421safari

Hi,

new here but hoping to tap resident knowledge I have a 2003 GMC Yukon XL 2500 4wd, so heavy!, with a 6.0 LQ4 that needs work. (burnt valves because of LPG conversion). Aside from the lpq conversion the engine is stock. As we're doing the work already I am looking to do some performance mods as well. So I was thinking to swap the original #317 heads with #243 heads and mill them to increase compression to about 11:1. Other planned mods: airaid intake tube, tbss/nnbs intake, camshaft. Exhaust/headers perhaps at a later date.

Using the original values:

4.0" bore
3.622" stroke
6.098" rod length
71.06cc chamber volume
6.7cc piston dish volume
9.24" deck height
0.006-7" piston above deck
(don't know what top ring down is, or if it is relevant here?)
gasket bore 4.01"
gasket compressed thickness 0.051"

and the http://www.diamondracing.net/tools/ calculator I get 59cc for ~11:1 CR. With ~0.006" per cc that's ~0.0327" milling.

Regarding a new camshaft the texas speed stage 2 high lift truck with LSA of 112 seems to be a good candidate (or alternatively stage 2 low lift of stage 1 low lift)

Would this be possible without getting PTV clearance issues?

I am reading that it would be better to mill less and use a thinner gasket for better quench? Using a cometic of 0.027" would leave 0.0087" to mill. Does such a thin gasket have any drawbacks?

Would the intake seal be compromised doing either the mill or the thinner gasket?

tech@WS6store

iTrader: (2)

You dont want too much cr on a heavy vehicle esp if you tow. Youll be fighting fuel qualit and knock alot. Put the 243s on as is with a clean up and run the stg2 high lift and call it a day. Dont worry about a thinner gasket imo but if you do use .040 as a min.

421safari

Note that I don't live in the USA, but in Germany. The normal fuel the engine will run is LPG. LPG is about RON 108-110. Regular gas (RON 95) here is about 90-91US, with premium RON98 (93US) and RON100-102 (94/95US) relatively easily available but more expensive. Fuel quality is fairly consistent here as well as far as I know. Car starts on gas and switches to LPG when warmed up a little, usually within one kilometer.

Maybe should have added that detail to the initial post

Does that alter your opinion?

tech@WS6store

iTrader: (2)

Using RON doesnt give you the actual quality of the fuel whether LP, CNG, or reg Gas. That onlt gives you the possible octane average. Whether that is true for every gallon of fuel that comes out of the pump or not...well thats doubtful. Plus every station is different. Most of the LP conversions i have driven were gutless. Hills were such a problem on them. Then again the old Moline Tractors with LP were great and still are pulling machines, you just cannot expect much rpm from them. Adding some cr with the 243 heads alone would be sufficient. Milling the head down that much drastically effects the flow characteristics of the head as well.
I would keep them as is or milled .005 to .010 max and use either stock gasket or .045 - .040 at most.

421safari

This conversion is a modern injected setup and I have not noticed that much, if any, difference between gas and lpg with it. On lpg it (logically) gets worse mileage, that's about it. But your recommendation is well noted, especially regarding flow characteristics. So if I would go with ~.006 and .040 would PTV clearance be ok?

tech@WS6store

iTrader: (2)

That would be fine yes.

421safari

Ok, that's great to know! Just to have the complete picture/know how much or little margin we're at, at which point (milling/reducing gasket thickness) would PTV clearance start to be a problem?

This would be the plan so far:

63cc #243 heads (http://www.texas-speed.com/p-1391-pr...ted-heads.aspx)
cometic .040 gaskets
tbss/nnbs intake
airaid intake tube
212/218 .600/.600 112LSA camshaft (http://www.texas-speed.com/p-4925-te...-camshaft.aspx)
new pushrods

What would be best for springs? single beehive? duals?

Anything else I shouldn't forget or take the opportunity to do? Timing chain/gears, waterpump, ..? Engine has around 200k miles on it. Or if it ain't broke..?

tech@WS6store

iTrader: (2)

Dont forget the pac1218 springs.
Id do ls2 chain at least and a 10295 melling pump.
The nnbs intake is up to you. Are you cable tb or drive by wire?

421safari

Drive by wire. Using the nnbs would need some minor modifications (fuel rail and relocating a sensor if I remember correctly)

any difference between 1218 vs 1219 springs? Asking because the 1219 are offered on the TSP head.

melling 10295 does not seem too costly, so check.

any recommendations for a timing chain? there seem to be as usual a lót of options.. or best to go with just standard GM LS2?

tech@WS6store

iTrader: (2)

1218 would be fine in your application. The 1219 has a bit more pressure and rate and is rated to .625 lift the 1218 is .600. Ls2 chain is all youd need.

The nnbs intake requires the 90mm tb AND the Dr X/TorqRush tb adapter. Otherwise it wont work. If your orig tb was a 3 bolt that is.

421safari

Yes, thanks, I had forgotten about that. I believe my TB is a 3 bolt 78mm with 8 pin connector. So I would need a new TB (should be available between $150 - $200 as far as I can see) and the adapter.

tech@WS6store

iTrader: (2)

Correct.
The throttle adapter was on backorder not too long ago. Before you order from any place make double sure they are in stock, otherwise youll be stuck with money paid but no part for a while.

421safari

thanks for the heads up! I will keep that in mind.

Regarding the intake tube, for a 2003 airaid lists the 200-912 as the one to use. But if the TB is replaced with a larger one (78 -> 87mm) this probably would not fit anymore? However, the 200-996 listed for the newer intake/TB (2008 model) probably would not fit either? I'll be inquiring with airaid as well, unless someone knows what will fit?