Troubleshooting help/suggestions

[Coloradocobra]

I'll try to be brief but thorough. I have a 428 that makes gobs of power up to 4000 RPM but then the HP curve goes flat and the torque drops off. It runs great but falls flat on its face. I know it's not a small block - but a 4000 torque peak cuts into the fun factor. This has been shown on two chassis dynos with two carbs. Here are some specs:

428 .030 over with stock crank, stock peened and polished rods.
Weisco forged pistons with dish for 10.8:1 compression
Dove F5 heads with bowl blending, 2.09 intakes
Dove rockers
Crower "hydraulic hauler" 280 HDP cam with 280/288 advertised, .550 lift, 108 degree lobe center. The cam was degreed and set "straight up".
Blue Thunder dual plane med. riser style intake
Holley 750 vac. secondaries - professionally built for Denver's altitude
Pertronix/MSD ignition setup with re-curved distributor - 32 degrees total - timing verified on dyno

The dyno test shows good fuel pressure and mixture throughout the rpm range.

The torque and HP curve would be great in a tow vehicle but with the cam specs, etc., it would seem that I should see peak torque at 5,500 or so, not 4000.

Plans this winter are:

1. Bench flow the heads and port match intake and exhaust
2. Switch over to solid lifter cam approximately 288 degrees
3. Possibly install larger diameter valve springs to eliminate possiblity of a float issue.
4. Replace the mechanical fuel pump with electric

Valve float often creates a "dip" in the curve over a short rpm range but this is just a steady drop.

Things I don't want to do: Hydraulic roller, solid roller, change carb

Am I missing anything? I'm not looking for thousands of HP but with the above combo, it seems that it should pull hard up to the mid 5000's. It will hit the rev limiter at 6,000 but it seldom sees that because when drag racing I short shift at 4500 to 5000 to get my best times.

Thanks in advance for your help.

Gary

[SCOBRAC]

Nope. peak torque typically occurs at 3400 rpm. Your experience is typical. You do seem restricted at your top end if it falls off at 5000. Most midly improved 428's fall over at 5500-5600. The 1968 428 CJ makes 335 hp at 5500 rpm and 440 ft lbs of torque at 3400.

This could be a result of valve size, carb size intake or (not typically) exhaust pressure. To improve your top end your bottom end will suffer enless you bump compression to compensate. You may want to start with finding out what heads you have. heads with valves less than 2.09/1.66 are pretty much going to slow you down no matter which intake manifold and cam you have.

Heads are easily available from Edelbrock for $1200. Less expensive ones can be had on e-bay. I have a set of rebuilt 1968 GT heads I got on line for $550. After that I would say a stout cam. Not too much but not too little if you plan on driving it in traffic. 6000 rpm is not a great task any good performance grind will get you there.

Roller rocker arms will free up horsepressure but do nothing for your torque. They generally require an adjustable (solid lifter cam) arrangement which you may not really want or need.

Carb selection is another sore spot. If you are running an 850 (with a 5000 rpm redline) you have 150% of the carb you need. Right now a single Holley 600 cfm unit is too much for your engine. Even at 6000 rpm you only need just under 600 cfm. for a typical street engine with a VE of 85-88%.

Good Luck... Hope this helps,

[SCOBRAC]

I really only read the problem you had before I posted. This will do nothing to improve where you see torque occur... your results are still typical. The horse power curve is another matter. Given your location I would say your biggest problem is O2. Oxygen at 5200' is at a premium. If you have the components you say you have I would say it's the atmosphere holding you back. I don't doubt you do. In my experience you have already done what I would do. As such 5000 is pretty low for HP. That said I would want to mechanically verify I had the cam I was told I had. After that...

I'd go to nitros. Seriously. If that isn't your thing I would increase the cam duration to at least what you are planning (288+) and maybe or maybe not go solid. Now your Dove rocker arms are solid cam units now right? (or do thay make a non-adjustable unit).

