So I have a 1970 something truck block with 4 bolt mains and it is bored out to a 385.5 to the best of my knowledge. It has a real big cam, retrofit hyd. roller lifters, trickflow AL. heads and Air-gap intake. 750 holley DP with new main body to have adj. air bleeds. Exhaust is big, long tubes and 4" cat back, no cats. This setup should be good for around 500 FWHP. So maybe 425 HP at the wheels.

I am getting 355 HP at the wheels....

I have told you just about all I know, the place that built it went out of business so I am at a loss there.

Here is my plan, real simple.

Get a local racing shop to take the whole thing appart, and build it back up with the same parts, but have them make sure it is all OK, (I don't even know an exact CR #s) I mean, maybe have them use different pistons or something, and of course new bearings, or maybe mill the block to get a nice surface.

Anyways, you get the idea and the perdicament I am in. The motor is going to be all NA, with a 250-275 Shot of N20 on it. All street with the juice for the track. I don't care how ruf my idle is, it is all ready just about as ruff as it can get and still stay on at a red light.

Thanks for any insight.

 

You start with the basics.

Check to make sure it's getting enough fuel.
Check the timing.
Do a compression and cylinder leakdown test.

Without any specifications, it's hard to give any kind of opinion.

Did you put it on a dyno to get 355 HP or are you guessing?

Maybe the engine is making as much HP as you think but the rest of the car isn't using all that power efficiently.

 

i agree with Stephen you need to check your motor first before spending all that money to tear it apart.

get a gauge and check the compression of each cylinder, check the timing, check your fuel, maybe the motor only makes 450 horsepower and with drivetrain loss is getting 355HP seems like it should make a big more but like what was said check the basics

 

Sorry that I didn't state that earlier.

I have checked all the basics over and over again... The 355HP was on two seperate dyno trips. So it is very confirmed. The car has a T-56, chromoly DS, and a Moser 12 bolt with 4.11s.

I have checked fuel and timing, so has the dyno shop. Twice. I have checked spark, running an MSD pro-billet with a digital 6+ box. I did compression tests on 2 cylinders and they showed normal.

I guess I could check the rest for starters...

Oh and the engine is definetly getting ebough fuel. Holley volumax pump (electric) moves 150 GPH. All lines are 8-AN all the way to the carb. Return style regulator. I am also running a 15 gallon fuel cell. I KNOW I have enough fuel. Problem is someplace else.

This is really all I know. I figured if I am going to get this thing tunned right, I might as well know the motor exactly, get it blue printed so I can start learning how much N20 I can spray.

Thanks for any help

 

Check the compression on all the cylinders to see if you have a bad one. Other than that you may have low compression all around. Some 70's something truck blocks had like 8.5 to 1 compression. Did you change the pistons?

 

Post the dyno curve.
It may give a clue.

Buy a degree wheel and a dial indicator, and get an idea as to what cam is in it. Measure the valve events and post it up. (or disassemble and get a part number...)

 

Fuel cell in the rear? Pushing fuel from the back of the car to the front under acceleration through a 1/2" line may need a bigger pump. that's a lot of fuel that the pump is trying to push. When I had a fuel cell mounted in the rear, I was using a BG400 pump to push the fuel to the front through a 1/2" line. Running alcohol, I also needed a lot more fuel.

Fuel pressure should be around 7 PSI. Is it maintaining that pressure at WOT through all the gears (1/4 mile run)?

The needle and seat in the float bowl may be too small and not enough fuel is getting to the jets. Your 750 carb, although should be more than enough, may be too small.

Exhaust may be too big but it's hard to know without having cam specs or even knowing exactly what heads are on the engine. There's a possibility that there's a bad combination of parts being used but you have no real specs to know.

 

Valve springs could be too light, and floating valves at the higher RPM where you'd make the HP, etc etc.
Dyno curve might give some indication. That's how you "sherlock holmes" it.

 

That's a big part of the "mystery" right there.

 

OK here are all the parts in the motor. It was a Trickflow setup that was sold over summit that was supposed to make 500 FWHP, at least thats what trickflow said.

