Low Compression Piston Solutions

[slownslurious]

stock rods are plenty stout for what I have planned, the stock pistons look like they will detonate if I look at them wrong.. too high of compression ratio, horrible hot spots all over the top.

[defrag010]

What kind of power and rpm are you looking for?

[slownslurious]

looking more for torque than power thats why I'm swapping in the v8..it will be blown with an eaton M112 spinning up around 12 psi of boost hoping to get at least 450 hp and about the same in torque. Has to run on 93 octane pump gas as well.

My vh45de block needs rebuilt anyway the compression is low all around.

I've got an adaptor plate for the Z32 TT trans, an os giken triple disc clutch, an expensive water to air intercooling setup between the blower and the block, and I'm planning on using mega squirt for ECU duties.

If I rebuilt with stock pistons and compression ratio and could control heat and detonation it would make a ton of power, but I just dont think it would be reliable for long trips on 93 octane. Its not like a turbo where I can turn down the boost, unless I swap pullies.

[defrag010]

at that power level, flat top pistons will be the least of your worries. Flat tops are some of the best kind of piston as far as resisting detonation, as hotspots only form on surface risers (casting roughness, scratches, bumps, etc). If you're worried about detonation, then stack your headgaskets or get some thicker ones made and polish the chambers on your heads. Getting your chambers enlarged and cc'd will also lower the comp ratio.I would just get some new VH45 pistons, cause it won't be any less prone to detonation if you just swap out stock flat tops for some dished pistons with less quench area that sit down further in the bore.

[slownslurious]

hot spots form around valve reliefs as well especially on a flat top piston. Lowering static compression lowers the chance of detonation if all other factors are equal. less compression means the gas is less likely to ignite due to the heat of compression until the spark plug tells it do. Less preignition, less detonation, less engine damage.

[slownslurious]

and if the pistons should be the least of my worries what should I be worrying about?

[John Dixon]

If you need custom, thicker head gaskets, http://www.ferriday.co.uk/ did mine. He'll still have the specs for the stock nissan gaskets I gave him and can do a few different thicknesses and any bore you want.

[defrag010]

and if the pistons should be the least of my worries what should I be worrying about?

Nothing.. bumping static compression down a point when you are still going to be running 12 psi at 450 horsepower isn't going to do much for detonation resistance. There are plenty of other things you can do to cure detonation like water injection, colder plugs, a good ecu tune, or polishing your combustion chambers. If you want to think flat top pistons are the worst for hotspots and that's the only thing you can do to prevent knock - go ahead, its clear that nothing is gonna make you think otherwise... but the only time a flat top piston is going to form hotspots on it is when there are carbon deposits that act as surface risers. The motor that comes with the blower you're using has the same design piston, and it has no trouble at power levels even more than what you're going to be running. If you want to make a mutt motor out of some other OEM pistons and get the same results, then be my guest..

[SuperHatch]

Just wanted to let everyone know...

I spoke with Chris at Ross Pistons. A Set of 8 custom pistons for the VH45 with wrist pins and rings is $1150USD

Also, if you wanted to try it, they would sell you 8 of the 6g74 pistons, so either option is available.

Chris obviously recommended the custom pistons over trying to make something else work.

[SuperHatch]

Question...

Can anyone measure the skirt to counterweight clearance at BDC between the pistons and the crank?

[SuperHatch]

block

John,

That picture of your block, is that before or after it was bored to 98mm?

- Steve

[Mettler]

Lets just clear something up before it causes further confusion.

When talking about HP, the only logical HP figure to mention is flywheel HP.

Why?

Drivetrain losses are different for every single vehicle... so to discuss what resultant wheel HP you'll get with XXX mods is retarded.

This brings me to my next point... 450HP with a supercharger... why? You can get 450NA HP with a good set of cams, exhaust, intake, and a mint ECU tune.

The supercharger may provide an increase in volumetric efficiency, but a definite decrease in reliability due to the hot air being compressed into the engine.

Not worth it for the ****ing around you have to do to make a supercharger setup work.

[sijoko]

John,

That picture of your block, is that before or after it was bored to 98mm?

- Steve

Those have to be aftermarket sleeves, since you cannot see the stock cast-in liners from above.

What I would like to know is: "How much metal is behind the liners?" Can the VH accept a dry sleeve setup that can support a 4.00 inch bore?

[John Dixon]

Yes, it's after. I think mirage took some VHs out to 4.0" with no problems.

[defrag010]

Sijoko, I cut up an old junk VH block at work on the bore bar, and there isn't enough meat to do a 4" bore dry sleeve. You can do a wet sleeve 4" bore setup, though, using 5/32" wall sleeves. All you would need to do is leave a small reciever lip at the bottom of the stock bore for the sleeve to sit in.. and then fill the bottom half inch or so of the block with blockrete along with flipping the block over and filling the bottom of the deck about 1/2" with blockrete. This will seal it, and also strengthen the block's deck and webbing area where the sleeve sits.

