Lotus Elan

Lets discuss dynamic compression ratio!

PostPost by: Davidb » Tue Feb 13, 2018 11:48 pm

I rebuilt my engine last year:- BDA crank, 4.826 rods, Carillo pistons, 84 mm bore. The camshaft is .385 lift, 280 deg. 35/70 - 70/35. The head has Sprint size valves and a bit of porting-so far as I can tell the head has never been decked. The engine goes very well! Pulls from 2000 rpm. However, it needs premium fuel and I add octane boost for good measure. Why? Because a compression test showed 225 lbs compression on all cylinders!
I was lazy when I ordered the pistons and gave all the dimensions to Tony Ingram and told him I wanted 10.25 or so of compression. He had the pistons made by Carillo (they are like jewelry). When I contacted Tony after the compression reading he did the calculations again and said it might be 10.7:1 but not more.
I am concerned with the compression readings. When I use this dynamic compression calculator I have to enter a compression ratio of 13.5:1 to get 225lbs compression :
http://www.wallaceracing.com/dynamic-cr.php

(Premium fuel in Canada is 94 octane--that is by the N.American method: ron over mon = pon if I remember correctly-this equates to close to 100 octane in the UK)

Comments?

Edit: The block is the original, low deck height.
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PostPost by: rgh0 » Wed Feb 14, 2018 10:38 am

In a standard unshaved head it is hard to get above 11:1 unless you have a substantial intruder on the piston. 13.5 typically requires a large intruder of around 6 to 10mm depending on other engine details. Were your pistons flat top ? if not how big was the intruder?

If Tony Ingram calculate 10.7 :1 max I would tend to believe him.

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PostPost by: nmauduit » Wed Feb 14, 2018 1:47 pm

thank you for bringing up that topic, which is of interest of many enthusiasts I'm sure.

My current street engine cranks about 215psi (one cylinder maybe a bit more towards 220, within the uncertainty of my compression meter I would say) and I run it on European premium with no additive, which is 98 RON = 93 PON, so apparently no more octane than your pump gas.

Have you experienced pinking or detonation issues with plain pump gas (or are you just being cautious) ?

___
I've just ran the simulator with the data you mentioned, including 70° ABDC 0 boost 600 feet altitude, and find 11.8 SCR to get 225psi cranking pressure, then 10.56:1 DCR

I've then run it for what I know of my engine, Q420 at 62.5° 215psi to find backward 11:1 SCR (not built by me) and 10.17:1 DCR
__


The way I understand Dynamic CR with respect to Static CR is that since the piston has moved up when the inlet valve closes (by the angle Inlet close ABDC), it does not sweep the whole volume of the cylinder into the chamber, hence DCR is lower than SCR.

As a result for a given engine, the actual way it is timed will also have an impact on DCR (since camshaft timing will change this angle at which inlet closes) - though one usually does not alter constructor specs a whole lot.

My next engine will have 2 detonation sensors (one at front, one at the rear), as I'm planning to experiment some about that (still waiting for the block, though), in order to prepare a race engine.

Question for Rohan, whom I've seen has posted while I was writing that : would a compression test on the starter lead to an accurate indication of DCR (using the actual engine to measure cranking pressure even though only at 100rpm, 200 tops - in other words, to what extent carbs/exhaust pulsing or other high rpm effects can add to quasi-static DCR as one measures with a compression meter) ?
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PostPost by: Davidb » Wed Feb 14, 2018 4:47 pm

Thanks for the responses. nmaudit I appreciate you taking so much trouble. I wonder why we get different results?

In answer to nmaudit-Yes I have had running problems on lower octane fuel-on one run in a remote area last summer we found that premium fuel was simply not available and had to use 'mid grade' with octane booster added. The engine ran very poorly as a result-misfiring under load.

Thanks Rohan, I did not think the engine has 13.5:1 compression- I should have made that clear-that is the number the calculator comes up with. Years ago I had a Hart 416B engine and that had about that much compression. It had huge intruders on top of the Mahle pistons-I know that because I blew a hole right through one! Hence the concern now.

The Carillo pistons have small intruders-I will try to find a photo.

I seem to have a convergence of high compression causes: Short duration cams, 1720cc, short rods and high static compression.

