Carbs (they're the work of the devil)

[MikeBz]

All the sound wisdom I can find suggests that you shouldn't run a bigger barrel carb than is necessary to take the choke size your engine needs to make max power. More specifically, all sound wisdom I can find suggests that for say a 170-190bhp 4-cylinder engine you're likely to want chokes in the range 35-38mm. So for a stock Vx red top there would seem no point in fitting a pair of Weber 48s, 45s will give just as much power and better driveability/mid-range. And yet people do fit 48s (and to 'stock' Zetecs etc.). Why oh why oh why?

Mike

[norman-normal]

Mike, the most important thing to remember is:The carb should be in harmony with the cam. Secondly aside from increasing the bore, stroke, compression or supercharging, any power you gain high RPM will be lost at low RPM.
So taylor your HP increase to the RPM range you will use most. Peak HP is allmost usless.

Its very easy to spend a lot of money and end up with a slower car than you started with.

[Al Weyman]

You are of course correct and never more so than the selection of a 4 barrel carb for an American V8. An 850cfm carb invariable will not work so well as a 750 unless you have one big engine (6.6 litre plus) simply because each cylinder sees 850cfm and cannot draw enough vacuum at lower speeds to work correctly, not so bad for out and out top end or drag racing but no good for circuit work. In fact the problem is far worse than your example of dellortos or webbers where each cylinder will only see one comparitively small small choke (compared with a 4 barrell Holley that is). I am going to swap the 850 off my car for a 750 as it works better.

[zefarelly]

I run 40's ( as homologated) on 34mm chokes, I could go to 36, and am unlikely to ever need much more without cheating, ie radically changing the rest of the engine to scream high enough revs to warrant bigger chiookes and 45's, it would then become virtually undrivable IMO under 6k rpm as it is it seems to sork well between 4-7500 which gives plenty of room for error with gear changes !!!

[graham bahr]

zef, 36 chokes would be a total waste of time, you would loose performance, once you get to a 33 choke the choke no longer restricts air flow through the carb, its the aux vent and throttle themselves that become the restriction, going too big on chokes only looses drivability,

if your were to down down a bit on your currently fitted 34's (you would probably need to go down a size or two on the mains also) your engine would probably come on cam earlier for no loss of topend

when we used to build x/flows we found once you reached 32mm choke you needed to go to a 45 carb to pick up more power

[zefarelly]

34mm seems to be the best so far, but in all honesty I've never had the time or money to experiment any further . . . . but soon I will

the ratio of inlet / exhaust is, in part at least down to comnpression though, I can ( theoretically) run bigger inlet valves and same/smaller exhaust valves with higher compression, although I doubt it would gain anything useful other than a bit at tht e top end at the expense of the mid range/bottom end

I have at home ( experiment No 37 in an ongoing series ! ! !) some vegantune VTA valves (41/35mm) I reckon I can squeeze in a precrossfloe head and run on a 1650 bottom end and get a good 12:1 maybe 36mm chokes and a KC254 or A6 might be a bit hairy for my road car.

[Al Weyman]

Just stick a small block in it Zef and stop messing around, you know it makes sense:-)

[dtype38]

Interesting point MikeBz, and I agree with Graham (I think) but I'm surprised at the sizes you are quoting for the HP values. I run a 6cyl and used to run 36mm chokes in 45mm barrels (Webers). With those and standard road cams I was getting about 215hp. I went to 39mm chokes and warmed the cams up to fast road and got up to about 230hp. After reading some stuff I realised that I couldn't go any further on the chokes without losing the bottom end, but I still seemed restricted at the top, so I've now gone up to 42mm chokes in 50mm barrels. I'm on the rolling road tomorrow, so I'll report back either in happiness or repentance :-)

[Al Weyman]

I thought the 4.2 Jag would have came from the factory with more power than that Dtype, thats very conservative for such a large engine.

[Tim Falce]

Originally Posted by Al Weyman

I thought the 4.2 Jag would have came from the factory with more power than that Dtype, thats very conservative for such a large engine.

I think it's a conservative rolling road he uses. My car makes around 250bhp on the local rolling road but only gets 221 on the one Dtype38 uses.
Providing he uses the same one each time it won't matter how conservative the figures are.

[Al Weyman]

Oh I see and I assume then that was an 'at the wheels' reading.

[dtype38]

Originally Posted by Al Weyman

I thought the 4.2 Jag would have came from the factory with more power than that Dtype, thats very conservative for such a large engine.

I have a very conservative state of tune on my engine Al. It's all aimed at the widest and biggest spread of torque I can get. The latest carb change, together with much longer intake tracts, is intended to bolster the higher end of the curve. I'm actually not that bothered about getting high peak horsepower figures as one of the race series I compete in has classes by power-to-weight ratio. That's not to say there aren't plenty of people out there tuning their 4.2 Jag engines for peak horsepower. Lots claim 300hp or more. Occasionally, if the straight is long enough, they can even overtake me

And no, Al, that's a flywheel figure calculated from the rundown drag on the rolling road. At the wheels I've got something more like 190-195hp, and yes, a Jag transmission really does lose that much!