[slownslurious]

for the record:1) M112 blower was removed from a car with dished pistons. The northstar normally has flat top but they lowered the compression using dished pistons for the -V variant because of the supercharger. 2) lowering of the static comression ratio is normally considered necessary when adding a roots style blower because adding a supercharger effectively increases the compression ratio because it is compressing air into the cylinder before a naturally aspirated cylinder would begin its compression.3) a 450 hp NA v8 would require more money than a supercharged one. The stock valve train won't support the engine speed required to acheive those kind of figures. You are looking at expensive custom ground cams, expensive forged high compression pistons, expensive forged rods, and expensive crank work (knife edging, chamfering) etc. In addition you will probably end up needing a standalone to tune an engine that close to the ragged edge. a v-8 it is, true, but at its meager 4.5 liters of displacement, a streetable 450 hp naturally aspirated engine on pump gas is not possible. And thats besides the fact that it would suck down low, which is what I'm building this engine for. If I wanted a screamer with no torque I would have left the ca18det and T04e in there. Been there done that, it sucked, I need more torque. Hence the v8 + eaton blower.For the power goals I'm shooting for I am looking for a lower compression piston.

You can argue with me about the purpose or ability in my project in another thread if you wish to do so but please limit responses in this thread to those helping towards finding a lower compression piston solution. If none is found soon the engine will be reassembled with factory pistons. And then we will test thier durability.

[Mettler]

3) a 450 hp NA v8 would require more money than a supercharged one. The stock valve train won't support the engine speed required to acheive those kind of figures. You are looking at expensive custom ground cams, expensive forged high compression pistons, expensive forged rods, and expensive crank work (knife edging, chamfering) etc. In addition you will probably end up needing a standalone to tune an engine that close to the ragged edge. a v-8 it is, true, but at its meager 4.5 liters of displacement, a streetable 450 hp naturally aspirated engine on pump gas is not possible. And thats besides the fact that it would suck down low, which is what I'm building this engine for. If I wanted a screamer with no torque I would have left the ca18det and T04e in there. Been there done that, it sucked, I need more torque. Hence the v8 + eaton blower.

Ok this is horse****, because I've been talking to a VH engine builder for the last couple of months about how to go about it. I'm getting stock VH45 internals, a cam grind + shims for NZ$440+gst (that's around 250-300 bucks US), and GTIR inner valvesprings. Combined with my custom headers and freed up intake, plus tuned & chipped ECU, 450HP is possible.

You don't need forged pistons, forged rods (which the factory ones are anyway), expensive crank work etc for a safe 8000rpm.

You don't need a standalone, I've said it over and over and I'll say it again, the factory computer can be completely remapped as much as an aftermarket computer, it's additional (useless) functions like launch control etc that an aftermarket computer grants, stuff that you don't need on a 5 speed sports/thrash car.

As for the low down torque thing, yeah you have a semi valid point but you exaggerate a bit much... stop making sweeping assumptions. A highly efficient, high compression, modified & tuned quad cam 4.5L V8 is not going to have no power down low. Have you also forgotten that it has VTC ?

You're trying to say a motor like this won't be streetable... hell at least I'll have a throttle pedal and not a 'burnout button', it'll be driveable with a very controllable power & torque curve.

[elwesso]

Great post mettler!!!

I think that youd be safe to say 400HP is going to be reasonable with the stock short block, with just modified cams and exhaust.... and of course a reasonable tune... On stock compression and pump gas??

Mettler, do you not think the stock plenum is capable of handling 400HP?? you think youd need to go with something like john's? maybe not ITB but something else?

[Mettler]

Hmmm what makes you mention the plenum as being a potential shortcoming of the setup ? I thought the intake manifold was a nice one with long tuned length runners. It is true that the air has to curve a bit, it's not just a straight shot into the ports, but it's not bad either.

I wonder whether it'd be worthwhile investigating the flow characteristics of the manifold to see whether any part of it acts as a bottleneck. I certainly don't plan to change it, when I say freeing up the intake I just mean removing the 50kgs or so of plastic boxes & expansion chambers and running a simple straight intake pipe from the filter.

[elwesso]

Ok, I was hoping you were referring to an "intake" as everything before the TB...

I think whats important is that after the injectors it is pretty much a straight shot... I think that really there was any part that was a bottleneck youd have to make an entirely new intake... unless you somehow turned the plenum 90* so that the TB was on the front and made new runners... Still i dont think itd be worht the work. The stock intake should be good to 500HP with no big restriction...

Id really like to make a really nice intake systme using the stock air filter.... Paper > oil for filtering!

As soon as I get me some heads im going to be sending them to defrag or getting his recommendations on what to do.... I think that a 400-450HP N/A VH would be nasty sexy... Plus, chances are that that setup would work pretty good with boost... if thats what you had to do!