1974 truck block, I think. Bored, honeds, and fully prepped. Just don't know what my final deck hieght was or things like that.

Crank:
Brand Eagle Specialty Products
Manufacturer's Part Number 435037506000
Product Line Eagle Forged 4340 Steel Crankshafts
Summit Racing Part Number ESP-435037506000
Rear Main Seal Style 2-piece
Engine Stroke (in) 3.750 in.
Crankshaft Material Forged 4340 steel
Engine Balance Internal
Rod Journal Diameter (in) 2.100 in.
Main Journal Diameter (in) 2.448 in.

Rods:
Brand Eagle Specialty Products
Manufacturer's Part Number 5700B3D
Product Line Eagle ESP H-Beam Connecting Rods
Summit Racing Part Number ESP-5700B3D
Connecting Rod Length Center To Center (in) 5.700 in.
Connecting Rod Beam Style H-beam
Connecting Rod Material Forged 4340 steel
Connecting Rod Finish Shot-peened
Wrist Pin Style Floating
Clearanced for Stroker Yes
Connecting Rod Bolt Brand ARP
Connecting Rod Bolt Diameter 7/16 in.
Connecting Rod Bolt Material 8740 chromemoly steel
Connecting Rod Bolt Head Style 12-point
Approximate Connecting Rod Weight (g) 630
Big End Bore Diameter (in) 2.100 in.
Pin End Bore Diameter (in) 0.927 in.
Advertised Horsepower Rating (hp) 700 hp
Advertised RPM Rating 7,500 rpm

pistons:
part number: 131387
Thats all I know about them.

Heads:
Cylinder Head Style Assembled
Cylinder Head Material Aluminum
Cylinder Head Finish Natural
Combustion Chamber Volume (cc) 72
CNC-Machined Combustion Chamber Yes
Intake Runner Volume (cc) 195cc
Exhaust Runner Volume (cc) 75cc
CNC-Machined Intake Runner No
CNC-Machined Exhaust Runner No
Intake Port Location Standard
Exhaust Port Shape D-port
Exhaust Port Location Standard
Spark Plug Style Angle
Intake Valves Included Yes
Intake Valve Diameter (in) 2.020 in.
Exhaust Valves Included Yes
Exhaust Valve Diameter (in) 1.600 in.
Valve Springs Included Yes
Maximum Valve Lift (in) 0.600 in.
Outside Diameter of Outer Spring (in) 1.460 in.
Damper Spring Included Yes
Number of Springs Per Valve Dual

cam:
Summit Racing Part Number TFS-31402002
Cam Style Hydraulic roller tappet
Basic Operating RPM Range 3,500-7,000
Intake Duration at 050 inch Lift 246
Exhaust Duration at 050 inch Lift 254
Duration at 050 inch Lift 246 int./254 exh.
Advertised Intake Duration 308
Advertised Exhaust Duration 316
Advertised Duration 308 int./316 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.558 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.558 in.
Valve Lift with Factory Rocker Arm Ratio 0.558 int./0.558 exh. lift
Lobe Separation (degrees) 112
Intake Valve Lash 0.000 in.
Exhaust Valve Lash 0.000 in.

 

This setup was not a custom setup, it was bought as a kit though summit and like I said, was advertised to be able to make about 500 FWHP. This was all done about 4 years ago, maybe 5. I have learned a lot since then and know a whole lot more about what I want out of my car. I am just trying to get this thing to where I want it to be power wise, and I know this cam setup is way too big for the street, but I want around 500 HP NA and be able to be around a high 10 second car with a lot of jusice. I think I willl need to swap cams for an N20 cam.

I will check compression on all cylinders I guess as a first step.

I don't have the space right now to be taking the motor appart myself. So degreeing it and so on is out of the question right now. Fuel pressure has never been a problem. I have a guage on my rail and another inside the car, it holds at 7 to 7.2 all the way though 6800 RPM without a flinch on the dyno.

I don't think the valve springs are too light since they were sold in the kit with the cam and heads and so on.

My only known problems are:
A dent in my exhaust from bottoming out on the ground on one of my Y-pipes halfs. This could be a restriction and might cost me a few horses.

