Reducing CR??

mgbv8 · Post by **mgbv8** » Tue Jul 23, 2013 8:48 pm

I know this has been done to death. But I have a crap memory.

4.6 running 9.35:1 CR or whatever the stock CR is using comp gaskets.

So if I double the gasket thickness to 80 thou what will CR drop to??

Comp gaskets are 40 thou arent they??

Hopefully those of you that can do sums will be able to work this out.

Ta! Pel

ian.stewart · Post by **ian.stewart** » Tue Jul 23, 2013 11:39 pm

http://www.csgnetwork.com/compcalc.html

bigaldart · Post by **bigaldart** » Thu Jul 25, 2013 7:18 pm

Very bad idea, the piston on a stock rover is usually around 20 thou down the bore, with 80 thou of gasket the squish are gets close to 0.10". This is a guarantee you will suffer detonation. Change the pistons or keep the compression up and use an octane booster, cheaper in the long run. The detonation effect will melt pistons, bend rods, break main caps and all kinds of other nasty things. Anything close to 0.10 is bad news in all engines except a true hemi which doesn't rely on squish.

Alan

mgbv8 · Post by **mgbv8** » Thu Jul 25, 2013 7:50 pm

bigaldart wrote:Very bad idea, the piston on a stock rover is usually around 20 thou down the bore, with 80 thou of gasket the squish are gets close to 0.10". This is a guarantee you will suffer detonation. Change the pistons or keep the compression up and use an octane booster, cheaper in the long run. The detonation effect will melt pistons, bend rods, break main caps and all kinds of other nasty things. Anything close to 0.10 is bad news in all engines except a true hemi which doesn't rely on squish.

Alan

Thanks Alan!
It was just a thought. It my Sons engine in his escort. Its a scrap 4.6 that he's playing with now. We can chuck some power pour in there I guess ?

Its all stock apart from forged pistons.

Pel

sidecar · Post by **sidecar** » Thu Jul 25, 2013 8:09 pm

A good way to drop the CR is to 'scoop' out material from the chambers using a high speed burr. The down side is that it is a lot of work and you need to burette the chambers often especially when the volume starts to get close to the target value.

bigaldart · Post by **bigaldart** » Thu Jul 25, 2013 10:35 pm

Perry,

This was a Dave Vizard discovery that I have seen proven out over the years, example is a nitrous pro mod, adding an extra head gasket turned the thing into a grenade, split the three central billet main caps on an aftermarket BBC, measured the squish with two gaskets and ended up 0.099, ouch! About 20 grand down on that one! I heard the story from the racer then had him measure after I said what had happened, one surprised racer! Another example was a supercharged altered.

Alan

bigaldart · Post by **bigaldart** » Thu Jul 25, 2013 10:36 pm

Easy solution is drop a big cam with lots of overlap and reduce the dynamic compression.

Alan

kiwicar · Post by **kiwicar** » Fri Jul 26, 2013 8:07 am

Mike

sidecar · Post by **sidecar** » Fri Jul 26, 2013 8:29 am

The problem with a 'big cam' is that it brings other issues which will need to be looked at and resolved. Namely valve to piston contact. Also if the cam wants to spin the engine up over 6k which is what a big cam will want to do then the stock rods, pistons, and valve train will complain by making an appearance 'outside' of the engine! (Although I see from your OP that the pistons are forged).

At the end of the day building engines is all about sorting this sort of stuff out, personally I'd never go down the route of fitting two head gaskets on top of each other though!

I also think that the Rover more or less runs with no squish anyway, the heads have the potential for squish but because the pistons are dished any squish band that could be formed will only be about 6mm wide which isn't very good.

Muscle Manta and I are building a 4.6 lump at the moment, the block has been decked and the pistons are flush with the deck (one or two are proud by a couple of thou) For what its worth this engine will be running with a squish clearance which is as thick as the head gasket (1.2mm).

It's surprising how much metal has to come off the engine if you want to run 10:1 with stock pistons....40 thou off the block and 10 thou off the heads. (The pistons were 37-40 thou down the bore before the machining).

With regards to your following statement "Hopefully those of you that can do sums will be able to work this out" …..Really the proper way to do this is to burette the engine, the only other way to do it is to rely on the CR stated by Rover and then reverse engineer the formula for working out the CR in order to obtain the various volumes and I think that is a really dodgy way of doing things!

However here goes:-

Assuming that the stated CR is 9.25:1 the total volume over the piston at TDC must be 69 CC, the swept volume is 568.7CC
Most head gaskets are 1.2mm thick but as your post talks about adding 40 thou to the gasket I’ll use that figure, (and you can get 40 thou gaskets). This would add 6.9cc to the volume above the piston. So with a new volume of 75.9cc above the piston at TDC the CR would drop to 8.49:1….I would not trust any of this!

mgbv8 · Post by **mgbv8** » Fri Jul 26, 2013 3:42 pm

bigaldart wrote:Easy solution is drop a big cam with lots of overlap and reduce the dynamic compression.

Alan

That was my next thought Al!

I can chuck a cam in the same as mine. The heads are modified for good flow and 1/2" lift. They have the real steel double springs and shims etc.

We know the engine wont last long, but while he's saving up for better internals and a new block Jason can have some fun with R&D

Blown v8 · Post by **Blown v8** » Fri Jul 26, 2013 6:57 pm

Why not get your old SDi heads and have the bowls matched to the bores ?

mgbv8 · Post by **mgbv8** » Fri Jul 26, 2013 8:43 pm

Thats an interesting idea?

I have plenty of spare heads to play with. I wont mess with the ones he has now because they are my old 4.6 heads which I dont want to alter.