Compression ratio of 11 and Cam choice???

[MPO]

If I have a compression ratio of 11;

When does increased cam overlap start to reduce the effectiveness of higher compression? Maybe having a higher CR lends its self to making a cam with more overlap a better choice…. Thoughts???

Also, how much overlap could a plenum take before cross contamination becomes too much?

I’m at a crossroads with a replacement cam and compression adjustment, I have valve cut outs and want to choose the most effective cam.

Anyone been here before or got any advice?


MPO

[MPO]

Thanks for your reply Rev-erend and dnb, I only use Shell Optimax and being a Pre CAT I’m able to keep the engine temperature between 75 and 80 and run my fans most of the time to ensure this. However, I don’t use a fuel cooler….. or run a intercooler…

From my understanding, the higher the compression the more effective a Cam can be that has ‘Overlap’ and with sufficient overlap the compression will fall so I should see a reduction in compression.

I’m not quite sure if the compression is reduced when the cam is “ON CAM”…

I’m trying to find the sweet spot where compression, Cam choice (with overlap) and a plenum can can work to the optimum.

MPO

[MPO]

The heads are ok, but will be reworked over the Winter as the valve seals need replacing. So, any carbon build up shouldn’t be an issue moving forward….

As far as the ECU is concerned, I will continue using the 14CUX (Open Loop) with an AFR logger. Changing the fuel requirements on the 14CUX will not be a problem.

Managing and mapping the advance curve will be, as I have a standard TVR Dizzy but for now, I will stick with it :-).

MPO

[MPO]

Brummmie said:

You are referring to dynamic Compression ratio, as said here your Clockwork ECU/Dizzy want chucking in the bin really, as you need to prop up flat spots with a bit more timing and back it off as it "comes on cam"..

http://wallaceracing.com/dynamic-cr.php

Thanks Brummmie

I know what your saying with the Dizzy but it will have to remain for now...

Thanks for the link :-)

MPO

[MPO]

blitzracing said:

Its all about the engines volumetric efficiency- A cam with large overlap wont breath well at low RPM, as the gas velocities are low in the inlet track, so you loose the ability for the charge to maintain its velocity into the combustion chamber during overlap. At this point the actual compression ratio will be low as the engine is not breathing well and cant fill the combustion chamber with a full charge. You wont get the full compression pressure until you are fully "on cam" when the volumetric efficiency is at its highest. Problem is many things come into play here- typically if the port size is too small to allow you to make use of the cam profile, you will never get near 100% volumetric efficiency as the port becomes the restriction when the cam is at its best point, so you will never reach your desired CR anyway. This is why we pay engine tuners lots of money so you dont have to re invent the wheel. Also the 14CUX- it really struggles with long duration cams- I think this is due to the AFM metering not being the best way to measure fuel requirements when you have poor airflow and reversion due to the cam profile. As for detonation- it takes place at a microscopic level- its not as simple as saying my engine runs cool- its all about shock wave that passes through the un burnt mixture just after ignition, that causes a chain reaction, or hot spots in the combustion chamber that are enough to detonate a highly compressed mixture. Just because the water is cool it does not mean the combustion chamber has no hot spots- heat just makes things worse.

Blitz

Nice…

You have taken my brain to the next level! :-)

MPO

[MPO]

Bluebottle said:

My old engine was 11:1 with ported and gas flowed head running std 14CUX. Rebuilt with mc2 cam really strong engine but would run lean over 5k RPM . You can see the graphs on my profile

Hi

I remember you telling me you were running 11… Can’t find the graphs you refer to, got a link to the topic?

MPO

PS The seat is still on my to-do list :-)

[MPO]

dnb said:

I'm not sure what more I can add after Blitz's excellent post, but here goes - most are more practical considerations rather than describing an engineering method for designing a powerful Rover v8.

