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Hi all: I have recently purchased a rebuilt 427. The engine was rebuilt and had sat in storage for a while, so the seller agreed to have the engine serviced and dynoed as condition of sale. Please see the dyno sheet attached here. The seller of the engine was conservative, only wanted to verify proper functioning, and so it's only run up to ~ 5400 rpm.
I sought a separate professional opinion on the data from
the Dyno and was told:
"THERE IS A PROBLEM" with the engine. (Valve train? compression? )
with reference to "dips" in HP vs RPM. I see a dip in the range of 2800 to 3100, but I just thought that may have to do with tuning/ setup of the tri powers...
Well....I'd be curious to know what's actually in it. It says hyd cam...which cam? What heads? What compression is it now? Was this with headers or manifolds?
Water/Oil temps? What brand of dyno? What weather conditions are listed on the sheet-usually at the top....it's got some awful big correction factors unless it was dyno'd on top of a mountain. Tuning data- timing? Did they record oil pressure?
All of that would be to question the results...but it shows a peak TQ around 4400 and HP was hanging in there at 5300...not sure if the drop at 5400 was letting off throttle or if it was dropping a little..but my bet is it was done at 5300. That would make me wonder again about cam/valvesprings. Maybe that's all the cam it has does?
It's not uncommon to see a little dip here and there in the curve...a carb can drip a drop of fuel sometimes or it's just what it sampled at that moment.
The fuel curve is a lean from 2,000 to 4,000 where it starts looking pretty good.
How many pulls were made on the dyno?
Did they take time to break in the cam and tune a little?
I am assuming this test was on a Engine Dyno and not on a chassis dyno with the engine in a car.
Tested with factory manifolds or headers?
What 427 is this advertised as? 390 HP,400, 435?
Jim had very important questions above that should be known before purchasing.
Well....I'd be curious to know what's actually in it. It says hyd cam...which cam? What heads? What compression is it now? Was this with headers or manifolds?
Water/Oil temps? What brand of dyno? What weather conditions are listed on the sheet-usually at the top....it's got some awful big correction factors unless it was dyno'd on top of a mountain. Tuning data- timing? Did they record oil pressure?
All of that would be to question the results...but it shows a peak TQ around 4400 and HP was hanging in there at 5300...not sure if the drop at 5400 was letting off throttle or if it was dropping a little..but my bet is it was done at 5300. That would make me wonder again about cam/valvesprings. Maybe that's all the cam it has does?
It's not uncommon to see a little dip here and there in the curve...a carb can drip a drop of fuel sometimes or it's just what it sampled at that moment.
JIM
Hi Jim; Thanks very much for your message: to answer your questions:
Cam: 396296 ISKY racing cam (but with hydraulic lifters)
Heads: 3919840 's
Block bored 0.60 over, with compression: 10.25 :1
engine designed to be lower maintenance, with an aggressive cam but with the hydraulic lifters (less valve adjustment) and Compression ratio suitable for pump gas; and TI lectric 1-wire points conversion.
Dyno : run on was a Dynomax pro 2000
"Correction Method" on the report says: "Standard"
this dyno was conducted in Albequrque NM (altitude ~ 5000 ft)
The report I was given does not provide any water temp or oil temps / pressures .
The seller advised me that the Dyno tech asked "Should I keep going" and felt the engine could easily make 450 or 460, but the seller didnt want to push anything further. and so I thought this meant perhaps some more tuning, and rerun up to into the 6000's . The seller payed for the dyno, so I assume he just wanted to validate operation, and got a basic report.
This engine was a professional build , done by an experienced Corvette shop, (I did speak to the builder and seemed like he knew his craft.)
but other than knowing the fundamentals of internal combustion , Im no expert . The seller, seems very a conscientious gent to me, (I've had nothing but good experiences so far from folks who are on this forum.) and so I wasn't too concerned until i received this 2nd opinion.
thanks again for your time and appreciate any advice.
The fuel curve is a lean from 2,000 to 4,000 where it starts looking pretty good.
How many pulls were made on the dyno?
Did they take time to break in the cam and tune a little?
I am assuming this test was on a Engine Dyno and not on a chassis dyno with the engine in a car.
Tested with factory manifolds or headers?
