Jump to content
Ultimate Subaru Message Board

formula to estimate hp increase with boost inctease?

Recommended Posts

Anyone know roughly the horsepower increase per extra pound boost over std 6 psi for our ea82Ts?

Share this post

Link to post
Share on other sites

Dyno the car and let us know! In a non turbo car theory is basic. Atmospheric pressure at sea level is 14.79 psi. Add 7 psi of boost. Pressure is now 21.79 psi. So if at sea level say an Acura Integra GSR makes 180hp. Now we are adding an additional 7psi. Pressure is now 21.79. so by adding 7 psi, a 48.8% power increase is possible. Making the total 260hp. Doesn't always equal out but increasing pressure makes more power as long as there is cooling and tuning. So crank it and let us know when she blows!

Edited by wagons

Share this post

Link to post
Share on other sites

There should be some old threads about tweaking the turbos on the forum.

The big problem is the stock turbo system regularly cracked heads, had barely adequate cooling. 

Any power increase will trade away reliability further.

Share this post

Link to post
Share on other sites

aha, so if we knew the power rating of an EA82 with 7.7:1 comp ratio , guess 77 HP, add 6 pounds boost we might just touch the 113 HP ?


I'm just trying to work out fuel needs and think I have it worked out - but to find a thread in any forum by search ?? ....phhhffft !


I ran 12 pounds boost for years no problems. The dodgy factory clock display was turfed for an indepenently powered Volt metre to monitor the knock circuit. Hardly evered needed activation with the 109 octane fuel - propane


The fuel dispensing gear has both CFM and HP ratings. The components that generally go hand in hand and work well in NA applications might need a rethink with turbo


The smaller convertor, the J or Cobra is rated at  5 to 100 HP which has done well, in fact at its sweet spot with EA81 with spfi inlet manifold or twin Hitachis and their manifold and tidy cams working with a 125 gas carby/mixer A standard 113 HP EA82T is just outside the small converters delivery, so I upped it to an L converter that is rated from 50 to 325HP.


The usual match to the larger L converter is a 200 or 225 mixer both rated very closely to each other in HP and CFM and at 12 psi boost, 6500 rpm on an EAs 1791 cc works out just covered


The smaller 100 , 125 series mixer/carbs rated at 123HP and 197 CFM, the HP delivery should be fine for std boost but falls short on CFM at 6500 rpm


12 psi boost at 6500 rpm needs 376 CFM which is close enough to the larger 200 series mxer/carbs


With the turbo under bost and in , say a bit of a hurry, I would get some weird bog or flat spot if I asked too much of it and at this stage no idea if it was boost or fuel issue. I thought it may have been double fuelling, but it may have been a lean bog. I need to get engine in and monitored when this happens.


Dyno shops have had their chances with me over the years so the HP figures for you just wont be happening anytime soon. Dynos seem to cloud peoples minds. Once had a dyno shop to work out speedo accuracy and they got it ronw according to the highway patrol. Another reported that my engine build pinged all the time on the dyno, blasted the invoice with COMPRESSION TOO HIGH FOR STREET USE across it I got it home gently to find they confused clearly marked timing numbers, stuffed up points dwell setting ..not their first stuff up, so you'd go back hey?


Then when I got to assist with a dyno run the tech could not get a proper reading for some reason, later worked out than when he asked me to take it up to 100 - he meant km, the old beast was in miles .. so here I was doing 100 mph on the dyno...so, I should have asked, and he should have checked with me ...so dyno does funny things to people



The 200 mixer is rated at 175 HP so not gonna hit that with 12 psi boost


Boost pressure sort of does not work with std gas ratings worked out for NA stuff either as the 200 is rated at 280 CFM falling short by about 100 CFM if you use NA figures, yet it worked fine up to about the 380 CFM mark boosted.


Think was at first to use two smaller gas carby/mixers ( 140 HP/224CFM each in NA terms)  off the bigger L converter. 325HP


but now thinking the bigger mixer 200 could feed off two smaller converters


My tinkering is happening on the NA 8.9:1 EA82 twin port in prep for the EA82T


The gas mixer that was tuned on the turbo engine, slapped on NA gave perfect readings despite the comp ratio change and no hair dryer, so pretty impressed withold Impco technology. I do ahave a lean flat spot bog because the whole combo is a bit to big for the NA EA82. Considering Peel Instruments LBL for enrichment hence my question elsewhere if newer, EJ TPS fit and work on EA throttle bodies

Share this post

Link to post
Share on other sites

It's relatively easy to calculate. You really don't need a dyno. You can derive a fairly accurate HP number from the grams per second airflow through the MAF. It's called a volumetric efficiency calculation. You know the crank HP of a stock EA82T - 115 HP. Take your stock peak MAF reading in g/sec and the RPM it crosses at and work backward using 115 to arrive at the VE. Then using the modified MAF peak and RPM cross and work forward again from those numbers to arrive at a fairly accurate crank HP figure for your modification. Give some consideration that the VE will likely increase some at higher levels of forced induction. VE beyond 100% is not unusual for turbocharged engines. 


I did this exactly on my Trans Am and came up with a HP figure that was less than 1% off the factory rating of my engine (it tested at 190 compression across all cylinders). Then did this same calculation after changing to 1.6 rockers and calculated that I picked up 3.6 HP from the additional valve lift. Not a worthwhile upgrade on its own but I was doing valve stem seals and already had the Comp roller tip 1.6 rockers left over from another build. I did it partially just to do the VE calculations for fun. 



Edited by GeneralDisorder

Share this post

Link to post
Share on other sites

Jono, you are using propane yes meaning a different country than USA? GD good call on Volumetric Efficiency, I forgot that even though that was used for my base map on my old 400hp Nissan. Jono I wouldn't say dynos cloud people, people cloud people. A proper set up Dyno and TUNER will make a car great. A bad one will end up like my turbo Dodge Daytona, in smoke! I have a good tuner here in AZ, and won't go to anybody else.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now