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new build feels under powered

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355 chevy
13K views 66 replies 15 participants last post by  Project89 
#1 ·
hey guys just got my nova road worthy however im not too happy about it while driving it for the first real drive i feel it is massively under powered for what it is...which is...
sbc 355
all forged rotating assembly 9:1 cr
stock gm heads
cam is .488 lift
summit high rise single plane intake manifold
full roller rockers 1.5 ratio
msd ecurve dizzy 15deg intial 30 total
profesional products throttle body efi 750 cfm
and a gt45 single turbo on top

the car take s little while to start i think may be due to the time it takes to build initial fuel preesure at start up
it idles at 15 inhg smoothly and it doesnt seem to miss at all on the road
 
#35 · (Edited)
I've got to know the real detail; which includes part numbers and manufacturer if you don't have the specs. A turbo or any blown or laughing gas motor is not a simple accomplishment. It becomes a very large set of compromises the direction being set by the major use, an automatic transmission just adds to the difficulty because building the engine toward the high performance side then controlling it with gear selection makes the overall task a lot simpler.

I just finished an injected, turbo 911 for an owner with no cash flow problems, it took a year to build including extensive chassis, suspension and transmission work but the engine alone used up as much time as everything else and probably chewed up 80 thousand of the 100 thousand this cost him.

On the surface you have what I perceive as a mild cam in that it has a lot of duration and not a lot of lift, so what is called its Hydraulic Intensity appears fairly mild. But having the part number and maker or its opening and closing specs would be really helpful. Blown or juiced engines like a lot of exhaust duration and lift and for the ratio of the intake to exhaust valve size to be closer together than found in a normally aspirated engine. This is because there is more exhaust product to get rid of. This cam would appear to me to be very rampy that is long areas of slow and low lift chewing up lots of duration degrees. This just isn't good with a turbo, but again I need to know the events, I could be wrong on my assumptions.

The 624 head is kind of a do it all, but none too well head. It was found on 350's and 400s and had valve sizes that range from 1.72x1.5 in heavy duty trucks and industrial engines, to 1.94x 1.5 in average use 350s and did show up with 2.02 x 1.6 valves in some 350s and 400s. So when you say these have 1.94x1.5 is that by actual measurement? Supercharged and juiced engines like big valves and good porting. The blower does not overcome porting deficiencies, it just increases pressure not flow, remember gases like air and vaporized fuel are compressible so you can get a pressure change without a flow change.

The 1.5 rockers aren't doing you any favors with this moderate lift cam and lack of valve size (1.94/1.50- (especially exhaust) you're just not getting enough lift for the needed flows with these valves. This in particular backs up the exhaust to where the residual pressure in the cylinder is near the pressure coming off the compressor so the net effect of flow into the cylinder is much less than the pressure shown on the boost gauge. Again, flow and pressure are independent of each other where the flow of gasses is concerned. To a huge extent back pressure in the entire exhaust system is an issue. You need two gauges to really determine true boost pressure delivered to the cylinder as a function of CFM. One you probably have that measures intake pressure. The other gauge needs to read exhaust back pressure. To simplify, the difference between these two gauges of exhaust subtracted from intake is the Effective Pressure going into the cylinder. You will discover that with just 5 pounds of boost it's really easy to have no net gain over atmospheric as getting exhaust pressure under 5 psi takes a lot of attention to detail. This is one reason why guys tend toward higher blower pressure, but even this begets more exhaust pressure but there usually is some space here to mess with before the whole exhaust system has to be redesigned.

I can and will go on as I’m barely scratching this issue. Out of everything i've read (and I’m not negating the contributions of others at all) certainly cooling the intake charge for example is a good way to get around problems of cam timing, port and valve flows and exhaust pressure. But that works so much better when everything else is properly assembled and tuned.

I’m aiming at where I think your biggest problems are, but I’m leaving for a week of vacation and will be off line from this evening through the morning of the 24th.
 
#36 ·
If it wont rev up past 4kRPM:
1) running out of air due to turbo/intake air limitations
2)restricted exhaust
3)running out of fuel for any number of reasons
4)mechanical problem in valve train due to loss of valve control, usually a spring problem

I have a car in the shop with the first PP TBI system I have touched. The edit software is kinda clunky, and this car refuses to run the same for 5 minutes at a time. It also has the PP Fuel on Demand returnless pump setup.
I think that an A/F of les than about 12.0 is going to cost you horsepower, plus you cant count on the O2 to respond like a dedicated A/F gauge is capable of.
 
