Rod/Stroke Ratio and torque

[cheekychimp]

Bear with me here, I'm trying to learn a few things.

I've looked at various engine configurations utilizing different configurations of cranks, blocks, piston sizes/bores and rod length.

It seems the 'best' rod to stroke ratio is obtained using an 87mm bore and 162mm rod in a 4G64 block. The stroke for that configuration is 88mm. This takes capacity up to near 2.1 litres. But Magnus also lists two 2.2 litre configurations using a 92mm stroke. So this raised a few questions.

(1) Where does the 92mm stroke come from? I thought OEM cranks were 88mm and 100mm? Is the stroke accomplished by varying the piston height and rod length?
(2) Any engine build utilizing a 4G64 block seems to use a minimum of an 87mm bore, I take it 4G64 bores are larger than 4G63 bores?
(3) If you sleeved a 4G64 block and used 85.5mm bore pistons could you further 'improve' stroke/rod ratio?

I started looking at stroke further. It's clear that there is only so far you can go in terms of increasing bore with a 4G63 block due to the material between the bores. In addition you require the additional deck height of the 4G64 block to accommodate a longer rod and a longer stroke if using larger diameter pistons. So the next questions that came up were;

(a) What if you utilized the same 2.1 litre configuration above using a standard 2.0 litre bore. Would you lose power or low down torque?
(b) And if so could you play around further with custom road lengths/piston heights to create a 'stroker' with a higher rpm limit?
(c) What effect does the CR have here? NA motors with high CRs can spin to high rpms so could you bring torque at lower rpms back by using a higher CR and still keep your RPM limit?
(Many 2.3s already use 9.0:1 CR or higher!)

 

[belize1334]

(1) Where does the 92mm stroke come from? I thought OEM cranks were 88mm and 100mm? Is the stroke accomplished by varying the piston height and rod length?

The 92mm crank is custom. It doesn't come from an OEM application.

(2) Any engine build utilizing a 4G64 block seems to use a minimum of an 87mm bore, I take it 4G64 bores are larger than 4G63 bores?

6-bolt 2.4L is typically a G4cs (not 4g64 which is 7-bolt) but either way the factory bore is 86.5mm

(3) If you sleeved a 4G64 block and used 85.5mm bore pistons could you further 'improve' stroke/rod ratio?

Now THAT'S an interesting idea. I don't see how the bore would affect the rod/stroke ratio BUT I've been trying to figure out how to do an all OEM stroker for the "cheap" build. Taking a G4cs to 0.020" over would make it 87mm. Then if you sleeved it back to 85.5mm you could run run 0.020" overbore 2G pistons which would put you at 2.3L exactly. Of course with the longer stroke the 8.5:1 cr would be bumped up to 9.7:1...thicker HG puts you at 9.5:1. If you ran 1G pistons then the CR would be 8.86:1. I like it. Is it possibly to sleeve an iron block though?

(a) What if you utilized the same 2.1 litre configuration above using a standard 2.0 litre bore. Would you lose power or low down torque?

Not sure I follow. The 2.1L comes from using 88mm stroke with 87mm bore. If you had the standard 85mm bore from a 2.0L then you'd in fact just have a 2.0L. You could still possibly do this with a long-rod setup but if you wanted to do it on a G4cs you'd have to sleeve the cylinders as mentioned. I doubt you'd get different torque character than from a normal 2.0 but you'd have a rev-happier motor. Of course the standard 4G revs just fine so I'm not sure what the point would be.

(b) And if so could you play around further with custom road lengths/piston heights to create a 'stroker' with a higher rpm limit?

I think for a stroker it doesn't get any better than 156mm long rods in a g4cs. Slugsgomoo was trying to sell a 100mm crank with 156mm rods and stroker pistons (wrist pin lifted 6mm just like with standard stroker pistons...but bigger bore for a g4cs block). It's a long-rod 2.4L but the rod ratio is only marginally better than a normal stroker and still not nearly as good as a 2.0L.

(c) What effect does the CR have here? NA motors with high CRs can spin to high rpms so could you bring torque at lower rpms back by using a higher CR and still keep your RPM limit? (Many 2.3s already use 9.0:1 CR or higher!)

