Low Compression Piston Solutions

[slownslurious]

OK I am building up a VH45 to accept some boost and I nee a solution for low compression pistons. I am trying very hard to track down any and all rumors but most are proving to be unicorns.

1) escort/other custom pistons. Cheapest I have found these is $2000/set. Not worth it

2) vq30det pistons. I have confirmed that while the bore for these is identical to the VH and they do have a nice dished design, they do not appear to be compartible because the overall piston high is dramatically shorter than the vh. Use of these will probably result in compression too low to do anything. UNLESS you use longer rods. TMS suggested that the 4.1 liter's rods might be a bit longer and use the same piston pin and crank journal diameter as the vh 45, but TMS later said that the difference in stroke between 45 and 41 is 6.7mm less so they will probably be too long. He said its possible to weld a rod to add or remove length but I haven't ever heard of having this done. It sounds very risky.

3) http://www.cardomain.com/ride/2133309Check out that page, that gentlement from newzealand claims to be using forged subaru low compression pistons in his VH with a bore. I have contacted him and asked him for more information on his setup but I am waiting for info that may never come.

4) After extensively googling and reading other forums another person has confirmed that certain toyota supra pistons will fit (same bore) as the Q but no word yet on piston height and pin diameter, also have not yet confirmed which pistons (though that would be relatively simple since the supra has only had 2 engines).

If anyone has any additional information, corrections, or assistance on this matter please post it here. Lets try and get this thread going with all the information the community has on getting low compression pistons in the VH45de. My search has revealed that, if nothing else, there is a HUGE potential market for this engine once these questions are answered (ie this question has been asked by many people on many boards, and a lot of those people have projects in the works as we speak).

If you wish to contribute anonymously or to keep your idea secret let me know via email at [email protected] and let me know which your preference is (anonymous I will post it up here for you or secret I will keep it a secret).

I need a low compression piston solution for my motor as soon as possible.

thanks

[defrag010]

I built a few 4g63 motors with 2jz supra pistons, and both the 7m and the 2jz are way too small of a bore to fit in a vh. The 7m is 84mm (I think), and the 2jz is 86mm. Ours is roughly 93mm.

For lowering the compression, have you looked into headgasket shimming and having the combustion chambers enlarged?

What is your target compression?

[Mettler]

VH41DE conrods are the exact same, identical item to VH45DE conrods. The 6.7mm difference in stroke is taken off the piston compression height.

[hannibal]

3) http://www.cardomain.com/ride/2133309Check out that page, that gentlement from newzealand claims to be using forged subaru low compression pistons in his VH with a bore. I have contacted him and asked him for more information on his setup but I am waiting for info that may never come.

I just looked around for some EJ20 1mm overbore pistons (stock is 92mm). Wiseco is the only company I found that makes them. CP, JE, Ross, or Arias dont make them. I'm dying to hear if these work...

[Ezekial]

CP pistons ... $1500ish AUD

[sijoko]

Have you considered dishing out the OEM pistons? Might be the cheapest route other than using a thicker head gasket.

On a side note: Has anyone verified the actual compression on the VH45?

Is it 9.x as others have posted on this board or is 10.2 the correct number? From what I have read, the dynamic compression is usually always less than the static.

[defrag010]

The EJ20 pistons have a 1.289" comp height, that's about .030" longer than the stock VH45.. but they have a -12.8cc dish. Assuming you can shim the headgasket up .030 to .040 and assuming the EJ20 has the same pin diameter, that should yield mid to high 8's static compression.

The 5VZ-FE toyota v6 also has a stock bore of 93.5mm, but I can't find the comp height or the pin size.

"early" rover 3.9 v8's also have a 93.5mm bore stock

6g74 mitsubishi v6 motors have a 93mm bore stock
Modified by defrag010 at 11:03 PM 11/14/2006

[slownslurious]

the bore isn't as big an issue... ideally you would want it a little bit over stock VH45 size so you can bore out to it (in my case anyway, rebuilding a motor with bad compression). Piston height, pin location, and pin size are as important as bore diameter.

Let the search continue! the information on the subaru pistons should be out there somewhere I'd think the subaru guys have this somewhere on nasoic or similar.

[defrag010]

The EJ20's use the same 22mm pin that the VH does, so number wise all you would need to do is use a .030" shim headgasket or just double stack them to run the EJ20 pistons.

[slownslurious]

any links to purchase a set and also what about valve clearance?

I am going to be rebuilding starting this week so I am in an immediate position to make these work. lol.

does anyone make a thicker headgasket for the vh that would make these work? I'm not sure stacking OEM style gaskets is a good idea with the VH. I do plan on trying to get the ARP headstuds but even then...

[slownslurious]

btw i wanted to add thanks everyone for the feedback and posts I think this is a very useful thread.

[defrag010]

I've stacked composite gaskets before, and have had good results.

Also, today at work I measured 4.7 sohc chrysler v8 pistons and they are identical to VH45 pistons in bore and comp height.. except the distance between the crown and first ringland is smaller.

You can get some forged blower pistons for the 4.7 from http://www.krcperformance.com.

