Low h.p. from a 360 / 408 Stroker

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molson_ab

molson_ab

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I recently had 1971 Dart Swinger Dyno'd at an event. Here is a rough description of what I have :

Car: 71 dart swinger

Gear ratio: 3.55

Tire diameter 14''

Engine CID: 360 /408ci

Head Type: eddy performer rpm 63 cc

Head Work: none

Cam Manufacturer: comp cam

Cam specs: 240 dur with 525 lift

Carb: holley dp 700

Jet sizes : stock

Plugs: are normal bosch plats

Intake: eddy air gap

Piston: kb dished

Compression Ratio: 9: 5 I believe?

Trans type: 727

Trans Mods: none

Convertor stall: 2800, wanted it mostly for street

Drive shaft: stock

Initial timing: not sure on number, but recall mech saying it was fairly high.


Rear end type: 8 3/4 posi



On the dyno I had 251 h.p. at the rear, and 300 f.p.t. I find this extremly low and was wondering with what little info i gave you guys...wtf. If there is any sugguestions or comments please let me know.
 
I'll bet your compression ratio is lower than you think, that would explain the very high initial timing needed. The cam may be retarded too.

Was the cam degreed when installed and was the compression ratio actually measured?
 
I honestly don't know, these are the questions I need to asked him! When it comes to internal engine questions , I am clueless.
 
Are you sure it's a 408? Your 251 HP at the rear wheel translates to approximately 315 HP at the crank. That would make more sense if it was a mild 360.
 
Is this dyno reputable? The numbers may be way off and your combo may be fine. Have you run the car down the 1/4? A timeslip will make it easier to see if dyno agrees with the dragstrip.

I don't see any mention of exhaust, I hope you're using headers. Manifolds would probably completely choke a 408.

The distributor probably should be recurved, if it hasn't been, and timing set around 34 to 36 @ 3000 RPM. I understand FBO does a great job with distributor recurving.

I'd start with the exhaust and on to the timing.
 
Ya, I have a scat stroker kit installed. The funny thing is that it feels like alot more power.
 
With so many IMPORTANT variables up in the air,there's no telling what your combination really is. It does not seem overcammed.....but until you figure the true blueprinted compression ratio.....not what the books list....but what it actually MEASURES, you're shootin in the dark. 240 degrees duration for a REAL 9.5 to 1 motor is a pretty good match......but, at 8.5, which wouldn't be hard to be at if it's not been blueprinted, makes that same cam too big.....by a pretty good margin. I'm puttin my money that the compression is lower than you "think". Easy to tell. Measure the cranking compression with a compression gauge. If it's under 150, that's too low for that camshaft. I bet you a dollar, a doughnut and a cup of coffee you ain't got 150.
 
molson_ab said:
Ya, I have a scat stroker kit installed. The funny thing is that it feels like alot more power.
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Then you need to get a second opinion on your HP. Take it to a different dyno and operator. Or run it at the track, you can use you 1/4 mile MPH to get a HP WAG.
 
I am going to get another one done by a different shop. For exhaust I have hooker headers and magna flow exhaust put together with 2 1/2''.
 
Thanks for all the tips!, if their is anymore suggestions please share them with me. I want to be prepared when I chat with the builder.
 
while on the dyno...check/change ignition timing....check/change jeting in carb...
 
I would get a second opinion too. It's gotta be makin more than that....at least you'd think.
 
I think it is at least 150 H.P short! It's weird cause it feels good and has great pick up. The thing is I never raced it yet, Just cruised with it.
 
I personally never use dynos for anything i build, the track tells me everything i need to know, now when you say it feels "good", whats the fastest car you been in down the track? A 13 second/ 12 second/ 11 second car?? I mean if it really feels good, those #s might be off considerably, I have a mild 360 magnum in my 71 dart with a bone stock bottom end, the calculators say i'm at 290 RWHP at 3200 lbs. for the ET & MPH, the car runs 12.5s at 105 & feels fairly good, doesn't set the world on fire, but it runs well for what it is, i've had a 10.8 duster for 2 yrs. trapping at 123 mph, so mid 12s isn't that fast to me.