#37 ·
old bogie you believe its a matter mostly of valve sizing not allowing air flow mainly the exhaust... i plan to remove the muffler i have on the car hopefully that will aid a little bit with exhaust pressure i do plan to put a better set of heads on the car a little later on.. i wanna know why it wont spin past 4800 even if i bring the rpm up slowly it wont spin higher
 
#42 ·
i plan to remove the muffler i have on the car hopefully that will aid a little bit with exhaust pressure i do plan to put a better set of heads on the car a little later on.. i wanna know why it wont spin past 4800 even if i bring the rpm up slowly it wont spin higher
Not to contradict anyone but I believe something is going on besides the heads having 1.94" x 1.50" valves. A turbo- if done right- will work w/them. Not as well as it could w/bigger valves and/or better heads but it should pull hard to at least 6K rpm if not higher, regardless. By 6K rpm the boost may continue to rise but there will be a plateauing of the powerband due to the head restriction past that point.

How much boost are you seeing at 4800 rpm?

Is the engine smooth all the way up to where it stops accelerating or is it missing? Is there any popping through the carb or exhaust? What

Has TDC been verified? Not just by looking at the timing tab/damper line- but by actually setting #1 or #6 to TDC and see what the line/tab says? If it's off, the entire curve isn't what you think it is. Have you tried temporarily advancing the timing just to see if it revs higher? As long as the engine isn't under a load, there's no problem w/advancing it further than you would if you were driving the car.

Has the cam timing been verified? Compression test? Leak down? What do the plugs look like, color-wise? Fuel fresh? What octane?

What about putting a carb on it temporarily, just for testing? You might be under fueled. You've verified the TB is opening fully, right?

You didn't leave a grease rag in the intake, did you?:p Somewhere along the way you do need to disconnect the exhaust to be sure there's no excessive restriction there.
 
#38 ·
not reading any of this, i say instead of the muffler remove the whole tailpipe. when meinekie first opened i had them do my first exhaust. they welded a new pipe right up to the old one with the old one extending into the new at an angle about three inches in!!! maybe you got something going on like that.. or maybe yer cams too far adanced, (or flat). hate to be dum but aint readin 4 pages. i can tell you this tho, with four pages on this subject you HAD to have somebody smart as hell tell you what to do!
 
#39 ·
Pupsvette76, As you have been told the heads you have are poor flowing. I know you have to have 76cc chambers to keep your compression down. Look around and try to find a set of these heads cheap. 3973487-76cc, 1.94/1.5, 3973487X-76cc, 2.02/1.6, 3932441-76cc, 1.94/1.5. You can check with a machinist to see if the 1.94/1.5 valves can be upgraded to 2.02/1.6 without problems. Get a book on porting heads so you can do this yourself. Here is an article on head porting. Do-It-Yourself Small-Block Cylinder Head Porting - Tech Article - Chevy High Performance Magazine You can also get David Vizard's book, "How to Build and Modify Chevrolet Small-Block V-8 Cylinder Heads" You definitely need heads that breath and a large diameter (3") free flowing exhaust.
 
#41 ·
I have a friend at my local speed shop who can work me a deal on a set of afr heads I have to see if my valve reliefs on my pistons are big enough first because I believe these heads house a 2.05 int valve... I believe it's a 215cc runner and still a 76cc head
 
#43 ·
thats the school of thought i was on... i can dial it up to 20 psi even before 4800 but i have dialed it down i never verified exact tdc its pretty smooth acceleration though ... never advanced it past 34 and i never thought to advance it past 34 just for idle no load no a bad idea... compression is ok and it runs on fresh 93 the only plugs i can use unfortunately are the accel shortys since i have extreme header clearance issues and the throttle is definitely oopen
 
#44 ·
I'm not necessarily saying you need more than 34 degrees. What I AM saying is the timing marks- if not verified- might be making you think you have 34 degrees when you may not have that much.

The timing marks/TDC should be verified. A quick test for a lack of advance is to do as I said above- crank in some more timing to see if the situation improves. If it does, this might be pointing to an erroneous timing tab/damper line, or a problem w/the cam timing.
 
#48 ·
Don't get your hopes up too much- the chances are good something else altogether will be the prob, but at least this will eliminate one thing as a possibility and it's easy enough to do...

