There were a couple of threads on this in the past but I never did see this avenue investigated any further, and I'm not quite sure why... perhaps a lack of information?

I measured a VH41DE piston last night and the compression height is roughly 36mm (that's using a tape measure and my eyeometer which is fairly accurate).

VH45DE is 6.7mm longer stroke, so assuming I've measured correctly, we have the following information:

- VH45DE pistons are 93mm, have a 22mm gudgeon pin, and 29.3mm compression height.

- VQ30DET pistons are 93mm, also have a 22mm gudgeon pin, 30.4mm compression height, and are of a dished design. (Information sourced from one of those Nissan piston dimension lists on the net.)

One small query I have, the compression height column of this list stated a value such as 30.4mm, but had a second figure after it saying -0.4, does anyone know what this second figure is? Is this the clearance between piston top and the deck at TDC or something?

There is only 1.1mm difference in compression height between the two. This leads me to conclude that VQ30DET pistons are a suitable low compression piston to drop in, requiring one of two machining operations to make them compatible.

1. Mill the top of the piston down by 1.1mm... this seems the least expensive way, but would require proper investigation to ensure the strength and material thickness is not compromised. Due to the piston's dished design, only a circular surface around the outside of it would have material removed... but doing so would reduce the dish and therefore raise the resultant compression (this may be desirable).

2. Offset grind the crank by 1.1mm and use undersize bearings. Doing this would reduce the stroke ever so slightly, losing you a few ccs of displacement but allowing you to run the pistons unmodified. (And who really cares when you have two bigass turbos to feed your engine 18PSi+ eh?)

Static comp of the VQ30DET is 9.0:1 (wikipedia) & 2,987cc, calculate its total combustion chamber size from that, and then find the head's combustion chamber volume for both VH & VQ to figure out how many ccs relief is in both pistons (barely any on a VH45DE, flat top and all), to calculate the amount a VQ30DET piston would decompress a VH45DE.

Lastly, I also need to investigate what allowances have been made in the VQ pistons for valve relief, as the valve reliefs on a VH piston go all the way out to the edge, and I don't think the tops of VQ pistons have valve reliefs, just a big dish. Piston/valve clearance is obviously as crucial as the rest of the equation.

If anyone is willing to pursue this further, please post what you know or what you can find out to get some concrete information together... genuine VQ30DET pistons are somewhat cheap and would be an effective & readily available means of decompression the VH to allow for plenty of boost.
Modified by Mettler at 4:36 PM 6/13/2007

Very interesting information. I wonder if anyone makes different pistons for the VQ30DET that may work better? One would think that the dish would make up for the valve reliefs in the VH pistons. maybe flycutting would need to be done if you were running cams..

Certainly taking some meat off the top seems like the easiest route as any machine shop would be able to do that. That makes me a lot less nervous knowing what I know now than offset grinding the crank.

Do you think we'd run out of space or if we'd HAVE to grind the 1.1mm off the VQ pistons? Meaning do you think that the side of the dish would hit something.

maybe im stupid but I dont see how removing material (and thus increasing CC volume) would increase the compression ratio?

Ok because of the difference in compression height, you have to do one or the other. Otherwise the piston will sit 1.1mm proud of the block deck (again, assuming my eyeometer measured the VH41 piston compression height accurately.)

Please understand that I am discussing compression ratio within the context of these machining operations, comparing the two to each other.

Now, milling the piston top down to get it to sit right is going to reduce the amount of dish compared to what it would have if you simply offset ground the crank, ergo the resultant compression ratio will be higher with a milled piston than it would if you went the other way.

I'd prefer to offset grind the crank to ensure a low enough CR is retained for decent boost if you're going to all this trouble to run low comp pistons for turbos to start with!

elwesso wrote:Very interesting information. I wonder if anyone makes different pistons for the VQ30DET that may work better? One would think that the dish would make up for the valve reliefs in the VH pistons. maybe flycutting would need to be done if you were running cams..

Well, if you're going to look at aftermarket VQ pistons, you may as well look at aftermarket VH pistons. Reason I'm talking about factory VQ pistons is they're cheap... tho perhaps combined with the machining ops required to get them to fit, won't be such a cheap option anymore.

