3900 Info

[mbirt]

Ive measured the OEM headgasket at .065" from my LZ9, since its already been installed and run for 24K miles, Im thinking that it should be compressed right around .060'?. Also been looking at piston/rod combos and was looking at the wiseco LS1 k449x3903 piston. With a 6.125" rod and the wiseco piston CH of 1.050", the piston will be proud of the deck at least .015" depending on if the block needs to be squared up. The quench will be right around .045".

Im not sure why the piston sits down in the bore that far, I measured about .040" as well. Whats the reason for so much piston-to-head clearance?

The piston sits so far down the hole in the LZ9 because it shares a common compression ratio, piston, cylinder head, and head gasket with the LZ4 despite the increased stroke. The extra TDC volume to maintain a constant compression ratio despite increased displacement thus comes from the connecting rod that GM had to change for the stroke change anyway--good thinking for minimizing part numbers on the part of GM bean counters. They publicly justified the huge piston to head clearance as something called "mist quench", but the only real engine engineering justification I see for it is reduction in crevice volume (quench area) for a reduction in unburned hydrocarbon emissions. But the real reason is to keep as many parts common between the two engines as possible.

I don't doubt that you measured your HG at .065", but .053" is the HG thickness I settled on after reading other threads like this one and a post by a Scoggin Dickey employee in one of them that voted for either .052 or .053. Here are my CR calculations for the two engines. A stock flat top LS1 piston with a 1.330" compression height will put the CR at 14.5:1 in an LZ9.

All units mm	LZ4	LZ9
Bore	99	99	(GM)
Stroke	76.0	84	(GM)
Comp height	32.89	32.89	(Silvolite)
Piston dish cc	18	18	(Silvolite)
Block deck height	224	224	(GM)
Rod length	153	148	(Pauter LZ9)
Down the hole	0.107	1.11	calc
Head gasket	1.3462	1.3462	.053(forum avg)
Quench height	1.453	2.45	calc
Chamber CC	36	36	forum avg
TDC vol CC	65.19	72.88	calc
Disp CC	585.03	646.61	calc
CR	9.97	9.87	calc

[34blazer]

Ive measured the OEM headgasket at .065" from my LZ9, since its already been installed and run for 24K miles, Im thinking that it should be compressed right around .060'?. Also been looking at piston/rod combos and was looking at the wiseco LS1 k449x3903 piston. With a 6.125" rod and the wiseco piston CH of 1.050", the piston will be proud of the deck at least .015" depending on if the block needs to be squared up. The quench will be right around .045".

Im not sure why the piston sits down in the bore that far, I measured about .040" as well. Whats the reason for so much piston-to-head clearance?

[mbirt]

Cam swaps on a 5.3
http://www.hotrod.com/how-to/engine/...st-comparison/

This dyno might read a smidge high, but the gains are real. 5.3 is close to displacement per cylinder as well as hp per cylinder as the 3.9. The 5.3 stock heads flow just a smidge less than the 3.9 heads.

If you look at that link, gains of 100 hp from a cam swap is possible. Break it down to additional hp per cylinder, and you can see that the 3.9 should be able to theoretically make about 310-330 crank hp with a great tune, with just a cam swap. Realistically its prob closer to 290-300 cam only

This is exactly what I've spent the last several months thinking about--squeezing LS "cam only" gains out of the LZ9 which, on paper, looks like a 3/4 LS engine. I want to build this theoretical 315 hp "cam only" 3.9, pair it to an F23, put it in my 1986 MR2, and compete at the GRM $2017 challenge.

I have GM's SAE paper on the development of the LS1 ASA racing engine in front of me. The LS1 in this configuration made 430-450 hp and I see it as clear recipe for achieving 300-315 3900 here:

A custom (no VVT) 60v6 cam similar to the following:
226/236 int/exh @ 0.05" lift, 0.525" / 0.525" lift, 110 LSA
LS3 valve springs with requisite increases in install height (eliminate spring base cup and use PAC +.050" locks)
1.75 for 11" stepped to 1.875 for 25" primary long tube headers
~10.5" intake runner length to match the LS intakes and no UIM restriction (I'm looking at you, LZ9 ramhorn)

I have a plan to increase compression ratio (to 10.8:1) and avoid longer pushrods with a reduction in cam base circle, but I'm not going to suggest that everyone removes the center shim of their head gaskets.

LZ9 Head
Chamber volume: 36cc

Lift_________[0.100]_[0.200]_[0.300]_[0.400]_[0.500]_[0.600]_

stock(int)____62.4___128.6___186.8__218.9__232.1__ 238.5_

stock(exh)____52.6___100.0__138.2__150.1__154.7__1 55.8_

Intake Valve Diameter: 1.87"
Exhaust Valve Diameter: 1.52"

LM7 5.3 Head

Chamber	61.15 cc	0.100	0.200	0.300	0.400	0.500	0.550	0.600
Intake	200 cc	63	128	179	210	218	221	226
Exhaust	70 cc	54	93	121	145	163	168	174
Valve Diameter Intake					1.89"
Valve Diameter Exhaust					1.55"