What 427 is this advertised as? 390 HP,400, 435?
Jim had very important questions above that should be known before purchasing.
Hi thanks for your input also! I forgot to add to the note above:
This was an Engine Dyno, not a Chassis dyno. Factory exhaust manifolds were used (not Headers)
Yes I am told they ran the oil pump a while before startup and ran the engine for 30 -40 minutes. There were a few other runs made, but I was told they had a fuel leak problem to fix . The Block is a 3904351, 4 bolt
and so this was billed as a 427/435 engine.
That's a hyd flat tappet cam. Says it's 234/234@.050" and .510" lift on a 114 LSA. Probably works out somewhat similar to the factory L71 cam...depending on actual valve events. I think the solid would be better...valve adjustment isn't a big deal and most folks don't drive them that much. CK them once a year and not worry about it. It's going to peak HP in that mid 5000 range anyway..so it's not going to show much more HP. For real performance, where the cam signs off is about where the heads really start to work. A different cam would certainly add power across the board.
That said...as mentioned I don't see anything out of line on that sheet. I've never used a Dynomax dyno..so not sure about their correction factors. It was at high altitude so that makes the corrections at least in the right direction even if it seems a lot. Since it was dyno'd with manifolds and no real tuning on it..and with lower compression...it's probably about right on HP.
Let us know how it runs once you get it in the car!
That's a hyd flat tappet cam. Says it's 234/234@.050" and .510" lift on a 114 LSA. Probably works out somewhat similar to the factory L71 cam...depending on actual valve events. I think the solid would be better...valve adjustment isn't a big deal and most folks don't drive them that much. CK them once a year and not worry about it. It's going to peak HP in that mid 5000 range anyway..so it's not going to show much more HP. For real performance, where the cam signs off is about where the heads really start to work. A different cam would certainly add power across the board.
That said...as mentioned I don't see anything out of line on that sheet. I've never used a Dynomax dyno..so not sure about their correction factors. It was at high altitude so that makes the corrections at least in the right direction even if it seems a lot. Since it was dyno'd with manifolds and no real tuning on it..and with lower compression...it's probably about right on HP.
Let us know how it runs once you get it in the car!
The secondaries starting to open at 3000 rpm could cause a little fluctuation in HP readings also (too lean or too rich).
Thanks for that. So looks like although a bit lower than yours, Im in the ballpark. The dyno was done at 5000ft, Im at Sea Level. I will be using headers on my car , so I may be close to your figures. In the end the absolute HP doesnt matter to me, (just wanted to be sure this engine was healthy.)
It's always a little difficult comparing hydraulic cams to mechanical cams but I think your hydraulic might be a little smaller than the stock L71 cam at 234 @ .050 inch and .510 lift. That's close to a hydraulic cam I used in a Ford 459 FE that peaked at around 5400 rpm and that seems supported by yours peaking at 5300 - although engine characteristics and design would account for some difference. The dyno run by 427435 peaked at around 5700 rpm. I suspect that accounts for why you didn't make more power.
Thanks for that. So looks like although a bit lower than yours, Im in the ballpark. The dyno was done at 5000ft, Im at Sea Level. I will be using headers on my car , so I may be close to your figures. In the end the absolute HP doesnt matter to me, (just wanted to be sure this engine was healthy.)
I see it's lean at low rpm.
I see the usual flat spot in tq @~3000 rpm that is typical with stock exhaust manifolds on a rectangle port BBC.
Tuning the jetting at the final altitude & exhaust back pressure will help the AF mix.
Headers & tuning the advance curve will address much of the flat spot.
Carb tuning the accelerator pumps and vacuum springs on the Tri-Power will likely smooth out and improve the way it feels.
Did the "expert' tell you what they read that was a concern (other than his thin wallet)?
I see it's lean at low rpm.
I see the usual flat spot in tq @~3000 rpm that is typical with stock exhaust manifolds on a rectangle port BBC.
Tuning the jetting at the final altitude & exhaust back pressure will help the AF mix.
Headers & tuning the advance curve will address much of the flat spot.
Carb tuning the accelerator pumps and vacuum springs on the Tri-Power will likely smooth out and improve the way it feels.
Did the "expert' tell you what they read that was a concern (other than his thin wallet)?