And I mentioned that the valve reliefs on the VH pistons go out to the edge for a reason... the dish in the VQ pistons is only small, from what I saw of the little cross section drawing, there's at least 10mm of flat piston top before the edge of the dish, I am concerned that this area could cause interference with the valves. If the factory flat top VH45DE pistons have reliefs right out to the edge, then they must do so for a reason, and that's with factory lift too. That's reason enough for me to believe you may need to flycut the VQ pistons.

elwesso wrote:That makes me a lot less nervous knowing what I know now than offset grinding the crank.

Eh? Why would offset grinding the crank make you nervous? If a machine shop f*cked that up, they'd be legally obligated to replace your crank anyway.

offset grinding the crank made me nervous, and that's what I do for most of the day every day.#1 - if you offset grind the VH rod journals, you will have one half of the journal with part of what's left of the inside radius (rolled fillet), and the other half with an outside radius. Not good, because the radius on the journals is a big factor in crankshaft strength.#2 - 1.1mm would be roughly 40 thousandths offset, so that would mean your journal would be at least 40 thousandths smaller than stock. No one makes any bearings other than .25mm/.010. I called our clevite rep specifically asking for any off-the-shelf bearing part numbers for the VH, and the clevite part number for rod bearings is i think cb-1665 or something but that they only do .25mm and any other size would have to be custom made and $$$.

The solution:If you're looking to lower the pistons, just make the rods shorter. Instead of grinding the cap, you take the bolts out and cut however much you want to make the rod shorter out of the rod side of the cap mating surface, then re-size the rod. I actually did this to some B16 rods when I was messing with my welded up L28 stroker crank, and it is a loooooooooong painstaking process but alot "safer" than offset grinding a VH crank.

Good post, cheers for that.

this is all fine and dandy, but how available are they? Got a customer thinking about this.

Well I priced up genuine ones from a Nissan dealer here in NZ, and it was gonna work out to be around NZD$1200, that's around USD$900.

Availability.... well, you order them, wait for them, and get them.

Dunno what it would be like trying to source them in the states.

What we really need to do is concretely confirm the compression heights of VH45DE & VQ30DET pistons. So far, all this has been going on the assumption that the big list I found on the internet is correct.

mettler, i measured compression height a long time ago, and it was around 1.25" roughly. how did you get 1.5"??

Also, is there anything special about the v6 pistons, or are they just another cast piston?

I used to have a big thread, "master measurement thread" or something where i took a bunch of pictures of a stock piston and rod, and some of a head, and measured everything humanly possible.... but after searching, it looks like it has been deleted???????????? mods??

Posted in your other thread, it got archived. zerothread?id=179262

Huh ? You need to re-read my first post m8, I measured a VH41DE piston (roughly) and based it on that. The 45 piston clearly has a difference in compression height. I will refer to your measurements from now as they are probably more accurate than using my eyeometer.

Ok so let me ask. If the 1.25in (or 31.75mm) is correct for the VH45 compression height, then the VQ30DET pistons should give us more room than the stock pistons have, right? Seems like theyd decompress a VH45 pretty easily with room for some sick cams. Am I missing something somewhere? The compression height on a VQ seems to be almost 2mm smaller which means it should go in with room to spare?

I really hope that Im not overlooking something terribly trivial.

elwesso wrote:Ok so let me ask. If the 1.25in (or 31.75mm) is correct for the VH45 compression height, then the VQ30DET pistons should give us more room than the stock pistons have, right? Seems like theyd decompress a VH45 pretty easily with room for some sick cams. Am I missing something somewhere? The compression height on a VQ seems to be almost 2mm smaller which means it should go in with room to spare?

I really hope that Im not overlooking something terribly trivial.

Here's a question. Why does it have to be the VQ30DET pistons? Are they forged? The VH45 pistons have a compression height of 32mm. The VQ30DE pistons have a compression height of 30mm, IIRC. That is 2mm more clearance which should lower the compression nicely. The only real concerns are how the quench area is affected by the VQ pistons and also the cutting of the valve reliefs if needed.


