DCR. What to aim for?

[DangerousDerek]

I am putting together a tasty 5.0 Rover V8 for my TVR.

Most of the spec is set but before I decide on head gaskets and maybe a little further head work I was wondering where I should aim for dynamic compression ratio.

Spec
ACR top hat block
96mm forged pistons with 20cc bowls
Offset ground crank to 86mm
6 inch Chevy I beam rods
a solid 50304 crower cam. Inlet closes 47 ABDC.
Deck height is 7.5 thou
I have Elring gaskets which are 1.28mm but thicker MLS are an option.
The will be measured this Friday. I believe they are 28cc for now.

So looking for advice on DCR in order to aim for a suitable static CR.

The car is mostly used for drag racing but will be used on the road also.
I intend to never put less than Vpower in the tank.
I also plan a 2nd fuel system to run VP racing fuel when adding 150 shot of nitrous.

[Stan Weiss]

Why on what sound like a race engine would you what to work backwards from a DCR?

Stan

[Pumaracing]

DangerousDerek said:

a solid 50304 crower cam. Inlet closes 47 ABDC.

74 ABDC.

[DangerousDerek]

Pumaracing said:

DangerousDerek said:

a solid 50304 crower cam. Inlet closes 47 ABDC.

74 ABDC.

spec sheet saya 47



I am just trying to establish where i need to be CR wise guys.
All advice taken thankfully.

[Stan Weiss]

One is talking advertised duration 108 ICL and one is talking 0.050" lifter raise duration 104 ICL.

If you check the 0.050" numbers the ICL is @ 104. Which it also states on that sheet.

As far as DCR / CR what cranking compression / cycle pressure are you looking for?

Just a simulation -- Bore = 3.5" Stroke = 2.8" Rod = 5.66" CR = 10.5 DCR = 7.9 Cranking Compression 196 PSI

Stan

[DangerousDerek]

Stan Weiss said:

One is talking advertised duration 108 ICL and one is talking 0.050" lifter raise duration 104 ICL.

If you check the 0.050" numbers the ICL is @ 104. Which it also states on that sheet.

As far as DCR / CR what cranking compression / cycle pressure are you looking for?

Just a simulation -- Bore = 3.5" Stroke = 2.8" Rod = 5.66" CR = 10.5 DCR = 7.9 Cranking Compression 196 PSI

Stan

Then maybe i should rephrase the thread title... What cranking compression to aim for?
On a learning curve here. Help me out.

[Pumaracing]

DangerousDerek said:

Pumaracing said:

DangerousDerek said:

a solid 50304 crower cam. Inlet closes 47 ABDC.

74 ABDC.

spec sheet saya 47

And therein lies quite a number of your problems. The spec sheet also says those are timings at 0.050" tappet lift and NOT seat timings which is what even those few people on the planet who have the slightest interest in DCR would need to know to calculate it.

The inlet valve close of a 292 degree cam at the seat on a 108 degree LSA timed straight up is (292/2) + 108 - 180 = 74 ABDC

It is merely a further minor coincidence that this is the reverse of the digits you quoted but the multiverse does like its occasional little joke.

So even if any of us had tried to answer your question about DCR any calculation from that about static CR based on 47 ABDC would have been hopelessly wrong. However as Stan said, why would you want to start from DCR in the first place? Most people don't understand it as you've just proved and I've always thought it does little more than add a further unnecessary level of complication into an already fraught subject.

If there's one question in here you can pretty much guarantee no one is going to stick their neck out on it's guesstimating CR for other people's unknown engines, let alone ones that are going to run N/A most of the time but with a big dose of nitrous at others.

[DangerousDerek]

There are very few that would stick their necks out when it comes to nitrous in general.
Maybe i should not have mentioned it. I used to use 170 in a 2 stage that worked well but will be winding the substance abuse down on this build (I promise).
I was thinking to getting the static CR to around 11:1.
Any thoughts on that number i plucked out the air?

[Pumaracing]

It sounds like an eminently reasonable number for a big bore 2v engine and a long cam. Not so high as to introduce too many unreliability problems. Not so low as to make the cam desperately unhappy.

