how much HP loss from crank to rear axle
 

I'm not happy after my last Dyno. How much HP is losed from crank to the rear. I was thinking 15 -20%. I have a 75 that had a L48 / 355 mild cam alum intake and Holley
650 carb and headers with a 3:70 rear gear that posted 202HP at the rear. Currently It has a Blue{print 383 with 435 HP I changed the intake and replaced the carb to a 770 Holley. So I was thinking that I would lose about 20% (86 HP) and would have about 345-350 at the rear. Last week Dyno topped off at 286 WTF? I thought the carb might be too much and replaced the prim jets 2 sizes. it was over 6 grand to replace for only 84 more HP. I think I should have keeped the 355 put alum heads and bigger cam and maybe a bottle and what would have been about the same gains in HP. I don't know

Welcome to the world of dyno racing. I experienced the same reaction after putting a lot of work into my 454 to gain only about 100 hp. Your estimate of 15-20 percent is about what most agree is the loss from flywheel hp to rear wheel hp. Does the car feel faster? That would be one way to deal with the numbers.

IIRC a magazine did a '72 with a 425HP engine and got 285 at the wheels. I did a 383 that was similar to some 475 HP published builds and got 327 at the wheels of a '69 with a T56 in it.

I talk about this issue of Gross to Net to RWHP all the time and most folks do not like to hear the facts. There is a big difference first between Gross HP which is how most crate engines are rated including your 383 BP and Net HP which is how all cars from the factory are rated today since 1972, as well. The conversion from Gross HP to Net HP, not Gross to RHWP, is roughly a reduction of 10-15% on our cars with a GEN 1 engine and the drivetrain losses. The conversion from Net HP to RWHP is an additional 10-15 % reduction. Total conversion from gross to RWHP on theses cars is roughly 25-30%, not 10-15%.

A modern car like my 10C6Z06 has 505 NET HP and generally dynoes about 450 RWHP which is a 10% reduction from Net HP to RWHP but a modern engine and modern drivetrain with less frictional loses.

I would expect your BP 383 435 Gross HP to dyno on a dyno jet (not a mustang dyno) about 300-325 RWHP, Gross to RWHP. You are a bit low for a dyno jet but would be in the range for a mustang dyno whose numbers are even lower based on the estimate algorithms. I figure my L-82 355 with AFR heads, Roller cam and 10.2 compression at 425+ Gross HP (RWHP 325 roughly) and that number is no way even close to the 450 RWHP LS7 HP from my Z06 ...not even close performance and you can see why when you look at the numbers and the losses. above^^^^^^^^^^^

BTW-A properly built 355 with top flowing heads (AFR/Dart etc) aggressively moderate roller cam with 10+ compression can easily match a crate 383's Gross HP numbers(410-430 Gross HP)....A max built 383 525+ Gross HP 383 is a different story.

Its amazing isn't it, the amount lost from front to back. And yes, the norm is 17% loss. A little higher with the automatics. That's a whole lot of friction as soon as something is attached to the end of the crank.

Makes you wonder what those little foreign 40 horse cars of long ago were putting down to the ground.

If you have access to a 1/4 mile track you can get your max speed and time and back figure the HP to the wheels and it wont cost you a dyno trip, plus its alot of fun

280 something is in my estimation about 40 horse off. I would believe you should be in the 310-320 range. Personally I would do the following.

1. Install some O2 bungs in either the header collector or exhaust pipe. Install a temporary wide band O2 setup then you’ll know what your air/fuel ratio is. Should be around 12.5:1 at WOT. Can be around 13:1 cruising. Don’t guess.

2. Make sure your timing is advancing as it should. Sounds silly but higher RPM is where horsepower shows up. If your timing isn’t advancing enough at higher RPM it will fall on its face when it should be stretching its legs.

You didn't say what cam is in your 383 but I would consider changing that to get the HP you are looking for. I'm happy with the Mustang dyno results for my 355 but it doesn't come alive until 3500+ rpm which doesn't take long to get there. Pulls strong to 6200 rpm.

Thanks for all the feed back , I was just thinking out load. I'm comparing my 2007 Trailblazer AWD that has 395 HP on the same dyno and it pulls 334 HP .
As for test driving before and after I think that the 355 was a little bit faster in and out of traffic. I'm going to call BluePrint and see what info they can provide then a call to Holley.

when on the dyno we adjusted the timing to 34 degrees total and the air/fuel ratio was at 11 and was change to 12.
Thanks to all that replied

Here's the some spec as stated by BluePrint .
heads- 64cc,2.02 intake/1.60 exhaust valves & 195cc intake/75 cc exhaust runners
Cam- roller .528 intake/.536 exhust,221 intake/226 exhaust duration @.050-110 degree lobe

auto trans 3.70 rear gear

Its a nice torquey combo, not a hp monster
This is why I pay no attention to dyno #s or the guys that brag up their sheets... it means nothing except a before/after if you are tuning or changing things.
How does it run overall? What kind of tq is it putting down and where?

once had a car tuned on an old Clayton water brake..#s were horrible but for being basically stock with a cam/headers it could still dip into the 13s and felt great.