Modified by sijoko at 5:48 PM 7/1/2007

too big of a quench height is a bad thing.. quench creates turbulence, which results in more power and less chance of detonating. bigger quench height = slower flame front
Modified by defrag010 at 1:45 PM 7/1/2007

Well I spent the weekend doing some 3d modeling and calculation, and found that I had initially figured wrong on a couple of points... I forgot that with the 6.7mm stroke difference, the compression height would only be changed by 1/2 of that >_<

sijoko wrote:Here's a question. Why does it have to be the VQ30DET pistons? Are they forged?

Here's my reasoning, I'm not a millionaire and am therefore trying to find the cheapest possible decompression piston solution. A factory piston suitable for the job would be ideal due to being mass produced and therefore cheaper, as opposed to a small batch of custom forged pistons.

On a street motor I'd choose cast hypereutectic pistons over forged pistons. Why? Because hypereutectic pistons are ultra lightweight, they've got a higher silicone content in the mixture than what the aluminium is capable of dissolving, resulting in a very light weight piston. As we all know, lighter piston = more revs with less strain on the rods and bearings.

If you tune the engine properly, there's no reason these pistons can't see massive, ridiculous amounts of power, without a compromise in reliability.

Sure if I was building a race motor with more than about 800 flywheel HP then I'd move to an H beam rod & forged piston combo, but I don't really think it's necessary with less power than that.

I'm starting to think that if the VQ30DET pistons sit 2mm lower in the bore, then they won't be suitable at all due to ruining quench effect. Maybe someone wants to run some calcs etc?

Actually, what I was wondering was why do the pistons have to be for the VQ30DET and not for the VQ30DE since the compression heights on both of them are less than the VH piston. Either way, you'd lower the compression and the VQ30DE pistons are very easy to get and cheaper.

Oh wait, you're right. I was confusing the VG30DE with the VQ30DE. Hmmm, where can we find specs on the VQ30DE piston? I doubt they offer as much decompression relief as the VQ30DET piston, but seeing as they sit lower in the bore due to the compression height, providing they don't ruin quench too much, perhaps the DE pistons would be more suitable?

The VQ30DE pistons are going to be flat top with the same specs as the VQ30DET. Again, the main concern is the quench area being changed drastically. Knowing, that the VQ30 pistons are 2 mm shorter, I could make the case for using a thicker head gasket instead.

Does anyone have a figure for the standard headgasket thickness when compressed?

Sorry to dredge this up but I can't sleep and have this engine stuck in my head at the moment.

Instead of using low compression pistons, why not take a loss in the displacement department and build a VH45 with a VH41 crankshaft and VH45 rods and pistons? That effectively lowers the piston height and compression but also lowers the displacement. I figure the loss in displacement will be offset by the 20+ lbs of boost people will be cramming in them anyways and will rev a lot harder.

Maybe this has been discussed a lot before but I missed it. Anyways...just a thought.

And again you lower the quench and so risk detonation again!

Gotcha. I knew there was a reason for not doing it but couldnt' remember why. Still learning about psi.

Yeah, you lose quench, but I don't think many people would notice/care once it was together and running. Straight from the factory Ford lightning trucks have no quench as the piston is over .100" below deck at TDC. Supra guys have went beyond .200" below deck to lower compression and it works just fantastic. Quench is the cherry on top if you can get it, but it won't ruin the sundae if you don't have it. With the relatively small bore of the VH45 and not a lot of quench area to begin with along with the forced induction (which also helps negate the need for quench) I think thicker head gaskets, full dish pistons, shorter crank stroke, shorter pistons... are all great ideas.

So, where do I get a set of 8 stock VQ30DET pistons for a good price? Used in great shape is fine but new preferred.
Modified by ScottJackson at 9:13 PM 8/10/2007

Sorry, but I disagree. That is misinformation, bad advice, and is not true.I have a 5.4 shortblock from a lightning in my garage right now at this moment, and I just went out and measured and the pistons are between .008 and .010" below deck. This is a picture I just took of my 5.4 shortblock. I have built alot of modular engines, and none of them have that kind of quench height. There are only 3 sizes of pistons that came in modulars (4.6 and 5.4 use the same piston, along with using the same heads).. flat top, ~-5cc dish, and ~-11cc dish.Also, there's no way a 2jz can run anything better than crap with the pistons almost a quarter inch in the hole. That's about a 4-5 point drop in compression even with a flat top piston, there's no way people are running 2jz's with a 6:1 compression ratio. You couldn't even get that motor to start with that low of compression. That's about like running a 327 crank in a 350 chevy, and using the 350 rods and pistons.. lol