You can spend a lot of money getting a RV8 to be excellent, but this must be weighed up against the cost of an engine swap to say LSx or AJP - so my experiences are based on using a significantly limited budget to the best effect I could, albeit with me taking on a lot of the risk for it not working as desired - this is not for the faint of heart and why we have engine builders! For my budget, I needed to get as a minimum new liners and pistons, do some head repairs and replace all the bearing shells. I ended up being able to do quite a lot more than this for the budget, although I would not ever want to add up my own cost of labour both for design and build for the project since I work slowly on things I haven't done before. (I tell myself it's not re-inventing the wheel if it is training or "career development")

I designed my new (home built) engine to have a static CR of 10.8:1. I took the view that the standard Rover heads, no matter what you do, are going to flow relatively badly compared to anything modern. So I want to squash the hell out of all the air I get in there. More CR would have been possible, but this appeared to be driven by a law of diminishing returns - the additional design complexity and head work required simply didn't look (on paper anyway) to provide value for money.

Fuelling and timing to support this isn't a problem for me - I have a nice mappable ECU with lots of bells and whistles. I currently have a H404 cam, so it's pretty wild but not completely silly. The engine will allow a wilder cam should I choose this, but the everything was planned around cams of this approximate lift and duration such that the dynamic CR comes out "sane". (I had a bit of help to construct a computer model of this to aid with component choices).

I am not worried about detonation at the moment. The original engine modified by thin hear gaskets had a bit of a problem with it at high RPM with the H404 cam even running with high octane fuel, so timing had to be compromised a bit. This I put down to the very large squish clearance (over 3mm). When I put in the new pistons and rods, I reduced the squish clearance significantly and have not seen detonation (yet - I haven't used the car in anger much this year because of work) in the even when running on pump fuel. The ECU has reasonable knock detection and control facilities.

I would suggest the original plenum is a severe limitation for both drivability in the mid-range (charge robbing when the cam isn't performing right) and for top end power (when the throttle is restricting the engine). A few solutions exist to help with both of these, but they aren't particularly cheap.

Lovely…

Just the sort of info and tips I need :-) Thanks

and yes I’m trying to do this rebuild myself too because I want to and fancy the challenge….

If I need to put a thicker gasket in to reduce the CR a little I will…

If I save some money all good and well, more to spend on something else…

The drive is to understand and enjoy the build :-)


It’s such a shame some of the cams available don’t have all the technical detail and on that basis, I’m more inclined to choose one that has…

I’m currently looking at;

Kent - 224
Kent - 234
Real Steal – Tornado

Anyone used these with a higher than average compression ratio and got any feedback? Any other brands/CAMS that can be recommended?


MPO

[MPO]

Guys

Have I missed the boat?

It looks like going LS in a Griff is the icing on the cake...

Hey Ho, maybe I'll leap frog, lets see what Les comes up with :-)

For now, it's TVR/RV8.


As there seems to be issues with poor drivability at low revs with some cams is it worth looking at Rhodes Lifters (I hear they do sound quite rough though!)?

But do they work?

MPO

[MPO]

Sardonicus said:

blitzracing said:

The Rhodes followers allows the oil to drain from them at low camshaft speeds, so it opens up a big gap between the follower and the push rod. The effect is the valves opens later and closes sooner, so you reduce the duration and some lift- so its like putting a low performance camshaft in for low speed use- but the price you pay is a lot of mechanical noise at low speed from the valve gear as you have massive clearances. Never actually seen anyone use them however so that tells you something...

+ running a flat tappet cam with excess valve clearances/loose is bad for the cam and follower cant see why these would be any different I heard a small block Ford on them and it sounded just fine but then I have also heard people fit them and regretted it soon after most I have heard run these bleed down tappets/Rhoads etc on the RV8 have not been successful

That’s interesting…

I would have thought with the correct Rhodes Lifter pre load clearance, this would take up the slack but not open the head valves so far…

And when the Rhodes lifter was fully open the head valves would open further

What causes the noise?

I was also thinking of using adjustable push rods to perfect the preload...


MPO

[MPO]

I like the sound of this there must be someone who has tried it???? I'll happily give it a a go. Is the wedge shop in the States?


MPO