Compression linked to cam?

[littlecampbell]

I've been trying to figure out why my 340 is putting out 135 psi on all cylinders. Dosent burn oil and runs strong. Then i figured it out...I think...its that 484 cam,right? With that long of a duration it just bleeds off compression. Its supposed to be a .030 over motor with U heads, a 71 and thats about all I know about it. Oh yeah, supposidly 10.5 to 1 pistons. This thing should be in the 180psi range. Would the 484 cam bleed off that much compression?...Thanks

[23T]

Yes it's possible, the more over lap the more compression lost due to both valves being open @ the same time

[littlecampbell]

I'm wondering how much low range power i'm losing because of this. Its a stick with an airgap, 750 AFB, hedman shorties, 2 1/2 mandrels, 3.55 SG. Its a street car so all this is no big deal i guess. Would hate to lose a backwoods race because of it. Not that I would ever do that ...Maybe over the winter I should take the heads off and see what i've got in there. Get them decked as well. That low psi just bugs me!

[Toddd]

Are you going to Mission next weekend?. Toddd

[littlecampbell]

Toddd....Unless i croak from this colonoscopy on tuesday Or its pissin rain. Hope this weather holds. I hope to enter the show and shine and hook up with "Big Daddy" and Mr.Mopar...Should be a hoot! You comming?

[littlecampbell]

Hey Toddd...I see you were born the same year as your Barracuda ...what am i grinnin about....i'm an old freakin man

[23T]

When you checked the # was the carb @ WOT, engine hot or cold, bores dry or oiled?
If the car runs strong through all the gears and RPM's I would'nt loose any sleep over the #s you posted. If you have the same on all cylinders sounds like the engine's in good shape

[littlecampbell]

It was warmed up for about 30 seconds due to open headers (neighbors)...I believe it wot...this was 4 or 5 months ago...dry bores. Thanks 23T

[dave571]

Whats your elevation?

Here we're 3500 ft. I measured the components and know my engine is 9.9- 10 :1 and mine measures out at 135 psi too.

did the check right, wot, strong batt, warm ect.
A smogger measures in at about 120 psi.

I expected mine to be what it is, so yours sounds ok to me.

I'll see you guys in mission, too

[Toddd]

Indeed I'm comin over, unless the weather changes everything. I think I'm pickin up a carb from Don for a friend who can't make it. I'm comin over Saturdat morning on the first boat.

Just got the lowergears in and can't wait to try them out. See ya there. Toddd

[littlecampbell]

Well i'm not that far from Mission and the raceway is 24' above sea level. So maybe 3 or 400'. Dave so if your psi was say 180 what would your compression be?.....Supposed to sunny and hot next weekend

[littlecampbell]

Well i'm not that far from Mission and the raceway is 24' above sea level. So maybe 3 or 400'. Dave so if your psi was say 180 what would your compression be?.....Supposed to sunny and hot next weekend
Toddd...what gears did you put in?

[dave571]

It's my understanding that compression psi to compression ratios are guidlines only. I'm not sure how it converts.

When running, the difference in the compression, caused by the cam is dramatic. The result is "Effective" compression. Big cams reduce effective compression, subsequently they require more compression to operate properly.

Big cam in a smog motor reduces lowend, to the point of making the car a dog.

Theoretically, compression test results don't show the difference as dramatically, as it is. The cranking speed is so slow, compared to running speed, that a full charge of air is still achieved, and psi is artificially higher.

Short answer, I don't know what your compression is, but it seems like less than the 10.5:1.

You'll Have to take some measurements in the winter to see.

It better not be too hot, next weekend. I carry a litle extra insulation, so I sewat like norm on cheers LOL

[Toddd]

4:10 with 26 * 9.5 * 14 Hoosier quick time pros.

[littlecampbell]

Good stuff Dave. Makes me wonder why anyone would put these long duration cams in in the first place. Even nowadays theres guys that want these things in there. They sound real horny at idle maybe thats it ....We were figuring my compression was more like 8 to 8.5......Go figure.....The forecast for next saturday is 30* C. It'll be more like 35 or 40 around the track. So there will be two Norms with puddles around them! How about speedos?

[littlecampbell]

Yikes Toddd! Are you driving the beast over? Maybe a wheelie launch...

[sanborn]

Back in the 50s, stock camshafts had no overlap, race cams had very, very little-mostly by accident even then. Why? In the early 50s, many engines were flathead in design. Any loss in compression due to cam design couldn't be made up without additional cost. In the later 50s, after overhead valve design became popular, the camshaft trend continued-little or no overlap even with race cams. It still worked well with the small bore/long stroke overhead valve engines of that era. A radical race roller in that era might have 260-280 degrees of TOTAL duration. Total valve lift was up to .480"(Gasp!!). If you had an engine that produced a real 1HP/cubic inch-it was on the cover of Hot Rod Magazine.

