Peak torque too close to peak hp

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robch
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Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 11:04 am

I bought this new old stock sbc that was dynoed and can't figure out why there is such a small split between pk trq and pk hp.
The motor has an eddy air gap manifold with a comp xe268h flat tappet cam.My dyno says 349 hp @ 4700 and 397 trq @ 4400 Compcams websited says this combo makes 340 hp @ 5000 and 413 trq @3800. I see they have my cam retarded 4*. Any ideas why there is only 300 rpm split?
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Last edited by robch on Wed Mar 14, 2018 11:20 am, edited 1 time in total.

Brian P
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Re: Peak torque too close to peak hp

Post by Brian P » Wed Mar 14, 2018 11:18 am

On a percentage basis, you still have 96.4% as much torque at 3600 rpm as you do at the maximum.

No experience with that specific engine, but as a general principle, late cam timing will tend to push the RPM of peak torque upwards. Reason: At lower revs, it would trend towards pushing more of the charge back out of the cylinder before the intake valve closes. At higher revs, there is more momentum in the charge to resist this.

Depending on what you are doing with the engine, you could try advancing the cam a little, and again as a general principle, I would expect to see the peak-power RPM move down a bit. Disregarding the tendency for everything to affect everything else, this might lose a little top-end power ... but if it so happens to be close to an intake-resonance tuning peak, who knows what would happen, it could go either way.

Or you could just leave it alone; it's not that bad.

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Re: Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 11:26 am

Thanks for the reply. I would prefer the hp to be at the lowest rpm possible, this engines in a jet boat with an impeller that will never rev passed 4100 rpm @wot at this hp.

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Re: Peak torque too close to peak hp

Post by Brian P » Wed Mar 14, 2018 11:40 am

Aye, OK. Try taking the cam timing back to straight up. You will want a mild cam in that application.

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Re: Peak torque too close to peak hp

Post by Zmechanic » Wed Mar 14, 2018 11:49 am

robch wrote:
Wed Mar 14, 2018 11:04 am
I bought this new old stock sbc that was dynoed and can't figure out why there is such a small split between pk trq and pk hp.
The motor has an eddy air gap manifold with a comp xe268h flat tappet cam.My dyno says 349 hp @ 4700 and 397 trq @ 4400 Compcams websited says this combo makes 340 hp @ 5000 and 413 trq @3800. I see they have my cam retarded 4*. Any ideas why there is only 300 rpm split?image.jpeg
You say they did retard it 4*, but just by the dyno sheet comment it seems like it's open to interpretation. DID they, or are they telling you that you should?

It's because the torque starts falling off a cliff after 4400. Torque/Horsepower is a linear relationship. They either did or are telling you to retard in hopes of holding on to some of the torque up higher, netting you more horsepower.

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Re: Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 12:04 pm

Good catch I don't know if was done or just suggested.The dyno operater has passed and the builder can remember anything so I guess I can pop the lid on the two piece timing chain cover.

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Re: Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 12:13 pm

I fired up the engine for the first time in 15 years to set timing,carb and check for leaks. It ran real nice and had 14" vacuum with just a bit of lump. The builder told me they replaced the valve springs with beehives and roller rockers after it was dynoed, I also added a Quickfuel marine 750dp carb. Do you think it could have been valve float that cause the small trq /hp split ?
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Re: Peak torque too close to peak hp

Post by pdq67 » Wed Mar 14, 2018 12:30 pm

You might consider if CC's old, "top of the page", Pure Energy hy-cam would help here?
I think that it might be the smallest they make so should keep peak power down low and peak torque good.

Spec's are: 246/203 -- 265/212, 110/106, .429"/.438" lift.

It is a decent step up from Chevy's great old -929 hy-cam spec'd at: 195/202, 112/108(?), .390".410" lift so it is small, but a great DD torque cam.

JMHO, and I am just thinking about a boat engine lugging all the time is all..

pdq67

PS., another good stock cam is the often forgotten about 305HO, L-69 hy-cam.

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Re: Peak torque too close to peak hp

Post by RCJ » Wed Mar 14, 2018 12:38 pm

I had that cam and intake on a 71 Camaro years ago.12.70 was my best times ,I shifted at 5200 or 5300, it did not like to be spun up. We thought it ran very good for what it was.

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Re: Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 12:41 pm

Ya this cam would not be my first choice but I have to "run what I brung" or a divorce will be in the works. Camking just put a small marine cam in his personal boat that Looked like it would work good here as well. But first I have to dial in this one as much as possible so I have a good base line to go from.

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Re: Peak torque too close to peak hp

Post by robch » Wed Mar 14, 2018 1:32 pm

Here's what I'm thinking might be happening, the retarded cam is giving peak trq the 600 rpm increase over Comps dyno run and the 300 rpm decrease in hp was the old springs were begining to loose control. Do you think this plausible?

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Re: Peak torque too close to peak hp

Post by MadBill » Wed Mar 14, 2018 11:49 pm

It's very easy to confuse 'retarded'. Is it retarded from the ubiquitous CC recommended 4°advance, or is it retarded from TDC? :-k
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Re: Peak torque too close to peak hp

Post by mag2555 » Thu Mar 15, 2018 6:36 am

I would have liked to have seen the motor run up to 5500 to see how the cross over numbers pan out on the dyno sheet!
That 670 Carb was starting to get a bit fat above 4600 also.

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Re: Peak torque too close to peak hp

Post by gmc406 » Thu Mar 15, 2018 10:19 am

Something seems off to me. I run that exact cam in a 406, and as per chassis dyno, it peaks at 5350rpm. I assume you have a 350, it should peak even higher than that.

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Re: Peak torque too close to peak hp

Post by robch » Thu Mar 15, 2018 8:14 pm

Exactly! I have searched the web for dyno results with the same cam head combo and found several.They all peaked 5000 or higher.There should be at least a 800 more rpm between peak trq and peak hp.

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