This is a funny question but one often over looked, What are you gapping your plugs to? In Denver I would go as much as I could, with the MSD I think you can go as much as .055" I would re-gap them to at least .045 and see if it makes a difference.

I used a standard gap right after I installed my MSD. It really improved the feel after going to .045" at 30' above sea level.

Regards,

[Coloradocobra]

Thanks, Michael

Perhaps my description of peak torque was vauge. It is relatively flat from 2000 to 4000 and then heads down at almost a 45 degree angle above 4000. The Dove rockers are adjustable and I have the appropriate push rods, etc. They are set at zero backlash per the crower specs. It almost looks and feels like the cam is 2 or 4 degrees advanced. Before I tear it all apart (I have to yank the intake anyway because I just developed an intake leak) I am going to verify the cam setup. Interesting on the plug gap. It is set at .035 based on suggestions due to compression - a lot of cars running 8.5 or 9:1 can use the bigger gap but I was told at 10.8 to keep it about 0.035. I have never tried to open them up but may give it a go. Nitrous is a popular add-on in the high country as adding 100 HP just about gets you to sea level output so the engines are not terribly stressed up here. Given the goofy power curve, though, it would be a band-aid.

Gary

[KobraKarl]

Gary/Micheal......

What about a poor port match between midrise intake and low rise heads???


KK

[Coloradocobra]

Hi Karl,

The Dove F5 heads are a medium riser based on the CJ heads. Local guy using the same intake on a 427 with Edelbrock heads - also medium riser - made over 520 HP so it does not seem to be intake limited. Depending on how you do your corrections for altitude and chassis dyno to flywheel, I am down around 350 flywheel at sea level. Maybe this is just realistic compared to some of the inflated values I see thrown around here but I was hoping for low 400's based on what was put into the motor. Right now, my dollars per horsepower ratio is pretty high.

Gary

[HighPlainsDrifter]

Gary,
1st thing to do is have someone step on the gas pedal all the way and look down the carb,engine not running, make sure the carb butterflies are opening all the way, if not adust the linkage,seen this a few times. ****** There is a choke interlock that prevents the vacuum secondaries from opening,find it at the base plate throttle linkage ,manually open the secondaries ,if they don't open then the choke rod needs to be ajusted.****

next......
Beg, borrow or steal a double pumper carb, bolt it on your engine and go directly to 5,500 RPM. This is the easyist way to find out if it's the vacuum secondaries screwing up, seen it happen on many cars.
Perry.

[Coloradocobra]

Thanks, Perry

The butterflies open all the way and we checked secondary opening on the dyno. I checked the butterflies early because that is just what it feels like. I know someone with a DP so I may try it out. At higher altitudes, most engines don't like all the air a DP can provide so vacuum secondaries are typically used. Generally, we lose about 15% at 5,000 feet - but this is across the RPM range, not just in the upper end.

Keep 'em coming!

Gary

[HighPlainsDrifter]

Gary,
Your engine will inhail the same volume of air whether you are there or here are 24' above sea level. I've run at tracks that the air was measured at 3,800 ' and it runs a little rich because of less oxygen, just go down 2 jet sizes, ran 2 tenths slower.A simple carb change may save you an expensive tear down.
Your engine should tolerate more timing, I'd up it to 36* total.
You have a dual plane intake so you can run a bigger carb than if you had an open plenum manifold. If you put on a 800 double pump carb any 1 cylinder would only 'see' the righ or left side of the carb at any one instant, it would 'see' 400 cfm of carb. EG: I run dual 600 carbs on a blue thunder dual plane dual quad intake. It never bogs and has crisp throttle response because any 1 cylinder only 'sees' the left or right bank of carbs at any one time.....300 from the front carb and 300 cfm from the back carb= 600 cfm.
Good Luck,
Perry.

[MJN]

Sounds like airflow restriction to me. Couple of suggestions. You may have already tried this but pull the air cleaner(s) off. The small air cleaners typically used on Cobras are very restrictive. Second, try pulling off the side pipes. The typical glass packs used in the side pipes can be extremely restrictive. If you can do this while on a dyno it may help to uncover the problem. My torque curve sounds exactly like yours and my problem is the side pipes. With my cam and heads the torque curve should hold flat until around 5000rpm but as you can see it takes a dive at around 4000rpm.