Appart from that I have spent FOREVER trying to fix this damn motor and I can't even get it to transition from its primaries to its secondaries smoothly. It falls flat on its face unless I mash the gas and go right to secondaries. This happens 50% of the time, the other 50%, there are no problems at all and the car drives great.

Due to all this confution, and the fact I have no space, I was considering just pulling the motor, haveing a race shop tear it down, build it up and dyno the motor and tune the N20 on the stand. Getting it running great, pay the bill, drop it in and call it done.


.....

What do you think,,,

 

dyno sheets can be seen here. They don't post like they used to.

sorry for the link, and the novel I just wrote.

http://www.cardomain.com/ride/782913/29

 

who tuned the carb? has it been tuned?

 

The first thing I see is the heads are not big enough for that cam even though the heads should be able to support 500 flywheel HP. That's a big cam with an operating range from 3500-7000. The dyno sheet shows HP really dropping off at 6000.

Did the heads come assembled? Are the springs installed at the recommended install height to give enough seat pressure to prevent valve float?

The 195 runners and 72cc chambers are really killing that engine. Those are flat top pistons so the compression ratio is too low for that camshaft to really work.

The 750 carb is too small for that engine. An 850 will make a big difference at high rpm.

The dent in the exhaust pipe won't make much of a difference unless it was very pinched. A couple of my primary tubes are dented to clear suspension components.

Fuel pressure on the dyno may stay constant because it doesn't have inertia working against it. In an accelerating car, the pump needs to push fuel against the forces pushing it back to the rear. Your dyno numbers are low even without the forces of a moving car against the fuel.

Did you play with timing or jetting while on the dyno?

 

The carb was tunned by the dyno shop. They tried many different jettings and timing.

New England Dyno and Tunning tuned it. I am not going back.


The heads came assembeled.

I sorta thought that about the chambers. Should I look into domed pistons?

I guess i could look into an 850, but man that seems like a ton of carb for a 385!

I fully undertand about the pump working harder on the street. I am just saying that the car has had all kinds of problems even on the dyno without the extra forces working against it.

UGH!!!!

Kinda makes me wanna just rip it appart and build a tubo setup or something. I dunno..... Maybe swap a cam that can make around this power that is made for N20. I dunno what to do....

Wish I wasn't moving right now or I would buy a procharger and build the engine up for booste. Expensive but at least I could have all the power I want and not have to have all these strange problems and rough idle.

 

I ran an 850 carb on a 383 that pushed me into the high 11's. A 750 is good for driving around on the street but if you want high performance, you need to feed the engine. Since that cam wants to go to 7000 rpm, it's going to want more fuel than the 750 can provide.

I'm not saying this is going to be the cure but a bigger engine and higher rpms need a bigger carb. Recommendation is a 800-950 CFM carb.

As for the original quoted HP numbers for the package, you should never believe quoted numbers. They'll always tweak something to get the best results. Getting 355HP to the wheels with that package is still pretty good. It's possible that if you put the engine on an engine dyno, it may make that quoted HP.

You can also put the car on 3 different chassis dynos and get 3 different results. The dyno is for tuning, not for bragging rights. Don't worry about the final numbers as long as it's tuned to give a peak number.

 

It has to be compression you need domed pistons or smaller chambers. Try running more advance 15 degrees initial.

 

I plumed your setup into my engine calculator and the best was 460hp/449tq. That is using 12:1 compression, 10.25:1 fell down to 437hp/426tq.

 

I know that a dyno is not to be used as a bragging rights tool. I just know that is my goal to hit. I want a car that is a certain speed and I know thats the area of power I need to shoot for. Thats all.

As for the bigger carb, maybe you got something there. I have no idea. I am in fact driving around on the street a LOT though. 99.9% of the time I am.

Should I forget about trying to make that kinda power NA, and start looking at turbos or SC, or should I aim lower and just try to shoot for the numbers I have NA and let the juice take me to the level I want?

One major problem my setup has it that is diesels every time I turn it off if it has had a chance to come up to temp. I changed my secondary idle by an 1/8 of a turn a while back and that helped get it running right. But now it diesels so I tried closing the secondaries back up and had to change my pump shot cam to compensate for the later opening secondaries. This cured my flat spot a lot. But even with that done, it still seems to run on if it is hot.