[DangerousDerek]

In a 1100kg TVR with a lightish flywheel (19lb) i think it should have acceptable enough road manners.
The aim is for a useable road car that can run 11's NA on the strip.

[Pumaracing]

DangerousDerek said:

Then maybe i should rephrase the thread title... What cranking compression to aim for?
On a learning curve here. Help me out.

If you search the archives for "cranking pressure" you'll find I've covered that along with CR and DCR in some depth in the past including here

http://www.pistonheads.com/gassing/topic.asp?t=147...

and here

http://www.pistonheads.com/gassing/topic.asp?t=144...

[DangerousDerek]

Measured the heads today and they were pretty much bang on 28cc as i suspected.
So ive calculated CR will be at 11.3:1

Check my maths
bore 96, stroke 86.3 = 624.66cc
piston bowls 20cc, top ringland 2cc, 7,5 thou deck clearance 1.38cc, gasket (1.28mm) 9.26cc, head chambers 28cc = 60.64cc

624.66 + 60.64 = 685.3 / 60.64 = 11.3

60 thou MLS gaskets would give me an extra 1.89cc which would drop the CR to 10.99

Decisions?
Any thoughts?

[Pumaracing]

Your top ringland volume is almost certainly wrong and should be close to zero anyway when the engine is hot as was discussed on here a while back. Your CR is 11.3 with the thicker gaskets by my calcs.

[DangerousDerek]

Pumaracing said:

Your top ringland volume is almost certainly wrong and should be close to zero anyway when the engine is hot as was discussed on here a while back. Your CR is 11.3 with the thicker gaskets by my calcs.

I must admit I thought less than 0.5cc for the top ring land but my engine guy said 2cc. Maybe another chat needed.
Now I am at 96mm bore maybe I have room to gain a cc or 2 from the chambers also.

[DangerousDerek]

Roland said the 2cc he listed for the top ring land was to include the valve cut outs which makes sense.
So I am at 11.3:1 with the Elrings.
I will take a little out of the chambers and remeasure but think I will be using 60 thou MLS gaskets and bring the CR down into the 10's.

[Pumaracing]

Valve cutout volume should always be part of of the bowl volume. The last place you want it confused with is ring land volume which IMO should never be included in a CR calculation anyway as it should be be very small when the piston is hot and leaving it out gives you a tiny bit of safety margin in your calculations.

[Pumaracing]

BTW, this 2cc sounds like nothing more than a guess anyway. With a flat top piston with a bowl and valve cutouts there's no way to measure the valve cutout volume separately from the bowl volume as they normally intersect. You put a piece of perspex on the crown and measure the lot in one go. I suspect the 20cc bowl volume is just a spec figure from the data sheet and the actual total volume with cutouts has never been measured at all.

[Stan Weiss]

After putting tape on the piston so it will not leak. I put the piston a fixed distances down the bore and then with a Plexiglas plate cc what the volume is. I then calculate the volume for that bore and stroke in cc's. Subtract one from the other and have the real answer. Which most times is not close to what I have been told it is.

Stan

PS - Let me add

In the way of calculations. If your bore = 96 mm and the top of the piston = 94 mm in diameter and the top ring is 6 mm down from the top you have 1.7907 cc's volume there.

Edited by Stan Weiss on Monday 31st August 16:26

[Pumaracing]

If you have a 96mm bore and a piston with a 94mm top land then you've got a hell of a problem with your piston designer because the top land on a forged piston that size should be 95.3mm.

Assuming a slightly more normal top ring 8mm down from the crown that creates a 0.84cc land volume at ambient, all running clearance taken up at a maximum land temperature of 390c and at an average top land running temp of 300c a land volume of 0.24cc. Inconsequential in any CR calculation.

This assumes high expansion 2618 piston alloy and cast iron liners. 4032 alloy could manage on a smidge less cold clearance. Approx 95.4mm top land at ambient.

[Boosted LS1]

The carbon will infill the space soon enough.