How much....

need4speed · Post by **need4speed** » Mon Oct 29, 2007 9:26 pm

Ive built a 4.6 using a 3.9 block. Im using standard 4.6 pistons. If i use composite head gaskets, how much will i have to skim off the 3.9 heads to acheive a compression ratio of 10:1?

Post by **katanaman** » Mon Oct 29, 2007 10:23 pm

impossible to say exactly without you supplying cc figures from heads and pistons but its around 1mm or less if you use 4.0 pistons.

need4speed · Post by **need4speed** » Tue Oct 30, 2007 8:01 am

I dont know the cc measurements. I thought maybe someone would know what these measurements were for bog-stock 4.6 pistons and 3.9 cylinder heads...Or do the stock components vary so much? Surely not...

Post by **katanaman** » Tue Oct 30, 2007 9:04 am

They vary a lot, 1mm could get you from 9.9-10.1 or even further out. Your heads could already have had a skim and although unlikely so could your block. This is what blueprinting is all about but if all you want is roughly 10:1 then 1mm will be in the ball park. Depending on your cam be careful with your valve-piston clearance with that amount skimmed off.

russell_ram · Post by **russell_ram** » Tue Oct 30, 2007 12:22 pm

Assuming you have std 3.9 36cc heads and std 4.6 piston bowls, then to get a 9.35 compression with composite gasket requires you to have cylinder heads volume of 28cc like the 4.0/4.6 head.

That 8cc head volume difference is required to compensate the increase of 26thou thickness in the composite gasket vs tin.

You require a head volume of 23cc to get to your desired 10.1 ratio.

By calc if 26 thou = 8cc then 13cc results from 42thou difference, hence you need to skim by 42thou to get 10.1 with your combination. (Not exactly true because the head bowl tapers and so the more you skim the less volume change results. )

Very good guess by Marki/Katanaman.

Russ

need4speed · Post by **need4speed** » Tue Oct 30, 2007 12:25 pm

Im kinda open to ideas regarding the cam. Im considering the stump puller from real steel.

5000SE · Post by **5000SE** » Tue Oct 30, 2007 2:20 pm

need4speed wrote:Im kinda open to ideas regarding the cam. Im considering the stump puller from real steel.

What's the vehicle, what are you using it for, and what inlet system will you be using?

need4speed · Post by **need4speed** » Tue Oct 30, 2007 2:55 pm

Landrover 90. Used 50/50 on-road and off-road. Induction looking more and more likely to be CBR600 throttle bodies with custom manifold...

5000SE · Post by **5000SE** » Tue Oct 30, 2007 3:11 pm

need4speed wrote:Landrover 90. Used 50/50 on-road and off-road. Induction looking more and more likely to be CBR600 throttle bodies with custom manifold...

They're not necessarily ideal for torque, and you'll have all sorts of fun and games getting them set up. If you're looking at a stump puller, V8 Developments' Hybrid 200 is a very good cam. I had a 4.6 with stage 3 heads and a twin plenum / twin airflowmeter flapper intake which made 367ftlb and 313bhp with a Hybrid 200.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 30, 2007 7:19 pm

Need For Speed.

Presumably you have used a 4.6 crank with the mains machined down to fit the 3.9 block, and are using the the 4.6 rods and high compression 9.35/1 pistons.

The estimates by others seem to be about right, but there is an interesting disparity in the Land Rover data sheet that gives a total length from centre of crank ,rod length and compression height of piston for the 3.9 at 264.5mm and 4.6 at 267.76mm and I'm curious to know where this 3mm+ is, and in fact what the height of your pistons is in the bore at exactly TDC. If the Land Rover data is correct your pistons could be 2-3mm out of the bore at TDC! which might actually be useful as you could have the pistons machined on the outer edge which would raise the bowl and increase compression without having to excessively machine the heads.

Please let me know about the piston height.

Kevin.

need4speed · Post by **need4speed** » Tue Oct 30, 2007 8:06 pm

wow. i never heard of that one! It'll be a few days until im able to look at my engine, but i think the pistons are level with the block faces. I think i would have noticed if they were protruding above the block - coz that normally isnt good news at all...but in my case could be. I'll let you know.

kiwicar · Post by **kiwicar** » Tue Oct 30, 2007 8:07 pm

I would be a little carefull of bringing the piston too far up the bore as you need at least 25 thou of piston to head clearance, even this is tight, and you would have to be certain all are the same. I'm going to use 32 to 35 to be safe on my build. Also you don't want to take too much off the piston above the top ring as if it is too thin it will nip the top ring and stop it making a proper seal (or worse still break up, the top of the piston can come out the bore a little but be sure of your clearances on all pistons
Mike