"Usual flat spot" ?? Thanks , THAT DOES IT!!! #$%&*@!!
Yes upon reading the dyno sheet above, the "expert" told me that the Dyno Tech should have advised that there is " A PROBLEM" with this engine. He didn't specify, other than to state that HP vs RPM should progress in an increasing fashion without any dips. I think the "expert" I consulted wants to talk me into "more work" to be done un necessarily on a new, fully rebuilt engine. He stated there may be a valve train or Cam problem, and it should have a compression test as well. The Expert also had an "Issue" with Cast Pistons. I was told by the builder of this engine that Cast Hypereutectic pistons are JUST FINE for an engine making 425- 450 HP. Obviously Forged would be be recommended if you're building a 700 HP beast.
X3 on it being too lean...but I'll say probably from idle to 4,000 rpm. AFR should never be in the 15:1 range on pretty much ANY car or engine under load.
There is a definite "hole" in the torque curve between 2900 and 3300. I've seen this on Holleys before and I attribute it to when the secondaries start to open, but on 4-bbl. Holleys it occurs over a narrower rev range. Also, note that it goes lean in this rev range, but not dangerously so.
I attribute this behavior to air and fuel distribution issues as the end carbs begin to open. There's some very complex physics going on in a carburetor-manifold system, and it's why emissions finally forced all the OEs to go to fuel injection. It's just too difficult, if not impossible, to precisely control the A/F mixture in a carb-manifold system. Dave McClellan said in his book, it's a wonder they work at all.
At 2000 torque is only 77 percent of peak. I like to see at least 80 percent on a high performance road engine.
The correction factor appears to be about 1.36. The farther it deviates from unity the more "iffy" the accuracy of the corrected results. I like to see it somewhere between 0.95 and 1.05, but when you run a test at high altitude, a big CF is unavoidable.
Given what has been offered about the configuration and test conditions, the output is in the ballpark of what I would expect, and I don't see any major issues.
Get it installed in the car and then work on optimizing the spark advance map. You may be able to improve low end torque and top end power if it will tolerate, detonation-free, a quicker centrifugal spark advance curve with total WOT advance in the range of 38-40 degrees.
Also, don't use light springs to open the end carbs too quickly. That will exacerbate the hole in the torque curve. Usually the OE springs are best on a stock or near stock engine, which this appears to be. If you can feel that hole in the torque curve on the road try stiffer springs in the end carbs to slow the opening rate.
The jetting looks fine. In fact the carbs must have been jetted down or it would be running very rich at high altitude. The 15 number at 2000 is no big deal because in the real world the engine passes through 2000 at WOT in a heartbeat. Also, this was the start point of the test, so it may be just a brief transient condition when the throttle was opened. If it accepts WOT at 2000 (and below) on the road without any hesitation, then no problem. In the mid to upper rev range at WOT I like to see it between 12.5 and 13.5:1, so IMO you're good to go.
This configuration basically works out of the box. Sometimes it takes nearly a day of dyno time and tuning just to get an engine to start making smooth pulls from low to max revs.
There is a definite "hole" in the torque curve between 3900 and 3300. I've seen this on Holleys before and I attribute it to when the secondaries start to open, but on 4-bbl. Holleys it occurs over a narrower rev range. Also, note that it goes lean in this rev range, but not dangerously so.
I attribute this behavior to air and mixture distribution issues as the end carbs begin to open up. There's some very complex physics going on in a carburetor-manifold system, and it's why emissions finally forced all the OEs to go to fuel injection. It's just too difficult, if not impossible, to precisely control the A/F mixture in a carb-manifold system. Dave McClellan said in his book, it's a wonder they work at all.
At 2000 torque is only 77 percent of peak. I like to see at least 80 percent on a high performance road engine.
The correction factor appears to be about 1.36. The farther it deviates from unity the more "iffy" the accuracy of the corrected results. I like to see it somewhere between 0.95 and 1.05, but when you run a test at high altitude, a big CF is unavoidable.
Given what has been offered about the configuration and test conditions, the output is in the ballpark of what I would expect, and I don't see any major issues.
