Compression Ratio v Cam,

[v8ian]

Just been checking the compression ratio on the new engine, 4.6 Rover V8, UBV TVR Challenge heads, Throttle bodies and Emerald M3dk management,
assuming tins are 0.015" and comps are 0.045"
the ratio works out at 15.00:1 or 13.2:1, now where do I go, to get away with the comp I need a good mechanical overlappy cam, but with plenty of lift. torque low down is not really an issue, I just want a good stonkin lairy cam, anyone with any suggestions -----relevant of course

[stevesingo]

There is a big difference between 15:1 and 13.2:1. You should confirm this before you proceed. It will depend on fuel! What is important as regards compression ratio is the dynamic compression, that is the compression in relation to the inlet valve closing event. You will need to choose a cam with sufficiently late inlet closing event in order to reduce the dynamic compression and therefore the cylinder pressure, sufficiently to avoid pinking/detonation/knock.

Check this calculator...

http://www.rbracing-rsr.com/comprAdvHD.htm

Steve

[PhillipM]

13:1 you'll just about get away with given good fuel and a lairy cam, but 15:1 is a whole new ball game...

[Boosted LS1]

Have you double checked those figures, they seem high? Have you factored in piston position relative to the deck at tdc or assumed the piston is at zero deck height?

[v8ian]

I fully understand the Static/dynamic compression bit as I allready run a 14-1cr 3.9 on a big overlap cam, which seems to work very well,
Mr Boosted, yep those compression ratios are right, In reality I cant run the 15:1 as the piston to head space is probably too close @ 7k the only thing not put into the calcs was the piston cutouts as they have not been machined yet, I was thinking about moving the pistons up the bore a bit by rebushing the little ends and re mechining them offset, Not any more, Ill just rebush the rods to suit my pistons and keep the centers stock @6.11

[Boosted LS1]

Interesting stuff Ian. What is it that's raised the compression so much? Do you have smaller head chambers or true flat top pistons or is the block decked. Valve cut outs are probably 1 cc each.

[v8ian]

The pistons are thick deck forged pistons designed for gas,
and if I have done my calcs correctly,
Bore, 3.7,
stroke, 3.2283
piston below deck 0.016"
Chamber size 36cc
Tins @ 0.015"
comps @ 0.045"

I am using 4.0 rods as they are longer @ 6.11", hopefully this should help with the burn with the piston dwell being increased and the rod angularity lessened, I was also looking at BBC rods, a 6.135 would work, using -0.02 bearings and tale 0.005 off the crank the bearings would fit, but the width at .0990 is probably too much to trim down to 0.850 for the rover journals, as Im not sure taking this much off the sides of the rods would impact on their strength

[Boosted LS1]

Given your figures I get 14.7:1 with the tin gaskets and 13.21:1 with the composites. I guess you'll want compression if you plan to rev it in order to raise the DCR. Is that your plan of attack?

[v8ian]

With my old engine, nobody would beleive I ran a 14:1 CR and drove it everwhere on 99ron, even on 97 it was ok, Im sure that was down to nobody understanding SCR and DCR and the ability to change SCR by cam overlap and cam timing, Ive never been known for being kind to an engine, even if I am an engineer, so 7k is not a worry,
I think overlap has to help these engines breath due to the poor positioning of the valves away from the bore centerline, and with overlap you can run silly SCRs, what cam to get, my last cam was a GrpA TWR WO6 European touring car cam very little lift at cam, about 0.33, but massive amounts of overlap and duration, I may go down to see my old cam grinder, Newmans, he is allways open to ideas, then either shoots them down or says maybe, might be worth trying the same duration and overlap, but with something like 0.38 lift, that will give around 0,61 net lift, which I feel is plenty, if not a tad too much,
I wonder if I could convert those figures into a roller cam, then make the ramps more aggressive, and use SBC cam followers.

[Boosted LS1]

With some engines they'll aim for low dcr to reduce pumping losses and get revs that way. I've looked into rollers and think you'd need to incorporate a rev kit to keep the lifters on the cam. It's all doable but not the sort of thing you'd try for a customer. That said, roller cam kits are available for the rover nowadays but I don't know anybody who's used one.

[v8ian]

I would prefer not to have to run a rev kit, and the asssociated spring pressures, I looked into this a while ago, not using silly springs and all the other bits and pieces, possibly not even a rocker train girdle, thats if im right, no triple springs or silly poundage, but use Beehives, im fairly certain I would need custom retainers, or at a very minimum collets, Paul Ivey??
This way you have far less friction on the cam and push rods, lighter valve train and much more willingness to rev without the worry of valve float,