'74 Corvette 454 dyno results- Comp Cam XE 274H questions

After doing some analysis on the dyno run this past Sat, I know that there is some work to do on the motor. The PEAK hp of 348 was achieved at 4800 rpm's... power dropped big time after 4800.

This motor, dynoed in a couple articles (March 2000 Chevy High Performance and another one) with the same camshaft had PEAK power at 5400 rpms...

I think the module in the HEI unit needs to be switched out with the performance one that I have (the Proform HEI that I bought initially).

Aren't stock HEI modules good only to 4500 rpms?? That's what might be in there considering how it performed.

Also, Keith at AZ Dyno Chip stated that he "may" have heard valve float (over 5500rpms) which tells me that I probably should have gone with the stronger springs from Comp Cams... He also mentioned that I should check the rocker arm adjustments as well...

The cam's power range- 1800-6000- Comp Cams XE274H 230/236 @.050" 110 LSA .552"/.555"

Demon 850 DP annular carb/Performer RPM intake/ full length Hedman headers/Magnaflow exhaust/HEI distributor/Harland Sharp rockers


When I bought the cam, I went with the "standard" 924 valve springs. In the Comp Cams catalog, the "premium" choice were the 930 springs with a significantly higher spring rate. That said, does anyone think I should switch to the higher rate springs??

I would appreciate any comments/ideas/advice on these dyno charts:

http://www.geocities.com/bobt1974/D...l?1116427646894

I think I can get more power out of this motor- at least 375 Rear wheel HP...

What do you guys think?

Thanks!

Bob


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Your cam isn't very big to be expecting big horsepower.

It seems you might have a spring or ignition problem at 4800 on up.

Do you have any wide band info or EGT's or any other info to chart? VE% is a good one to have. Manifold Vacuum is also god to have.

If you're using those lighter valvesprings, I wouldn't be surprised at the possible valve float you're experiencing.

Can you post a dyno graph?

A stock HEI will spin 6000 NO PROBLEM and even as high as 7000 rpm.

What carb did this have?

Your cam has some decent lift but the duration numbers have something to be desired. they are small and therefore the RPM will be low and HP is only a function of RPM so the HP numbers will also be low.

You need more duration to pull the HP numbers. Let me guess that it also peaked around 345 Ft/lbs TQ.
Consider that the same 345 ft/lbs @ 6500rpm would equal about 425 HP.

==============================================
My bad, on slow internet i didn't see the TQ table and engine pics! LOL, what size is that BG? What fuel pump? What heads?

Why do you guys think it's a VS problem?

* 552/555 230/236 @.050 (CompCams XE274H)
* Holley 850DB carb
* FlowTech 1-7/8 headers


Dyno result with 3-inch exhaust & pumpgas: 526ft/lbs@3500rpm, 468hp@5100rpm (over 500ft/lbs torque from 2800rpm to 4800 rpm!)

You can't believe everything you read but in the above ^^^ link/paragraph you will see that his combo drops off at 4800 rpm also( using a comp274)

Thats what I was thinking as well from looking at the graphs. I believe it to be a spring problem though. From my experience on poor ignition in relation to dyno graphs the problem areas are usually more "wavey".

What is your base time and full timing? Have any denotation? I ran a similar cam and it didnt like to much timming, but I am on crap california gas

Here is a quick link with the HP graph:

http://www.geocities.com/bobt1974/H...l?1116441476142

Thanks for looking at it!

Bob

johsongrass1-

I agree- I originally put this motor together because of an article I found put this combo with this cam at 480 HP at the flywheel at 5400 rpms....

I wanted somewhat of a streetable big block. But, when power dropped big time after 4800, I figured something was wrong. My thought is that I could probably get 365-370 rear wheel hp out of this combo.

Based on what you have said and some of the others, I think I may swap out the springs. But, I will swap in the Proform HEI ignition module first and see what happens....

Thanks!

Bob

I am not sure of the base timing but I know that the full timing is 33 degrees at this point (from the dyno shop)..... No detonation at all!! I could probably advance it a tad.... maybe to 35-36.....

Thanks for the feedback on the springs- If this cam's power range is 1800-6000 supposedly, then it shouldn't be dying out at 4800 IF the igntion is fine....

Bob

What heads are you running, and what heads were used in the motor you built this one after. If you are running different heads that could make all the difference in the world.

Adam

heads, what heads?

In this virtual dyno sheet above note the VE drops to 99%@4500 rpm but really begins to fall of very fast @5000rpm and above.

Attached Thumbnails

 

Thanks for you input as well! I am running the Demon 850 double pumper with an Edelbrock Mechanical fuel pump. Someone from the Corvetteforum had suggested that it COULD be fuel.... I am thinking more on the springs at this point...

Bob

Thge dip in the curve from 5100-5500 tells all. This might be from the igniton crapping out but then you wouldn't see the curve pick up agian. The dip might be atributed to that paticuler VS freqeuncies. Valve bouncing off the seat. You really need the other info to rule out fuel siphoning, or mixture control, or about 1o different things to be sure.

Very good. I will continue to research. Here is an opinion of the springs from the Corvetteforum that I am a member of:

Bob,

I looked up the specs of the 924 spring and I don't think it will cut it.

115@1.900 seat pressure is fine but the rate is only 296 lbs/inch. I don't know what your max lift is but if it is 0.550" you only have 278 lbs open pressure. You want (need) 330-350 lbs.

A better choice would be the 925 spring. A 122 lbs seat wth a rate of 395 lbs/inch would give you 339 lbs open. Just about perfect.

Please let me know your valve lift ad the installed height and I can make a better recommendation.


The 930 has good open pressure - 337 lbs @0.555 lift. IMO the closed pressure is a little high at 142 lbs. I like the 925 better from looking at the spec. Maybe you should give comp a call and ask them about the 925.

Also make sure your installed height really is 1.900".



Bob