[slownslurious]

is the piston pin diameter the same?

and if I have to stack gaskets I may but I thought somewhere someone said that a company/person was making some taller ones (or maybe someone had one made?)

[slownslurious]

I dont see those pistons on their site maybe you have to call for more info. Do you have any idea what the resulting compression ratio would be when used in a Q motor? I dont actually know how to calculate that.

Also any clue as to price?

thanks for the info.

[John Dixon]

Wossner made my 98mm ones so they should have some data on the stock piston they were sent. They'd certainly be able to make you some. Mine were about £1800UK for the set of 8 but I guess there is some design time in that too.

http://www.woessner-kolben.de/....html

[slownslurious]

for that much I could get custom built ones from escort or cp.

98 is a pretty large overbore for the vh too.

[John Dixon]

These were custom because of the 98mm bore and they're slipper design.98mm is big, but still plenty of meat left in the block.

[John Dixon]

block

[slownslurious]

so anything between 93 and 98 would fit if you bored it out enough... good to know. but custom pistons are out of my budget range at this point, thats why I'm trying to find something OE that would fit.

[SuperHatch]

VQ35DE Rods have the same big end and small end diameters as the OE VH Rods. They are 2mm shorter though. If you used these rods (assuming the width is correct, or they can be narrowed if too wide) with the suby pistons you would end up with a crown height 1.2mm lower than stock. I haven't done the math to figure out what that would do to the compression ratio, but it might be workable.

[SuperHatch]

6G74 Pistons have a Compression Height of 1.254" according to Ross's catalog. They are 93mm bore, and through some google searching they have a 22mm pin diameter. The stock compression height on the VH45 is 32mm (1.2598"). The Ross pistons have an 11.1cc dish. With stock rods, these pistons should work, but the CR will be real low I think...

I wish I knew the Combusiton Chamber Volume of the VH heads and I could calculate it for sure.

[SuperHatch]

Ok, one last post.

I used the calculator on http://www.zealautowerks.com, I know it's a honda calculator, but if you select "other" for everything it works out.

Bore - 93mmStroke - 82.7mmDeck Height - 220.35mmComb Chamber Volume - 54ccPiston Dome - -2cc (to account for valve reliefs)Comp Height - 32mmRod Length - 147mmHeadgasket Thickness - 0.030

That gives me 10.18:1 CR which is exactly what the FSM calls for stock.

Now if I change Piston Dome to -11.1cc and Comp Height to 31.851mm (the specs for the Ross 6G74 pistons)

It comes out to 8.99:1 CR, which is pretty boost friendly if you ask me.

I'm not sure if the valve reliefs are the right size and in the right positions, but those pistons might just work out.

Ok, the newbie is done now...

[elwesso]

Welcome to NICO!! THANK YOU FOR THOSE POSTS..

lets see what the other VH guys think!

[mtcookson]

http://www.atomicspeedware.com or http://www.brcperformance.com

Cheaper than the ones that are already made (Escort) and you can have them made to any specs that you could possibly dream of.

[defrag010]

so how much are the 6g74 ross's? Usually, if you want say 8 and they come in sets of 6, they will make you buy two sets of 6..

[slownslurious]

I'm sure if you ordered them directly from the manufacture you could get as many as you needed. Sourcing them from a speedshop you may have to get 2 set of 6. blah.

My internet is acting up I am very eager to find out how much a set of those would cost and buy them and see if they will fit. I'm going for a couple mm's of overborer to make sure my cyl walls have the scoring removed.

[slownslurious]

damn $672 for 6 for the mitsubishi pistons, taken off the ross website.

and you would possibly still have to have the machinist cut the valve reliefs into the piston.

[slownslurious]

I dont get it, according to their website they have custom v8 pistons for around $650-750 for a set of 8 but the 6g74's are about that much for 6...I could see paying around $650 max for 8 pistons even that is pushing my budget.

[SuperHatch]

I dont get it, according to their website they have custom v8 pistons for around $650-750 for a set of 8 but the 6g74's are about that much for 6...I could see paying around $650 max for 8 pistons even that is pushing my budget.

Call them up and ask them for 8 custom pistons for an import V8. When they ask for the specs, tell them to make them exactly like the 6G74 pistons, see what they say.

As far as the valve reliefs, the stock VH45 pistons are a flat top with a very small relief cut in them. The 6G74 pistons are a dished piston, and their compression height is slightly lower than the stock pistons. You most likely won't need reliefs at all.

[defrag010]

I wouldn't worry about valve reliefs either unless you're going to be having cams with alot more lift and valves that are bigger than stock.

Instead of having them make you 6g74 pistons, why not just send them a stock VH piston that way whoever makes them will have the specs and anyone who is looking for drop-in VH45 forged pistons would be able to buy them from the part number they will give you once they make them.. You can tell them what compression ratio you want, and you give them the VH45 specs so they can calculate how big they need the dish to be.

If you're just going to go ahead and get a custom made set of pistons, a re-make of the stock VH45 piston would be better because it will have a better quench than with the 6g74 piston that sits further down in the bore.

what are you going to use for rods? If you're going to go the trouble of upgrading to forged pistons and spend $$, it would be senesless to just use stock rods when you can also get some H beams made.