I am running 17* initial and 36 total I believe. I might be off by a couple degrees max. I have not checked in a while.

Maybe I will be fine just dropping the car off and getting it tunned WELL. Or maybe just swap over to a N20 cam that is a better fit for my compression and combustion chanmber specs.

Oh one last thing. If I swap cams and go boosted, would I have to swap pistons if I am at 10:1 right now? how is it that guys with Cobras can run 15 lbs. with stock parts?

 

Ford engines also blow a lot of head gaskets from too much boost because of the poor clamping force of the head bolts. 15 pounds of boost is simulating an engine with twice the displacement. That 302 is now effectively a 604.

10:1 with boost from a turbo or blower is pushing it. All that hot air going into the engine under pressure causes lots of problems. Using a big intercooler to get the temperatures down helps a lot. NOS is different because it's so cold going into the engine. A NOS engine can still have high compression.

 

mine used to diesel a lot...figured out that a couple worn off cam lobes were the culprit. Come to think of it that is where u might be loosing some of your power too. I would pull the cam or better yet just crank the motor over by hand with the valve covers off and plugs out and make sure all the back of your rockers come up about the same height.

 

I could do that and let you know what happens. As for the hot charged air, I would def run an intercooler. But after thinking about things I think I will stick with N20 and just make what I have run right. Should cost a whole lot less.

I think I will drop the car off at a shop and let them come to the conclution that the motor wants more carb. Right now I don't think it is a good idea to drop that kinda money on a maybe. A new 850 or 950 would be too much if it don't fix the motor for sure.

Also bigger carbs are harder to get to run nicely on the street.

Any other ideas as to what is causing the dieseling? I will check compression and check the cam lobes are moving the same. Anything else???

 

try advanving the timing, I had an 86 K5 that dieseled alot, when I advanced the timing, it stoped, it also made a huge difference in performance.

 

I dont have a problem with 355hp, Ill trade you my 305. Haha. My 87 diesels often. I might try your timing method or just leave it for a bit till I swap engines. Dont know yet.

 

hey, have you ever taken the car to the track? Just curious as to what your 1/4 is. mmm 4.11s

 

no track time yet, but once I figure these motor issues out and the N20 up and running (200+ shot) I will be heading to the track.

No idea what the car will run though.

Will probably take me a while to get the car to hook and learn how to shift at the right time with the N20 going and all.

I have played with timing a lot with no real luck. The reason I started this thread is precisely because I have tried a lot of the thngs people seem to suggest. Any other ideas? I will check what I have the timing at right now and post back here in a couple days so that we can get exact numbers here.

 

Ok well I decided that the $ is not worth the power right now. I will be keeping the motor exactly how she is. I will be dropping it off at a reputable dyno shop that will tune it and maybe get another carb, small stuff like that. No cams, or any internal work. With that said, I will post back here when I get numbers. The N2O will finally get tunned as well. I assume this will cost me a little over a grand when all is said and done. That is the kind of price range I can swing these days.

Wish me luck, SC will have to wait.

 

Updates?

 

Honestly I think your exhaust is killing the power, 4 in on a under 500 hp car isnt good, id drop down to 3 in or 2.5, believe it or not this will make a big difference

 

I ended up changing direction again. lol.
SC is back on the table. I bought an F1!
I also got a 31x24x4 intercooler.
I am looking for a bypass valve now and will be sending the carb out to be rebuilt for the SC. I am having the entire engine torn down and built back up for boost. New pistons, cam and springs. I have found a shop to do the work. only around 1400 in labor, then price of parts on top of that. SC is almost payed off. Tax returns will more than pay for labor so now I am just waiting to get my car brought to my gf parents house, they have a big garadge for me to work in. This should be happening in a couple of weeks. Then soon after I will be pulling the motor and off to the engine shop.

By mid spring to early summer, should be back on the road. I will be shooting for 700HP at the wheels, maybe a little less. Not sure. Going to be turning around 15 PSI.

Anwyays, that the plan for now!