I don't think CR affects the position of the power band very much except for spool characteristics. I think the high CR and high rev-limit of n/a motors is a two-front assault at trying to get more power. High CR raises torque in general and high rpm allows that torque to be applied where it'll amount to more power. Remember power is torque*rpm.

I don't have it with me but a while back I sat down and did all the math for rod/stroke ratios of 2.0, 2.1(destroked 2.4), 2.3(stroker), 2.4(standard), and 2.4(long-rod). The 2.1(destroked 2.4) has by far the best rod/stroke ratio but I just don't see the point when 2.0 will do 10k already. The 2.3(stroker) has the worst rod ratio. This amounts to a slight increase in rod force across the board but the biggest problem is that you get about 20% more cylinder side-load throughout the stroke. The 2.4(long-rod) reduces this effect to about 15% over stock. So, provided that you have strong rods, the biggest effect of the stroker is that you're going to oval your cylinders faster (by 20% with a standard stroker and 15% with a long-rod G4cs). But, IMHO if you keep it under 7k it just doesn't matter.

 

[H05TYL]

4D68 diesel engine has a 93mm stroke and may go into a 4G63 block...

 

[mrnvit]

according to dirk it will

Quote:
Yes I have done this.
The 4D68 crank fits into 7-bolt Evo blocks.

 

[cheekychimp]

The 92mm crank is custom. It doesn't come from an OEM application.

That is what I originally thought as well, but Magnus lists two 2.2 litre engines on it's website one using a 4G63 block and the other a 4G64 block and for the latter it states that you can use either a Magnus or an OEM crankshaft. /ubbthreads/images/graemlins/dunno.gif

THAT'S an interesting idea. I don't see how the bore would affect the rod/stroke ratio BUT I've been trying to figure out how to do an all OEM stroker for the "cheap" build. Taking a G4cs to 0.020" over would make it 87mm. Then if you sleeved it back to 85.5mm you could run run 0.020" overbore 2G pistons which would put you at 2.3L exactly. Of course with the longer stroke the 8.5:1 cr would be bumped up to 9.7:1...thicker HG puts you at 9.5:1. If you ran 1G pistons then the CR would be 8.86:1. I like it. Is it possibly to sleeve an iron block though?

I don't think there is any problem doing it, I think it just has never been done because it isn't necessary. People usually sleeve aluminium blocks to make them stronger the 4G63 is already strong enough to take more than most of us can throw at it. The purpose of sleeving the block here is different.

Given what you have said I have probably cocked up on my assumptions about the rod/stroke ratio and bore. I thought that a larger diameter bore would exacerbate the cylinder side load throughout the stroke. If it doesn't then there would be no benefit in doing THIS:

(a) What if you utilized the same 2.1 litre configuration above using a standard 2.0 litre bore. Would you lose power or low down torque?

What I was getting at here was that I thought the smaller 2.0 litre bore would create less cylinder side load.

My question to loss of power or low end torque was aimed at the difference between 'normal' 2.0 litre motors and long rod ones. Normally you don't get something for nothing and since the Mitsu engineers chose not to use long rod configurations that gave a higher rpm limit, I assumed that this affected not just the rpm limit but perhaps the power band, pushing the usable power to higher rpms not utilized as much on the street.

As you stated You could still possibly do this with a long-rod setup but if you wanted to do it on a G4cs you'd have to sleeve the cylinders as mentioned. I doubt you'd get different torque character than from a normal 2.0 but you'd have a rev-happier motor. Of course the standard 4G revs just fine so I'm not sure what the point would be.

If bore has no effect on rod/stroke ratio, there would be no point. You would be making a huge amount of work to lose 70cc of displacement without gaining any rpm advantage.

(b) And if so could you play around further with custom road lengths/piston heights to create a 'stroker' with a higher rpm limit?

(c) What effect does the CR have here? NA motors with high CRs can spin to high rpms so could you bring torque at lower rpms back by using a higher CR and still keep your RPM limit? (Many 2.3s already use 9.0:1 CR or higher!)

This is where my understanding stopped!!! I was under the impression that displacement in the 2.3 stroker was achieved not only through using a larger bore, but by using a 'shorter' piston that allowed more air/fuel to be drawn in. What I cannot get my head around, is that if the stroke remains constant, by doing that surely when it came to the compression cycle you wouldn't be able to achieve the same compression ratio because you'd have a bigger 'gap' at the top of the stroke. Obviously I'm missing something here.

