Thread: new top end and no more power???

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I went through the exercise of calculating your DCR for both straight up and 2* retard installation. straight up (intake close 41 ABDC) DCR=8.27:1... 2* retarded (intake close 43 ABDC) DCR=8.20:1. At 4500' altitude, barometric air pressure (at 68*F and 20% relative humidity) is 25.37inHg or 12.46psi (compared to 14.7psi at sea level). Given your DCR of 8.2:1 and 12.46psi your cranking compression should be 102psi... given whatever the prevailing weather was during your test, this could easily account for 2psi.

techinspector knows more about what a good DCR is than I do but I do know that 8.2:1 is pretty low if you want any kind of performance. I think 8.5-9.0:1 is a good target for pump gas friendly power. And at higher altitude you can push that a bit further... BTW: with the specs you gave here, my compression ratio calculator says you have 8.98:1 SCR.

Edit: I was using 64cc chamber... with 62cc chamber SCR is 9.16:1

Good luck...

-Chris

my exhaust is 1 5/8 inch header tubes to 3 inch collector to dual 2 1/2 side exits with an H pipe after the collectors. i have dynatech split flow mufflers.

i thought it might be the valve seats too, but the cylinders hold pressure they just don't get much psi. and with the stock setup i was getting 120psi and holding it so it couldn't be the rings. i'll try advancing the cam staight up here in a few days and see if that will help.
about spark everything from the ignition switch to the spark plugs is brand new.
and i did swap out the springs that were on the head for Lunatis that were good to just over .600 lift with the installed height they're at. they have 120lbs. seat pressure and around 310 open.

what is DCR? and how could i have less cranking compression with higher static compression? the cam isn't that big. is 2* cam retard really that much?
i was getting 120psi with the stock cam and heads and head gasket and all that "low compression smog" stuff at this same elevation and everything else.

i also did a vacuum test and i'm only pulling 10in at my 700-800rpm idle????

i'm just curious, which g tech do you have? my friend has the old one. it's just a little black box with red numbers on it. and his has been pretty accurate thus far. with manual and automatic cars. low performance, high performance, all wheel drives, turbos, etc. i work at summit racing over in sparks, nv and we have dyno'd everyones hot rods who works there and g teched right after, so weather was the same, and yeah you lose a little because it measures pull and if you can't get all the power to the ground it shows, but the car that was most off number wise, was a 69 chevelle 396 with a 4 speed. his car ended up being about 20 hp short. now i have bought the new one for me because i thought they were all great, but the latest one out, with all the graphs and everything is so erratic and unreliable it's truly unbelievable. on the same road run after run after run with my cars and my buddies the numbers have varied almost 100hp. all i'm trying to say is i know it's possible for the g tech not to work, but i am a true believer in my friends'. and like i said we have used it on dyno'd cars, my truck for example and it was only 4hp off!

thank all of you for your help so far

DCR is dynamic compression ratio which is dependent upon the timing of the cam specifically the intake closing point.... the intake valve closes some time after BDC so that the amount of mixture that gets compressed is less than the full volume of the cylinder (in a cranking test that is... with engine running the amount of mixture compressed depends upon the volumetric efficiency at different RPMs which is getting into more of the science that I don't fully understand)... if your stock cam had an earlier intake closing point, your compression test would have a higher reading... but, at 9.17:1 compression the highest possible cranking compression you would have is in the ballpark of 9.17*12.7(air pressure at 4000')=116psi with intake close at 0* ABDC (maybe higher if it was a really cold day and this doesn't account for a slight increase from the heat generated during compression)... so I think it's unlikely that you had 120psi unless the stock static compression ratio was higher than what you have now. 10" of vac seems a little low but not too far off considering this is a somewhat "hot" cam and we pull less vac at altitude but you should make sure to check for vac leaks... a high speed lean out (caused by a vacuum leak) would nose you over similar to what you describe. Is the idle smooth at 700-800rpm? When you checked/set your timing did you have vacuum advance disconnected and plugged? I would start from 36* full advance (no vac) then increase it by 1-2* increments and see if that picks up the power. You should read your plugs for mixture and timing (search for threads on here as there is a bunch on plug reading).

