Compression ratios

[zefarelly]

Michael, its an FiA App K historic saloon race car . . .
soI run to original geometry with restricted valve size and carburation etc.

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

this site gave me this for the new cam, which doesn't read so well, I can get 12:1 CR static without weakening the head too much, but with a small compromise to flow (130 CFM instead of 138) at full lift, 11.3mm its all a bit of a compromise, until I make/find a thick floor head!

Your engine summary is as follows: Bore 3.228 inches, stroke 2.864 inches, rod c-c length 4.83 inches, with a static compression ratio of 12 :1. Your camshaft specifications call for an inlet valve closing of 88 degrees ABDC (after bottom dead center).

Your chamber volume is 34.92 cc's. With this camshaft your dynamic, or effective stroke is 1.70 inches. Your dynamic compression ratio is 7.53 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 146.93 PSI

[zefarelly]

oops, I got that wrong . . . . timing is at 050 lift!


8.349 is a bit better . . . if I can get to 12.5:1 static then its 8.8

[SidewaysFeltham]

OK, I need to think about this for a bit.

However, obviously, the critical thing is the inlet valve size, more than the exhaust.

Are the size of the carb choke tubes restricted (to a max)?

Is position of the ports restricted to original?

[zefarelly]

39.6 inlet, 34mm chokes for twin 40's, I could go 36, although not sure thats strictly speaking legal, but everyone else cheats much worse!

34.5 ex, although I rarely bother worrying about ex side, it looks after itself to a large extent

position is restricted . . .ie I can't go downdraught, I tend to port up the head face as much as I can to straight line the inlet if you like, its negligible but every little helps !

[SidewaysFeltham]

The devil is in the detail, Zef!

I absorbed the strategy of the legendary Freddie Dixon in the 1930s, the ace tuner and builder of Riley racing cars.

He presented his car for scrutineering (at Brooklands) and the chassis was meant to have a cruciform member roughly in the middle.

It had apparently been removed. The scrute rejected the car. Dixon protested and provided the scrute with a copy of the regs which stated that one could lighten the chassis but the material of construction must remain the same. Dixon pointed out the fine cross made from mild steel wire...............

He raced!

I copied this strategy with an 850 cc Mini formula, which stated that the original 1 1/4" SU carb must be retained.

I grabbed a spare carb, dropped it in the demon de-greaser and then built up the middle section, all round with Plastic Padding Liquid Metal.

Then onto the lathe: and turned out the interior choke to 1 1/2". And could then fit a 731 cam, which could breathe!

Coudn't bother to make up a jig to turn up a new oval throttle plate to fit, so it ticked over at about 4,000 RPM! So what?

Stuck it in gear before firing up the lump!

So, I'll think on a wee bit!

[zefarelly]

Its debatable whether bigger chokes on 40's help . . . 32,33, and 34 are on my list, as well as a DCD 28/36 ( it doesn't say how many!)

but me being me has special pistons made at 82mm rather than what everyone else seems to do and go straight to TC or 85mm!

[SidewaysFeltham]

Agree about the choke size.

We used to select total carb spec by the simple expedient of running the car on the rolling road and finalising carb tuning from there.

Despite what expectation and Urban Myth might predicate, greater power often came from smaller choke tubes. OK, I was lucky (or organised!) as I was a dealer under Chris Montagu Carburettor Company, at the time and had a large stock of 40 and 45 chokes, auxiliary venturis and jets.

That said, we also regularly turned out chokes to half m.m. size increases for optimum perormance.

All about gasflow, really. If you become hung up on sheer size this doesn't necessarily produce more power: often, quite the reverse.

That said, it is wholly pointless sporting out for mega-sized inlets and a wild cam if you are restricted to overall inlet bore size.

Back to compromise! On that note, the 28/36 DCD is of course a progressive carb and a huge compromise! OK for (e.g.) Cortina GT MKI, to which it was originally fitted, but "Untunable", basically. Stick with the twin DCEO 40s!

This was really reinforced to me by a young lad who walked into the shop one day and ordered brand new twin SU 1 1/2" and a BMC suitable manifold.

I explained, gently, that there was no way his slightly tweaked 850 c.c. Mini could suck enough! "Oh yes it can!" he stoutly maintained, "I read it in this month's "Mislead the Gullible Tuning for Minis", magazine!"