What about the turbo? Any chance the bearings or shaft is binding, and not letting the turbo spool up like it should? Because if it's not spinning up, it becomes a big restriction in the exhaust system.:pain:
 
#51 ·
pups is there anyway u can take a video of u driving the car, sometimes being able to see and hear is worth 1000 words

whats ur plug gap? noone has even asked , ur going to need around a .022-.026 plug gap

if ur at .032-.045 thats going to be a big part of ur problem

gear gears gears this is going to be another issue u have way to much gear plain and simple

u know the turbos is not an issue specially if it takes 30 seconds to spin down, the ex housing on that gt45 is massive and a 355sbc even with outstanding heads wont choke it

ur ex system on the other hand could choke it

what is ur timing under boost? ur only going to want about 20-22* is were it would be at with intercooling and aditional 2-3* if running meth injection, if no form of intercooling and using pump gas 18-20* from 5 psi and up is a good place to start
 
#52 ·
Timing is locked at 34 but I hear not ping I been having an issue with my intercooler whenever I run the charge thru it the motor produces no boost 0 psi for some reason with the charge pipe right to the bonnet of the tb I can crank it to whatever I want... I also have water inj not fully operational yet though spark plug gap is at .030 I planned to gap them down to .025 and yeah my gears are not a match for this motor I know plan to swap them out around Christmas time
 
#54 ·
are u still having trouble reving over 4k? if so what does it do , does the motor just fall on its face, or is it starting to miss and pop and bang?

if needed i can post a video of what it sounds like if the plug gap is to big and the ignition system isnt upto the task.my friends car i tuned would have the same symptoms at around 4,500 it wouldnt rev past there at all and would bang and pop.

went from .032 plug gap to .022 and its been flawless ever since.

what intercooler are u using is it something out of a junkyard car?if not whats the core size ?

genrally ur going to want something in the 32x12x4 range though a 32x12x3 will work for moderate hp levels

remind me again what car this is in and what trans ? 3:55 or one step down would prolly be a really good match . i do recall u saying no more then 5 psi in first gear. once u do a gear swap u will get alot more boost in first
 
#55 ·
Its a 70 nova with a turboed sbc 355 and a turbo 350 trans and 4.88 gears on a spool It doesn't pop or bang at all it just ceases to keep climbing in rpm and in fact if I hold the pedal Down even when it doesn't climb it will actually start to drop In rpm plug gap is .030 it's a new intercooler not sure exactly what core size I believe it's 27in long and 3in deep
 
#56 ·
ok let me post some videos for u in a lil bit, keep in mind the downpipe is only about 2.5 ft long so the noise u do hear would be very hard to hear if it had a full ex.

see if it kinda acts like what ur engine is doing.


have u checked the same for wiped lobes, possible rocker stud pulling out.bent push rod etc

will it rev in nutral or park?

whats the fuel system have for line size /pump what size injector
 
#57 ·
I have checked for wiped lobes and the cam is fine and all rockers were tight so that indicates no loose studs and I didn't notice and bent pushrods it does the same thing in neutral it will rev to 4800-5 and then stop no banging no rev limit sound just stop and then if I hold it then it will actually go back down the fuel system is a 255gph pump I bought the better pump with the kit 62lbs injectors with -8 feed line -6 return
 
#58 ·
what is ur afr when it does this, right off the bat u can eliminate the turbo and plug gap, since it does it in nuetral as well as gear, the turbo wont build boost during free rev.

still go ahead and tighten the plug gap anyways though

i would look into fuel system issues, for sure, get a fuel presure gauge and rev the motor up and see if fuel presure is droping


even with crappy heads he thing should deff rev up over 4800, hell even the cfi camaro motors will rev up higher then that and they have very very tiny ports
 
#59 ·
Ok just an update I messed around with the fi today and got it pretty dialed in didn't do much tuning just kinda changing some preset settings and it will now hot like 5300 still low since the rev limiter is at 7k... But anyway I went into my log files and my afr at WOT will hit 9 for like a millisecond and then rebound and search for the target afr which is about 12 at WOT fuel pressure never drops off lowest number I saw was like 42 and I have it set to 45 boost refernced
 
#60 ·
theres ur problem now with a wideband u have to be carefull if the motor is going way lean it can give a false rich reading on the wideband.

same goes for the other way around.

so go into ur ve map and take away a lil fuel in the area were its going into 9afrs and see if it helps , if u take away some fuel and it gets worse then u know its a false rich reading and u have to add fuel instead

what fuel injection system did u say u were using, if u need it i canprolly lend u a hand getting the thing tuned and looking at the datalogs
 
#61 ·
I think you need a cam with zero or negative overlap. As rpm builds the pressure in your exhaust increases to the point that it's higher than the intake boost. When the intake opens during overlap it's reversing flow because of the high pressure inside the cylinder. Another thing to think about is increasing the strength of your valvesprings because of the greatly increased pressures that their dealing with. The World Products heads that you mentioned would be a good choice, the 74cc heads that Engine Quest makes for IMCA racers would work well too.
 
#63 ·
After spending some hours with a PowerjectionIII the last few days, I would suggest you spend some tuning it as it is not nearly as self-tuning as they claim. Also, you have to go their website and read through the forum cause there is some stuff posted there that is absolutely necessary to know that is not in the instructions.
 
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