I do agree that your compression must be low if your zinging up the timing, i would think you should be around 165-170 cranking pressure, what KB pistons did you use? I see there dished, i'd check cranking pressure 1st. before getting it dyno'd again.
 
With in a week , I'll go and get a compression test done. If the compression is too low, what is the next step? Do I have to change my pistons? Cam? . Thanks for all the input. I am positive this thread will help more people then just myself!
 
molson, I'll tell you about a 71 Dart I had years ago and let you think about it. I took a '71 360 straight out of a junkyard, tore it down, hosed it off, honed it out with a dingleberry, cleaned the crank up, rerung it STANDARD and put standard bearings in it with a stock oil pump. I cleaned the stock J heads up, lapped the 1.88 and 1.60 valves in, put crane springs on to match the cam i picked out. The cam btw was the 464/494 222/234 114 lobe center cam. AWESOME cam. I used the stock rockers and shafts and pushrods. I also put in the matching crane roller timing set. I had Mopar electronic ignition with a stock IRON 340 intake and an 850 thermoquad and headers. I built a hot 904 with a STOCK converter behind it with 3.23 gears. that was a atock 8.5:1 motor....or whatever they were rated at in 71. firstime out on the track that car ran 13.99 spinnin like a ***** in church off the line. 2nd time around, I found a friend there who let me bolt on some extra slicks he had. that time the car went 12.72. LMAO that was the second time down the track! that car ran a best of 12.46. That motor had like 175 PSI cranking pressure, but was still only 8.5:1. Cylinder pressure is where it's at. If you have low cranking PSI, I'd recam it.
 
StrokerScamp said:
molson, I'll tell you about a 71 Dart I had years ago and let you think about it. I took a '71 360 straight out of a junkyard, tore it down, hosed it off, honed it out with a dingleberry, cleaned the crank up, rerung it STANDARD and put standard bearings in it with a stock oil pump. I cleaned the stock J heads up, lapped the 1.88 and 1.60 valves in, put crane springs on to match the cam i picked out. The cam btw was the 464/494 222/234 114 lobe center cam. AWESOME cam. I used the stock rockers and shafts and pushrods. I also put in the matching crane roller timing set. I had Mopar electronic ignition with a stock IRON 340 intake and an 850 thermoquad and headers. I built a hot 904 with a STOCK converter behind it with 3.23 gears. that was a atock 8.5:1 motor....or whatever they were rated at in 71. firstime out on the track that car ran 13.99 spinnin like a ***** in church off the line. 2nd time around, I found a friend there who let me bolt on some extra slicks he had. that time the car went 12.72. LMAO that was the second time down the track! that car ran a best of 12.46. That motor had like 175 PSI cranking pressure, but was still only 8.5:1. Cylinder pressure is where it's at. If you have low cranking PSI, I'd recam it.
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Not to sound like a start-azz, but are you "sure" about those cranking pressure figures, i've done up a few stock bottom end 360s with J heads (un-milled) & NEVER saw those kind of #s, & i ran very mild cams on 2 of them, the only 360 that saw good #s was one i did useing KB190s (quench dome), it was .060" over, .040" gasket & un-milled J heads with the XE 284H cam, i had right around 175 pressure & those pistons stuck out of the block, did up 1 useing KB107s (if memory serves), they were FTs sitting down about .020" rated at 9.5.1 or abouts with stock Js, with a milder cam then yours it had 150-155 PSI, most stock 360s i've checked were right around 130-135 psi, thats with a stock cam.

Also, what kind of 60ft. times were you seeing with a "stock" converter & 323 gears, they would have to be good to pull off mid 12s.
 
