CR assumptions. Check my Maths please

dbv8 · Post by **dbv8** » Tue Dec 16, 2014 10:42 am

Ive only guessed and still am at my compression ratio.
I realize there are many assumptions without proper measurements but i would appreciate comments on my sums. I am soon to be fitting 40 thou Cometic head gaskets and worried i may be heading a little too high CR wise.

My engine specifics are as follows.
4.6 landrover crossbolted block. This was fitted with top hat liners a couple of years ago by Chesmans in Coventry. I am pretty sure they said the block did not require significant decking.

So it’s a standard 94mm bore and 82mm stroke for 4552cc.

The pistons are 4.0 shallow bowl so sit at the same deck height as 4.6 but only have 13cc as opposed to 22 for 4.6 pistons.

The cylinder heads are TVR 500 big valve and extensively ported. They have been skimmed somewhat and the chambers were originally 28cc. I have polished the chambers and taken most of the ‘bump’ away to try and eliminate hot spots (seemed a good idea at the time).

I had previously used Elring composite head gaskets that I thought were 1.2mm thick fitted but some people think they could be thicker at maybe 1.28 or even more.

I have a pair of 40 thou Cometic MLS gaskets to fit and wondering if I may be taking my CR too high.

I am not sure at all how much is accounted for piston height

So here’s my Maths with a few assumptions
Base original spec would have been 9:35:1 i.e. 569.062CC swept volume per cylinder / 8.35 gives 68.151 CC. Take away 28cc heads, 22cc pistons, 8.88cc comp head gasket leaves 9.27cc for deck height clearance = 1.335mm.
Does this look feasible upto now?

If so then taking my numbers.
Previously on Elring composites.
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 8.88 comp gasket and the above 9.27 for piston height = 59.15 = Total 628.212 / 59.15 = 10.62:1

When 40 thou Cometics are fitted
569.062 swept volume + 28cc for the chamber, 13 for the 4.0 bowl, 7.05 MLS gasket and the above 9.27 for piston height = 57.32 = Total 626.262 / 57.32 = 10.93:1

DEVONMAN · Post by **DEVONMAN** » Tue Dec 16, 2014 12:43 pm

You say that the cylinder heads have been skimmed somewhat and the chambers were originally 28cc.
Are they still 28cc?

Can't find fault with the maths but the effective CR will depend on the camshaft you are fitting.

Will you need pockets in the pistons given the large TVR valves?

dbv8 · Post by **dbv8** » Tue Dec 16, 2014 12:55 pm

I kept the chamber at 28cc in the calcs as i have taken the bump out and polished the chambers open slightly. I will figure a way of measuring the chambers when the heads are off soon.

Ive ran these non cut out pistons without issue on the comp geaskets but will be checking clearance with a dry build on the 0.2mm thinner Cometics in place.

The cam is a real steel Typhoon.
Gave me 329 bhp at 6000 rpm last time on the rollers.

mgbv8 · Post by **mgbv8** » Tue Dec 16, 2014 8:02 pm

Whats worrying you Del ??

Is it just CR ? The valves will be 10 thou closer to the pistons which shouldnt be an issue unless you are too close to start with !

I cant see 0.3 of a CR point being an issue. Its only a 3% increase !

Pel

dbv8 · Post by **dbv8** » Wed Dec 17, 2014 6:59 am

Just wanting to check the numbers.
I will try my best to measure up as accurately as i can as i am intrigued to know where i am with this engine now.

I guess i will be needing to some more rocker shims for the rebuild too. Be nice to fit a set of roller rockers but not in the budget. New MTs and driveshafts is all i can afford this winter.

mgbv8 · Post by **mgbv8** » Wed Dec 17, 2014 7:18 pm

Cool!
I understand now.

Pel

unstable load · Post by **unstable load** » Thu Dec 18, 2014 6:26 am

Mate uses a sheet of perspex with vaseline to seal it on the head and then a burette to fill via a modified spark plug of known internal volume. When the plug is full to the top, subtract the internal volume of the plug and you have your head volume.

minorv8 · Post by **minorv8** » Thu Dec 18, 2014 9:37 am

Drill a hole in the perplex sheet and use a normal plug. You´ll get the volume without doing any math.