This was the same mag, BTW, that convinced another gullible young and impecunious lad to demand that we convert his 850 c.c Mini to 2 Litre..........................

And of course, back he came, to complain that there was summat wrong with the new carbs we'd sold him!

Eventually reason won and I swapped them for a secondhand set of Cooper s 1 1/4" and matching manifold.

And his car ran very nicely!

There has to be something else, Zef.

Thinks.......................................

[zefarelly]

well 40's 34 chokes A6 cam works well, but theres more to come, I'm working on new heads and cam profiles, as thats all I can play with, which gets us back on topic, which is COmpression ratios

theres a very direct link between the 2, and gas flow can be improved at the expense of CR . . . its finding the happy medium, which gets me back to the olde 100 BHP/Litre . . it is the holy grail !

[SidewaysFeltham]

OK; so it has to be gas velocity and weight of charge. Which can only be cam and porting/inlet layout.

Let me think for a bit about standing waves and gas velocity.

Few years since I've done this, Zef!

[phoenix]

Originally Posted by zefarelly

gordon has ran similar (ish!) with no restrictions and had 161 english shire horses, I've yet to break 140 with my shopping cams!

Was Gordon on 40mm carbs with 34mm chokes?

And at what engine speed did you see 140 bhp?

[GORDON STREETER]

Hi . fancy getting me away from the swimming pool !
I was using 45 DCOE.s with 39mm chokes. I wasn't limited to any spec at the time only cc (1489) and a non crossflow head.

[zefarelly]

FANCY DRAGGING ME AWAY FROM WORK !!!

I was told 140 on the rolling road ( at just over 7000 RPM) but I think its more like 135.

it races between 5-7500, will rag to 8k but pointless really most of the time as I have a CR box

[phoenix]

Originally Posted by GORDON STREETER

Hi . fancy getting me away from the swimming pool !
I was using 45 DCOE.s with 39mm chokes. I wasn't limited to any spec at the time only cc (1489) and a non crossflow head.

Thanks Gordon - I was looking at likely gas speeds in the manifold/carb bodies and chokes and I think it might be tough for Zeff to get the power he wants on 40s. In which case, playing with heads and cams will not help.

Zeff, have you checked the head flow with the manifold and carbs mounted? Of course on a flow bench you won't see the gas speeds that could cause the restrictions I am concerned about. I certainly don't see any gas speed issues at the valve, as you have plenty of curtain area.

[zefarelly]

good question, the answer is no! although head port face diameter is 33-34mm, so I think we have venturi effect as the inlect tract progresses . . .40.34,33, around 31 at the turn in the head, 35 throat ?

we're now discussing luxury developement territory, which for bottom of the garden workshop engineers is hard work . . . not to say I won't try

as michael said earlier, gas speed and charging may be the way to optimise, and . .back on topic . . . CR may well be optimised, as I think it shold be, in my case, by a combination of all factors, including cam timing ?

either way I'm sticking with it, its challenging and fun!

[phoenix]

Originally Posted by zefarelly

good question, the answer is no! although head port face diameter is 33-34mm, so I think we have venturi effect as the inlect tract progresses . . .40.34,33, around 31 at the turn in the head, 35 throat ?

we're now discussing luxury developement territory, which for bottom of the garden workshop engineers is hard work . . . not to say I won't try

as michael said earlier, gas speed and charging may be the way to optimise, and . .back on topic . . . CR may well be optimised, as I think it shold be, in my case, by a combination of all factors, including cam timing ?

either way I'm sticking with it, its challenging and fun!

I would have thought that the duration on the latest cam you are looking at will push maximum power up towards (or above) 9000 rpm. As this is way above your target speed, might a shorter cam, with maximum power in the 7000-7500 rpm range and which didn't hurt your CR be a better choice, as those last few degrees that the inlet valve are open really hurt dynamic CR?

[bdwoody]

Originally Posted by phoenix

I would have thought that the duration on the latest cam you are looking at will push maximum power up towards (or above) 9000 rpm. As this is way above your target speed, might a shorter cam, with maximum power in the 7000-7500 rpm range and which didn't hurt your CR be a better choice, as those last few degrees that the inlet valve are open really hurt dynamic CR?

