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Cylinder head Intake port size requirement for LS 427

Junker

Junker

Well-known member
Currently researching and planning a LS 427 build, I have the block and rotating assembly sourced and now researching cylinder heads.
Expecting to turn 6k RPM.

As a base number I believe the stock truck 6.0L lQ9 intake port is 210 cc
Most aftermarket aluminum 7.0L heads have the large square intake ports are anywhere from 255 cc to the sky is the limit with casting and porting.

I have concerns that most of the aftermarket heads are geared toward building the most HP at higher RPMS vs torque at a lower RPM range.
I have to wonder if a lower cc head with higher velocity to atomize the fuel would be more beneficial vs rolling a hot dog down a hallway.

I do realize comparing intake ports for a 6.0L to the requirements for a 7.0L will vary but do not want to overflow the heads to a point to were there are no sufficient gains in the lower RPM range.

As with cams, bigger is not allows better.

So I reckon’ what I’m asking is what cc intake port would be suggested for a LS 427.

Or am I overthinking this …..?
 
Junker said:
Currently researching and planning a LS 427 build, I have the block and rotating assembly sourced and now researching cylinder heads.
Expecting to turn 6k RPM.

As a base number I believe the stock truck 6.0L lQ9 intake port is 210 cc
Most aftermarket aluminum 7.0L heads have the large square intake ports are anywhere from 255 cc to the sky is the limit with casting and porting.

I have concerns that most of the aftermarket heads are geared toward building the most HP at higher RPMS vs torque at a lower RPM range.
I have to wonder if a lower cc head with higher velocity to atomize the fuel would be more beneficial vs rolling a hot dog down a hallway.

I do realize comparing intake ports for a 6.0L to the requirements for a 7.0L will vary but do not want to overflow the heads to a point to were there are no sufficient gains in the lower RPM range.

As with cams, bigger is not allows better.

So I reckon’ what I’m asking is what cc intake port would be suggested for a LS 427.

Or am I overthinking this …..?
Click to expand...

Is the 427 an LS3 or LS7?
 
Its a New Dart LS NEXT steel block, standard deck, 4.125 Siamese bore, clearance for a 4.100 stroke, sleeves extend down from the holes 0.375.

So yeah I guess it would be considered a LS7 hybrid.

Compstar 4" Crank and 6.125" rods
12 degree Diamond pistons - dish TBD once heads are sourced
 
Junker said:
Its a New Dart LS NEXT steel block, standard deck, 4.125 Siamese bore, clearance for a 4.100 stroke, sleeves extend down from the holes 0.375.

So yeah I guess it would be considered a LS7 hybrid.

Compstar 4" Crank and 6.125" rods
12 degree Diamond pistons - dish TBD once heads are sourced
Click to expand...

I would go with some good LS7 heads on that motor. HSP is what determines how much prop you turn. Not torque. I doubt you will give up much low end snap with good aftermarket LS7 heads. Also the 12 degree LS7 heads will work well with the 12 degree pistons you have.
 
Thanks for the input CMB...…
So back to the original question ….

I believe the stock LS7 head intake runners are 260cc with a 70cc combustion chamber resulting in a 11:1 compression ratio.
What if any should I deviate from that may result in over all performance.

Will be running a carbureted set up to begin with, possibly EFI later.
 
Junker said:
Thanks for the input CMB...…
So back to the original question ….

I believe the stock LS7 head intake runners are 260cc with a 70cc combustion chamber resulting in a 11:1 compression ratio.
What if any should I deviate from that may result in over all performance.

Will be running a carbureted set up to begin with, possibly EFI later.
Click to expand...

260cc on the intake runners sounds pretty good. You could also go to a 265cc or 285cc. I just googled PRC LS7 265cc and 285cc heads. Their flow numbers beat stock LS7 heads even at lower lift. So I don't think you will lose any bottom end snap. Lots of after market heads to choose from. I would let your wallet be your guide. LOL

What static compression are you shooting for? What type of fuel? Any idea about potential cam specs? All of this stuff is related and results in the dynamic compression for the motor. Also it is always a good idea to call a cam company and see what recommendations they have.
 
