Anyone know how much compression drop there is when you fit 380 heads and cams to a 6g74 .What i have read there is a compression drop when fitting these heads to a 6g74 due to the combustion chambers does anyone know what the difference is cc wise between the 2. I am boosting my engine so a compression drop is desireable just need to know by how much. I have sourced a pair of heads and cams.

Around 7.2-7.4:1. The 380 chamber is about 69cc, 3.5 chamber is 43cc, 3.5 piston dish is 22.5cc. Custom piston would solve the problem

Cheers Dave

Using the specs
Bore 93mm
Stroke 85.8mm
Piston Dish 22.5cc
Combustion Chamber 69cc
I come to 7.1

I do not have the specs for the Head Gasket bore size, compressed head gasket thickness or the Deck Clearance so there will be quite a large variable there however it is still a long shot from the std 3.5

Using the specs
Bore 93mm
Stroke 85.8mm
Piston Dish 22.5cc
Combustion Chamber 69cc
I come to 7.1

I do not have the specs for the Head Gasket bore size, compressed head gasket thickness or the Deck Clearance so there will be quite a large variable there however it is still a long shot from the std 3.5

The total head volume per cylinder of the 3g75 (ie dish + gasket + cyl head vol) must be:

3828/(10*6) = 63.8 ml

Does the 380 have a dished piston? if so what vol?

If not then new CR for 3500 engine should be (assume same gasket thickness etc) (3497/6)/(63.8+22.5) = 6.75

If it does have a dish then (3497/6)/((63.8 - dish) + 22.5)

The total head volume per cylinder of the 3g75 (ie dish + gasket + cyl head vol) must be:

3828/(10*6) = 63.8 ml

Does the 380 have a dished piston? if so what vol?

If not then new CR for 3500 engine should be (assume same gasket thickness etc) (3497/6)/(63.8+22.5) = 6.75

If it does have a dish then (3497/6)/((63.8 - dish) + 22.5)

OR maybe I am wrong!!

I used CR = swept volume/chamber volume

Should it be (Swept Vol + Chamber Vol)/ Chamber Vol??

If so the calc CR comes to 7.24 for the 75 head on the 74 block

how about 3.5 block, 380 heads, and pajero 10:1 pistons?

75 uses a flat-top piston.

how about 3.5 block, 380 heads, and pajero 10:1 pistons?

Are you saying that Pajero is 3g74 with less dish in pistons to achieve CR = 10?

If so I will redo the calcs for that configuration

I wonder if you could do do 380 heads and drop in forged pistons, how much boost and compression you would end up running safely?

Seems like a good way to drop the compression for FI if you want to. Could always bring it back up a little bit with different pistons.

6G75 head on 6G74 block + Ralliart pistons = ?
Might be a way for OP to go?

Are you saying that Pajero is 3g74 with less dish in pistons to achieve CR = 10?

If so I will redo the calcs for that configuration

Not sure which part is different as to where its changed, http://www.aussiemagna.com/forums/showthread.php?t=64729

Hera we go like I said before standard 3.5 with 3.8 heads 7.2-74 depending if there is any facing
Ralliart 3.5 with 3.8 heads 7.4-74 " " " "
Pajero 10:1 3.5 with 3.8 heads 7.8-8.0 " " " "
Standard 380 6g75 3.8 with 3.5 crank 8.0-8.2 " " " "
The lower numbers are with standard block, heads & gaskets the high numbers are allowing for facing of the block and heads.

Hope this helps Cheers Dave