No factory motors have pistons any more than .020" in the hole, and this is coming from someone who has torn down and rebuilt alooooooot of motors over the years. I've built enough old and new motors to know that quench is everything on a performance motor, and if you lose it, you will Definately notice. I've had motors of my own where you could tell certain cylinders were detonating after teardown/inspection, and after measuring, those cylinders had pistons with excessive quench which can come from crank deflection or crank twist. Here is a picture I just took of what 3/8 of the pistons looked like in the motor I just mentioned. Quench has everything to do with you not detonating.The best way to lower compression is with dished pistons or enlarging combustion chambers, anything else is a bandaid.


Modified by defrag010 at 10:17 PM 8/10/2007

Hmmm, where I read that the lightning motors had pistons so far down was on the turbomustangs forum (I'm pretty sure) and those guys generally know their stuff. I don't think they were talking about the 5.8 windsor engines, but maybe. A 6:1 engine will start, as most any flathead or Briggs & Stratton owner can verify. And quench is nice but I still say it's not needed. If it were, all the Chrysler hemis would have burnt up their pistons like that picture you linked.

I'm going to try not to be abrasive, but in the kindest way possible I seriously urge you not to spread anymore information that you read on forums from people who "usually know their stuff" as your own. The VH section of nico doesn't need any parrots.Sure, lack of quench area was a drawback to the old hemi, but don't get quench area and quench height mixed up. The new hemis have radiused chambers and dome pistons, so there is a "3d" quench area that extends all the way around the piston. They have alot more effective quench area than pretty much any modern engine where the only quench area is between the deck of the head and the flat part of a piston. That's why hemi's have been dominating in professional racing... if they used wedge heads, they wouldn't be ahead. You don't need quench area to run, but you do need quench height to promote turbulence. With pistons .200" in the hole, you will have practically no turbulence at all, which leads to horribly inefficient combustion.

Again, this isn't flaming, but please refrain from making statements about quench when you don't know how it affects combustion.

I always thought there was a difference between quench and tumble (turbulence). Just as there's a difference between pre ignition and detonation. I thought quench only starts to occur around .060" from piston top to head surface and closer. I thought quench was where the air and fuel is forced out into the open part of the chamber very quickly and the flame travel doesn't get into that area until the piston starts going back down the bore. I thought tumble was when the air and fuel gets pushed from one area into another yet the area wasn't tight enough to keep the flame out of it. How is the new hemi even a hemi if it has quench pads? I thought hemi meant a hemispherical combustion chamber. And hemi heads don't make big power because they have a lot of effective quench area, it's because their canted valves allow huge valve sizes in the bore and a very straight shot from the runner to back of the valve. And the 2 valve hemis have dominated lots of U.S. drag racing because rules don't allow 4 valve heads and even top fuel dragsters are very rule restricted. There are also what are known as "semi-hemi" engines like the 351C that have canted valves but a typical quench pad of a wedge head. Of course the cleveland was dominant when it first came out and rule changes against it quickly ended that.

Say, was that second picture of the bad piston a KB hyper? It looks like one to me. I agree that when you're running decent compression for the fuel you're using, quench goes a long ways toward "quench"ing detonation. I've only built a few small block chevies, 2 ford 302s, 2 351W, 3 351Cs, and one 514. I am certainly a beginner and it appears I was wrong about the lightning engines. I must have either mis-read or mis-remembered something. I haven't done but one back-to-back test with quench and it did seem tomake a noticeable difference. It was a basic 350 chevy with 6" rods, manley forged flat tops, pocket ported Vortec heads, .530" lift hyd roller cam... yadda yadda. I went from .055-.060" quench to .030" (pistons out the hole .010 with .041" gasket). It did seem to maybe give it a tiny bit more low rpm power, but that was it. EVERYTHING else stayed exactly the same except for new gaskets and bearings. I used the old pistons and rings even. I'm not sure if the throttle response at low revs was improved from the tighter quench or the slight bump in compression. Granted, that's not the same as going from .200" to .030", but I'm still not convinced that in a forced induction engine with less than 4.5" bore size, it's that big of a deal when most of us are just wanting to make the most power for the least $$$. Of course I would take a 8.5:1 VH45 with custom dished pistons that give me quench over one with a piston with no quench and the same compression, but for the $800+ price difference, I'll deal with the small hit in low rpm torque and couple psi less max boost I can run without detonation. I'd also rather have an 8.5:1 quenchless VH45 with boost than a stock 10:1 VH45 with quench and boost. If you've got a good quench distance, I don't think it's worth giving that up for half a point of compression. But for 1.5+ pts of compression drop from 10:1, sure.
Modified by ScottJackson at 12:22 AM 8/12/2007