My Father in Law has been building conventional SBC for strip and dirt track for decades and is pretty notable in East Texas thru Tennessee area. Last year brought him down for his first airboat experience and he got to experience first hand the constant loads that we put on our rigs.
He has since became interested in the LS platform thus the current build. I am currently running a healthy conventional 383 with a 2.3 gear and have always wanted a big inch LS, most likely will change gear to a 2.55.
As he is a vendor for Comp, RHS, Dart and a few others it allows me to save in a few areas.

What static compression are you shooting for?
Was thinking in the mid 10's to play it safe. Looking for reliability and longevity thus the 427 vs. 441.
I really want EFI and a 2.9 Whipple but that is just not in the cards at the moment.

What type of fuel?
93 non-ethanol is what I currently run and plan on running moving forward

Any idea about potential cam specs?
All of this stuff is related and results in the dynamic compression for the motor. Also it is always a good idea to call a cam company and see what recommendations they have.
My FIL will be contacting Comp for recommendations, but I will be researching other avenues as well, so I am open to any input and recommendations of whom to speak with

Thanks again for the comments, it all helps with me moving forward.
 
I am curious as to what you guys end up with and the results. I have built sbc unlimited modified and late model dirt motors for years in Texas as well. Reher Morrison always did my head work and I was custom grinding cams from Crower with great results. However, in airboats, this is new learning for me, so please keep us posted.
 
Junker said:
My Father in Law has been building conventional SBC for strip and dirt track for decades and is pretty notable in East Texas thru Tennessee area. Last year brought him down for his first airboat experience and he got to experience first hand the constant loads that we put on our rigs.
He has since became interested in the LS platform thus the current build. I am currently running a healthy conventional 383 with a 2.3 gear and have always wanted a big inch LS, most likely will change gear to a 2.55.
As he is a vendor for Comp, RHS, Dart and a few others it allows me to save in a few areas.

What static compression are you shooting for?
Was thinking in the mid 10's to play it safe. Looking for reliability and longevity thus the 427 vs. 441.
I really want EFI and a 2.9 Whipple but that is just not in the cards at the moment.

What type of fuel?
93 non-ethanol is what I currently run and plan on running moving forward

Any idea about potential cam specs?
All of this stuff is related and results in the dynamic compression for the motor. Also it is always a good idea to call a cam company and see what recommendations they have.
My FIL will be contacting Comp for recommendations, but I will be researching other avenues as well, so I am open to any input and recommendations of whom to speak with

Thanks again for the comments, it all helps with me moving forward.
Click to expand...

What prop are you planning to run with this engine? I will say that if you are worried about engine load and you are planning to turn the motor around 6000 at WOT, then you need to add more gear such as a 2.68. More gear takes load off of the engine and allows the prop to stay in the RPM range where it is most efficient.

My current setup is a 419 LS3 (4 inch stroke) using the Holley HP EFI turning a 4 blade 83.5" R with a 2.88 Ox Box. I turn the motor 5800 to 6200 depending on how much pitch I have in the blades. I do cruise at higher RPMs because of the 2.88, but there is less load on the engine and the fuel usage is really good. I can get over 3 MPG if I stay in the water mostly.

I am also running 11:1 static compression and have no signs of detonation. I run 93 octane, but the 11:1 compression is supposed to be able to run on 91 octane. One of the advantages of an LS over a SBC is that the LS heads are more efficient. You can run more compression. The 403 LS2 that I am building now has 12.5 static compression. I plan to run this on 93 octane also. Might have to play with the timing and possibly the cam duration to accomplish this. 12.5 static compression is on the edge. :shock:
 
I currently run a 80" 3 blade maximus water walker with a 2.37 gear, not so sure the blade profile would be conducive for the LS7, but may try it in a 4 blade configuration.
2250 is the sweet spot / 6000 rpm divide by 2.55 = 2,350

I really like the push of these blades, dozer like quality. But I'm sure there is a better blade profile for the combo I'm building.

Most likely once the engine is built, will build a completely new boat around it and offset the cost by selling my current setup.

Again thanks for the input, looking forward to any comments you may have in regards the build.
 
I think 2200 RPMs is the maximum RPM for that prop. If you plan to turn the motor at 6000 at WOT, you need to use at least a 2.68 ratio. Especially with a 4 blade. A 4 blade SW will put significant load on the motor.
 
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