421 SBC - something holding it back at the track !

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SBC-68-FIREBIRD
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421 SBC - something holding it back at the track !

Post by SBC-68-FIREBIRD »

Hi Guys,

Will start from the beginning.
I have a 68 Firebird that I run a SBC in. The car is 3617lbs with me in it. Turbo 400 with a modified valve body. 12 bolt 3.73 posi. 275/50/15 MT drag radials. Bolt in subframe connectors. Caltracks. Calvert shocks front & rear. 1.75" 4 into 1 headers - 3" collector - 3" X pipe - 3" Hooker Aerochamber mufflers dumped in front of diff ( mandrel bend off the head then press bends down to the collectors ) .
It is a 95% street driven weekend toy that goes to the track a few times a year just for some fun here in Australia.

I had a 406 in the car -
GM block & crank, Eagle 5.7 rods, TRW flat tops. 10.7 comp ( this bottom end was built in the late 90's ).
AFR195 street heads out of the box. 550 lift. 1.5/1.5 rockers.
Comp Cams 242/248@50 HR on a 108LSA.
Super Victor match ported to heads & plenum ported.
850DP Holley.
Engine was never dyno tuned but had been on a chassis dyno a few times to get jetting right & it made 400RWHP.
I had a 9.5" x 4000 converter in the car & it ran consistant 11.0-11.1@ 121-122mph.

The engine was getting on so I had been collecting gear for a long time to build a new engine.

Dart Little M block.
3.875 & 6" Callies rotator with CP pistons. 4.155 bore so 421 cubes. 11.7 comp.
AFR210 Competition heads out of the box. 600 lift. 1.6/1.5 rockers.
Crane 246/250@50 HR on a 108LSA.
Super Victor match ported to heads & nice plenum porting.
950QFT carb.
Engine dyno'd @ 580HP @ 6000rpm & 550TQ @ 5200prm with 1 7/8" dyno shop pipes.

The converter was no longer right for this engine but gave it a run to see how it went.
It ran 1.60 x 60' - 11.0@122mph.

I ended up getting the stator in the 9.5" converter changed & got it up to 5000 stall.
Back to the track -
It picked up half a tenth in the 60' but nowhere else & lost mph. Went 1.56 x 60' - 10.92@121mph.

I now have an 8" 5000 converter in it & took it for a run last weekend.
60' - 1.51
330' - 4.422
660' - 6.909@98.62mph.
1000' - 9.074
1320' - 10.920@121.63mph. ( it was a very windy crappy day so there may be a little bit left in it yet ).
https://www.youtube.com/watch?v=tPt6aWZzwUo

I am finding it hard to accept that I have changed engines & the 421 should have been making quite a bit more power than the 406 & still run virtually identical times & added 1000rpm stall & picked up a tenth in the 60' but nowhere else. I feel like there is something holding it back & my thoughts are -
Press bent 1.75" primaries are maybe at their limit.
210 intake runners are too small for 421 cubes but it does not feel like it is nosing over at all.

Does anything stand out to you guys ?

Thanks.
Scott.
cardo0
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Re: 421 SBC - something holding it back at the track !

Post by cardo0 »

I think your ready for 2" primaries so it would be a custom set of headers for your car. But tube size determine the amount of your torque peak where tube length determines RPM torque peak. And Smokey Y. used to run 2" tubes on a lot less motor than yours.

I never liked air cleaners in the wind. Don't know what that does to the flow pattern in the carb. A scoop there would reduce the turbulence into the carb as it kinda becomes a high pressure area.

3600lbs is kinda high I think. My 4th gen camaro is like 3300lbs stock.

And for my last wild azz guess maybe a 2sp powerglide would have less power loss and lose weight to.

Just my notes so take them for what they're worth and please don't get flamed about them. I'm close to being the biggest amateur on the forum. :lol:
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Geoff2
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Re: 421 SBC - something holding it back at the track !

Post by Geoff2 »

It needs more cam to raise peak hp for the higher 5000 stall c'ter.
KnightEngines
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Re: 421 SBC - something holding it back at the track !

Post by KnightEngines »

Yep, needs more cam, lots more cam!
HR suck, bleed down at higher rpm, even with good lifters it'll flatline above 6500.
Stick a SR in it - don't be scared, just check your lash every 1000k's & swap out lifters at 5000 for peace of mind.
Something like 264/270 on 108 will get it humming.
Probably need 1 7/8" headers.
Also will want more carb - 950 will work.

It's a big motor, let it eat.
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Re: 421 SBC - something holding it back at the track !

Post by KnightEngines »

Ps - the crane .420 & .438 endurance racing lobes are a good bit more agressive that the street roller stuff but still pretty easy on the lifters. Use 1.6 ratio rockers, .438 intake lobe & .420 exhaust lobe.
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Re: 421 SBC - something holding it back at the track !