I am thinking that I should fit a thicker head gasket anyway!
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PostPost by: Davidb » Wed Feb 14, 2018 5:02 pm

I find that if I simply retard the inlet cam by 5 deg the dynamic compression comes down below 200 psi. I wonder if that is a usable answer or if the longer period will make the engine less flexible?
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PostPost by: nmauduit » Wed Feb 14, 2018 6:28 pm

Davidb wrote:I find that if I simply retard the inlet cam by 5 deg the dynamic compression comes down below 200 psi. I wonder if that is a usable answer or if the longer period will make the engine less flexible?


well, I've just tried to run the simulator again and do not get the same results... could be a parameter issue with the web page, I will try to get the formula at some point to be sure.

in any case, I would not alter one camshaft by as much as 5 degrees without extensive checks (risk of valve clash) and fear of serious performance loss... If the engine runs fine on premium and you usually can find premium, I would just stock up some octane booster additive for the odd occasion where you would only find regular

then if the option of a thicker head gasket has to be considered, though while the head is out and you are positive that the SCR is too high there may present itself an opportunity to rectify that (machining piston tops, or enlarging the chambers), it does not seem that easy to source thicker composite head gaskets (Ajusa composite fibermax being about 1mm compressed height and the extra tall version does not seem available anymore, while the MLS type Cometic etc that come in many thicknesses does not always seal well with the flexibility of the Lotuc TC).
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PostPost by: pauljones » Wed Feb 14, 2018 7:16 pm

I work this out as,

1720÷4=430
CC vol = 41

430+41=471
471÷41=11.48 scr

Thats not taking into effect gasket volume or piston relieves.

So assuming 10.7 is correct i would look at ignition timing.

If im barking up the wrong tree im sure ill be corrected
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PostPost by: Davidb » Wed Feb 14, 2018 7:28 pm

Thanks Paul and again nmaudit.

Paul, we played with the timing. I started out with the original distributor and the engine pinged-I read in Miles Wilkins book that that particular distributor is not recommended due to an inappropriate advance curve so I fitted a new Pertronix distributor-much better! No pinging and smoother running so the advance curve is different. I think we have the timing right.

I looked for another Dynamic Compression Simulator and found this one:
http://www.rbracing-rsr.com/comprAdvHD.htm

It is basically the same one but with one Huge difference-the inlet closing is entered at the point of 50 thou lift! It looks like it has come from the same template as the one I linked to earlier in which case it may solve the problem of the 13.5:1 compression--or at least, it brings the required static compression, required to get 225 psi of cylinder pressure, down to 'only' 12:1 which seems within possibility.
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PostPost by: nmauduit » Wed Feb 14, 2018 7:51 pm

Davidb wrote:Paul, we played with the timing. I started out with the original distributor and the engine pinged-I read in Miles Wilkins book that that particular distributor is not recommended due to an inappropriate advance curve so I fitted a new Pertronix distributor-much better! No pinging and smoother running so the advance curve is different. I think we have the timing right.

I looked for another Dynamic Compression Simulator and found this one:
http://www.rbracing-rsr.com/comprAdvHD.htm

It is basically the same one but with one Huge difference-the inlet closing is entered at the point of 50 thou lift! It looks like it has come from the same template as the one I linked to earlier in which case it may solve the problem of the 13.5:1 compression--or at least, it brings the required static compression, required to get 225 psi of cylinder pressure, down to 'only' 12:1 which seems within possibility.


For a given camshaft properly timed, provided that the ignition curve is correct (eso. not too much advance) playing with ignition timing may only do so much to counter a tendency to pinging that is mechanically engineered, and that by retarding (de tuning) mainly : that is also what I'm planning to investigate, using a 123.nl distributor and a detonation/pinging counter to switch to a retarded curve when problem arise.
But that would work only marginally, if you have a dragster type engine you'll need high octane.

thank you for the other simulator, will check it out later.
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PostPost by: Davidb » Thu Feb 15, 2018 2:05 am

I found several more simulators and all tend to confirm that the static compression reading is 11:1 or higher.
So, this afternoon I took the head off. Everything looks really good the intruders on the pistons are small-I found Tony Ingrams response about the pistons and he said he had contacted Carillo and they and said they are only .5cc!
I have decided to fit a thicker head gasket for peace of mind-I would like to lower the compression by half a point or so-Dave Bean's guys have the gasket in stock.
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PostPost by: 2cams70 » Thu Feb 15, 2018 3:33 am

If you have already got the head off you may as well just measure/calculate the actual compression ratio yourself using a burette.
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PostPost by: Davidb » Thu Feb 15, 2018 3:37 am

Yes! That is what I plan to do tomorrow.
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