[Coloradocobra]

Thanks for posting your curve. Mine is similar but even more dramatic. The HP curve climbs to 4000 but then is absolutely flat, so you can imagine what the torque does. The Midstates sidepipes are a 2.5 inch inlet inside a 3.5 inch pipe and you may be right about restriction. I have run open "shorties" (I welded in a 3 bolt flange on each side for a quick-change) and to be honest, it did not seem, by seat of the pants, to help much. I hope to run the engine on a dyno while it is out if I can scrape up the funds and will test it open and with mufflers. I plan to install new mufflers this year since I am going to get the headers and side pipes coated.

I am hesitant to change carb or manifold due to cost and also manifold vacuum looked good on the dyno indicating that any restriction does not seem to be on the intake side.

Gary

[KobraKarl]

Gary,

Oops ,...didnt notice you had Dove heads when I read your original post...

How many inches of vac. @ idle?

KK

[wtcobra]

32 degrees total advance will make an FE run as you describe. Are you sure cam was set in straight up?

wt

[Coloradocobra]

Hi Wayne,

It runs about 10 inches vacuum at idle. I have a 6.5 power valve and the power valve circuits were leaned out for altitude. Evergreen Performance Systems did the last dyno test/tune. Walt Hane is an old Ford engineer and campaigned Shelbys way back when. They checked the timing and the optimum settings they got were at 32 degrees. Walt and his son Chip know Fords and our altitude so I would be surprised if they got it wrong.
The engine builder's calc's also indicated 32 as optimum.

When I bump it, I get into starting problems. It has 20 degrees mechanical advance so when I go to 36 or 38 total it wants to kick back. What do you suggest/recommend?

Thanks,

Gary

[Coloradocobra]

Oh, and the cam was supposed to be set straight up. They degree'd it and then set it +4 and then checked it again and it was right on compared to the cam card. The builder showed me his form from the test - he keeps records on all his motors. The plus 4 has me wondering but the builder has built a lot of motors and I am sure/hope he got it right. First check after pulling the intake will be to check the cam.

[Jim Downard]

Just a thing or two more. I haven't seen what your cam duration at .050 is. Just because you have an advertiesed duration of 280 or so, doesn't mean duration at .050 isn't something like 204. I've seen this in a cam someone sold me. Super nice lowend torque, but wouldn't breath. Now I have a cam with the same advertised 280 duration, but at .050 it's 246 degress. NOW she breaths on top. Bear in mind that I'm running a windsor of 393 cubes. Also, I read where someone mentioned a new air cleaner. The Edelbrock Pro Flow tri oval has NO restriction below 1000 cfm. My dyno runs proved that. Also, you do have too much Carb. There's my two cents worth. Good Luck.

Jim Downard

[Coloradocobra]

Hi Jim,

The cam is 224 and 232 degrees @ 0.050. This seems fairly typical for a hydraulic flat tappet cam. This may be good reason to go with a solid lifter cam as the duration at 0.050 is wider, as you mentioned in your post. Although I might improve throttle response some with a 600 or 650, I don't think this is limiting my higher RPM performance. The mixture measured during the dyno test was good. The shop I went to engineers a 600 Holley that they generally recommend for 428's in Denver but based on the results, they felt that they could not improve on my carbs performance.

I currently run a 14" X 3" round air cleaner. I put on the S&H for shows. The carb sits on a 1/2 inch phenolic spacer with a 1/4 inch heat reflective gasket. At our altitude, the fuel vaporizes much easier than at sea level, so I need to keep the bowls as cool as possible. They dyno runs were conducted without an air cleaner.

Wayne - can you elaborate more on timing?

[MJN]

I think your cam is pretty mild for an engine that large. The cam in my 331 is 221/225 and it is really too mild for my combo. I wouldn't be surprised if the cam is contributing to limiting the top end.