 - 212.39 = 10.973mm which is the height I need for the block riser, correct?

 

[Downshift]

I don't understand, I got your response "153 - 137.9 + 10.973" this equals 26.073. But what is this number for? Second question, not particularly for any one person: what manufacturers make block risers for our motors, or is it a custom piece?

 

[Downshift]

on the other hand...
If I use the crank from a B16 (77.4mm) and the stock length connecting rod (137.9mm) and use JE Pistons 11.5:1 compression, 81.5mm bore ITR pistons (30.353 compression height) and the stock deck clearance is 0.76mm...
(77.4/2) + 137.9 + 30.353 + 0.76 = 207.713
this is only 4.677mm short of the stock deck height of 212.39 AND it gives a rod ratio of 1.78 (very close to the 1.75 goal)
This combination is very close to all stated goals, and it uses commonly available components (this is a very good thing!) The only downside that I can see is that pesky 4.677 (or 0.184134 in) that I would need to get rid of.
What do you think?

 

[Jacksont001]

use domed pistons to compensate, but bore that Fooker out! 84.5mm!longer stroke bigger bore 1.75 rod ratio 2.1 L monster!?!?!?! Hell yeah!

 

[Downshift]

Um...okay, domed pistons will help a little, I can go with any bore I want, I was considering an 85mm bore with a sleeved block. If I go with a longer stroke, then that means a custom crank, which I am trying to avoid if possible. I'm not concerned with the final displacement, I'm looking for the best overall of all possibilities. Thanks though, now I need to get my calculator back out. How do I calculate displacement?