ScottJackson wrote:
Say, was that second picture of the bad piston a KB hyper? It looks like one to me. I agree that when you're running decent compression for the fuel you're using, quench goes a long ways toward "quench"ing detonation. I've only built a few small block chevies, 2 ford 302s, 2 351W, 3 351Cs, and one 514. I am certainly a beginner and it appears I was wrong about the lightning engines. I must have either mis-read or mis-remembered something. I haven't done but one back-to-back test with quench and it did seem tomake a noticeable difference. It was a basic 350 chevy with 6" rods, manley forged flat tops, pocket ported Vortec heads, .530" lift hyd roller cam... yadda yadda. I went from .055-.060" quench to .030" (pistons out the hole .010 with .041" gasket). It did seem to maybe give it a tiny bit more low rpm power, but that was it. EVERYTHING else stayed exactly the same except for new gaskets and bearings. I used the old pistons and rings even. I'm not sure if the throttle response at low revs was improved from the tighter quench or the slight bump in compression. Granted, that's not the same as going from .200" to .030", but I'm still not convinced that in a forced induction engine with less than 4.5" bore size, it's that big of a deal when most of us are just wanting to make the most power for the least $$$. Of course I would take a 8.5:1 VH45 with custom dished pistons that give me quench over one with a piston with no quench and the same compression, but for the $800+ price difference, I'll deal with the small hit in low rpm torque and couple psi less max boost I can run without detonation. I'd also rather have an 8.5:1 quenchless VH45 with boost than a stock 10:1 VH45 with quench and boost. If you've got a good quench distance, I don't think it's worth giving that up for half a point of compression. But for 1.5+ pts of compression drop from 10:1, sure.

the piston in the picture is actually a probe forged piston that came out of a 4.6 dohc lincoln motor. I hate kb hypers with a passion.. lolYou are right, you won't notice a small difference in quench height like that when you are just making average power on a smallblock chevy, but whenever you are getting the most you can out of an engine with forced induction and your tuning is at the limit, the motor with more quench area and a smaller quench height (without banging the piston against the head) will be the one that won't give you any problems. A 400hp vortec head smallblock chevy, you won't notice... but on a 600+whp kenny belle supercharged 4.6 ford, it can mean the difference between being able to take the abuse or destroying pistons (picture). Forced induction adds a whole new variable to everything. Have any of those engines you built ran high psi at a high power level from a turbo or blower?That's all just pretty much rambling anyways, the main point of my posts have been that running a piston a quarter of an inch down in the hole is bad.

Well I agree with you then. I would much rather have good quench and as much as possible than to have little or none. I agree that running the piston a quarter inch below deck is bad compared to buying a custom piston with the proper size dish that has quench surfaces. However, in the interest of dropping compression and opening up more volume to cram in more air/fuel on a budget, I'm still not convinced it's a terrible thing. Nope, I've got some turbos and some schemed up projects, but have yet to actually put them on anything. I've just been reading and reading a bunch about it. I thought quench was LESS of a benefit on a forced induction engine than a N/A one. Although, a little detonation on a NA engine probably won't kill the pistons while a bunch of boost with a little detonation very likely will. So yeah, I guess if you're pushing the limits as far as boost with the fuel/compression/air temp/timing, then keeping the quench is the only way to go. However, if I wanted to put about 12psi boost on a VH45 with pump gas and I didn't have the $$$ for custom pistons, I think it would be better to have 8-8.5:1 compression and no quench compared to 10:1 with stock quench. Is this not right?