I have a g-techRR that is about 3 1/2 years old. The 1/4mile times and speeds and split times it figures are quite accurate with what I get on my time slips but the power curves read in the 250hp range while it takes ~400hp (uncorrected to the wheels) to run a 3300# car to an 11.7s 1/4mile. Dynosim puts my motor at ~550hp less ~17% for altitude and ~12% for drivetrain losses that puts you about ~400hp to the wheels up here in Denver.

-Chris

"i also did a vacuum test and i'm only pulling 10in at my 700-800rpm idle????"

Again, too much cam.
Now, you can listen to me or not listen to me, that's up to you, but I don't spend my time answering these problems because it makes me any money. I'm trying to bring 50 years of puttzing with this stuff to light in order to help you and others from making the same dumb mistakes that I and others have made.

Pull the timing cover and advance the cam 4 degrees past straight up. When you find that the motor has awakened a little from its snooze, you'll realize that you have installed a cam with a too-late intake valve closing event for the available static compression ratio, which, by the way, is 8.81:1, no matter what you think it is or what you wish it to be. I've poured these early smog pistons and they're 22 cc's. Go back to post 9 of this thread and read my answer again.

With 8.81 scr, you would need a cam that closes the intake valve at around 15-18 degrees ABDC @ 0.050" tappet lift.

I would also put a dial indicator on the retainers and hand-turn the motor through a couple of revolutions to check lift at the valves. Those behemoth valve springs could be turning your cam lobes into mush.

If everything checks out, follow skid's tutorial concerning ignition lead.

[QUOTE=techinspector1]"i also did a vacuum test and i'm only pulling 10in at my 700-800rpm idle????"

Again, too much cam.
Now, you can listen to me or not listen to me, that's up to you, but I don't spend my time answering these problems because it makes me any money. I'm trying to bring 50 years of puttzing with this stuff to light in order to help you and others from making the same dumb mistakes that I and others have made.

Pull the timing cover and advance the cam 4 degrees past straight up. When you find that the motor has awakened a little from its snooze, you'll realize that you have installed a cam with a too-late intake valve closing event for the available static compression ratio, which, by the way, is 8.81:1, no matter what you think it is or what you wish it to be. I've poured these early smog pistons and they're 22 cc's. Go back to post 9 of this thread and read my answer again.

With 8.81 scr, you would need a cam that closes the intake valve at around 15-18 degrees ABDC @ 0.050" tappet lift.

I would also put a dial indicator on the retainers and hand-turn the motor through a couple of revolutions to check lift at the valves. Those behemoth valve springs could be turning your cam lobes into mush.

If everything checks out, follow skid's tutorial concerning ignition lead.[/QUOTE YES YES AND YES i would never retard a cam for street with low cr have done it in the past with very hi CR bbc were low end just was not what we needed

oh ok. i understand now. i have checked for leaks and it checked fine. i think i'm going to install the cam 4* advance from straight up like techinspector1 said. i'm sure you know more than i do i'm 19!!! so i hope you know more than me but seriously thanks for all your help guys i really appreciate it.

i would swear on my truck that at this altitude, when the engine was warm i did a compression test and 6 of the cylinders were over 120psi and the other 2 were about 115. and i know the compression wasn't higher than it is now. i honestly don't know what's going on with that but after i advance the cam i'll let you all know how it goes and what the new test measures. idk how soon it will be. hopefully this weekend or the next, whenever i can get my other car running again. does anyone know anything about 1990 tubo lasers??? hahaha just kidding i'm on another forum for that......

the idle is fairly smooth. after a few seconds or so you can hear it lope a little then pick up the beat again.

thanks again for all your help. you guys are awesome!

ok sorry it took longer than i thought but i FINALLY got around to advancing my cam. i advanced it 2 degrees from straight up (4 from what i was at). the only reason i didn't go 4 is because i had no ride to summit to get advance bushings and my crank gear only had a slot for 2*. now my cranking compression is at 130 to 140 psi around all cylinders. which is a nice improvement, BUT THERE IS NO CHANGE IN PERFORMANCE! no better throttle response, low end pull, top end pull, no better gas millage....it's all the same. i just took it to my dyno friend and i am still making around the same horsepower and torque. 5 more ft lbs and 5 less hp at 300 rpm less then what it was at when it was 2 degrees retarded.