I don't have it with me but a while back I sat down and did all the math for rod/stroke ratios of 2.0, 2.1(destroked 2.4), 2.3(stroker), 2.4(standard), and 2.4(long-rod). The 2.1(destroked 2.4) has by far the best rod/stroke ratio but I just don't see the point when 2.0 will do 10k already.

Magnus quotes 11,000 rpms but yes I take your point. Is all that work worth it to make the 2.1 litre gaining in fact just 70cc displacement, if a long rod 2.0 utilizing the same block can rev just as high?

I just wonder how much longer the bores would remain intact with that better rod/stroke ratio? I appreciate people will say I am mad for thinking about this but my direction on this was why not build a rock solid 2.0 litre super long rod engine with a valve train and head that could flow to 8,000 rpms. You could theoretically rev to 8K all day and still have a 3K buffer to your actual threshold. As I said I was just concerned whether lower end torque would be affected. I'm interested to do this with a slightly higher compression.

Finally, if the 4D68 crank fits in a 7-bolt 4G63 would it fit in a 4G64 7-bolt? That might give you the option of a 2.2 litre stroker utilizing all OEM parts and that idea I do like!

2190cc and a theoretical 10K plus rev limit. I would almost guarantee that means the answer to my above question is NO!

 

[belize1334]

I don't think there is any problem doing it, I think it just has never been done because it isn't necessary. People usually sleeve aluminium blocks to make them stronger the 4G63 is already strong enough to take more than most of us can throw at it. The purpose of sleeving the block here is different.

That's exactly the problem. Because the 4g63 can take massive power at up to 0.060" (1.5mm) overbore nobody seems to have experience with sleeving it. For me the purpose of sleeving would be to bring a G4CS (86.5mm bore) down to a bore that would accommodate OE replacement 1G pistons which only go up to 86mm bore. I talked to a local machine shop today who said that they could sleeve it with 3/16" sleeves which they'd then bore out to the desired bore. They quoted $400 + $20 per sleeve. For comparison they quoted $100 for a standard bore and hone. That puts the machine work disparity at about $400 which isn't alot less than a set of Weisco stroker pistons. Given that, I'm convinced that it could work and it could be awesome but you'd need to get a hook-up at the machine shop to make it worth it.


This is where my understanding stopped!!! I was under the impression that displacement in the 2.3 stroker was achieved not only through using a larger bore, but by using a 'shorter' piston that allowed more air/fuel to be drawn in. What I cannot get my head around, is that if the stroke remains constant, by doing that surely when it came to the compression cycle you wouldn't be able to achieve the same compression ratio because you'd have a bigger 'gap' at the top of the stroke. Obviously I'm missing something here.

When you talk about a shorter piston you're referring to the stroker pistons which have lifted wrist pins. This serves only to make the piston sit lower on the rod so that it doesn't slam through the head when the bigger crank puts the small end of the rod closer to the head. This CAN affect the wear of the side of the piston on the cylinder wall but it won't affect things like displacement and compression ratio.

The added displacement comes first and foremost from the longer stroke. 2.0L*(100mm)/(88mm)=2.27L. As a secondary effect you get a little more displacement from the increased bore. 2.27L*(86.5mm^2)/(85mm^2)=2.35L. To get an idea of how displacement goes up with diameter observe that, in my calculation, a key factor is the ratio of the square of the actual diameter to the square of the original (4g63) diameter. If you write the actual as the original plus a small correction, and then expand the square, you get ((d+s)^2)/(d^2)=(d^s+s^2+2sd)/(d^2). Now, for s much smaller than d, s^2 is MUCH smaller than d^2 or even 2ds so we can approximate this as (d^2+2sd)/(d^2) or 1+(2s/d). Thus, for small adjustments from stock, the displacement increases linearly with the increase in diameter as d2=d1(1+2s/d).