Yup. I'm sure about the pressure. 60 ft times were in the 1.7 range. it was a stock factory high stall which would flash to almosr 3K because of the gear and slicks. normally, they flash to around 24-2700. I just hit on a good combination be complete chance. I'm trying to duplicate it with a big block scamp....lotsa luck there.
 
StrokerScamp said:
Yup. I'm sure about the pressure. 60 ft times were in the 1.7 range. it was a stock factory high stall which would flash to almosr 3K because of the gear and slicks. normally, they flash to around 24-2700. I just hit on a good combination be complete chance. I'm trying to duplicate it with a big block scamp....lotsa luck there.
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Ok, it was a factory high stall, the factory intake & TQ helped on those 60fts. no doubt.
 
If you can give the kit part numbers, or piston part number specifically that would help. It sounds low even for the parts which I think are mismatched so something's iffy. I would have expected another 360-380hp and 430-450 pound feet there with what you have. Shutting off early.
 
I have a very similar combo in my Duster. Eagle crank and rods, KB dished pistons, and Edel 65cc heads, 236/242 .541 hyd roller cam. My cylinder pressures were around 170 for all 8 cylinders. Static comp was at 9.6, dynamic was at 8.7 (cam is dropping it). Running through full 3" dual exhaust and using a Thermoquad on an M1, 18 initial 35 degrees all in. 4200 9.5" dynamic stall into Manual Valve Body 727, into Dana 60 3.73:1.

My chassis Dyno runs were 278 hp and 305 tq. (VERY similar to yours!)

I have a best time of 12.09 @ 110 on MT street tires (295-45-17's), That's with a 1.639 60' time, and 7.648 1/8 mile.

Idles in gear at 800 rpm with around 10 inches vacuum.

I was hoping for mid 11's, but I don't have enough compression for the cam and aluminum heads. I have read many opinions that suggest that due to the heat absorbing properties of Aluminum, you effectively "lose" 1 point of compression. Can't say if it is true, but I know I need more compression, regardless of cylinder pressures. To wit, I can run 87 octane on 100 degree days and no pinging, even at WOT up a hill!

You need to run it on the track to see what it can really do. Dynos a prone to error, IMHO, but good for before and after comparisons, if your using the same one.

For instance, one calculator online yields these results:

Pic05.jpg
 
I built a similar engine. You can't take advantage of the added stroke and 240 duration cam with stock heads. The compression is likely low too as mentioned already. I would think about adding better flowing heads with a smaller combustion chamber, zero decking and getting the compression up to 10.5 (with aluminum heads). The timing has to be right too around 34 degrees total.
 
My cam is 20-227-4
grind: xe275hl-10
intake: 231
exhaust:237
lift:525

I tried looking for my piston info, but came up with nothing.
 
Hmmm my 408(410)is an Eagle h-beam kit with Eddy heads with some P&P work.I have an M1 intake with a 850HO carb,my cam is custom grind solid roller valve lift is .631 [email protected] 252/258 and is [email protected],m running a 8 inch 4500 stall with 4.30 spool.Car is consistant at 11.20@118mph 60fts are 1.56 on a hot day.Your #,s just seem low,get a second opinion or go the cheaper route and take it to the track.Good luck.


Oh my car is a 66 Dart,I,m thinking weighs in around 3000lbs.
 
pettybludart said:
Hmmm my 408(410)is an Eagle h-beam kit with Eddy heads with some P&P work.I have an M1 intake with a 850HO carb,my cam is custom grind solid roller valve lift is .631 [email protected] 252/258 and is [email protected],m running a 8 inch 4500 stall with 4.30 spool.Car is consistant at 11.20@118mph 60fts are 1.56 on a hot day.Your #,s just seem low,get a second opinion or go the cheaper route and take it to the track.Good luck.


Oh my car is a 66 Dart,I,m thinking weighs in around 3000lbs.
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brag brag brag blah blah blah brag brag brag LMAO :cheers:
 
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