now i am totally lost. what kind of horsepower numbers would you guys expect from this motor if it was yours??? all i know is i had these heads on my old motor and it was a lot faster then and what DYNOSIM tells me. is there any chance that my losses are from my bottom end? i know that sounds kind of dumb but i am all out of ideas. no exhaust leaks. ignition timing is right at 14* initial/34* total advance, air fuel ratio is still at 13 to 13.5:1 all the way through redline, oh but now i'm only getting 7 to 8 in of vacuum at my 700 rpm idle?????

please help me

thank you

Did you check the actual cam lift at the valve to ensure the lobes are still at 100%? Not a chebbie guy, but isn't the stock rocker arm ratio 1.5? I see you are running 1.6 rockers......maybe the combination of too much duration and the higher ratio is making for way too much cam?????

you need some dang compression and a different cam

no i didn't check the valve lift. there is no ticking and no lash in my valvetrain though. i thought going to 1.6 from 1.5 rockers is about .030 lift and about 2* duration @ .050, correct? i guess that would only make matters worse.....what kind of compression would this motor need to run correctly?

compression can make that big of a difference? i was always taught that compression can make or break 10% of a motors power....is it more than that? if i changed the cam how much smaller of a cam should i go? 210* @.050? would i really make more power with a smaller cam? i didn't think this cam was even that big. like i said before, my friend has a pontiac 428 with the performer RPM cam and intake, with stock heads, stock bottom end and he's making 30whp more than me. wouldn't that be too much cam for him too?

thanks for all your help on this.

Compression is horsepower...plain and simple. Compression has a lot to do with what cam the engine wants and how it reacts to it.... I don't do cam reccomendations, I have my favorite cam company, when I need a cam I gather up all my engine data as well as the rest of the drivetrain and call the companies tech reps--not some mail order phone rep, but a guy who really knows cams. Some on here know cams well, I don't so I'll defer to them....

Your friend with the big inch Poncho? Big inch engines need a way different cam... If I ran the same specs on a cam in a sbf that I run in my 514 stroker, the engine wouldn't work well at all....

+thats the same principlae with my motor a really mild cam in a 400sbc may be a monster in a 283.

I lost 100 ft-lbs torque changing from a 255* to a 270* with 10.8:1 compression... changed it back. You need a cam that is compatible with your compression... a big cam will make more power but you need more compression in order to run the big cam. And as said, a big motor will tame a wild cam and vice-versa

-Chris

Originally Posted by skids72

I lost 100 ft-lbs torque changing from a 255* to a 270* with 10.8:1 compression... changed it back. You need a cam that is compatible with your compression... a big cam will make more power but you need more compression in order to run the big cam. And as said, a big motor will tame a wild cam and vice-versa

-Chris

Good note on cam selection, Chris!!!! Cam and Heads is not always a case of bigger is better.... The cam you choose has to compliment the rest of the engine and work with it, not hold it back. The two most common errors in any engine build is the wrong cam and too much cylinder head flow. What works on an 8,000 rpm drag engine with 12.5:1 compression and everything else tweaked to the max is almost guaranteed to turn your street engine into a pooch.....

ok i understand now. i tried matching my intake, heads and cam selection together but i honestly didn't know compression ratio had such a big effect (and i thought it was over 9:1 anyway). i was always told and read otherwise, but i guess that would make sense now. i mean i've done some research on cams and head selection, not enough to be an engineer or anything, but even the basics of cams and heads excite me i know how overlap, intake/exhaust opening/closing, LSA, cross sectional area, valve size, spring pressures, rocker ratio, and how too much flow and duration can be detrimental to power BUT i did not know that compression played such a big role. thank you guys very much for pointing that out. i don't know how i could miss that, it makes so much sense now it's weird i didn't see it before. i feel REALLY stupid.

and the whole reason i started with the cam retarded in the first place was not only because DYNOSIM said it would help out a little, but because i know the heads i have are pretty big (about perfect for my 360 at 6500rpm correct?), and didn't want to go too wild on the cam (under 6500rpm because of my stock bottom end) but i did want to open up some more top end power.....but i've never tried retarding a cam before and didn't know exactly how any cam or engine combination would react with that kind of adjustment. i figured because most cams are ground with advance built into them, and are intended to be installed straight up, that i would still have enough advance for my combo to work. ha ha boy did that backfire!

also would being at a high elevation change compression enough to make a difference? now if i did put another camshaft in my truck, is it possible i may get more power, or would it just be more efficient?

thank you guys again for all your help.