The compression has no effect on displacement BUT the change in displacement does affect compression. Remember that compression ratio is literally the ratio of the chamber volume at BDC to that at TDC. If you take the same piston out of a 4g63 and put it in a 4g64, you have a deck that's 6mm taller and a crank with 12mm more stroke so the piston rises to the same final position. But, by virtue of the longer stroke the chamber volume at BDC is increased. Thus the ratio rises. As an example, consider an OEM 7.8:1 piston. If I sleeve my G4CS to make it 85mm bore, then the only thing that changes is that the displacement goes from 2.0L to 2.27L. Now, for 7.8:1 compression on a 2.0L I need the total TDC volume (dome + head) to be 0.294L so that (2.0L+0.294L)/(0.294L)=7.8. If I change the stroke but the piston still rises to the same height then the head volume is still 0.294L but the BDC volume goes from 2+0.294=2.294 to 2.27+0.294=2.564 and the new compression ratio is (2.27L+0.294L)/(0.294L)=8.72:1. So, that's the compression that you'd get running OEM pistons as strokers in a sleeved G4CS.

Now, to be fair, a higher compression ratio may help raise the volumetric efficiency of an engine which will help it act like it has a slightly higher displacement, but that's going to be a small effect by comparison to raising the displacement.

 

[cheekychimp]

Quoting belize1334:
That's exactly the problem. Because the 4g63 can take massive power at up to 0.060" (1.5mm) overbore nobody seems to have experience with sleeving it. For me the purpose of sleeving would be to bring a G4CS (86.5mm bore) down to a bore that would accommodate OE replacement 1G pistons which only go up to 86mm bore. I talked to a local machine shop today who said that they could sleeve it with 3/16" sleeves which they'd then bore out to the desired bore. They quoted $400 + $20 per sleeve. For comparison they quoted $100 for a standard bore and hone. That puts the machine work disparity at about $400 which isn't alot less than a set of Weisco stroker pistons. Given that, I'm convinced that it could work and it could be awesome but you'd need to get a hook-up at the machine shop to make it worth it.



I see your point, if you are determined to do a stroker. But for my purposes then, running a destroked 2.4 (G4CS based 2.1 litre) would actually be a cheaper option than a 2.0 litre super long rod in a sleeved G4CS block. 70cc is not much in my opinion but as you stated why pay $480 to lose it when you can keep it and save yourself $380 towards other parts of the build. The real question is if going the G4CS block route is cheaper than just dropping 156mm rods in a standard 2.0 litre. I guess that means pricing out rod/piston combos, machining of wrist pins, honing of bores etc

 

[belize1334]

It's gonna come down to how long a long-rod setup you want. To get 2.1L you need to go to 87mm bore but that's just 0.020" overbore for the g4cs and you can bore your 4g63 out to that too if you want. With a 4g you can get 156mm rods and run stroker pistons (with lifted wrist-pins) and a standard crank and that'll get you your first-level longrod. With the G4cs you can do the same thing but with the added deck height you won't need the lifted wrist-pins. On the other hand, if you do get stroker-style pistons then you could run a 162mm rod and have a "super-long-rod" motor. The ratios of the two setups are:

4g Normal -> r/s = 1.7
4g Long -> r/s = 1.77
G4 S. Long-> r/s = 1.84

Honestly though, I still don't think you gain that much over a normal 4G. The side-loading is non-linear with shorter ratios but once you get to "good" there isn't a big difference between good and great. Also, 162mm rods are probably quite a bit pricier than the regular 156mm long-rod. If I were thinking 2.0L or 2.1L I think I'd either do regular 4g or else destroked G4 but stick with the basic 156mm long-rod.

 

[cheekychimp]

Or build a really good 2.0 litre long rod block using standard bore pistons to start and just rebore to 0.060" over further down the road. That should guarantee 150,000 miles of fun /ubbthreads/images/graemlins/grin.gif

 

[BrandonEchols]

Quoting cheekychimp:

Finally, if the 4D68 crank fits in a 7-bolt 4G63 would it fit in a 4G64 7-bolt? That might give you the option of a 2.2 litre stroker utilizing all OEM parts and that idea I do like!



This actually sounds like a very promising build...one I'd be interested in down the road.
I always wanted a 2.3 stroker, but after reading about the rod ratios and side-loading of the cylinder walls, I wasn't so sure.
The 2.1 seems like a lot of work for minimal gain (particularly on the street), but a 2.2 would potentially be a perfect compromise between those two.

Is there a similar possible solution for the 6-bolt 4G?