Get it installed in the car and then work on optimizing the spark advance map. You may be able to improve low end torque and top end power if it will tolerate, detonation-free, a quicker centrifugal spark advance curve with total WOT advance in the range of 38-40 degrees.
Also, don't use light springs to open the end carbs too quickly. That will exacerbate the hole in the torque curve. Usually the OE springs are best on a stock or near stock engine, which this appears to be. If you can feel that hole in the torque curve on the road try stiffer, not light springs in the secondary opening vacuum motors.
The jetting looks fine. In fact the carbs must have been jetted down or it would be running very rich at high altitude. The 15 number at 2000 is no big deal because in the real world the engine passes through 2000 at WOT in a heartbeat. In the mid to upper rev range at WOT I like to see it between 12.5 and 13.5:1, so IMO you're good to go.
This configuration basically works out of the box. Sometimes it takes nearly a day of dyno time and tuning just to get an engine to start making smooth pulls from low to max revs.
Duke
Thanks Duke; I have to say now I am PISSED. I will be asking my "expert consultant" just WHAT he saw in this dyno that tells him this engine has a PROBLEM. based on the sum commentary of all of you
he's full of $#!+ I appreciate this forum, and all your comments (which are unbiased and not motivated by personal profit. ) thanks again!
I'll just throw this out there...make SURE there are no detonation issues...even mildly with the hypereutectic pistons. The originals were forged...and hyper's don't do well with any pinging etc. Hyper's have their place...they can be installed tighter, better oil control and quieter...but the ring gaps must be set correctly to deal with the heat in the top groove area. You might ask the builder what specs he used for ring gaps just to be sure.
Jim, your Hyper information is correct for Keith Black Hyper pistons.
The Sealed Power version of the same Hypereutectic pistons need no special ring gaps. Use standard gaps on Sealed Power Hyper pistons. .004 per inch of bore. Keith Black raises the Top ring groove which increases the heat the top ring is exposed to. This practice is supposed to increase power slightly and reduce unburned fuel above the top ring but it certainly does increase the amount of gap needed on the top ring for that design. Mahle, Speed Pro, Sealed Power and others do not need the extra clearance on the gaps.
Keith Black Pistons were so well known and had so many top ring issues that the industry started believing this was a Hypereutectic problem when all along it was unique to Keith Black Hypereutectic pistons.
The Hypereutectic designs are very strong, stronger than a forged but in failure a forged will bend and mold and keep running where a Hypereutectic piston fractures and destroys the whole block. Most OEM pistons today are Hypereutectic so they have a place and are very popular but forged are a very good upgrade.
Jim, your Hyper information is correct for Keith Black Hyper pistons.
The Sealed Power version of the same Hypereutectic pistons need no special ring gaps. Use standard gaps on Sealed Power Hyper pistons. .004 per inch of bore. Keith Black raises the Top ring groove which increases the heat the top ring is exposed to. This practice is supposed to increase power slightly and reduce unburned fuel above the top ring but it certainly does increase the amount of gap needed on the top ring for that design. Mahle, Speed Pro, Sealed Power and others do not need the extra clearance on the gaps.
Keith Black Pistons were so well known and had so many top ring issues that the industry started believing this was a Hypereutectic problem when all along it was unique to Keith Black Hypereutectic pistons.
The Hypereutectic designs are very strong, stronger than a forged but in failure a forged will bend and mold and keep running where a Hypereutectic piston fractures and destroys the whole block. Most OEM pistons today are Hypereutectic so they have a place and are very popular but forged are a very good upgrade.
I agree the main culprit was Keith Black....but there are so many "claimer" off brand hypers being used that I would be leery especially since it's an engine being bought from someone else.
I can say there is no way I'd use any hyper in a 427/435 if I was actually going to have fun with it. If it was just riding around...maybe...but I doubt it. I'd get a nice forged 4032 and fit it nice and tight in the bores.
Certainly the OEM's are using hyper's in just about everything..including some boosted applications, but those dudes have serious engine management to make sure you stay out of trouble.
And I know there's some folks getting by with them in non-managed applications...but I'm not sure most folks can tune that good or recognize when they are getting near the edge. Lots of variables with fuel, timing, mixture, altitude, coolant temp etc, etc....
03-26-2016, 03:02 PM
1967 427 Dyno - interpretation?