[dtype38]

OK, just reporting back on the results of my rolling road test. Replacing my DOCE45's on 39mm chokes with DCO50's on 42mm chokes gave me an 8% increase in peak power (18-19hp) and a 12% increase in peak torque (29-30lb-ft). There was no loss in low end tractability, but that could be because I also added a couple of inches to the length of my intake bellmouths and fitted a mappable ignition system. Anyway, I'm pretty pleased with that for a £650 outlay.

[Al Weyman]

Now you can kick ass:-)

[dtype38]

Can't wait! :-)) Well, all except those 600hp Cobras I'll be racing, oh and the Crossle's and..... :-( Probably best not to think about it and just go and have fun :-)))

[graham bahr]

Originally Posted by dtype38

OK, just reporting back on the results of my rolling road test. Replacing my DOCE45's on 39mm chokes with DCO50's on 42mm chokes gave me an 8% increase in peak power (18-19hp) and a 12% increase in peak torque (29-30lb-ft). There was no loss in low end tractability, but that could be because I also added a couple of inches to the length of my intake bellmouths and fitted a mappable ignition system. Anyway, I'm pretty pleased with that for a £650 outlay.

excellent result Dtype, you should really notice that on circuit

we all knew you really needed bigger carbs, every one can breath a sigh of relief now,

now all you need is some bigger cams!

[zefarelly]

never happy are you Graham !

I'm waiting to plonk an engine on a Dyno and play, I still think I can get 100 BHP per litre legally. and hopefully reliably, but the revs will hit 8k in doing so I reckon

[dtype38]

Originally Posted by graham bahr

now all you need is some bigger cams!

And I'm told I'm already pushing my luck with my standard pistons and conrods! Thanks for the support though... maybe its a good time to start saving for next winter

[Al Weyman]

Originally Posted by zefarelly

never happy are you Graham !

I'm waiting to plonk an engine on a Dyno and play, I still think I can get 100 BHP per litre legally. and hopefully reliably, but the revs will hit 8k in doing so I reckon

Well according to our Rob (sorry Racing 59, I could not resist ) on the SCV8 thread you should be able to get 125 bhp out of a two valve engine, I personally don't buy it the same as I don't really buy 750 bhp small block Chevy circuit race engines (maybe single run Nitros drag race but not circuit race) only my opinion of course.

[R59]

Al,

30 years ago, if you got 190bhp out of a 2L Pinto, that was about your lot.

Note that RS2000 we mentioned in the Escort thread, turns 230bhp on Peter Baldwin's rollers. An optomistic wheel figure, but a splendid flywheel figure.

Ideas have moved on, technology marches at an alarming rate these days.

Take the engine in your black car Al, it's a 23deg 350ci, and you say it turns out 550bhp? A 18deg engine should turn out about 70-100bhp more, and yours is far from "wild" - what's it rev to - 6500? 7000?

Big block engines are not as efficient as SBC's. The 8.2L BBC in the March 707/717 Can-Am car was turning a genuine 997bhp on a british dyno.

Rob.

[Al Weyman]

and yours is far from "wild" - what's it rev to - 6500? 7000?

Bit more wild than you may think actually, it has fully ported race heads, roller cam, followers, and rockers, all steel Manley rods, lightweight race pistons, steel crank and will rev to 7800rpm. Lets see what happens when I get this custom built electronic dizzy fitted because I think the old twin point standard coil system is letting it down a bit, we will see as soon as these bits arrive it is out again in TRC or Classic Thunder which ever round comes first.

I know one thing I will have to build a better engine for the yellow car if I am going to get on the pace, I need the one thats in it for the other project anyway. I and have a few bits together, 4 bolt block, steel crank, roller cam, rockers and followers, pair of pukka and legal F5000 period heads etc. Just need some steel rods, some pistons and an inlet manifold and I should be ready to build.

[graham bahr]

Originally Posted by racing59

Al,

30 years ago, if you got 190bhp out of a 2L Pinto, that was about your lot.

Note that RS2000 we mentioned in the Escort thread, turns 230bhp on Peter Baldwin's rollers. An optomistic wheel figure, but a splendid flywheel figure.

Rob.

arrh peter baldwin, yes hes great at tuning, its a shame he give out pie in the sky figures as thats a real 210 flywheel then !

[Al Weyman]

I took a Sunbeam Talbot up to him once years ago, tuned it nice I must say, can't remember the BHP it made.

[Richy_Rich]

I use Pete, his figures are a bit high but consistent and he's pretty awesome at tuning the old carbs...

[R59]

From memory, here are some comparisons:

Formula First had 84bhp at the flywheel and showed 73 on his rollers.
Pete's BDA mini had 180 at the flywheel and showed 160 on his rollers.

Peter Last, the man from Sun, who calibrated Pete's rollers always said that they were spot on when he'd finished, but Pete would take them up two clicks to get his familiar figure for the Mini!! They are less than flywheel power, but not as much as they should be. With a syncro box, you should expect to lose between 25 and 30% of your flywheel power, where a dog box should only lose about 10%. (Figures for front wheel drive, taken with wet finger in air).

Now, taking Pete's figures, expect 8-10% difference between wheels and flywheel.

I worked it out with a comparison between Pete's and Bill Blydensteins (which had, at the time, just been serviced and gave a "corrected flywheel" figure). Somewhere I have the workings out... probably in a box in the loft at my parent's house.

Rob.