Post by JoePorting »

Both engines would have made more power with a solid roller cam. Bigger heads would help too.
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Re: 421 SBC - something holding it back at the track !

Post by CGT »

In my opinion your seeing what i would expect to see from, a few more inches, similiar cam, manifold, compression, similiar "style" of head.......a few mph.

If a SR cam, glide, headers etc help your new engine, they would likely have helped your old one also.
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Re: 421 SBC - something holding it back at the track !

Post by 77cruiser »

Peak hp at 6000, I would've expected higher. What were you shifting at?
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Re: 421 SBC - something holding it back at the track !

Post by bigmike »

What RPM is it running through the lights? Front end alignment and suspension been checked recently?
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Re: 421 SBC - something holding it back at the track !

Post by af2 »

Cam and what ever intake is the problem. A 355 I built runs the exact same 60' and runs 10.90 at 122 not the fastest but the cam is 270@ .050 and .600 with OLD heads and Intake built in 1980. 242 ? humm
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Re: 421 SBC - something holding it back at the track !

Post by prairiehotrodder »

I'm not an expert but it should have went faster with the new motor. I have dumped in fancy parts that went slower and it sucks. Are you confident in the tune up with the new motor ? Like the timing and jetting ? I don't see any big problems with the combo, of course if you put on bigger yet ,heads, cam ,headers etc... you will just be deeper in. I think it will want more gear, even the old motor would have liked more gear. The air cleaner may be a restriction. I would encourage you to try little things and sort out what you have before spending big money or taking things apart much.
Get some older racers to look under your hood and listen to what they say. The car is fairly heavy.
Brian

Edit : were the passes all through the full exhaust ? I've seen major restrictions in some mufflers. Also is your fuel system up to it ?
Last edited by prairiehotrodder on Sun Aug 26, 2018 5:39 pm, edited 1 time in total.
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77cruiser
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Re: 421 SBC - something holding it back at the track !

Post by 77cruiser »

What are the valve springs, does it have enough spring?
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Re: 421 SBC - something holding it back at the track !

Post by BrazilianZ28Camaro »

All good suggestions, listen to this guys OP.

But I'd would start with the basics. Fuel system. What fuel pump, line sizes , its a return style fuel system?

Catch tank installed?

Check your ignition and double check the tune, before do any changes. Put your car on a diet.

Looks like your car combo is well balanced for street some strip usage, but if you'd like to go faster, everything but the converter is on the small side.

Good luck
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
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Re: 421 SBC - something holding it back at the track !

Post by GARY C »

I think MCS, carb and cam.
Once you go over 400" most 23 degree heads are restrictive.
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SBC-68-FIREBIRD
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Re: 421 SBC - something holding it back at the track !

Post by SBC-68-FIREBIRD »

Hey Boys,
Thank you for taking the time to reply, much appreciated.

I should have maybe put some more info in my first post but it was already getting long enough to lose peoples attention 😂

When I built the car it had a plain 350 hood, I did not want a plain hood on the car but $ was getting tight so cut a hole in it to have the air cleaner sticking out to giving it something. I have thought about putting a scoop on it but have never seen any scoop on a Firebird hood that looks any good.
When I started collecting parts for the new engine the class I run in was an 11.00 cut off before a cage was needed. I was not & am still not going down that path. So the new engine was built very similar to the old one & I expected it might go 3-4 tenths quicker. The cut off for a roll cage has since been dropped down to 10.50 so just trying little things to work at the times.
My old engine was over 15 years old when I sold it, had many street miles & lots of 1/4 mile passes on it & I never touched it maintenance wise. I wanted the new engine to be the same, I drive it from home to the track, dont even open the hood & drive it back home.
I do understand I could have gone bigger heads, solid roller etc etc & made more power but then needed to change rear gears & probably toughen up the tranny, where does it all stop.
I do a lot of miles in this car & am not prepared to lose the streetability of it, I just want to get the best out of what I already have.
It runs 1/2" fuel lines from the tank to carby. It runs a Holley mechanical pump.
The valve spring were put on at AFR as an upgrade & then checked by engine builder & all okay.
I generally shift 1-2 @ 6000 & 2-3 @ 6500. It crosses the line at 62-6300.
There nothing I can do to reduce weight in the car, except maybe the driver 😂. Its a real nice car & am not prepared to start pulling stuff off it.

I do want to drop the exhaust off it & give it a run just to see if there is any difference.
I dont see the 1.75" crush bend headers will be doing me any favours but dont know how much if any gain there will be.
I am thinking there is probably only a tenth or two to be had with leaving the engine & gears the way they are.
I have not played with jets or timing in it yet. It is still how it come off the engine dyno.
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