[slownslurious]

with the modifications you have listed I would wager that you will never see 450 hp at the wheels. I measure hp at the wheels because it is the only reliable way to measure HP. Most of us will never use an engine dyno. I need an engine that has the torque to kick the rear wheels out, a low enough redline that I can bounce it off the redline for extended periods of time, and reliable enough that it will not need rebuilt after being abused.For my needs my plans will suffice.

Now back to this piston thing...

[Mettler]

Ok, well now that we know you're talking about power at the wheels, I'm not going to bother discussing it with you again. A slipping torque converter, different diff ratio, different gearbox ratio ALL affect the power & torque at the wheels figure. I'm talking about getting 450HP at the crank.

[elwesso]

you really cant objectively measure WHP due to that every dyno is a bit different. When tuning its most important to use the same dyno every time!

[sijoko]

Sijoko, I cut up an old junk VH block at work on the bore bar, and there isn't enough meat to do a 4" bore dry sleeve. You can do a wet sleeve 4" bore setup, though, using 5/32" wall sleeves. All you would need to do is leave a small reciever lip at the bottom of the stock bore for the sleeve to sit in.. and then fill the bottom half inch or so of the block with blockrete along with flipping the block over and filling the bottom of the deck about 1/2" with blockrete. This will seal it, and also strengthen the block's deck and webbing area where the sleeve sits.

How much more will it cost to do the wet sleeves vs. the dry sleeves? I am seeing a price of $80-100 for each dry sleeve. So that means at least $640 to enlarge bore with the dry sleeves. Then, we need to add the cost of pistons, say at the minimum of $850 for a set. It seems that a large displacement VH is going to be pricey.

[Ezekial]

leave the pistons as is i say

pull the heads off, clean up the combustion chambers, unshroud the valves, bla bla ... drop it to 10.0:1 ... 68 ABDC closing is LATE ... dynamic comp is low as because of it ... set of NGK 8's and 18psi

its a good theory right?

[SuperHatch]

Question...

Can anyone measure the skirt to counterweight clearance at BDC between the pistons and the crank?

This might have been missed on page 1, so I'm just bumping it...

[slownslurious]

one that will be tested soon enough. I dunno about the 18 psi though.

[Clifton]

Has anyone tried JE. I know someone that had a set of 6 made to his specs and it wasn't anymore than others on the shelf pistons. Maybe a group buy if there was enough people?

To help stir up the pot. To get 450 crank out of a 4.5l will cost much more (cams, head work, pistons) than adding 10 psi to a stock 4.5 and easliy getting 450 HP plus a HUGE bump in torque across the board that an n/a 4.5 couldn't hope to ever see. The area under the curve is what matters. RWHP is fairly accurate per dyno brand and it is what is actually getting to the pavement. Seen alot of stock cars dyno and the hp is usually pretty close to 18%(typical drivetrain loss) of what the factory claims. As far as turbo or supercharger and engine reliablity, that can be argued for days. Is the said engine really going to be expected to live a long 200,000 mile life anyway? Not that a supercharger one couldn't. I'm running 27 psi on a 3.0 (stock short block), not sometimes but all the time. No reliabilty issues yet after 2 years and has more torque than any n/a motor under 6l.

[slownslurious]

w3rdlolthats my whole thing, I'd like lowered compression for the increased resistance to detonation (lowering static compression will increase resistance to detonation regardless of other claims) but other than that the stock motor should be strong enough. the eaton superchargers are cheap and reliable... as proven on the lightning, the 03-04 cobra, the cts-v and the new jaguars as well (and thats just the same 112 unit as I'm using). OEM's are turning to boost because its better. more driveable, better packaging, etc. oh well. I guess to each their own.

I think a good thing for someone to do would be to call all the major manufacturers of pistons and tell them what you need with approx. measurements and get a price on them... if you were really motivated you could get a price for 8, and ask how many you'd have to get to get a price drop. If they say 32, then you know a 4 person group buy is gonna get us the price drop. This week is probably a bad time to try it though most places are gonna be closed.

[defrag010]

slow, I haven't been trying to argue with you at all in this thraed - just point you in the right direction. I'm just saying that you can lower the compression ratio effectively without having to mess with hodgepodge pistons and that flat tops aren't detonation prone.

here is a 4.6 dohc cobra shortblock.. and you know what kind of power those can make!!! look at the pistons

Seriously, john dixon linked to a place that will make you thicker headgaskets. According to the compression ratio calculator, if you get some gaskets from john's source that are .050" compressed thickness, and get whoever has your motor now to enlarge/shroud your combustion chambers (and polish them) out to 60cc... you will have about 8.8:1 compression.

I'm only suggesting this because I have worked in a machine shop for years and built looooooooooooooots of engines and have seen what works and what doesn't..