LS1 Head

Chamber	66.67 cc	0.100	0.200	0.300	0.400	0.500	0.550	0.600
Intake	200 cc	67	122	178	215	219	223	227
Exhaust	70 cc	52	97	133	156	170	176	180
Valve Diameter Intake					2.00"
Valve Diameter Exhaust					1.55"

http://www.smokemup.com/tech/ls1.php

Things working for the 3900:
-Forged crank and generally more oversquare than LS engines
-Shorter valves and pushrods for reduced valvetrain mass (6400 rpm stock fuel shutoff, LS3/525 crate engine has 6600 limiter, so 7000 rpm rev limiter may be possible in LZ4/LZ9 with LS3 springs)
-Better intake port flow than 5.3 and LS1
-Even firing order per bank for better exhaust collector scavenging effect
-Even firing order per bank impact on intake plenum design
-Low cost, light weight, GM Metric bellhousing pattern

Things working against the 3900:
-The lower intake manifold is an unknown to me as a 60v6 outsider and it's the only thing I feel like I don't (cheaply) have control over in this build
-The assembled camshaft being difficult to convince shops to do a regrind--anyone know any cam grinders friendly to hollow camshafts?
-"Mist quench"... With such a small combustion chamber and a common piston design with the 3500, GM had to put the 3900 piston .044" down the hole at TDC, or .097" piston to head including the .053 head gasket
-The cylinder offset with each bank in a different thrust direction. Do I need to care about this for anything other than piston design?

I think the good outweighs the bad and I think I'll make close to 300 whp with less than $500 in the engine build. What am I missing here?

it is wide but if you have looked at the cams coming from GM factory lately, the majority of LS cams have wide LSAs The zr1, zl1, L99, ls9 for example. Even the ls1, 2 and 3 are all over 115 lsa

Prob has something to do with emissions and a smooth idle.

Cam swaps on a 5.3

http://www.hotrod.com/how-to/engine/1307-ls-cam-test-comparison/

This dyno might read a smidge high, but the gains are real. 5.3 is close to displacement per cylinder as well as hp per cylinder as the 3.9. The 5.3 stock heads flow just a smidge less than the 3.9 heads.

If you look at that link, gains of 100 hp from a cam swap is possible. Break it down to additional hp per cylinder, and you can see that the 3.9 should be able to theoretically make about 310-330 crank hp with a great tune, with just a cam swap. Realistically its prob closer to 290-300 cam only

Joshua R on this board made 238 whp with a small cam swap, 75mm tb and headers through an auto trans. His cam only had 214 duration on the exhaust. With 15% drivetrain loss thats about 280 crank and with an 18% loss its about 290 crank hp.

huge potential with this engine.

That's pretty wide. Makes a strong argument for the cam to be constantly adjusted between advanced and retarded as the engine is running and not just changed as the RPM changes.

Exh centerline is right around 116.5 so LSA should be right about 119ish or so on my cam. CNC measured 120 or so, so I think my measurement is about right if GM kept things the same.

What's the separation between I and E?

Another update for those following this. I got out my digital indicator, made a degree wheel and checked the valve events with the actuator in the default position (total advance). I plan to do some more measurements as something seems very strange to me. I checked all the numbers 3 times each and got consistent readings.

The valve timing checked at .050 lift is as follows:
Intake Open: 1 ATDC
Intake Close: 30 ABDC
Intake Duration: 209
Exhaust Open: 59 BBDC
Exhaust Close: 29 BTDC
Exhaust Duration: 210

The duration looks pretty good but what seems strange to me is what is what I calculate the LSA at. By comparing the opening positions of the valves, the LSA calculates to 120 degrees. I know that it isn't the most acurate way to do it but using the Crane Cam spec cards, the numbers usually come out within a degree or two. I did not seen any cams with this much LSA on Cranes site.

I am going to try another method of measuring that I found and see if it comes out differently. It seems as though this would create an issue when it comes to regrinding the cam. Thoughts?

Ressurecting really old thread. I just did some measurements from a 2009 3.9 LGD engine. This differs from an lz9 as it was designed for flex fuel if wanted. This might make a difference in cam specs but not sure.

I also checked measurements with cam in default advanced position.

Measurements were made on the lifter body not the plunger.
Duration at .050.
Int 194
Exh 195

Lift
Int .272
Exh .286

Cam events @ .050
Int opens 12 ATDC
Int closes 26 ABDC

Intake Centerline checked in at 123

Exh opens 38 BBDC
Exh closes 23 BTDC

A genius it does not take to see this is a peanut for a cam. Gm did lower the HP output I believe of this engine in the later years by like 8 hp or so, which could be because of the cam?

[ericjon262]

I've been following this thread for a while now. I'm very interested how it turned out because I'm thinking of putting the same engine in a sunbeam. With your shortened timing cover, what's the length x width x height of this engine? Sorry if I'm posting in the wrong place. Thanks.
Robbie

is that another vote for stickie?

[socorob]

I've been following this thread for a while now. I'm very interested how it turned out because I'm thinking of putting the same engine in a sunbeam. With your shortened timing cover, what's the length x width x height of this engine? Sorry if I'm posting in the wrong place. Thanks.
Robbie

[CNCguy]

Regretfully, no. Ran out of time and money and needed to get the MG running. I do plan to dyno it whenever I get around to finishing the tune. I will be disappointed if it doesn't hit at least 225whp. At the moment the car is sitting with a busted rearend.

[ericjon262]

Did the engine in post #175 make it to the dyno?
I'm curious as to how well it did.

Ken

[Kenova]

Did the engine in post #175 make it to the dyno?
I'm curious as to how well it did.

Ken

[CNCguy]

I have been wanting to replace the pictures but my problem is a lack of time. I will try to squeeze some time in for this.

[zinX11]

got to agree, would lots of good info and pictures... should be easier to find.

Mike