Thats a relatively small cam with lowish lift for a 383 IMO. Should be more in the 232 range or more with a good .550 lift on a 108 or 107 LSA with the 370 out back. Is it flat tappet?

My 350 ihas a 219/219 roller with .549 lift on a 108 LSA, so a 383 is easily capable of more duration and lift, especially with a 370 rear diff.

why use a 195 cc intake if it gonna tap out early due to the cam?

You mentioned that the cam was a roller and those lift numbers are very typical for a 383 crate engine and for Blueprint engine with .528/.536 lift, very common. The duration, as mentioned, is a little low for a 383 at 221/226 (typical duration for those motors to make strong Gross HP numbers is 230/236), which is more suited for peak mid range torque for a 355 than a 383. The BP 383 engines do publish a 430 Gross HP engine with your specs and 10:1 compression but the dyno numbers seem low. I would think to generate 430-450 Gross HP with that 383 motor you would need better flowing heads (than the BP brand) at least with that cam and slightly more compression. A .550 lift roller cam with 230/236 duration and the best flowing heads with 10.5 compression would get you closer to the higher Gross numbers.

Are you sure that your 383 has those specs?

Just wondering, was that on a Dyno Jet dyno? I does seem a bit on the low side and why did you replace the carb on a new motor that came that way? What trans? However, if you really want disappointing results, put it on a Dyno Dynamics dyno sometimes called the heart breaker. My tuner in Lake Havasu uses only a DD dyno and that's also what I learned on at EFI University tuning school. IMO, it is a more realistic results dyno instead of inflated numbers which can be influenced by an operator. Once you go on a DD and if you want more to brag about, put it on a Dyno jet. Which means I need to put mine on a DJ at my friends shop here and see the difference. I got 343 rwhp, but I only have two injectors. :hide:

Also, your WOT A/F is off to make power, too fat. You should shoot for at least 12.6 and go up from there to around 13-13.3 some like a little more. Feed the motor what it wants to run well, running fat does not make more power. Did you try 36* and see what it does? I run 36* total, all in at 3,500, pulls hard all the way to 6300. If you look at the graph, it carry's the power all the way out, but my cam is a bit larger than yours.

My .02 is that I would not buy an engine with any level of mods until I could run it on an engine dyno first. Not even a GM crate motor. So many builders out there that "estimate" HP numbers. I think the 15-18 percent loss is a good base line. Some what depends on auto or manual.
I was just at a car show and a guy had a 67 Nova with a 500 HP 383 small block. Never on a engine or chassis dyno but he was sure it was 500 at the rear wheels. I don't think so.
My small "LS1" in my C10 is a 10.5 compression engine with good AFR heads and small cam "216I/220E .590 lift did 380 rwhp. 14-16 inches of vacuum to run power brakes. T56 Manual trans. So about 450 at the crank?
It's a bummer that so many sbc are compared to the latest engines. Just isn't fair.
Do a chassis tune with a good shop and then go from there. If it runs good just drive it.

Don't forget that most Crank HP numbers are with pretty good open shop headers, I did the compare thing for fun on my 460. Crank 560hp, Dyno Jet 481 RWHP (manual trans and Hooker side pipe headers). I think the RWHP is optimistic at best, I hope to get to the strip in the next 2-3 weeks to see how it does compared to the tired 427/435hp from two years ago (14.06@97mph). That will be the true test for gain amount.

What are you running for exhaust, and air intake?

I suspect I have the same Blueprint engine (BP38313CTC1) in my 75 Vette with a 700R4 tranny, headers, 750 Holley and 3:73 rear. The chassis dyno sheet delivered with the engine showed 450/450.

For reference, I put it on a dyno before any significant tuning and got 310HP/323TQ. Afterwards i tuned out a rough stutter under load so I feel confident the numbers got much better but did not put back on the dyno.

Not sure what dyno it was but here is a link that shows the dyno and chart at the end.

That is huge amount of loss IMO even going through a 700r4 which is what I have. Too bad you couldn't have put it on an engine dyno first to see if their values are correct.

Obas, that's the same engine I have in my 75. mine was rated at 435/450 . I'm running a 400 trans along with a 3:70 rear, long tube headers along with EPS #2701 intake and a 770 Holley Street avenger car.
I rechecked the dyno sheet for both. my old mild 355 showed 202 max HP at speed=87.01 vs the 383 at 286 max HP at speed=101.80

Thanks to all that replied...John

As mentioned by jb I think our cars have a much higher drive train loss % than is typical. I remember from my C5 days that a GM power train engineer said the C5 was 13% for the M6 and 16% for the A4. If a C5 manual is 13% you can bet our cars are easily into the 20%+ range. Dig around on the 'net and you'll estimates up to 30%.