Pheonix thats exactly the way i looked at it and i wont keep knocking the bottom end out coz of the high RPM , to finsh first , first you gotta finish as some wise person once said !... woody

[zefarelly]

I'm not so sure, we know the higher lift gives us more torque, and the flow increases up to 1/2" lift, despite not using all that

the cam lift @ 1mm is 274 degrees, this is a far more usable gfigure than total duration, some people won't even tell you half the figures, which makes me think their ramp angles atc aren't all theyre cracked up to be.

also the general efficiency of the old design of engine probably hinders some of the theory! . . . we'll soon find out

[SidewaysFeltham]

A higher Compression Ratio is not necessarily the ultimate way to produce power.

Both super and turbo-charged engines (as I mentioned before) use much lower than normal CRs. Also as I stated before, if super/turbo wants ultimate power it uses what is called Blow Down Scavange, which means the inlet and exhaust valves are overlapped even more, in order that the forced induction can literally blast every last vestage of burnt and partially burnt charge from the cylinder!

Even greater valve overlap means, obviously, that the effective (Dynamic) CR is again lower! (Since the piston will have already reached a position well past BDC and be well on its way back up the bore: ergo thre is far less swept volume left!). And since the static CR has already been purposefully reduced.................

The reasons this is done, BTW, is that with the far greater weight of charge, too high a static CR and you would melt the piston and keep blowing head gaskets, since the BMEP (Brake Mean Effective Pressure) starts going up, throught the roof!

Personally I believe that with the regulation restrictions placed upon the class Zef competes in, the only approach left (for gaining a few more BHP and performance) can be improving the velocity of gasflow and thus the effective weight of charge per cycle. (Basis, greater gas velocity = more weight for any given period).

[GORDON STREETER]

The long duration cam that I use with its closed up LCAs and 50DCOEs does make it a "little" difficult to drive with only a 4 speed box. But it reaps HP up the top end, its just a different way of driving that I have got used to in my dotage !

[SidewaysFeltham]

Off topic: heard from my chum, yesterday, Gordon,: he's currently at his villa in Torox, near Nerja. 35 degrees Friday apparently. Since Almeria is only sort of just up the road a tad, how hot are you?

[GORDON STREETER]

About the same (hot) I might go to Torrox next week to see a mate, although that "tad" is about 3 hrs down the road from Mojacar !

[phoenix]

Originally Posted by Michael C Felth

Also as I stated before, if super/turbo wants ultimate power it uses what is called Blow Down Scavange, which means the inlet and exhaust valves are overlapped even more, in order that the forced induction can literally blast every last vestage of burnt and partially burnt charge from the cylinder!

Even greater valve overlap means, obviously, that the effective (Dynamic) CR is again lower! (Since the piston will have already reached a position well past BDC and be well on its way back up the bore: ergo thre is far less swept volume left!). And since the static CR has already been purposefully reduced.................

As I understand it, blow down scavenging relies on opening the exhaust at any time before BDC on the power stroke while the gasses are still expanding. To increase blow down scavenging the exhaust valve is opened earlier, which would reduce overlap. As the exgaust gasses rush out under their own power, so to speak, this reduces pumping losses and also ensures a strong negative pressure at the exhaust valve around TDC. Overlap does not have to be increased to take advantage of this.

However, even if this did require and increase in overlap around TDC I don't understand how this would reduce the dynamic CR. You say that the piston will be 'well past BDC' - please explain?

[SidewaysFeltham]

Not ignoring you, Phoenix! Under the hammer at present!


Later..........................

[zefarelly]

another coversation I had with someone, they suggested that due to the less then perfect design of the engine we'd be better off with the best volumetric efficiency possible, at the expense of a little compression . . . . which certainly wouldn't hinder relaibility

I'm now thinking Tig welding the floor of the combustion chamber might be a good idea, get the best of both !

[phoenix]

Absolutely you need a high VE!

To attain your 'holy grail' target 100bhp/litre you will need a VE of 100% at 8000 rpm or a VE of 106% at 7500 rpm or a VE of 112% at 7000 rpm.

Getting such high VE on a 2 valve per cylinder engine is not going to be a walk in the park. You need to get the intake and exhaust tuning spot on as well as overlap timing and gas speeds to get this kind of cylinder filling. I think flow through 40 DCOEs will become restricted over 7500 rpm so bear that in mind and aim to have peak power just below that engine speed.