In the 60s, we discovered that overlap could help suck the incoming intake charge better. And, since practically all engines were overhead valve by then, we could cast dome pistons to raise compression to make up for the loss of running compression due to the increased overlap. Valve lifts were up over .500"-but not by much. A very good race engine had 1.3HP/cubic inch.

In the 70s, we racers almost went to the extreme. Durations went wild-up in the 320-330 range, this created extreme overlap, but we could make up for the compression loss by using domes as large as my fist. Mechanical compressions of 15 to 1 weren't uncommon on race engines. HP was about 1.5/cubic inch.

In the 80s, we discovered flow benches; and began to figure out that if we used very high cam lifts coupled with shorter durations, and had high flow heads, we could produce more HP. Domes began to shrink- and we made more HP. We also discovered higher ratio rockers- all the way up to 1.7 to 1. Good HP was about 1.8/cubic inch.

In the 90s, we learned that decreased valve angles produced more power. GM went from 23 degree, to 20 degree, to 18 degree, down to 14 degree. Ford did the same all the way to 10 degree. Mopar went from 18 degree, to 15 degree, finally to 12 degree. To make up for the decreased valve angle, we raised the intake ports. The exhaust was already better just due to the valve angle change. The flatter valve angle let us use smaller combustion chambers, which eliminated the need for domes, we even found out that a precise dish could produce more power. rocker ratios kept going up-to 1.9 to 1 to let us run even shorter durations. Good HP was 2HP/cubic inch. Big bore/short stroke was the ticket.

So here we are today. Where are we going? To heads with staggered valves(GM SB2, Mopar P5 and P7, Ford will bring one out next year). Duration is getting shorter, rocker ratios are getting higher(2.0 to 1), chambers are getting smaller(down in the low 30s), dishes are getting deeper, HP and RPM is going up. HP is already at 2.2/cubic inch and rising.

Where will we be 10 ten years from now? Who knows.

I know this goes beyond your question-but I was on my fourth cup of coffee- I felt a burst of energy. I'm over it now, need a nap, ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.

[wedge440]

Originally posted by littlecampbell
I've been trying to figure out why my 340 is putting out 135 psi on all cylinders. Dosent burn oil and runs strong. Then i figured it out...I think...its that 484 cam,right? With that long of a duration it just bleeds off compression. Its supposed to be a .030 over motor with U heads, a 71 and thats about all I know about it. Oh yeah, supposidly 10.5 to 1 pistons. This thing should be in the 180psi range. Would the 484 cam bleed off that much compression?...Thanks

The cam could bleed off that much compression if your compression was low to start with...
My 340 with 9.46 to 1 messured compression had 135psi. per. cyc. with the .557 mech. cam.....Changed it to the .474 hyd. and cyc. pressure jumped to 210psi.....
The 340 ran good with the .557 in it but it ran much better after the .474 was installed.....
Sounds like to me your compression is in the 9 to 1 or lower area..
I would try a smaller cam..
Joe

[MitchB]

Should be the intake closing event which most affects cranking (and dynamic) pressure. No? And this is most closely linked to duration.

Mitch

[sanborn]

Mitch, the intake closing does have an impact; but, maybe not as much as you might think.

A properly designed intake system(heads, intake manifold, etc.) has a flow velocity at valve opening. If the system has a good flow velocity, the cylinder will continue to be filled with incoming mixture past bottom dead center. In other words, the cylinder can be "overfilled". Now, if the intake valve is held open too long, velocity drops to zero and part of the incoming charge can actually be pushed back up the intake manifold. In that case, not only is compression reduced, but you get black residue back up the intake. Bad situation=poor intake system or poor cam choice.

[jelsr]

This is like Sanborn's post, kinda food for thought. Along with the camshaft and CR changes we now have multiple intake and exhaust valves, 4 per hole is getting commonplace, and I read of a Ferrari with 5. The latest, greatest Formula 1 car (Ferrari, I think) turns 19,000 RPM. The motorcycles, which have always been at the cutting edge, have racing models available to anyone with the cash, that produce 5+ HP per inch on gasoline, naturally aspirated. It's changing fast, folks!

[drag-n-cuda]

littlecampbell,
I've calculated my 70 340 at 9.8 - 9.9:1 mechanical compression ratio and get 160 psi dry, wot at 1200 ft elevation with my 284 MP SOLID cam. I don't recall the overlap, but I believe it was in the lower 60's°. Also, how sure are you that your gage is accurate? Hope this helps.

drag-

[b-1ken]

It's the intake closing angle which will cause the cranking pressure to be low, not the overlap. As was stated, the air will be pushed back out of the cylinder on the compression stroke until the intake valve closes at cranking speeds. At higher engine speeds, the late intake closing angle will help cylinder filling and will increase the compression pressure.

[littlecampbell]

Thanks for all the replies. I'll print them all out. Also try another gauge. Elevation here is actually 30 feet. Sanborn, thanks for the history lesson ...Maybe try decaf or a shot of Seagrams VO in that java!