When you have the heads off check the piston height at TDC. Helps when calculating the true CR.

mgbv8 · Post by **mgbv8** » Fri Dec 19, 2014 9:32 pm

unstable load wrote:Mate uses a sheet of perspex with vaseline to seal it on the head and then a burette to fill via a modified spark plug of known internal volume. When the plug is full to the top, subtract the internal volume of the plug and you have your head volume.

Thats how i did mine using Kiwicars stuff

dbv8 · Post by **dbv8** » Tue Feb 03, 2015 5:46 pm

The heads are on the way back from Dave Knights with matched 28.5cc chambers.

So thats 28.5 + 13 (piston bowl) + 7.05 (40 thou gasket)

Still to measure deck height but think it will be about 1mm below.

That adds up to 55.6cc + 569.062 (ie 82 stroke x 94 bore) = 624.662 divided by 55.6 = 11.235:1 CR.

The cam is a real steel Typhoon.

What do we think? Have i gone too far on the CR?
I can get a pair of 50 thou gaskets but that would only take the CR to 10.93:1

kiwicar · Post by **kiwicar** » Tue Feb 03, 2015 6:55 pm

Hi
Stick the RS234 in it and retard it 2 degrees would better match the engine and CR, with only 220 degrees on a single pattern cam it will be rather under cammed, even the tornado would be better but still a bit tame. With the one you have you will need to drop the CR to about 10.25 to 10.5 :1 to do it using thick gasket about another .5 point
best regards
Mike

sidecar · Post by **sidecar** » Tue Feb 03, 2015 7:06 pm

If I've read this right your CR is up over 11:1 with 40 thou gaskets and 10.9:1 with 50 thou gaskets. In my opinion this is way too high for an old open wedge chamber with poor squish. A tame cam will push up the peak cylinder pressure at low to medium RPM right around where peak torque RPM which will make the peak cylinder pressure too high. I've heard that people have detonation issues with the P6 lump running at 10.5:1 and your CR is higher.

You might get away with such a high CR if you pull back the ignition by quite a lot but my understanding is that an engine in with such a setup will not perform as well as an engine with less CR and more igntion advance.

I don't run the RV8 over 10:1, I was chatting with Roy Burrell ages ago and he had similar views on the CR limit of these engines. All just my humble of course! (I know that there are people running higher CR's but what cam have they got fitted, and what fuel are they running?)

Alot of people seem to think that there are large gains to be had by pushing up the CR, in reality the gains are not all that big unless you are starting with a really low CR engine! Interesting read.... http://victorylibrary.com/brit/compression-c.htm

sidecar · Post by **sidecar** » Tue Feb 03, 2015 8:15 pm

minorv8 wrote:Drill a hole in the perplex sheet and use a normal plug. You´ll get the volume without doing any math.

When you have the heads off check the piston height at TDC. Helps when calculating the true CR.

That's how I measure the heads, I also use a perspex plate over the bore with the piston at TDC sealed with a smear of vaseline around the bore. That way you can measure the piston dish and the volume due to the deck height all in one go.

SimpleSimon · Post by **SimpleSimon** » Tue Feb 03, 2015 9:14 pm

sidecar wrote:
minorv8 wrote:Drill a hole in the perplex sheet and use a normal plug. You´ll get the volume without doing any math.

When you have the heads off check the piston height at TDC. Helps when calculating the true CR.
That's how I measure the heads, I also use a perspex plate over the bore with the piston at TDC sealed with a smear of vaseline around the bore. That way you can measure the piston dish and the volume due to the deck height all in one go.

Thats how I did mine but used a smear of silicone grease round the bore for a good ring seal gas oil/red diesel dont break it down even over long periods, this way Derek your measuring the volume above the top ring rather than just using the piston crown dish known CC which is about 14cc IIRC

dbv8 · Post by **dbv8** » Wed Feb 04, 2015 2:21 pm

I will get better measurements and dry build to determine where i am regarding piston to valve clearance.

Now the cam issue is in the mix it could be a significant problem to add significant inlet lift and duration. My shallow 4.0 pistons are without cutouts. The heads are skimmed although the recut seats and valves may lift the valves somewhat to make up.

I think a 234 may be pushing it a little. The Tornado looks a better compromise to me but i know Real Steel had stopped stocking them. If ones not available then maybe the Crower 50233 would be an option